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RELIABILITY | RESILIENCE | SECURITYNERC | Report Title | Report DateI

Reliability GuidelinePerformance, Modeling, and Simulations of BPS-Connected Battery Energy Storage System and Hybrid Power Plants

August 2020

Table of ContentsPreface.......................................................................................................................................................................... iv

Preamble.......................................................................................................................................................................v

Executive Summary......................................................................................................................................................vi

Introduction.................................................................................................................................................................vii

Fundamentals of Energy Storage Systems................................................................................................................ix

Fundamentals of Hybrid Plants with BESS..................................................................................................................x

Co-Located Resources versus Hybrid Resources..................................................................................................xiii

Chapter 1 : BPS-Connected BESS and Hybrid Plant Performance..................................................................................1

Recommended Performance and Considerations for BESS Facilities.........................................................................1

Topics with Minimal Differences between BESSs and Other Inverter-Based Resources........................................4

Capability Curve.....................................................................................................................................................5

Active Power-Frequency Control...........................................................................................................................6

Fast Frequency Response.......................................................................................................................................6

Reactive Power-Voltage Control (Normal Conditions and Small Disturbances).....................................................8

Inverter Current Injection during Fault Conditions................................................................................................9

Grid Forming..........................................................................................................................................................9

System Restoration and Blackstart Capability......................................................................................................10

State of Charge.....................................................................................................................................................10

Recommended Performance and Considerations for Hybrid Plants........................................................................13

Topics with Minimal Differences between AC-Coupled Hybrids and standalone BESS Resources.......................16

Capability Curve...................................................................................................................................................17

Active Power-Frequency Control.........................................................................................................................17

Fast Frequency Response.....................................................................................................................................17

Reactive Power-Voltage Control (Normal Conditions and Small Disturbances)...................................................18

System Restoration and Blackstart Capability......................................................................................................18

State of Charge.....................................................................................................................................................19

Operational Limits................................................................................................................................................19

Chapter 2 : BESS and Hybrid Plant Power Flow Modeling...........................................................................................21

BESS Power Flow Modeling.....................................................................................................................................21

Hybrid Power Flow Modeling..................................................................................................................................22

AC-Coupled Hybrid Plant Power Flow Modeling..................................................................................................22

DC-Coupled Hybrid Plant Power Flow Modeling..................................................................................................24

Chapter 3 : BESS and Hybrid Plant Dynamics Modeling..............................................................................................26

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Table of Contents

BESS Dynamics Modeling.........................................................................................................................................26

Model Invocation.................................................................................................................................................27

Scaling for the BESS Plant Size and Reactive Capability........................................................................................27

Reactive Power/Voltage Controls Options...........................................................................................................27

Active power control options...............................................................................................................................28

Current Limit Logic...............................................................................................................................................29

State of Charge.....................................................................................................................................................29

Representation of Voltage and Frequency Protection.........................................................................................29

Hybrid Plant Dynamics Modeling.............................................................................................................................30

AC-Coupled Hybrid Modeling...............................................................................................................................30

DC-Coupled Hybrid Modeling...............................................................................................................................31

Chapter 4 : BESS and Hybrid Plant Short Circuit Modeling..........................................................................................33

BESS Short Circuit Modeling....................................................................................................................................33

Hybrid Short Circuit Modeling.................................................................................................................................34

Chapter 5 : Studies for BESS and Hybrid Plants...........................................................................................................36

Interconnection Studies...........................................................................................................................................36

Transmission Planning Assessment Studies.............................................................................................................38

Other Studies...........................................................................................................................................................39

Appendix A : Relevant FERC Orders to BESSs and Hybrids......................................................................................43

Appendix B : BESS Dynamic Model Parameterization.............................................................................................47

Appendix C : Hybrid Plant Dynamic Model Parameterization.................................................................................49

Appendix D : References.........................................................................................................................................53

Appendix E : Example Hybrid Plant Configurations.................................................................................................54

Appendix F : Modeling Different Hybrid Plant Operations......................................................................................55

Contributors................................................................................................................................................................56

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Preface

Electricity is a key component of the fabric of modern society and the Electric Reliability Organization (ERO) Enterprise serves to strengthen that fabric. The vision for the ERO Enterprise, which is comprised of the North American Electric Reliability Corporation (NERC) and the six Regional Entities (REs), is a highly reliable and secure North American bulk power system (BPS). Our mission is to assure the effective and efficient reduction of risks to the reliability and security of the grid.

Reliability | Resilience | SecurityBecause nearly 400 million citizens in North America are counting on us

The North American BPS is divided into six RE boundaries as shown in the map and corresponding table below. The multicolored area denotes overlap as some load-serving entities participate in one Region while associated Transmission Owners/Operators participate in another.

MRO Midwest Reliability OrganizationNPCC Northeast Power Coordinating CouncilRF ReliabilityFirst

SERC SERC Reliability Corporation

Texas RE Texas Reliability Entity

WECC WECC

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Preamble

The NERC Reliability and Security Technical Committee (RSTC), through its subcommittees and working groups, develops and triennially reviews reliability guidelines in accordance with the procedures set forth in the RSTC Charter. Reliability guidelines include the collective experience, expertise, and judgment of the industry on matters that impact bulk power system (BPS) operations, planning, and security. Reliability guidelines provide key practices, guidance, and information on specific issues critical to promote and maintain a highly reliable and secure BPS. Each entity registered in the NERC compliance registry is responsible and accountable for maintaining reliability and compliance with applicable mandatory Reliability Standards. Reliability guidelines are not binding norms or parameters; however, NERC encourages entities to review, validate, adjust, and/or develop a program with the practices set forth in this guideline. Entities should review this guideline in detail and in conjunction with evaluations of their internal processes and procedures; these reviews could highlight that appropriate changes are needed, and these changes should be done with consideration of system design, configuration, and business practices.

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Executive Summary

Text

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Introduction

The BPS generation resource mix across North America is trending towards increasing amounts of variable and renewable energy resources, most of which are inverter-based resources. According to the U.S. Energy Information Administration (EIA) Annual Energy Outlook (AEO) 20201, the wind power capacity in the United States (U.S.) more than doubled in the past decade, from 39.6 GW in 2010 to 107.4 GW in 2019, and solar generation has increased significantly from 2010’s 2.7 GW to 2019’s 67.7 GW in the U.S. (25x). Wind and solar generation supplied nearly 7.2% and 2.7%, respectively, of U.S. electricity demand in 2019. The EIA and many other organizations have projected continued rapid growth of both wind and solar energy in the next several decades. Specifically, the EIA AEO 2020 projected fractions of 12.5% wind energy and 17.5% solar energy in 2050 in the U.S., which correspond to 205.7 and 449.8 GW of installed capacity, respectively. As projected, power electronic inverters that would be installed along with wind and solar generation would account for about 30% of the U.S. electricity need in 2050. To put this into perspective, today’s shares of hydro- and coal- based electricity in the U.S. are 7 and 24%, respectively.

This evolving generation mix at both the BPS and distribution system is posing new challenges for reliable operation of the BPS but also can have significant benefits to BPS planning, operations, and design. One of the primary challenges with high penetrations of inverter-based resources is that most of these resources are variable in nature (i.e., renewable energy resources), leading to uncertainty in the planning and operations horizons. The need for flexibility services coupled with improving economics has led to an influx of requests for interconnection of BPS-connected energy storage resources and hybrid power plants utilizing energy storage.2

Areas across North America are seeking low- or zero-carbon power systems in the future. For example, California requires3 by the end of 2045 that eligible renewable energy resources and zero-carbon resources supply 100% of retail sales of electricity to California end-use customers and 100% of electricity procured to serve all state agencies. Further, the bill requires that this be achieved without increasing carbon emissions elsewhere in the Western Interconnection and not through resource shuffling. As such, the California Public Utilities Commission (CPUC) has seen a surge of new energy storage contracts and achieved its 2020 1,325MW energy storage goal ahead of time4, with projected 55,000 MW of new storage by 20455. The CPUC has also approved budget allocation for energy storage in their Self-Generation Incentive Program.6 Further, with the breadth and extent of wildfires in the region, California is planning for a resilient and flexible future grid in the face of rapid evolutions on the BPS. Energy storage systems, both BPS-connected and distribution-connected, are being considered as one of many solutions to help improve reliability and resilience of the BPS in the face of potential power shutoffs and other extreme contingency events. BESSs and hybrid plants offer short-term energy delivery as well as flexibility services such as ramping and variability control, voltage and frequency regulation, and other features. While battery technology is not the only form of energy storage, battery energy storage systems (BESSs) are the predominant type of energy storage resource seeking interconnection to the BPS; therefore, this guideline focuses primarily on BPS-connected BESSs.

Battery energy storage facilities have historically made up an immaterial amount of capacity connected to the BPS; however, today, requests and construction of these facilities have scaled up to

1 U.S. Energy Information Administration (EIA), “Annual Energy Outlook 2020 with projections to 2050,” Jan. 2020. [Online]. Available: https://www.eia.gov/outlooks/aeo/pdf/aeo2020.pdf. 2 Hybrid plants combine multiple technologies of generation and energy storage at the same facility, enabling benefits to both the plant and to the BPS. The majority of newly interconnecting hybrid resources are a combination of renewable energy and battery energy storage.3 California Senate Bill No. 100: https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml?bill_id=201720180SB100.4 https://www.cpuc.ca.gov/General.aspx?id=3462 . 5 Phil Pettingill, “Ensuring RA in Future High VG Scenarios – A View from CA”, ESIG Spring Workshop. April 10, 2020. 6 https://www.cpuc.ca.gov/General.aspx?id=5935

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Figure I.1: Review of CAISO Interconnection Queue for Hybrid

Resources and BESSs

Ryan Quint, 07/07/20,

Make additional nod to these being IBRs, and need for guidance on performance, modeling, studies for industry clarity.

Huang, Zhenyu (Henry), 08/16/20,

Added some wind/solar background information. Hope it is useful.

https://www.cpuc.ca.gov/General.aspx?id=5935

https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml?bill_id=201720180SB100

https://www.eia.gov/outlooks/aeo/pdf/aeo2020.pdf

: BPS-Connected BESS and Hybrid Plant Performance

match solar PV and wind plant sizes. For example, the Gateway Project being constructed and interconnected to the San Diego Gas and Electric (SDG&E) area consists of a 250 MW BESS providing energy and ancillary services in the California Independent System Operator (CAISO) market.7 The Stanton Energy Reliability Center is an example of a hybrid power plant, consisting of two General Electric (GE) LM6000 natural gas combustion turbines (with synchronous condenser capability) combined with a BESS component, totaling 98 MW of capacity. 8 Southern California Edison currently has several hundred megawatts of BESSs deployed and in their interconnection queue.9 Many more of these types of requests have been observed in transmission service providers’ interconnection queues across North America and will need to be studied as part of the interconnection study process, similar to all interconnecting resources to the BPS. Figure I.1 shows a cursory review of the CAISO interconnection queue, where most new interconnection requests are either stand-alone BESSs or hybrid plants consisting mainly of wind or solar PV combined with a BESS component. Refer to Appendix C for links to other transmission service providers’ generation interconnection queues.

Energy storage is now commonly being applied to existing and planned renewable energy resources for a multitude of resources. The Energy Information Agency (EIA) has tracked the existing and proposed amounts of hybrid and co-located power plants where energy storage is being applied, as illustrated in Figure I.2. These plots demonstrate that gigawatts of hybrid or co-located plants already exist and that the penetration of these types of resources is likely to increase rapidly in the coming years. Further, EIA details that currently “more than 90% of the total operating hybrid capacity (renewable generator plus energy storage) in the country is located in just 9 states. Texas alone holds 46% of the current total…Although nearly 25% of the total U.S. battery capacity is installed as part of a hybrid system, only 1% of total wind capacity and 2% of total solar capacity is part of a hybrid system.” The planned BESS installations are becoming increasingly larger as the cost of energy storage becomes cheaper.10

7 https://ww2.energy.ca.gov/sitingcases/stanton/8 https://www.eia.gov/todayinenergy/detail.php?id=437759 https://cdn.misoenergy.org/20190109%20PAC%20Item%2003c%20Storage%20as%20a%20Transmission%20Asset%20Phase%20I%20Proposal%20(PAC%20004)307822.pdf10 There are other types of reliability studies such as harmonics and electromagnetic transient (EMT) simulations that are often performed during the interconnection study process; however, those studies are outside the scope of this guideline.

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https://cdn.misoenergy.org/20190109%20PAC%20Item%2003c%20Storage%20as%20a%20Transmission%20Asset%20Phase%20I%20Proposal%20(PAC%20004)307822.pdf

https://cdn.misoenergy.org/20190109%20PAC%20Item%2003c%20Storage%20as%20a%20Transmission%20Asset%20Phase%20I%20Proposal%20(PAC%20004)307822.pdf

https://www.eia.gov/todayinenergy/detail.php?id=43775

https://ww2.energy.ca.gov/sitingcases/stanton/


Figure I.2: Existing and Proposed Renewable Plants with Energy Storage Capacity [Source: EIA]

Generation interconnection queues are often inundated with requests of all types, and many of the queue requests are often withdrawn over time as the study process proceeds. However, TPs and PCs need the capabilities to model and study these resources per the interconnection study process. While BESSs have been primarily proposed for energy arbitrage and mitigating renewable resource variability, there has been more recent interest in installing BESSs for providing transmission services such as voltage support under “storage as transmission facility”11 programs. Therefore, it is imperative to have clear guidance on how BESSs and hybrid power plants should perform when connected to the BPS, and also to have recommended practices for modeling and studying BESSs and hybrid power plants for power flow, stability, and short-circuit studies.12 These topics are the focus of this guideline.

Fundamentals of Energy Storage SystemsEnergy storage can take many different forms, and some are synchronously connected to the grid while others are connected through a power electronics interface (i.e., inverter-based). Examples of different energy storage technologies include, but are not limited to, the following:13

Battery Energy Storage: There are many types of battery energy storage systems (BESSs) – lithium-ion, nickel-cadmium, sodium sulfur, redox flow, and other types of batteries. 14 Batteries convert stored chemical energy to dc electrical energy, and vice versa. AC-to-DC power electronic converters are used to connect the battery to the ac power grid.

Pumped Hydroelectric Storage: Pumped hydroelectric power is one of the most mature and commonly used large-scale electric storage technologies today. Water flowing through a hydroelectric turbine-generator produces electric energy to be used on the BPS. Energy is then stored by sending the water back to the upper reservoir through a pump. These resources can quickly and efficiently balance generation and load, respond to price signals, and provide other essential reliability services.

Mechanical Energy Storage: Mechanical systems store kinetic or gravitational energy for later use as electric energy. An example of mechanical energy storage includes flywheels that accelerate a rotor to very high speed and maintain rotational energy using the inertia of the flywheel, which can then be delivered to the grid when needed.

11 https://energystorage.org/why-energy-storage/technologies/solid-electrode-batteries/12 https://energystorage.org/why-energy-storage/technologies/hydrogen-energy-storage/13 https://www.siemensgamesa.com/products-and-services/hybrid-and-storage/thermal-energy-storage-with-etes14 Note that hybrid natural gas-BESS plants may be desirable in some areas where capacity shortages have been identified.

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https://www.siemensgamesa.com/products-and-services/hybrid-and-storage/thermal-energy-storage-with-etes

https://energystorage.org/why-energy-storage/technologies/hydrogen-energy-storage/

https://energystorage.org/why-energy-storage/technologies/solid-electrode-batteries/


Hydrogen Energy Storage: Hydrogen energy storage involves the separation of hydrogen from some precursor material such as water or natural gas and storage of the hydrogen in vessels ranging from pressurized containers to underground salt caverns for later use. The stored energy can then be re-electrified in fuel cells or burned in combined-cycle power plants.15

Thermal Energy Storage: Thermal energy storage involves heating or cooling a material with a high heat capacity and recovering the energy later using the thermal gradient between the thermal storage medium and the ambit conditions. For example, electric energy could be used to heat volcanic stones, which can then be converted back to electric energy using a steam turbine. 16 Concentrated solar plants use molten salt as thermal storage medium and steam turbines to convert heat to electric energy.

Compressed Air Energy Storage: Compressed air storage uses either a geological feature or other facility to store pressurized air by converting from electrical energy. Energy can be delivered back to the grid at a later time by heating the air and sending it through a turbine to generate power. Compressed air storage facilities can be scaled up to large utility-sized facilities with minimal losses.

Supercapacitors: Supercapacitors are high-power electrostatic devices with fast charging and discharging capability (order of 1-10 seconds) and low energy density. There are no chemical reactions occurring during charging and discharging, which can result in low maintenance costs, long lifetimes, and high efficiency. These devices are scalable; however, their fast response can generally not be sustained due to the low energy density.

There are multiple benefits of energy storage systems connected to the BPS including, but not limited to, the following:

Providing balancing and fast-ramping services to help manage variable energy resources on the BPS and distribution systems

Mitigating transmission congestion

Enabling energy arbitrage to charge during low price periods (i.e., middle of the day with excess solar PV generation) and discharge during high price periods (i.e., evening peak demand conditions)

Providing essential reliability service such as frequency response and dynamic voltage support

Each of the energy storage technologies described can provide benefits to BPS reliability and resilience. As we focus on BESS, the interaction between the battery energy storage device and the electrical grid is dominated by the power electronics interface at the inverter-level and plant controller level, specifically on small time scales (from microseconds to tens of seconds to minutes). This is the primary focus of this guideline, and how industry can model and study specifically BESSs connecting to the BPS.

Fundamentals of Hybrid Plants with BESSHybrid power plants are also becoming increasingly popular due to federal incentives, cost savings, flexibility, and higher energy production by sharing land, infrastructure, and maintenance services. Hybrid power plants (“hybrid resources”) are defined here as:

Hybrid Power Plant (Hybrid Resource): A generating resource that is comprised of multiple generation or

15 ERCOT has drafted a concept paper specifically on DC-coupled resources, which may be a useful reference: http://www.ercot.com/content/wcm/key_documents_lists/191191/KTC_11_DC_Coupled_2-24-20.docx16 In ERCOT, a BESS was added to a quick-start combustion turbine for participation in ERCOT’s Responsive Reserve Service. The combustion turbine is normally offline, and if frequency falls outside of a pre-defined deadband, the BESS will provide fast frequency response until the combustion turbine is turned on to sustain the provided response.

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Key Takeaway:Hybrid power plants are comprised of multiple generation or energy storage technologies controlled by a single entity and operated as a single resource behind a single point of interconnection (POI). The most predominant type of hybrid power plant observed in interconnection queues across North America is the combination of renewable energy (solar PV or wind) and battery energy storage technologies.

http://www.ercot.com/content/wcm/key_documents_lists/191191/KTC_11_DC_Coupled_2-24-20.docx


energy storage technologies controlled as a single entity and operated as a single resource behind a single point of interconnection (POI).

There are many types of hybrid power plants that combine synchronous generation, inverter-based generation, and energy storage systems;17 however, the most predominant type of hybrid power plant observed in interconnection queues across North America is the combination of renewable energy (solar PV or wind) and battery energy storage technologies.18 Due to this fact, this guideline focuses primarily on hybrid plants combining renewable (specifically inverter-based) generation with BESS technology.

The conversion of direct current (dc) to alternating current (ac) occurs at the power electronics interface. However, how this conversion occurs within a hybrid plant impacts how the resource interacts with the BPS, how and if it can provide essential reliability services, how it is modeled, and how it is studied. Hybrid plants can be classified as either of the following:

AC-Coupled Hybrid Plants: An ac-coupled hybrid power plant couples each form of generation or storage at a common collection bus after it has been converted from dc to ac at each individual inverter. Figure I.3 shows a simple illustration of one possible configuration of an ac-coupled hybrid power plant where a BESS is coupled with a solar PV or wind power plant on the ac side. The BESS may be charged either from the renewable generating component or from the BPS, if appropriate contracts and rates are available.

DC-Coupled Hybrid Plants: A dc-coupled hybrid power plant couples both sources at a dc bus that is tied to the grid via a dc-ac inverter. There are often dc-dc converters between the individual units and the common dc collection bus. Figure I.4 shows a simple illustration of one possible configuration of a dc-coupled hybrid power plant, where the energy storage component is coupled to each individual inverter on the dc side. The inverter can be unidirectional where the BESS can only be charged from the renewable resource or bi-directional where the BESS can also be charged from the BPS (depending on interconnection requirements and agreements).19

Figure I.3: Illustration of AC-Coupled Hybrid Plant

17 https://www.nerc.com/comm/PC_Reliability_Guidelines_DL/Inverter-Based_Resource_Performance_Guideline.pdf18 The capability curve is almost symmetrical because when the BESS is operated in the second and third quadrant (consuming active power), a rise in dc voltage could limit the amount of power generation where reactive power also has to be consumed.19 NERC, “Fast Frequency Response Concepts and Bulk Power System Reliability Needs,” March 2020: https://www.nerc.com/comm/PC/InverterBased%20Resource%20Performance%20Task%20Force%20IRPT/Fast_Frequency_Response_Concepts_and_BPS_Reliability_Needs_White_Paper.pdf

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Put this in executive summary.

https://www.nerc.com/comm/PC/InverterBased%20Resource%20Performance%20Task%20Force%20IRPT/Fast_Frequency_Response_Concepts_and_BPS_Reliability_Needs_White_Paper.pdf


https://www.nerc.com/comm/PC_Reliability_Guidelines_DL/Inverter-Based_Resource_Performance_Guideline.pdf


Figure I.4: Illustration of DC-Coupled Hybrid Plant

Different technologies may deploy ac- and dc-coupled systems for different reasons. For example, it may be economical for a solar PV and BESS system to be coupled on the dc-side whereas it may be more cost effective for wind turbine generators to be coupled with a BESS on the ac-side. Each newly interconnecting hybrid will have its reasons for using ac- or dc-coupled technology, which ultimately comes down to which configuration provides the most value for the given installation.

Hybrid plants combine many of the benefits of stand-alone BESSs with renewable energy generating resources, including but not limited to the following:20

Cost Efficiencies: Co-locating different technologies at the same location enables a developer to save on shared electrical, controls, and communications equipment; simplified siting; shared personnel; improved maintenance schedules; and other relevant operational costs.

Energy Arbitrage: The storage element in a hybrid plant can be used to charge during low-priced hours and discharge during high-priced hours, shifting energy production to those hours where energy is needed. Current arbitrage for hybrids (and BESSs) is on the order of hours and days; future technologies may be able to further shift energy storage and production based on system needs.

Excess Energy Harvesting: Hybrid plants have the added benefit of being able to capture any excess solar or wind production that would otherwise be lost or “clipped” (e.g., due to curtailment or oversizing of PV panels compared to inverter size). Capturing excess energy increases plant capacity factor, enabling it to continue operating when the generating resource output decreases.

Frequency Response Capability: Most renewable energy resources (i.e., wind and solar PV) operate at maximum available power since there are presently little to no market-based mechanisms to incentivize their operation at a reduced power output. Therefore, these resources typically do not have the operational ability to increase power output during underfrequency conditions. Adding energy storage to a renewable facility can enable the ability to respond to underfrequency events while still operating the renewable component at maximum available power. Addition of storage to a synchronous generator may allow the hybrid plant to deliver fast frequency response.21 The energy storage component can initially charge or discharge rapidly while the synchronous generator turbine-governor provides a longer-term sustained response.

Reduce Generating Fleet Variability: As higher penetrations of renewable energy resources enter the BPS, higher levels of uncertainty and variability are occurring. This requires additional flexible resources able to

20 https://www.nrel.gov/docs/fy19osti/74426.pdf21 https://www.nerc.com/comm/PC_Reliability_Guidelines_DL/Item_4a._Integrating%20_Inverter-Based_Resources_into_Low_Short_Circuit_Strength_Systems_-_2017-11-08-FINAL.pdf

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https://www.nerc.com/comm/PC_Reliability_Guidelines_DL/Item_4a._Integrating%20_Inverter-Based_Resources_into_Low_Short_Circuit_Strength_Systems_-_2017-11-08-FINAL.pdf

https://www.nerc.com/comm/PC_Reliability_Guidelines_DL/Item_4a._Integrating%20_Inverter-Based_Resources_into_Low_Short_Circuit_Strength_Systems_-_2017-11-08-FINAL.pdf

https://www.nrel.gov/docs/fy19osti/74426.pdf


quickly ramp to meet sudden changes in generation output. Hybrid plants, with the BESS component, can help manage these quick ramps in the short-term, if needed.

Co-Located Resources versus Hybrid ResourcesAs described above, a hybrid power plant is “a single generating resource comprised of multiple generation or storage technologies controlled as a single entity and operated as a single resource behind a single POI.” Similarly, some transmission entities22 are differentiating co-located power plants from hybrid plants due to their key differences. Co-located power plants can be defined as:

Co-Located Power Plants (Co-Located Resources): Two or more generation or storage resources that are operated and controlled as separate entities yet are connected behind a single point of interconnection.

The key difference here is that the units are operated independently from one another even though they may be electrically connected identically to a hybrid resource. This distinction is important when considering how and when these resources will operate, as well as how to model and study these resources in planning assessments.

22 https://www.nerc.com/comm/PC/InverterBased%20Resource%20Performance%20Task%20Force%20IRPT/Fast_Frequency_Response_Concepts_and_BPS_Reliability_Needs_White_Paper.pdf

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Chapter 1: BPS-Connected BESS and Hybrid Plant Performance

BESSs and hybrid plants have similar recommended performance to other BPS-connected inverter-based resources (e.g., wind and solar PV plants). However, there are unique operational and technological differences that need to be considered when describing the recommended performance for these facilities. The NERC Reliability Guideline: BPS-Connected Inverter-Based Resource Performance23 provided a foundation of recommended performance for BPS-connected inverter-based resources, including BESSs and hybrid plants; however, it did not go into the technical details for these resources. This chapter describes in more depth the specific technological considerations that should be made when describing the recommended performance for these resources.

Recommended Performance and Considerations for BESS FacilitiesTable 1.1 provides an overview of the considerations that should be made when describing the recommended performance of BESS facilities compared with other BPS-connected inverter-based generating resources. The following sub-section elaborates on these high-level considerations in more detail.

Table 1.1: High Level Considerations for BESS PerformanceCategory Comparison with BPS-Connected Inverter-Based Generators

Momentary CessationNo significant differences from other BPS-connected inverter-based generating resources; momentary cessation should not be used to greatest possible extent24 during charging and discharging operation.

Phase Jump Immunity No significant difference from other BPS-connected inverter-based generating resources.

Capability Curve

The capability curve of a BESS extends into both the charging and discharging regions to create a four-quadrant capability curve. The shape of many individual BESS inverter capability curves are almost25 symmetrical for charging and discharging. From an overall plant-level perspective, the capability curves may be asymmetrical.

Active Power-Frequency Controls

Active power-frequency controls can be extended to the charging region of operation for BESSs. The conventional droop characteristic can be used in both discharging and charging modes. Further, a droop gain and symmetrical or asymmetrical deadband can be used. Active power-frequency controls should seamlessly transition between charging and discharging (i.e., there should not be a deadband in the power control loop for this transition), unless interconnection requirements or market rules preclude such operation. As with all resources, speed of response26 of active power-frequency control to support the BPS should be coordinated with system needs. The fast response of BESSs to frequency deviations can provide reliability benefits. Consistent with FERC Order 842, there should be no requirement for BESS resources to provide frequency response if the SOC or set point is very low or very high, though that service can be procured by the BA.

23 https://www.nrel.gov/docs/fy19osti/74426.pdf24 Unless there is an equipment limitation or a need for momentary cessation to maintain system stability. The former has to be communicated by the GO to the TP while the latter has to be validated by extensive studies.25 https://www.nrel.gov/docs/fy19osti/74426.pdf26 https://www.nerc.com/comm/PC_Reliability_Guidelines_DL/Inverter-Based_Resource_Performance_Guideline.pdf

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Ramasubramanian, Deepak, 08/12/20,

This is a relative term. Would it be possible to define ‘very low’ say in terms of a percentage such as 10% SOC?

Matevosyan, Julia, 08/13/20,

From Sid Pant: I haven’t found it so far, but was wondering if BESS (as a system, not just battery + converter) is defined or described in this document, except by implication? I think by BESS we mean the battery storage, the converters, any transformers, etc., in other words, the full Battery Energy Storage plant similar to how IBRs are described. Is such a description required? BESSs and Hybrid plants are both IBRs






Fast Frequency Response (FFR)

BESSs are well-positioned for providing FFR to systems with high rate-of-change-of-frequency (ROCOF) due to not having any rotational components (similar to a solar PV facility). The need for FFR is based on each specific Interconnection’s need.27 Sustained forms of FFR help arrest fast frequency excursions but also help overall frequency control. BESSs are likely to be able to provide sustained FFR within their state of charge constraints.

Reactive Power-Voltage Control

BESSs have the capability to provide dynamic voltage control during both discharging and charging operations, and this capability should be employed to support BPS voltage control during normal and abnormal conditions on the BPS. TOPs should provide a voltage schedule (i.e., a voltage set point and tolerance) to the BESS that can apply to both operating modes.

Reactive Current-Voltage Control

No significant difference from other BPS-connected inverter-based generating resources. BESSs can be configured to provide dynamic voltage support during large disturbances both while charging and discharging.

Reactive Power at No Active Power Output

No significant difference from other BPS-connected inverter-based generating resources.

Inverter Current Injection during Fault Conditions

BESSs should be configured to provide fault current contribution during large disturbance events that can support legacy BPS protection and BPS stability. 28 Inverter limits will need to be met, as with all inverter-based resources; however, state of charge may not be an issue for providing fault current for BESSs since faults are typically cleared in fractions of a second. Regardless, state of charge limits can still apply and restrict current injection, as needed based on the BESS operating point. Additionally, limits on dc voltage magnitude can apply.

Return to Service Following Tripping

No significant difference from other BPS-connected inverter-based generating resources. BESSs should return to service following any tripping or other off-line operation by operating at the origin (no significant exchange of active or reactive power with the BPS), and then ramp back to the expected set point values, as applicable. This is a function of settings and any requirements set forth by the BA (or TO in their interconnection requirements).

Balancing

No significant difference from other BPS-connected inverter-based generating resources. BAs, TPs, PCs, ISO/RTOs, and other applicable entities will need to understand what services are being provided from BESSs; however, the capability to provide balancing for the BPS should be available from BESSs.

Monitoring No significant difference from other BPS-connected inverter-based generating resources.

27 For example, PJM has requirements for blackstart resources to be operational for 16 hours: http://www.pjm.com/-/media/markets-ops/ancillary/black-start-service/pjm-2018-rto-wide-black-start-rfp.ashx?la=en28 In addition to any requirements imposed by the TO or BA regarding acceptable charging behavior, the structure of investment tax credits may also contribute to the charging characteristic. For example, currently a hybrid plant may need charge the BESS by renewable energy for more than 75% of the time for the first five years of commercial operation, and the tax credit value for the storage component is derated in proportion to the amount of grid charging between 0% and 25%.


Patel, Manish, 08/14/20,

Conflicting statements?We need to get clarification from OEMs and ensure that this matches with chapter 4. I have submitted a draft for chapter to Ryan in a separate email.


Should we say there is no significant difference, except that BESS can operate in both discharging and charging modes. Similar to the next item – reactive current-voltage control.

http://www.pjm.com/-/media/markets-ops/ancillary/black-start-service/pjm-2018-rto-wide-black-start-rfp.ashx?la=en



Operation in Low Short-Circuit Strength Systems


Grid Forming (defined below)

BESSs have the unique capabilities to effectively deploy grid forming technology to help improve BPS reliability in the future of high penetration of inverter-based resources. Key aspects that enable this functionality include availability of an energy buffer to be deployed for imbalances in generation and load, low communication latency between different layers of controllers, and robust dc voltage that enables synthesis of an ac voltage for a wide variety of system conditions. In grids where system strength and other stability issues are of concern, BESSs may be required or incentivized to have this capability to support reliable operation of the BPS.

Fault Ride-Through Capability

No significant difference from other BPS-connected inverter-based generating resources. BESSs should have the same capability to ride through fault events on the BPS, when point of measurement (POM) voltage is within the curves specified in the latest effective version of PRC-024. This applies to both charging and discharging modes; unexpected tripping of generation or load resources on the BPS will degrade system stability and adversely impact BPS reliability. Ride-through capability is a fundamental need for all BPS-connected resources such that planning studies can identify any expected risks. However, the behavior during ride-through while discharging and charging may be different.

System Restoration and Blackstart Capability

BESSs may have the ability to form and sustain their own electrical island if they are part of a blackstart cranking path. This may require new controls topologies or modifications to settings that enable this functionality. Blackstart conditions may cause large power and voltage swings that must be reliably controlled and withstood by all blackstart resources (i.e., operation under low short circuit grid conditions). For BESSs to operate as a blackstart resource, assurance of energy availability as well as designed energy rating that ensures energy availability for the entire period of restoration activities. At this time, it is likely that most legacy BESSs can support system restoration activities as a stand-alone resource; however, they may be used to enable start-up of subsequent solar PV, wind, or synchronous machine plants.

Protection Settings No significant difference from other BPS-connected inverter-based generating resources.

State of Charge (new)

The state of charge (SOC) of a BESS affects the ability of the BESS to provide energy or other essential reliability services to the BPS at any given time.29 In many cases, the BESS may have SOC limits that are tighter than 0–100% for battery lifespan and other equipment and performance considerations. SOC limits affect the ability of the BESS to operate as expected, and any SOC limits will override any other ability of the BESS to provide essential reliability services (ERSs) or energy to the BPS. These limits and how they affect BESS operation should be defined by the BESS owner and provided to the BA, TOP, RC, TP and PC.

29 See References section of this document for more details.



Made changes for consistency with Gary’s comments in the text explanations below and in the hybrid table (since same is applicable to hybrids and BESS with regard to black start).


Sid Pant modified based on the above comments


I think it should be an optional functionality which should be available if required. I don’t think all BPS connected IBRs (including BESS) should mandatorily have this capability.

Srinivasan, Lakshmi (US), 08/11/20,

I do not think this should be a requirement. Merely an allowed functionality.


Is this requirement going to be for every BPS connected inverter based resource? If so, is every synchronous machine plant in the system also going to be designated as a blackstart resource?


From Sid Pant

Maysam Radvar, 08/14/20,

I agree with Manish. I believe we shall make it clear here that PRC-024 provides no-trip zones for both voltage and frequency and those boundaries are slightly different in various entities.


We all already know this. Boundaries in PRC-024 are not ride-through curves, instead, represent voltage and frequency trip setting boundaries. The P2800 is working to put together true voltage ride-through curve and are wider than PRC-024 boundaries. Should we recognize these two points here?


Does the edit help?


From Sid Pant: Is this a necessary requirement? If so, we should clarify – round trip between what points.



Oscillation Damping Support

BESSs can have the capability of providing damping support similarly to synchronous gnerators and HVDC/FACTS facilities. BPS-connected IBGs could also provide damping suport. A major difference than BPS-connnected IBGs is that BESSs can operate in the charging mode in addition to the discharging mode, which provide greater capabilities of damping support.

Topics with Minimal Differences between BESSs and Other Inverter-Based ResourcesThe following topics have minimal difference between the recommended performance of BESSs and other BPS-connected inverter-based resources:

Momentary Cessation: BESSs should not use momentary cessation as a form of large disturbance behavior when connected to the BPS. Any existing BESSs using momentary cessation should eliminate its use to the extent possible and its use for newly interconnecting BESSs should be disallowed by TOs in their interconnection requirements. Sufficiently fast dynamic active and reactive current controls are more suitable.30 If voltage at the POM is outside the curves specified in the latest effective version of PRC-024, then momentary cessation may be used to avoid tripping of the BESS. However, inside the curves, momentary cessation should not be used, subject to equipment limitation exemptions allowed under PRC-024-3. This recommendation applies for both charging and discharging operation.

Phase Jump Immunity: BESSs should be able to withstand all expected phase jumps on the BPS during both charging and discharging operation.

Reactive Current-Voltage Control (Large Disturbances): Fundamentally, there are no significant differences between BESSs and other BPS-connected inverter-based resources with respect to reactive current-voltage control during large disturbances. During discharging and charging, the BESS inverters should maintain inverter stability, adhere to inverter current limits, and provide fast dynamic response to support fault current during BPS fault events. Transitions from charging to discharging during large disturbances should not impede the BESS from dynamically supporting BPS voltage and reactive current injection. Studies should ensure stable performance for charging and discharging.

Reactive Power at No Active Power Output: BESSs should have ability to provide dynamic reactive power to support BPS voltage while not discharging or charging active power. This is one of the benefits of inverter-based technology and can be utilized by grid operators to help regulate BPS voltages, with appropriate cost-based compensation. Every BESS should have the capability to perform such operation, and the actual use of such capability should be coordinated with the TOP and RC regarding any voltage regulation requirements and scheduled voltage ranges.

Return to Service Following Tripping: A BESS should adhere to any requirements set forth by its respective BA. In general, BESSs should return to service starting at its origin point on the capability curve (i.e., operation at no active or reactive power loading), and then ramp back to its expected operating point based on recommendations or requirements provided by the BA.

Balancing: The capability to provide balancing services to the BA for the purposes of ensuring BPS reliability should be available from all BESSs. BAs, TPs, PCs, and other applicable entities should understand what services are being provided from BESSs. Those services are typically coordinated through ancillary services markets or dispatch.

30 Some BESSs may have more than one substation transformer, and each should be explicitly modeled.


[email protected], 08/11/20,

Same comment as above, that this requirement should not be imposed through mandatory standards.


From Sid: Deleted key to align with Deepak’s comment and clarified what ‘this” was referring to.


To my understanding a synchronous generator resource can operate with Pgen = 0 (net power) while providing Q. It may not be efficient to do so, but it can technically be done. Similarly a synchronous condenser provides reactive power while consuming a small amount of active power from the grid to meet losses.


I removed the word “should” to clarify that NERC is not specifying this should be a standards or interconnection agreement requirement for inverter-based resources. Because provision of reactive power incurs a cost from the consumption of active power, and because inverter-based resources can provide this service while synchronous resources cannot, this service should not be imposed through a requirement that only applies to inverter-based resources.


I don’t think this document is the correct place for this phrase. I feel this document should focus on the technical considerations and leave the compensation narrative out.

Ropp, Michael Eugene, 08/12/20,

I suggest not using this wording. It is not common nor desirable to require controls tuning for every local BPS.


Why transition from charging to discharging during a disturbance? Can reactive current be injected during a charging mode?


This could vary depending on system. TO should define this. I am hoping that P2800 is able to address this as well.


From Sid Pant: Is there a reference that can be added to show typical requirements? Who specifies the requirement? The TO? Or, if someone else, should the entity be stated here?


Do we have quantification of this? (Or are we relying on IEEE P2800 for that?)


In principle, I agree. In P2800 there are arguments made that when voltage at POM is extremely low, entering MC may be beneficial. The loss of synchronism and temporary overvoltage upon fault clearance may do harm then provide benefit. We should recognize these issues here and let system studies determine best approach. (Something of order of footnote 34 but needs to be expanded with a little more detail)


Monitoring: All BESSs should be equipped with digital fault recorder (DFR), dynamic disturbance recorder (DDR), and sequence of events recorder (SER) capability.

Hybrid resource stability: Appropriate studies should be conducted to ensure that the hybrid resource will operate stably in its electrical environment. For example, if the short-circuit strength is low, operation of the hybrid resource should be studied in detail by the TP and PC using EMT simulations, as appropriate. Studies should also be conducted to ensure that no instability modes exist at higher frequencies. In addition, the ability of newly interconnecting BESSs to operate with grid forming technology (described below) enable BESSs to operate in very low short-circuit strength networks and further provide BPS support beyond other grid-following inverter-based resources. Refer to recommendations from NERC Reliability Guideline: BPS-Connected Inverter-Based Resource Performance as well as NERC Reliability Guideline: Integrating Inverter-Based Resources into Low Short Circuit Strength Systems.31

Fault Ride-Through Capability: BESSs, like other BPS-connected inverter-based resources, should have the capability to ride through voltage and frequency disturbances when RMS voltage at the POM is within the curves of the latest effective version of PRC-024, subject to equipment limitation exemptions allowed under that standard. Ride-through performance requirements should apply to both charging and discharging modes, since unexpected tripping of any generation or load resources on the BPS will degrade system stability and adversely impact BPS reliability. Ride-through capability is a fundamental need for all BPS-connected resources such that planning studies can identify any expected risks.

Protection Settings: BESSs should have appropriate protection to safely and reliably operate the BESS. Any applicable protection settings should be clearly documented and provided by the BESS owner to the TP, PC, TOP, RC, and BA to ensure all entities are aware of expected performance of the BESS during planning and operations horizons.

Refer to the recommendations outlined in NERC Reliability Guideline: BPS-Connected Inverter-Based Resource Performance32 for more details on each of the aforementioned subjects. The following sub-sections outline the additional topics from Table 1.1 that warrant additional details and where BESSs have specific considerations that need to be taken.

Capability CurveBESS are generally four-quadrant devices that extend into the charging region. Individual BESS inverters may be nearly symmetrical due to effects of the BESS dc voltage and inverter de-rating due to temperature/altitude impacting both reactive and active power generation (see Figure 1.1). From an overall plant-level perspective, the capability curves may be asymmetrical and further impacted by collector system losses. Capability curves for the overall BESS should be provided by the GO to the TO, TP, PC, TOP, and RC to ensure sufficient understanding of the capabilities of the BESS to provide reactive power under varying active power outputs.

31 During the interconnection study process.32 REEC_D is still under development. It enhances the modeling capability of REEC_C.



From Sid Pant: Deleted this, as a pumped hydro unit coupled through an inverter is also a 4-quadrant device.


Same comment as above.


Same as earlier comment regarding PRC-024


Are all distributed sources required to have these?


Figure 1.1: Example of 5.3 MVA BESS Capability Curve [Source: SMA America]

Active Power-Frequency ControlBESSs should have the capability to provide active power-frequency control that extend to the charging region. The conventional droop characteristic can be extended into this region, and operation along the droop characteristic can occur naturally. Deadbands, droop settings, and other response characteristics should be specified by the BA based on studies performed by TPs and PCs. The droop characteristic and deadbands may be asymmetrical, meaning different settings for charging and discharging modes. Any transition between charging and discharging modes of operation should occur seamlessly (i.e., a continuous smooth transition between charging and discharging). The speed of response should also be coordinated with the BA based on primary frequency response needs. Consistent with FERC Order 842, there should be no requirement for BESS resources to provide frequency response if the SOC or set point is very low or very high, though that service can be procured by the BA. Any active power-frequency control should be sustained unless the BESS state of charge limits power consumption or injection from the resource. However, the capacity and energy needed to support interconnection frequency control is relatively small and for short period of time. Sustaining times may be specified by the BA.

Fast Frequency ResponseAs the instantaneous penetration of inverter-based resources continues to increase, on-line synchronous inertia may decrease and rate-of-change of frequency (ROCOF) may continue to increase. High ROCOF systems may be faced with the need for faster-responding resources to ensure that unexpected underfrequency load shedding (UFLS) operations do not occur.33

BESSs have the capability of providing FFR to rapid changes in frequency disturbances on the BPS. Similar to solar PV, there are no rotational elements and therefore the active power output is predominantly driven by the controls that are programmed into the inverter. BESSs should have at least the following functional capabilities that may be utilized if the BESS is within SOC limits:

Configurable and field-adjustable droop gains, time constants, and deadbands within equipment limitations; tuned to the requirements or criteria specified by the BA

33 The repc_b module in PSLF is equivalent to the combined PLNTBU1 and REAX4BU1/REAX3BU1 in PSS®E.



Following Lakshmi’s comment, I think we are mixing requirements and implementation. Should we focus on requirements such as bullet #3 below? How a specific BESS meets the requirements is an implementation issue that might be different for different BESSs.


There may be some limitations imposed by hardware on time constants.


Similar question as before.


Real-time monitoring of BESS SOC to understand performance limitations that could impose on FFR capabilities

Ability to provide a response with a specified power vs time profile, in coordination with primary frequency response, as defined by the BA

Many different simulations can be performed to show the benefits of utilizing BESSs for improving frequency response, particularly improving the nadir of system frequency following a large loss of generation. Figure 1.2 illustrates one study demonstrating these affects. The blue trace shows the response following a large generation loss for a synchronous-based system. The red plot shows the same system (with same amount of reserves) with the synchronous generation replaced with BESSs (with one flavor of frequency control enabled). The green plots show the system with BESSs with a different frequency control logic and tuned appropriately. The system dominated by synchronous machines exhibits an initial inertial response followed by a slower turbine-governor response. On the other hand, while the BESS system does not have physical inertia like a synchronous machine, its controls can be tuned to provide a suitably fast injection of energy such that the initial ROCOF remains nearly the same (or even improved) and the frequency nadir is significantly improved.



Agree with Deepak regarding control. An additional point is that “fast” in FFR is a relative term. We could consider the red curve already has FFR control though its response is same as the synchronous generator, but they both are “fast” enough for the system. Another situation is that conventional synchronous generator is too slow to provide frequency response, while BESS can provide much faster response – which is the real advantage of BESSs over synchronous generators.


I do 😊 To me both the red and green traces are acceptable. In the simulation, the red and green traces are actually just different forms of BESS controls and time constants. Hope the edits I have made to the text clarify this, Mike?


Hmmm… the graph shows that the BESS (red trace) actually does have a similar response to the synchronous system even before we put in FFR. Does anyone have familiarity with this EPRI study and how the IBRs were controlled in the red-trace case?


There might be a conflict here as primary frequency response is also defined as sustained from FERC Order 842 perspective.


What is meant by sustained – how long is it? 15 min? 1 hr?


I suggest making this a little broader. “Sustained response” tends to imply “constant response”, even if that’s not intended, and we want to maintain the ability to do sustained but time-varying responses (i.e. a long ramp-out).


Figure 1.2: Demonstration of Impacts of a BESS on Frequency Response [Source: EPRI]

Reactive Power-Voltage Control (Normal Conditions and Small Disturbances)There are no significant differences between BESSs and other BPS-connected inverter-based resources with respect to reactive power-voltage control during normal grid conditions and small disturbances. In essence, BESSs should have the capability to provide reactive power-voltage control in both charging and discharging operation; however, it is useful to separate out the recommendations into each mode of operation:

Discharging Operation: There are no significant differences between BESSs during discharge operation and other BPS-connected inverter-based generators with respect to reactive power-voltage control. BESSs



If there are no significant differences, should this section be moved up to the “Minimal Differences” section?


should have the ability to support BPS voltage control by controlling their POM voltage within a reasonable range during normal and abnormal grid conditions. Refer to the recommendations from the NERC Reliability Guideline: BPS-Connected Inverter-Based Resource Performance.

Charging Operation: BESSs should have the capability to control POM voltage during normal operation and abnormal small disturbances on the BPS while operating in charging mode. The ability for resources consuming power to support BPS voltage control adds significant reliability benefits to the BPS, and may be required by TOs as part of their interconnection requirements or by BAs, TOPs, or RCs for BPS operations.

As the resource transitions from charging to discharging modes of operation, or vice versa, the BESS should continuously have the ability to control BPS voltage throughout the transition.

Inverter Current Injection during Fault ConditionsBESSs should behave similarly to other inverter-based resource during on-fault conditions in terms of active and reactive current injection during fault conditions on the BPS. Active and reactive current injection during severe fault events should be configured to support the BPS during and immediately following the fault event such that legacy BPS protection can operate as expected and the BPS can remain stable during and after the event. Inverter-based resources, including BESSs, should ensure that the appropriate voltage-current relationships of magnitude and phase angles (i.e., appropriate positive and negative sequence current injection) are applied. Inverter current limits should be adhered to in order to avoid unnecessary tripping of inverters during fault events. Injection of current during and immediately after faults should be configured to enable the inverter-based resource to remain connected to the BPS and support BPS reliability.

BESSs will need to ensure adherence to SOC limits. BPS fault typically persist for fractions of a second, and SOC should typically not be a concern; however, the SOC limits are always in effect and closely monitored by BESSs.

The reactive current injection during fault conditions while the BESS is charging or discharging will depend on the BESS PQ curve and its symmetry. In either case, dynamic reactive current injection should support BPS voltages in both operating states. Further, controls should be configured for each specific installation such that voltage control (i.e., reactive current injection) has priority and the BESS can stably recover active current output very quickly. Typically, this should occur in less than 1 second; however this will need to be studied by the TP and PC, and configured accordingly.

Grid FormingModern inverters require an external source to provide a reference voltage to which the inverter phase-locks. These inverters are termed “grid-following”. If short-circuit strength falls too low, i.e. the fundamental-frequency impedance of the grid source becomes too high, then the sensitivity of the POM voltage to the IBR’s P and Q injection increases and grid-following inverters can be susceptible to instability or control malfunction. There are multiple mitigation options for these low short-circuit strength issues to help stabilize the ac voltage. One mitigation option is to control the BESS in a way that it does not rely on external system strength for stable operation (i.e., termed “grid-forming”). While there is currently no standard industry definition for grid forming technology, a broad definition can be:

Grid Forming: An inverter operating mode that enables reliable, stable, and secure operation when the inverter is operating on a part of the grid with few (or zero) synchronous machines along with the possibility of weak or non-existent ties to the rest of the bulk power system.

Four key aspects that enable achieving this operation mode are:

1. Availability of an ‘energy buffer’ to be deployed for imbalances in generation and load

2. Ability of the inverter to contribute towards regulation of voltage and frequency



I feel this can be ambiguous. Because the ability to independently regulate frequency can have implications with regard to size of inverter. Suppose we have a 3 MW BESS which claims to have grid forming control, it may not be able to effectively regulate frequency in a larger system. It might need other devices helping it along. Do the edits help?


This may need some wordsmithing, but to me this is a key part of what makes a grid-forming inverter grid-forming: it has to be able to regulate the frequency of the system it’s powering.


Offering a broader definition to encompass that grid-forming inverters may also be useful for weak grid/low short circuit strength issues.


I removed ‘and’ because the scenario is valid even without weak ties. An example scenario is a small island system without any synchronous machines but also without any long lines.


I deleted this because from a circuit theory perspective, there is nothing to prevent a current source from serving isolated load.


From Sid Pant: To avoid confusion, it may make sense to decouple grid forming from low SCR systems. I would like to propose that the highlighted sentences be removed from the main text (they could be a footnote, if desired) and the sentence “….option is to” be modified to “An alternative option is to …”


But is the behavior same regardless of mode of operation? For example, if BESS injects 50% additional reactive current during a discharging mode, then should same be expected when BESS is in charging mode?


From Sid Pant


3. Low communication latency between different layers of controllers

4. A robust dc voltage that enables synthesis of an ac voltage for a wide variety of system conditions.

BESSs have these attributes and can effectively employ grid forming technology to improve BPS performance in the future as penetrations of inverter-based resources continues to grow.

System Restoration and Blackstart CapabilityIn the event of a large-scale outage caused by system instability, uncontrolled separation, or cascading, system operators are tasked with executing blackstart plans to re-energize the BPS and return electric service to all customers. This process is relatively slow as the blackstart plan identifies the boundaries of outage conditions, system elements, critical loads, etc.; reconnects pre-defined generators and load points to the overall BPS; and carefully resynchronizes regions or portions of the BPS. Throughout this entire process, grid operators are closely balancing generation and demand as well as managing BPS voltages within operating limits.

Generate its own voltage and seamlessly synchronize to other portions of the BPS.

Stably operate during large frequency, voltage, and power swings, and reliably operate in low short-circuit strength networks. Detailed EMT studies demonstrating the ability to operate under these conditions should be conducted.

Provide sufficient inrush current to energize transformers and transmission lines and start electric motors. Note that BESSs, like other inverter-based resources, have limited ability to provide high levels of inrush current. This necessitates the need to coordinate the BESS resource with the blackstart load.

Have assurance that the BESS will be available immediately after a large-scale outage requiring system restoration activities. BESSs will need to demonstrate to their RC and TOP they can be available at any point in time to be considered as a blackstart resource.

Have sufficient energy to remain on-line and operational for the time required to ensure blackstart plans can be fully executed.34 Therefore, BESSs energy ratings should be designed to achieve the required time frames. And their states of charge should be maintained above a limit to ensure enough energy is available for blackstart purposes.

Be able to quickly respond to and control fluctuations in system voltage and frequency.

Be able to start rapidly to minimize system restoration times.

Have redundancy to self-start in the event of any failures within the facility.

In order to ensure proper integration into the overall system blackstart scheme and coordination between resources via appropriate engineering studies, all control design, settings, configurable parameters, and accurate models should be made available to the BA, TP, PC, TOP, and RC.

Have remote start and remote operational control capabilities to avoid requiring dispatch of personnel to the field.

State of ChargeState of charge (SOC) represents the present level of charge of an electric battery relative to its capacity, within the range of fully discharged (0%) to fully charged (100%). The SOC of a BESS affects the ability of the BESS to provide energy or other essential reliability services to the BPS at any given time.35 In many cases, the BESS may have SOC limits that are tighter than 0–100% for battery lifespan and other equipment and performance considerations.

34 Such as capturing different control algorithms and any possibility of additional short circuit current from BESSs due to additional energy on the dc bus.35 FERC Order No. 841, paragraph 4.



Should we reference FERC Order 841 here?


Import to include, as batteries exceed the capability of other resources on this metric.


The word “Low” is vague. How low is low enough? And the layers of controllers also need to be clarified.


Is the modification better?


From Sid: Is this needed?


I’m unclear on what this means. Reduced from what? Round trip between what and what?


In terms of performance, the following should be considered for capability and operation of a BESS:

Provision of ERSs to the BPS: All BESSs should have the capability to provide ERSs such as voltage support, frequency response, and ramping capabilities to support BPS operation. However, each BESS will be configured to provide any one or multiple ERS during on-line operation, based on real-time dispatch, SOC, and system needs.

Nearing SOC limits: As a BESS approaches its SOC limits, the BESS will ramp down its charging or discharging. This ramp should be clearly defined by the owner of the BESS and communicated to the BA, TOP, and RC.

SOC Limits and Frequency Response: Consistent with FERC Order 842, there should be no requirement for BESS resources to provide frequency response if the SOC or setpoint is very low or very high, though that service can be procured by the BA.

SOC Limits and Reactive Power Support: The ability for a BESS to provide reactive power is limited by its dc bus voltage and SOC. Lower dc voltage results in a reduced amount of reactive power the inverter can produce. Any possible limitations on reactive power support caused by dc bus voltage limits should be clearly defined, if applicable.

SOC Limits and Blackstart Capabilities: SOC should be maintained above a limit to ensure there is energy to fully execute a blackstart process as designed.

SOC limits affect the ability of the BESS to operate as expected, and any SOC limits will override any other ability of the BESS to operate. These limits and how they affect BESS operation should be defined by the BESS owner and provided to the BA, TOP, and RC. For planning assessments, this information is also important to the TP and PC as they establish planning cases.

The SOC of any BESS depends on the past operating conditions of the BESS and the services it is providing to the BPS. To study BESS SOC, a time series (or quasi-dynamic) study can be used. Figure 1.3 shows an example of a BESS providing two services: peak shaving (charging in morning and discharging at night) and transmission line congestion management around a set of wind power plants. The magnitude and duration of any other service provided by the BESS (such as voltage control or frequency support capability) revolves around the two primary services. Figure 1.3 shows the evolution of the BESS SOC over two days, evaluated at half-hour time steps but with tracking of the dynamic evolution of the SOC.

Figure 1.3: Example Time Series of BESS State of Charge



Same comment as above.


Similar question as before


[Source: EPRI]

Tracking SOC is important for determining the initial BESS SOC for a transient stability simulation. For example, frequency response capability of the BESS at 20:00 on the first day will be different from the capability at 20:00 on the second day. In the example above, SOC at 20:00 on the first day is around 80%; however, SOC at 20:00 on the second day is around 50%.

It is the responsibility of the BESS owner/operator to manage SOC to ensure that the BESS can provide all services that it is contracted to deliver to the BPS, such that the BESS can be counted on as a reliable BPS asset. SOC should be monitored in real-time and reported to the BA, TOP, and RC for wide-area situational awareness purposes.

Oscillation Damping SupportMany synchronous generators are equipped with power system stabilizers (PSSs) that provide damping to system oscillation typically in the range of 0.2 Hz to 2 Hz. As these resources become increasingly limited (either retire or are off-line during certain hours of the day), there is a growing need for oscillation damping support in certain parts of the BPS. For example in the West Texas area of the ERCOT footprint where significant amounts of renewable generation resources connect to, synchronous generators in West Texas may be off-line under high renewable output condition and could lead to insufficient damping support required to maintain stability for high power long distance power transfer during and after large disturbances. Currently, renewable generation resources are not required to provide damping support in ERCOT and synchronous condensers typically are not equipped with PSS. A study conducted by ERCOT in 2019 identified oscillatory responses around 1.8 Hz between synchronous condensers in the Panhandle area and other synchronous generators far away from the Panhandle region under a high renewable generation penetration condition with large power transfers to electrically distant load centers.36

Newly interconnecting BPS-connected IBRs should have the capability to provide power oscillation damping controls. A major difference than BPS-connnected IBGs is that BESSs can operate in the charging mode in addition to the discharging mode, which provide greater capabilities of damping support. TPs and PCs may identify a reliability need for this type of control as the penetration of IBRs continues to increase. At that time, requirements should be developed by TOs to ensure that the capability is activated and actively damps out power oscillations typically in the range of 0.2 Hz to 2 Hz when the resources are on-line and operational. Each facility will need to have these controls tuned during the interconnection study process and may require re-tuning as the generation mix continues to change. These types of studies are critical to ensure reliable operation of the BPS over time. TOs should ensure interconnection requirements suitably address this functionality such that the capabilities can be utilized when and if needed.

36 FERC Order No. 841, paragraphs 209 and 246.



I’d like additional discussion of this. What controls retuning do we typically require of synchronous generators? How would such control retuning be done?Control retuning is not a subject to be approached lightly—it’s difficult in massively interconnected systems like this. The IBRs need to be designed with robust controls, but retuning them on the fly may create a lot more problems than it solves.


Recommended Performance and Considerations for Hybrid PlantsText

DC-Coupled Hybrid PlantsText

AC-Coupled Hybrid PlantsTable 1.2 provides an overview of the considerations that should be made when describing the recommended performance of ac-coupled hybrid plants compared with other BPS-connected inverter-based generating resources. The following sub-section elaborate on these high-level considerations in more detail.

Table 1.2: High Level Considerations for Hybrid Plant PerformanceCategory Comparison with BPS-Connected Inverter-Based Generators

Momentary CessationNo significant differences from other BPS-connected inverter-based generating resources; for BESS part of the hybrid, momentary cessation should not be used to the greatest possible extent37 during charging and discharging operation.

Phase Jump Immunity No significant difference from other BPS-connected inverter-based generating resources.

Capability Curve

The capability curve of a hybrid is a combination of capability curves of the generating resource(s) and BESS(s), which are part of the hybrid. The capability curve extends into the BESS charging region to create a four-quadrant capability curve. The curve is not symmetrical for injection and withdrawal. On the injection side the capability curve will be equal to the sum of capability curves of a generator and capability curve of BEES during discharging. On the withdrawal side capability will be equal to BEES capability curve, when charging.

Active Power-Frequency Controls

No significant difference from other BPS-connected inverter-based generating resources and BESS. The conventional droop characteristic can be used in both generating and charging modes of the hybrid. Active power-frequency control capability may be limited by total active power injection and/or withdrawal limit of the hybrid plant at POI, that may be set lower than actual capability of the plant. Due to the presence of the BESS, a hybrid plant can also have the capability of providing frequency response for under frequency conditions.

Fast Frequency Response (FFR)

FFR capability will depend on the resources making up the hybrid plant. BESSs are well-positioned for providing FFR to systems with high rate-of-change-of-frequency (ROCOF) due to not having any rotational components (similar to a solar PV facility). However if BESS is combined with wind generation facility coordination between resources within the hybrid may be needed to achieve sustained FFR. Additionally, hybrid plant FFR capability may be limited to total active power injection and/or withdrawal limit of the hybrid plant. The need for FFR is based on each specific Interconnection’s need.38 Sustained forms of FFR help arrest fast frequency excursions but also help overall frequency control. BESSs are likely to be able to provide sustained FFR within their state of charge constraints. Consistent with FERC Order 842, there should be no requirement for hybrid resources to reserve headroom to provide frequency response, though that service can be procured by the BA.

37 FERC Order No. 842, paragraph 183.38 FERC Order No. 842, paragraph 180.



I’ve deleted most of the details.


Is all of ths significantly different from standalone BESS? Apart from the ratings.


My assumption here is capability curve talks about what hybrid plant is capable of doing, not what it is required to do? If we are talking about requirement than active power self-limit, if any, will limit the reactive requirement, at least in ERCOT?


Cen we expand on system stability aspect of the footnote 34? Does the system here mean grid or control of inverters?


In Table 1.2 I assumed AC-connected hybrid is BESS + another IBR (wind or solar)


Julia volunteered.


Relate back to section above on BESS (and table).


Deepak:Make sure to mention again that this applies to BESS + IBR.


Mark – For DC-coupled, there is nearly no distinction between table above and here. Need to make that note.Deepak:For AC-coupled, there are nuances that need to be accounted for. E.g., state of charge, directionality of charge (charging from other resource or from grid or mixed?), etc.Mark:Coordination at plant-level controller needed.


Deepak:Performance w.r.t. inverter, will be very closely aligned to table above. Same capability. Overall plant itself – will need some coordination across the different AC inverters.


For this section and one above, make nod to P2800 and requirements being developed there.


Reactive Power-Voltage Control(Small Disturbance)

No significant difference from other BPS-connected inverter-based generating resources. The dynamic voltage support capability of a hybrid is a combination of capability of the generating resource(s) and BESS(s), which are part of the hybrid. BESSs portion of the hybrid have the capability to provide dynamic voltage control during both discharging and charging operations. TOPs should provide a voltage schedule (i.e., a voltage set point and tolerance) to the hybrid that can apply to both operating modes (injection and withdrawal).

Reactive Current-Voltage Control(Large Disturbance)

No significant difference from other BPS-connected inverter-based generating resources. BESSs portion of the hybrid can be configured to provide dynamic voltage support during large disturbances both while charging and discharging.

Reactive Power at No Active Power Output No significant difference from other BPS-connected inverter-based generating resources.

Inverter Current Injection during Fault Conditions

No significant difference from stand-alone BPS-connected inverter-based generating resources and BESS.

Return to Service Following Tripping

No significant difference from other BPS-connected inverter-based generating resources. Hybrid plant should return to service following any tripping or other off-line operation by operating at the origin (no significant exchange of active or reactive power with the BPS), and then ramp back to the expected set point values, as applicable. This is a function of settings and any requirements set forth by the BA (or TO in their interconnection requirements).

Balancing

No significant difference from other BPS-connected inverter-based generating resources. BAs, TPs, PCs, ISO/RTOs, and other applicable entities will need to understand what services are being provided from a hybrid plant; however, the capability to provide balancing for the BPS and should be coordinated through ancillary services markets or dispatch.

Monitoring No significant difference from other BPS-connected inverter-based generating resources.

Operation in Low Short-Circuit Strength Systems


Grid FormingBESSs portion of a hybrid plant have the unique capabilities to effectively deploy grid forming technology to help improve BPS reliability in the future of high penetration of inverter-based resources.

Fault Ride-Through Capability

No significant difference from other BPS-connected inverter-based generating resources. A hybrid plant should have the same capability to ride through fault events on the BPS, when point of measurement (POM) voltage is within the curves specified in the latest effective version of PRC-024, subject to the equipment limitation exemption. For the BESS part of the hybrid this applies to both charging and discharging modes. Unexpected tripping of generation or load resources on the BPS will degrade system stability and adversely impact BPS reliability. Ride-through capability is a fundamental need for all BPS-connected resources such that planning studies can identify any expected risks.



Same as above


Same comment as earlier.


Consistent with point above about not mandating this.


Once chapter 4 matures, revisit to ensure consistency.


We should start recognizing that a response during a large system disturbance and three-phase fault is same.


I removed some of the repeating details


Is there significant difference here between hybrid plant and standalone BESS? Apart from the Mvar value provide because of the ratings?


Again, I am assuming we are talking about capability here not the requirement. If there’s a set limit at POI for the hybrid Reactive requirement will be lower than capability.


System Restoration and Blackstart Capability

Hybrid plants may have the ability to form and sustain their own electrical island if they are a part of a blackstart cranking path. This may require new controls topologies or modifications to settings that enable this functionality. Blackstart conditions may cause large power and voltage swings that must be reliably controlled and withstood by all blackstart resources (i.e., operation under low short circuit grid conditions). For the hybrid to operate as a blackstart resource, assurance of energy availability is needed as well as designed energy rating that ensures energy availability for the entire period of restoration activities. At this time, it is unlikely that most legacy hybrids can support system restoration activities as a stand-alone resource; however, they may be used to enable start-up of subsequent solar PV, wind, or synchronous machine plants and accommodate fluctuations in supply and demand.

Protection Settings No significant difference from other BPS-connected inverter-based generating resources.

Power Quality No significant difference from other BPS-connected inverter-based generating resources

State of Charge (new)

Similarly to the standalone BESS, The state of charge (SOC) of a BESS portion of the hybrid may affect the ability of the hybrid to provide energy or other essential reliability services to the BPS at any given time.39 These limits and how they affect BESS operation should be defined by the hybrid owner and provided to the BA, TOP, RC, TP and PC.BESS’s SOC will be optimized by the hybrid plant controller in coordination with other parts of the hybrid (wind or solar), based on irradiance and/or wind conditions, market prices, energy and ESR obligations of the hybrid. In addition, the manner in which the BESS would charge is to be communicated by the GO. Here, system loading conditions and generation from other parts of the hybrid plant will play a role. For e.g. , in a Wind-BESS hybrid plant during low load high renewable scenarios, the BESS may be charged directly from the wind output. In this scenario, the hybrid plant will not appear as a load on the system. Alternatively, the plant may be directed to charge from the network in order to increase the loading on the system to satisfy stability considerations.

Operational Limits (new)

Based on economics or design considerations, BESS portion of the hybrid may be operated to only charge from other wind and/or solar part of the hybrid or to charge from the grid as well. This information should be provided by the hybrid owner to the BA, TOP, RC, TP and PC. Hybrid plant owners may choose to set on injection/withdrawal at the POI that is lower than actual capability of the hybrid. This information should be provided by the hybrid owner to the BA, TOP, RC, TP and PC. Where such limit exists, the studies as well as voltage support and frequency support requirements may apply only up to the limits.

Damping Support

BESSs can have the capability of providing oscillation damping support, similar to synchronous gnerators, HVDC/FACTS facilities, and other BPS-connected inverter-based resources. BESSs can operate in the both charging and discharging mode, which provides greater capabilities for damping support.

39 Ibid.



I think it looks ok now


It is similar, and I’ve added a blip in the beginning of the sentence to say that. I am not sure if anything could be deleted from here?


This is similar to a standalone BESS

Gary Custer, 08/11/20,

Systems are currently being designed and installed with blackstart capability. Consider adding “legacy”.


I rephrased to hopefully take care of Gary’s concern. I’ve also corrected the language in Table 1.1 regarding black start for consistency.

Gary Custer, 08/11/20,

This is a limitation of any generating resource. Rather, the needs of the generating resource are part of the plant design.


Made changes based on Sid’s suggestions for BESS Table to address Deepak’s comment


Is this going to be needed and required for every hybrid plant?


Topics with Minimal Differences between AC-Coupled Hybrids and standalone BESS ResourcesThe following performance characteristics have practically no difference between ac-coupled hybrid plants and standalone BESSs:

Momentary cessation

Phase jump immunity

Reactive current-voltage control during large disturbances

Reactive power at no active power output

Return to service following tripping

Inverter current injection during fault conditions

Balancing

Monitoring

Operation in low short-circuit strength systems

Fault ride-through capability

Grid forming40

Protection settings

Damping support

Refer to the recommendations outlined in NERC Reliability Guideline: BPS-Connected Inverter-Based Resource Performance41 for more details on each of the aforementioned subjects. The following sub-sections outline the additional topics from Table 1.2 that warrant additional details and where AC-Coupled hybrids have specific considerations that need to be taken.

Capability CurveAs in was mentioned in BESS section, BESS are four-quadrant devices that extend into the charging region.Capability curve of the AC-Coupled Hybrid plant is a summation of BESS capability curve and other IBRs that are part of the hybrid (i.e. wind or solar PV). Thus, the overall plant-level capability curve will be asymmetrical with more capability in the injection region than in the withdrawal region (see Figure 1.X). Capability curves for the overall AC-coupled hybrid plant should be provided by the GO to the TO, TP, PC, TOP, and RC to ensure sufficient understanding of the capabilities of the hybrid plant to provide reactive power under varying active power outputs.

[FIGURE]

Figure 1.X: Example of AC-Coupled Hybrid Plant Capability Curve

Active Power-Frequency ControlActive power-frequency controls can be extended to the charging region of operation for BESSs part of the hybrid, as described in detail in standalone BESS section above. The overall active power-frequency control capability of the hybrid is equal to combined capability of all resources that are part of the hybrid plant. The overall capability may be limited by total active power injection and/or withdrawal limit of the hybrid plant that may be set lower than actual capability of the plant.

40 FERC Order No. 842, paragraph 185.41 FERC Order No. 842, paragraph 187.



Revisit once chapter 4 matures. Same applies to Reactive Current – Voltage Control (Large Disturbances)


This point was not on “similar to all IBRs” list in the BESS section and it is discussed in more details later in the BESS section, but since this subsection here is on topics that are similar between AC-coupled hybrids and stand-alone BESS, I think this item can also be just added to the list without any more details, but feel free to pull it back out below if you disagree.


Fast Frequency ResponseBESSs and solar PV have the capability of providing FFR to rapid changes in frequency disturbances on the BPS. Since there are no rotational elements, the active power output is predominantly driven by the controls that are programmed into the inverter. Wind generating resources can provide FFR through tapping into kinetic energy of rotating mass of a wind turbine.42 Such response, however, cannot be sustained. To obtain sustained fast frequency response from hybrid plants containing wind/solar PV generating resources along with BESS the FFR capability of the AC-coupled hybrid plant is equal to combined capability of all resources that are part of the hybrid plant. The resources within the hybrid can be coordinated to optimize total FFR and achieve required sustain time. The overall capability may be limited by total active power injection and/or withdrawal limit of the hybrid plant that may be set lower than actual capability of the plant.

AC-coupled hybrid plant should have at least the following capabilities (which may be utilized based on BA requirements and BPS reliability needs):

Configurable and field-adjustable droop gains, time constants, and deadbands; tuned to the requirements or criteria specified by the BA

Real-time monitoring of BESS SOC to understand performance limitations that could impose on FFR capabilities from the hybrid

Ability to provide a sustained response, in coordination with primary frequency response, as defined by the BA.

Consistent with FERC Order 842, there should be no requirement for BESS resources to provide frequency response if the SOC or setpoint is very low or very high, though that service can be procured by the BA.

Reactive Power-Voltage Control (Normal Conditions and Small Disturbances)There are no significant differences between AC-coupled hybrids and BPS-connected inverter-based resources with respect to reactive power-voltage control during normal grid conditions and small disturbances. In essence, the hybrid plant should have the capability to provide reactive power-voltage control both during power injection at the POM and power withdrawal (during BESS charging); however, it is useful to separate out the recommendations into each mode of operation:

Power Injection: There are no significant differences between hybrid plants during power injection into the grid and other BPS-connected inverter-based generators with respect to reactive power-voltage control. Hybrids plant should have the ability to support BPS voltage. Voltage control needs to be coordinated between all resources within the hybrid plant to control hybrid plant’s POM voltage within a reasonable range during normal and abnormal grid conditions. Refer to the recommendations from the NERC Reliability Guideline: BPS-Connected Inverter-Based Resource Performance.

Power Withdrawal: Hybrid plants should have the capability to control POM voltage during normal operation and abnormal small disturbances on the BPS while BESS part of the hybrid is operating in charging mode. The ability for resources consuming power to support BPS voltage control adds significant reliability benefits to the BPS, and may be required by TOs as part of their interconnection requirements or by BAs, TOPs, or RCs for BPS operations.

As the resource transitions from injecting to withdrawing modes of operation, or vice versa, the BESS should continuously have the ability to control BPS voltage throughout the transition. Generally, the output voltages of inverter-based renewable energy resources vary severely due to large fluctuations and rapid changes in the availability of their energy resources. Therefore, if used individually, these resources have difficulty controlling their voltage. In a Hybrid power plant, however, this issue is resolved. Since the output voltage variation of the BESS from

42FERC Order No. 842, paragraph 188.



Similar question as before.


a fully charged to a discharged state is typically less, this variation can be easily controlled to maintain a stable output voltage. In addition, the battery is capable of balancing the power fluctuations either by absorbing the excess power from the renewable energy resources during charging or by supplying the power to satisfy the load-demand changes, during discharging. As the resource transitions from charging to discharging modes of operation, or vice versa, the Hybrid power plant should continuously have the ability to control BPS voltage throughout the transition

System Restoration and Blackstart CapabilityIn the event of a large-scale outage caused by system instability, uncontrolled separation, or cascading, system operators are tasked with executing blackstart plans to re-energize the BPS and return electric service to all customers. This process is relatively slow as the blackstart plan identifies the boundaries of outage conditions, system elements, critical loads, etc.; reconnects pre-defined generators and load points to the overall BPS; and carefully resynchronizes regions or portions of the BPS. Throughout this entire process, grid operators are closely balancing generation and demand as well as managing BPS voltages within operating limits.

Pre-determined generating units and BPS elements are identified in a blackstart cranking path, and some of these elements may require additional capabilities to be considered blackstart-capable resources. BESSs should have the following capabilities to be considered as part of system restoration activities:

Generate its own voltage and seamlessly synchronize to other portions of the BPS.

Stably operate during large frequency, voltage, and power swings, and reliably operate in low short-circuit strength networks. Detailed EMT studies demonstrating the ability to operate under these conditions should be conducted.

Provide sufficient inrush current to energize transformers and transmission lines and start electric motors. Note that BESSs, like other inverter-based resources, have limited ability to provide high levels of inrush current and this may limit their ability to be used as a blackstart resource.

Have assurance that the BESS will be available immediately after a large-scale outage requiring system restoration activities. BESSs will need to demonstrate to their RC and TOP they can be available at any point in time to be considered as a blackstart resource. However, it is more likely that BESSs may be able to support system restoration if and when available.

Have sufficient energy to remain on-line and operational for many hours to ensure blackstart plans can be fully executed.43 This may be a significant challenge for BESSs since their energy ratings are often substantially shorter than these time frames.

Be able to quickly respond to and help control fluctuations in system voltage and frequency.

Be able to start rapidly to minimize system restoration times.

Have redundancy to self-start in the event of any failures within the facility.

All control design, settings, configurable parameters, and accurate models should be made available to the BA, TP, PC, TOP, and RC.

Have remote start and remote operational control capabilities to avoid requiring dispatch of personnel to the field.

State of ChargeState of charge considerations for the BESS portion of the ac-coupled hybrid plant are similar to those of a stand-alone BESS discussed above. The state of charge (SOC) of a BESS portion of the hybrid may affect the ability of the

43 FERC Order No. 845, paragraph 279.



If (by consensus as a group) we decide there are minimal differences between standalone BESS and hybrid plants for restoration and blackstart (apart from MVA ratings), then this section can also be deleted and added to the list of no significant difference.


I didn’t touch this section, just copied what was there for BESS. Please add/edit as you see fit.


From Maysam Radvar


BESS to provide energy or other essential reliability services to the BPS at any given time.44 These limits and how they affect BESS operation should be defined by the hybrid owner and provided to the BA, TOP, RC, TP and PC. BESS’s SOC will be optimized by the hybrid plant controller in coordination with other parts of the hybrid (wind or solar), based on irradiance and/or wind conditions, market prices, energy and ESR obligations of the hybrid.

Operational Limits Based on economics or design considerations, the BESS portion of a hybrid plant may be operated to only charge from the generating component or to charge from the grid as well. Technical, economic, and policy considerations will dictate whether the hybrid plant charges from the grid or only from the generating component. 45 TOs and BAs should clearly define the acceptable charging behavior from the hybrid plant. Characteristic of charging and any operational limitations should be provided by the hybrid plant owner to the BA, TOP, RC, TP and PC.

Hybrid plant owner for various economic consideration may choose to set on injection/withdrawal at the POI that is lower than actual capability of the hybrid plant. This information should be provided by the hybrid owner to the BA, TOP, RC, TP and PC. Where such limit exists, the studies as well as voltage support and frequency support requirements may apply only up to the limits.

44 FERC Order No. 845, paragraph 285.45 Ibid.


Chapter 2: BESS and Hybrid Plant Power Flow Modeling

BPS-connected BESS and hybrid plants are modeled very similarly to other BPS-connected inverter-based resources such as solar PV and wind power plants. This chapter provides a brief overview of the presently recommended power flow modeling practices. Refer to other industry references for more details.46

BESS Power Flow ModelingAs mentioned, the power flow representation for a BPS-connected BESS is similar to other types of BPS-connected inverter-based resources. Figure 2.1 shows a generic47 power flow model for a BPS-connected BESS facility. The power flow representation of a BPS-connected BESS facility will include the following components:

Generator Tie Line: Where the BESS is connected to the BPS (to the POI) through a transmission circuit (i.e., the generator tie line), this element should be explicitly modeled in the power flow to properly represent active and reactive power losses and voltage drops or rises.

Substation Transformer: Any substation transformers48 (also referred to as “main power transformers”) should be explicitly modeled in the power flow base case. All relevant transformer data such as tap ratios, under-load tap changer (ULTC) controls, and impedance values should be modeled appropriately.

Collector System Equivalent: Based on the cabling and layout of the BESS facility, some GOs may choose to model an equivalent collector system to capture any voltage drop across the collector system. However, BESS facilities are not geographically and electrically dispersed like wind and solar PV facilities, so BESS collector system equivalent impedances are likely much smaller. Therefore, this may or may not be included in the BESS power flow model.

Equivalent Pad-Mounted Transformer: Each of the inverters interfacing the battery systems with the ac electrical network will include a pad-mounted transformer. An equivalent pad-mounted transformer is typically modeled, which is scaled to an appropriate size to match the overall MVA rating of the aggregate inverters at the BESS facility.

Equivalent BESS: An equivalent BESS generating resource is modeled to represent the aggregate amount of inverter-interfaced BESSs installed at the facility. The capability is scaled to match the overall capability of aggregate inverters. The equivalent BESS is modeled as a generator in the power flow, and appropriate voltage control settings (and other applicable control settings) should be specified in the model. In situations where different inverter types (e.g., make and model of inverter) are used49 within the BESS, each different inverter type is typically separately aggregated. GOs should consult with their TP and PC for recommended modeling practices.

Shunt Compensation and Reactive Devices: The plant may include shunt reactive devices to meet reactive capability and voltage requirements defined by the TO and TOP. These may include shunt capacitors and reactors, FACTS devices, or synchronous condensers, as applicable. If these devices are installed, they should be modeled appropriately. Figure 2.1 also denotes that these installations could even be located at the POI, within the boundary of the GO and GOP, and those devices should also be modeled appropriately.

Plant Loads: The plant may include a small load to represent station service load, as deemed necessary based on the TP and PC modeling requirements.

46 Hybrid plants combine multiple technologies of generation and energy storage at the same facility, enabling benefits to both the plant and to the BPS. The majority of newly interconnecting hybrid resources are a combination of renewable energy and battery energy storage.47 https://www.cpuc.ca.gov/General.aspx?id=3462 . 48 Phil Pettingill, “Ensuring RA in Future High VG Scenarios – A View from CA”, ESIG Spring Workshop. April 10, 2020. 49 https://www.cpuc.ca.gov/General.aspx?id=5935



From Himanshu Jain:Power Flow – One of the main issues that we have encountered in some of our projects, particularly under high variable renewable generation is around modeling of BESS under long duration outages. As you all are well aware of, typical contingency analysis as performed today assumes that each online generator can stay online at its pre-contingency power level for the entire duration of a contingency. The inherent assumption is that fuel supply to the generator is essentially uninterrupted and unlimited. With BESS, this assumption breaks down even for moderately long contingencies that may last for more than 10 odd hours. So, a key question to consider for modeling BESS in steady-state contingency analysis is how do we model its capacity during the contingency analysis. Should we discount it by a factor to account for typical duration of outages, or should we consider a time series type of analysis with tighter integration of scheduling and power flow tools for bulk power systems as is done in distribution systems. The latter is certainly possible to do, but significantly increases the data that is generated from contingency analysis and adds an additional burden of creating cases with changing resource mix from hour to hour (or every 5 minutes if that is the resolution of the scheduling tool used).


Chapter 3: BESS and Hybrid Plant Dynamics Modeling

Elements in Figure 2.1 shown in red are denoted as those elements that may or may not be represented in BESS models based on each specific installation’s modeling needs, with the goal of capturing all the needed electrical effects. Those elements described in black should be modeled in all BPS-connected BESS facilities. Common voltage levels are shown in Figure 2.1 only for illustrative purposes.

Figure 2.1: Generic Power Flow Model Example for BESS

The GO, TP, and PC will need to consider the following aspects of steady-state power flow modeling for BESSs:

Charging Operation: Charging capability can be modeled by setting the equivalent BESS generator with an appropriate negative value for the active power limit, Pmin. Note that the maximum charging limit (Pmin) may be different than the maximum discharging limit (Pmax). These Pmin and Pmax limits in the equivalent BESS generator record should be set to any limits imposed by the plant and inverter controllers in coordination with the capability of the inverters. Also, the BA, TOP, RC, TP, and PC should ensure they understand how the other BESS facility components (e.g., shunt compensation) operate during charging operation such that the overall BESS model can be set up correctly in both charging and discharging modes.

Point of Voltage Control and Power Factor Mode: As with other generating resources, the generating resource (i.e., the equivalent BESS) can be configured to operate either in a power factor control mode or a voltage control mode with a specific control point in the grid (i.e., the POM or POI). This should be configured appropriately in the generator record voltage controls. Newer models may enable advanced controls such as voltage droop characteristic to be represented.

Hybrid Power Flow ModelingThe configuration of hybrid plants will likely vary more than BESS facilities, based on the size of the plant, the type of technologies used, and the overall layout of the facility. Regardless, each hybrid plant should be modeled according to the expected50 or actual facilities installed in the field. Further, hybrid plants may be modeled differently depending on whether they are ac-coupled or dc-coupled facilities. GOs should consult with their TP and PC to determine the appropriate modeling approach based on whether the facility is ac-coupled or dc-coupled.

AC-Coupled Hybrid Plant Power Flow ModelingFigure 2.2 illustrates a generic model representation for an ac-coupled hybrid plant. Since the BESS and the generating resource are connected through the ac network, then each component should be represented accordingly, as shown in Figure 2.2. An equivalent BESS generation and equivalent pad-mounted transformer should be represented, as well as an equivalent collector system (if needed to properly represent the electrical

50 https://www.eia.gov/todayinenergy/detail.php?id=43775


https://www.eia.gov/todayinenergy/detail.php?id=43775


effects). For the example shown in Figure 2.2, where the ac-coupling is at the low-side of the substation main power transformer, the inverter-based generating resource is coupled to the BESS at this point. The inverter-based generating resource also has its own equivalent generator model, equivalent pad-mounted transformer, and equivalent collector system modeled appropriately. The substation main power transformers and plant generator tie line are also modeled explicitly. Any shunt compensation such as shunt reactors, capacitors, FACTS devices, or synchronous condensers should be modeled as well. Again, elements shown in red may or may not be represented in the model based on each specific location, and elements shown in black should be modeled for all facilities. Common voltage levels are shown only for illustrative purposes.

Figure 2.2: Generic Power Flow Model Example for AC-Coupled Hybrid Power

Plants

The GO, TP, and PC will need to consider the following aspects of steady-state power flow modeling for ac-coupled hybrid power plants:

Plant Configuration: AC-coupled hybrid plants can have significantly different configurations on the ac-side of the inverter interface. Therefore, special attention should be given to ensuring that the power flow model accurately represents the overall configuration of the plant (which may be different from Figure 2.2).

Coordinated Operation of BESS and Generating Component: Since the BESS is explicitly modeled, charging and discharging capability can be represented by setting the equivalent BESS generator Pmin and Pmax values appropriately. The Pmin and Pmax limits in the equivalent BESS generator record should be set to any limits imposed by the plant and inverter controllers in coordination with the capability of the inverters. BESS operation should be modeled by setting active power output, Pgen, accordingly. The BA, TOP, RC, TP, and PC should ensure they understand how the BESS is expected to operate in relation to the inverter-based generating component within the plant, such that the output of both resources is coordinated.

Maximum Overall Plant Power Output (Plant Pmax): The maximum power output of the overall hybrid facility may be limited by interconnection agreement, plant controller, or other means. While the



From Jeff Billo:This is fine if I only have one or two, but engineers are not going to be able to remember how each one works if there are 100+ on their system. I feel we need to expand this section such that we have a standardized way of categorizing the operation of hybrid plants. However, we need input from GOs on this.

Favela, Roberto, 03/27/20,

I agree Jeff, maybe we can create a template that would capture that information so that it can be used for reference and coordination between GO and TPs. For operational studies


nameplate rating of the individual BESSs and generating resources may exceed the limit, the power output of the overall facility may not. Therefore, it is important to understand what the maximum operational output of the plant will be and set up each component such that they do not exceed this operational limit.

BESS Charging from BPS or from Generating Resource: Depending on the interconnection agreement, the hybrid plant may or may not be able to charge from the BPS. If allowed, the BESS may be able to charge power from the BPS with the generating unit dispatched off. If not allowed, the BESS will only charge using energy produced by the generating component of the plant.

Coordinating Voltage Controls for BESS and Generating Component: The hybrid power plant will have obligations per VAR-002-4.1 to control voltage at its POI or POM, and the power flow base case should be configured to ensure similar voltage control strategies as used in the field. In an ac-coupled hybrid plant with the BESS and generating component modeled explicitly, the voltage controls will need to be coordinated among both devices. Both equivalent generator records for the BESS and generating component can be coordinated using the reactive power sharing parameter in each unit.51

The Western Electricity Coordinating Council (WECC) Renewable Energy Modeling Task Force (REMTF) is developing recommendations for software vendors to improve the capability for modeling BESSs and hybrid plants, particularly for representing overall plant-level active power limitations as well plant-level coordinated voltage controls in the power flow base case. This will enable more effective modeling of hybrid plant dispatch scenarios as well as overall plant voltage control.

DC-Coupled Hybrid Plant Power Flow ModelingFigure 2.3 illustrates a generic model representation for a dc-coupled hybrid plant. For dc-coupled plants, the BESS and inverter-based generating resources are coupled on the dc-side of the inverter. Therefore, these are not modeled in power flow simulation tools, and the coupled BESS and inverter-based generating resources are aggregated to a single aggregate generator model. Since the coupling occurs at each individual generating resource, there is no BESS inverter, pad-mounted transformer, or equivalent collector system represented. Only the equivalent inverter-based generating resource (including the battery aspect), the ac-side equivalent pad-mounted transformer, and the equivalent collector system are represented. Similar to ac-coupled hybrid plants and other BPS-connected inverter-based resources, the substation main power transformer and generator tie line are modeled explicitly. Any shunt compensation such as shunt reactors, capacitors, FACTS devices, or synchronous condensers should be modeled as well. Again, elements shown in red may or may not be represented in the model based on each specific location, and elements shown in black should be modeled for all facilities. Common voltage levels are shown only for illustrative purposes.

51 There are other types of reliability studies such as harmonics and electromagnetic transient (EMT) simulations that are often performed during the interconnection study process; however, those studies are outside the scope of this guideline.



From Jeff Billo:May want to have this as a recommendation that industry software evolve to include this type of limitation (see Julia’s comment above).


Julia:We are currently getting a lot of requests with such plants wanting to self-limit their combined output to a certain value the question than is how to capture this limit in power flow studies but also how to even get this limit passed on from the resource registration information into powerflow cases overall


Figure 2.3: Generic Power Flow Model for DC-Coupled Hybrid Power Plants

The GO, TP, and PC will need to consider the following aspects of steady-state power flow modeling for dc-coupled hybrid power plants:

Charging Operation: Charging capability can be modeled by settings the equivalent BESS generator with an appropriate negative active power limit, Pmin. Note that the maximum charging limit (Pmin) may be different than the maximum discharging limit (Pmax). These Pmin and Pmax limits in the equivalent BESS generator record should be set to any limits imposed by the plant and inverter controllers in coordination with the capability of the inverters. Also, the TP and PC should ensure they understand how the other BESS facility components (e.g., shunt compensation) operate during charging operation such that the overall BESS model can be set up correctly in both charging and discharging modes.


Ryan Quint [2], 03/10/20,

From Bill Quaintance:I’m starting to see bi-directional solar inverters that technically allow charging of DC batteries from the grid when the solar panels are not producing. Might come into play in this paragraph.

Ransome Egunjobi, 03/05/20,

No mention of Discharge Operation and point of voltage control/PF mode


With an appropriate power flow representation for the BESS or hybrid plant, a set of dynamic models can be used to represent the behavior of these resources during BPS disturbances. The dynamic modeling practices again are very similar to other types of BPS-connected inverter-based resources, with some unique characteristics to capture the four-quadrant operation of energy storage devices as well as a state of charge. This chapter describes these concepts as they relate to dynamic model preparation. Refer to Appendix B for more details.

BESS Dynamics ModelingThe modeling structure of BPS-connected BESS is the same as BPS-connected solar PVs and type 4 WTGs. It consists of a converter control module, an electrical control module and a plant control module. In addition, the frequency ride-through settings and voltage ride-through settings are modeled with generator protection modules. Figure 3.1 shows the modeling structure.

Figure 3.1: Block Diagrams of Different Modules of the WECC Generic Models [x]

REGC (REGC_*) Module: Used to represent the converter (inverter) interface with the grid. It processes the real and reactive current command and outputs of real and reactive current injection into the grid model.

REEC (REEC_C/REEC_D)52 Module: Used to represent the electrical controls of the inverters. It acts on the active and reactive power reference from the REPC module, with feedback of terminal voltage and generator power output, and gives real and reactive current commands to the REGC module.

REPC (REPC_*) Module: Used to represent the plant controller. It processes voltage and reactive power output to emulate volt/var control at the plant level. It also processes frequency and active power output to emulate active power control. This module gives active reactive power commands to the REEC module.

Table 3.1 shows the list of BESS simulation modules used in two simulation platforms commonly used. Although the internal use may differ across simulation platforms, the modules have the same functionality and parameter sets.

Table 3.1: Dynamic Models used to Represent BESSs in PSLF and PSSE

Module GE PSLF Modules Siemens PTI Modules

Grid interface regc_* REGC*

Electrical controls reec_c or reec_d REECC1 or REECD1

Plant controller repc_* REPC*/PLNTBU1

Voltage/frequency protection lhvrt/lhfrt VRGTPA/FRQTPA

52 https://energystorage.org/why-energy-storage/technologies/hydrogen-energy-storage/


Zhu, Songzhe, 08/13/20,

Reference to WECC documentA lot of the material below are from the WECC guideline, tailored for BESS. What is the best way to show the reference?


From Himanshu Jain:Dynamics: The issues related to dynamics modeling have been discussed in previous NERC reports/guidelines on modeling of inverter-based resources. I see two key issues that I believe are not yet fully addressed. One is the lack of BESS dynamic model standardization, where the key question is how comfortable we are about the dynamic response of WECC type BESS inverter models under high inverter-based generation penetration. The other related issue is about the dynamic modeling algorithms and software. It is well known that electromechanical transient programs are not designed to simulate high bandwidth dynamics of power systems, which becomes important to consider in BESS and other IBRs given the concerns around the accuracy of idealized phase-locked-loop (PLL) behavior in current BESS dynamic models under weak grid conditions. Hybrid EMT-Transient modeling approaches look promising, particularly in conjunction with the recent work being done in the co-simulation space (e.g., HELICS from DOE). Full EMT modeling is also being attempted. A cost-benefit trade off analysis of these approaches should be considered before settling on a standard approach for a system with increasing IBR and BESS penetration. The cost being the amount of data and memory requirements for detailed modeling and benefit being the increased modeling accuracy.



Model InvocationThe model invocation varies according to the software platform. Users must follow the instructions provided with the model documentation. Attention should be paid to the regulated bus and monitored branch in the repc invocation. They should align with the control modes in the repc model. For example, if voltage droop control through droop control gain kc is intended, the monitored branch should be specified in the model invocation.

Scaling for the BESS Plant Size and Reactive Capability Model parameters are expressed in per unit of the generator MVA base except in repc_b. The specification of MVA base is implementation-dependent. For example, in the PSLF implementation, if the MVA base for those modules is zero, the MVA base entered for the regc module applies to the electrical controls (reec) and plant controller (repc). The user may specify a different MVA, if desired. In the PSSE implementation, the MVA base is set in the power flow model.

To scale the dynamic model to the size of the plant, the generator MVA base parameter must be adjusted. It should be set to sum of the individual inverter MVA rating. The active and reactive range are expressed in per unit on the scaled MVA base.

The MVA base for REPC_B model is always 100 MVA. The per unit parameters of RECP_B model should be expressed on the 100 MVA base.

Reactive Power/Voltage Controls OptionsThe plant-level control module allows for the following reactive power control modes:

Closed loop voltage regulation (V control) at a user-designated bus with optional line drop compensation, droop response and deadband.

Closed loop reactive power regulation (Q control) on a user-designated branch, with optional deadband.

Constant power factor control (PF control) on a user-designated branch active power and power factor. This control function is available in repc_b, not in repc_a.

In the electrical control module, other reactive control options are available:

Constant power factor (PF), based on the generator PF in the solved power flow case.

Constant reactive power based on either the equivalent generator reactive power in the solved power flow case or from the plant controller.

Closed loop voltage regulation at the generator terminal.

Proportional reactive current injection during a user-defined voltage-dip event.

Various combinations of plant-level and inverter-level reactive control are possible by setting the right parameters and switches. Table 3.2 shows a list of control options, and the models and switches that would be involved. The entry "N/A" for vflag indicates that the state of the switch does not affect the indicated control mode. The entry “N/A” for refflag means repc model is not present. Although all the options in the table are mathematically valid, the options that are unlikely for BESS operation are greyed out. These unlikely options are due to 1) the plant controller is used in all newly interconnected inverter-based power plants and 2) the plant level Q control option and the plant level PF control option do not meet the automatic voltage regulation requirement in most, if not all, of the transmission planning areas.



Table 3.2: Reactive Power Control Options for BESS Generic ModelsFunctionality Required Models pffflag vflag qflag refflag

Constant local power factor control REEC 1 N/A 0 N/A

Constant Q control REEC 0 N/A 0 N/A

Local V control REEC 0 0 1 N/A

Local coordinated Q/V control REEC 0 1 1 N/A

Local coordinated PF/V control REEC 1 1 1 N/A

Plant-level Q control REEC + REPC 0 N/A 0 0

Plant-level V control REEC + REPC 0 N/A 0 1

Plant-level Q control & local coordinated Q/V control

REEC + REPC 0 1 1 0

Plant-level V control & local coordinated Q/V control

REEC + REPC 0 1 1 1

Plant-level PF control REEC + REPC (_b and above)

0 N/A 0 2

Plant-level PF control & local coordinated Q/V control

REEC + REPC (_b and above)

0 1 1 2

* Greyed out options are not likely to be deployed; although they are mathematically feasible.

Active power control optionsThe plant controller models includes settable flags for the user to specify active power control. Table 3.3 and Table 3.4 show the active power control modes, the models, and parameters involved, respectively. These types of controls include:

Constant active power output based on the generator output in the solved power flow case

Active power-frequency control with a proportional droop of different gains for over- and underfrequency conditions, based on frequency deviation at a user-designated bus

The BESS is expected to provide frequency response in both upward and downward directions. The no response and down only options are greyed out because they are unlikely to be approved by the transmission planning entity. In the WECC proposed modeling enhancement for hybrid power plants, the base load flag in the power flow model could override the frqflag setting in the dynamic model. The frqflag/ddn/dup are meant to reflect the inverter capability while base load flag represents the availability of the operational headroom. It is important to set base load flag to 0 for BESS generators regulating frequency.


Zhu, Songzhe, 08/13/20,

Will provide reference once it is published


Table 3.3: Active Power Control Options with REEC_C and REPC_A/BFunctionality Required Models pffflag vflag

No frequency response 0 0 0

Frequency response, down only regulation 1 > 0 0

Frequency response, up and down 1 > 0 > 0

No frequency response 0 0 0

Table 3.4: Active Power Control Options with REEC_D and REPC_*Functionality BaseLoad flag* frqflag ddn dup

No frequency response 2 0 0 0

Frequency response, down only regulation 1 1 > 0 > 0

Frequency response, up and down 0 1 > 0 > 0 *BaseLoad flag is set in the power flow model.

Current Limit LogicThe electrical control module first determines the active and reactive current commands independently according to the active power control option and reactive power control option. Each command is subject to the respective current limit, 0 to Ipmax for active current and Iqmin to Iqmax for reactive current. Then the total current of √ Ipcmd2+ Iqcmd2 is limited by Imax. In situations where current limit Imax of the equivalent inverter is reached, the user should specify whether active or reactive current takes precedence, by setting the pqflag parameter in the REEC module.

State of ChargeThe REEC_C module includes simulation of BESS’s SOC (see Figure 3.2). An initial condition SOCini is specified. Then Pgen is integrated during the simulation and added to SOCini. When SOC reaches SOCmax, i.e. fully charged, charging is disabled by adjusting ipmin from a negative value to 0. Similarly when SOC reaches SOCmin, i.e. depleted of energy, discharging is disabled by adjusting ipmax from a positive value to 0. This requires the user sets SOCini based on the dispatching condition being analyzed. It has been a common source of error that the BESS is in the charging mode with SOCini = 1 and the Pgen is forced to 0 in the simulation. Given the timeframe of transient stability simulation, change of SOC throughout the simulation is negligible. For this reason, the SOC is removed from the REEC_D module. For details of REEC_D, refer to Appendix XXX.

Figure 3.2: Block Diagram of the Charging/Discharging Mechanism of the BESS

Representation of Voltage and Frequency ProtectionFrequency and voltage ride-through are needed for transmission-connected solar PV plants. Because they are simplified, the generic models may not be suitable to fully assess compliance with the voltage and frequency ride-through requirement. Voltage ride-through is engineered as part of the plant design and needs far more sophisticated modeling detail than is possible to capture in a positive-sequence simulation environment. It is best to



use a standardized (existing) protection model with voltage and frequency thresholds and time delays to show the minimum disturbance tolerance requirement that applies to the plant. Also, the frequency calculations in a positive-sequence simulation tool is not accurate during or immediately following a fault nearby. It is best to use the frequency protection relay model in a monitor-only mode and always have some time delay (e.g., at least 50 ms) associated with any under- and over-frequency trip settings.53

Hybrid Plant Dynamics ModelingThe dynamic modeling approach to hybrid plants again depends on whether they are ac-coupled or dc-coupled.

AC-Coupled Hybrid ModelingFor an ac-coupled hybrid plant, each type of the resources is modeled explicitly by a set of equivalent generator(s), equivalent pad-mounted transformer(s) and equivalent collector system(s) in the power flow. Each generator has its set of REGC and REEC models. It is recommended that REPC_B is used as the master plant controller to coordinate electrical controls among all generators and apply plant level active and reactive power limits. It is also recommended that REEC_D is used for the non-BESS inverter-based generators for the reason discussed later in active power control.

Table 3.5: Models for AC-Coupled Hybrid Plants (in PSLF and PSSE)Functionality GE PSLF Module Siemens PTI Module

BESS Grid Interface regc_* REGC*

BESS Electrical Controller reec_c or reec_d REECC1 or REECD1

Plant-Level Controller repc_b54

PLNTBU1

Auxiliary Controller REAX4BU1 or REAX3BU1

Voltage/Frequency Protection lhvrt/lhfrt VRGTPA/FRQTPA

Non-BESS Generation Component of Hybrid Facility

Use appropriate modules for the generation type (i.e., applicable models for wind, solar, synchronous generation, etc.)

The BESS modeling follows the same principles discussed in the previous section. This section provides additional considerations unique to the hybrid power plants.

Model InvocationIn PSLF implementation, REPC_B is invoked from one of generators in the plant. It is important to have REPC_B invoked from an on-line generator. Also the regulated bus and the monitored branch must be specified for REPC_B.

Reactive Power ControlEach individual generation type in the hybrid power plant has its qmax and qmin specified in REEC module. The qmax and qmin in the REPC_B module represents the reactive capability limits at the plant level. Depending on the specific generation interconnection requirement, the plant level limit could be contractual instead of physical. The qmax and qmin should reflect how the plant operates. It should also be noted that qmax and qmin in REPC_B are provided on 100 MVA base instead of the generator MVA base.

53 Note that hybrid natural gas-BESS plants may be desirable in some areas where capacity shortages have been identified.54 In ERCOT, a BESS was added to a quick-start combustion turbine for participation in ERCOT’s Responsive Reserve Service. The combustion turbine is normally offline, and if frequency falls outside of a pre-defined deadband, the BESS will provide fast frequency response until the combustion turbine is turned on to sustain the provided response.



The reactive power capability requirement is generally specified at the high side of the substation transformer(s). For a hybrid power plant, an individual generation type may not have the capability to meet the requirement. Instead different generation types supplement each other to provide required var capability. Depending on the dispatch condition, one type may have little reactive capability available and the other has full capability. The weighting factors of voltage/var control, kwi, need to be tuned for different operating conditions.

Active Power ControlMost of the hybrid power plant has a contractual plant level Pmax less than the sum of the individual generator Pmax. Pmax and Pmin in the REPC_B module represents the contractual plant level active power limits. Pmax and Pmin in REPC_B are provided on 100 MVA base instead of the generator MVA base.

The frequency response is only modeled in REPC_B for the entire plant and pref is distributed among generators by the weighting factors kzi. Kzi may need to be tuned for different operation conditions. But more often, the hybrid plant relies on BESS for upward frequency response. REEC_D module should be used in conjunction with REPC_B to block or enable frequency response at the generator level. See an example in Table xx. The gen type that does not have headroom for upward frequency response has base load flag set to 1. REEC_D module will set Pmax to initial Pgen during the initialization, thus the blocking upward frequency response. The BESS has base load flag set to 0 and will respond to the active power command from REPC_B.

Table 3.6: Active Power-Frequency Control Settings for Hybrid Configurations

Component BaseLoad Flag Module

Solar PV - Frequency response, down only regulation 1 reec_d

BESS - Frequency response, up and down 0 reec_c or reec_d

Plant controller Repc_b withFrqflag=1, dup > 0, ddn > 0

DC-Coupled Hybrid ModelingFor a dc-coupled hybrid plant, one equivalent generator represents the inverters for multiple DC side sources, typically solar PV and battery storage. One set of REGC, REEC and REPC models is needed for the equivalent generator. The electrical control module suitable for the battery storage (REEC_C or REEC_D) could always be used for this type of inverters. In case the battery does not charge from the grid, one may choose to use the electrical control module suitable for the other DC side energy source, e.g. REEC_A module.

Table 3.7: Models for DC-Coupled Hybrid in PSLF and PSS®E

Component PSLF Module PSS®E Modules

Grid Interface regc_* REGC*

Electrical Controls

May Charge from Grid reec_c or reec_d REECC1 or REECD1

DC-Side Charging Only reec_a or reec_d REECA1 or REECD1

Plant Controller repc_* REPC*/PLNTBU1

Voltage/Frequency Protection lhvrt/lhfrt VRGTPA/FRQTPA

The modeling considerations for dc-coupled hybrid plant are the same as the discussed in BESS modeling above.



What goes in baseload flag column? N/A?


Reference used in the section:1) WECC Solar Photovoltaic Power Plant Modeling and Validation Guideline

https://www.wecc.org/Reliability/Solar%20PV%20Plant%20Modeling%20and%20Validation%20Guidline.pdf

2) White Paper on Modeling Hybrid Power Plant of Renewable Energy and Battery Energy Storage SystemTo be published

3) WECC Memo Proposal for New Features for the Renewable Energy System Generic Modelshttps://www.wecc.org/Administrative/Memo%20RES%20Modeling%20Updates_121719_Rev15.pdf


https://www.wecc.org/Administrative/Memo%20RES%20Modeling%20Updates_121719_Rev15.pdf

https://www.wecc.org/Reliability/Solar%20PV%20Plant%20Modeling%20and%20Validation%20Guidline.pdf

Chapter 4: BESS and Hybrid Plant Short Circuit Modeling

As with other BPS-connected inverter-based resources, BESSs and hybrid plants should be modeled in short circuit programs both during the interconnection process and during ongoing planning, design, and protection setting activities. TPs, PCs, TOs, and other entities will need to develop modeling practices for BESSs and hybrid plants to execute necessary short circuit studies. At a high-level, recommendations for modeling BESSs and hybrid plants are no different than the current guidance for other full-converter inverter-based generating resources (e.g., Type 4 wind plants, solar PV plants, voltage source converter (VSC) HVDC, and other FACTS devices). 55 For BPS short circuit modeling, using similar practices between BESSs or hybrid plants and other IBRs will capture the key performance characteristics of all resources because the inverter configurations are similar. Considerations such as appropriate ratings and other nuances56 involved with modeling each device will need to be considered.

While the state-of-the-art inverter-based resource short circuit modeling practices continue to evolve, the IEEE PSRCC WG C24 recently published a report titled “Modification of Commercial Fault Calculation Programs for Wind Turbine Generators” [1]. The intent of this report is to advise industry with modifications needed to commercial short circuit programs to allow correct modeling of wind turbine generators (WTGs) and wind power plants. Even though the report does not specifically discuss modeling solar PV, BESS, or other inverter-based resources, the recommendations in the report for modeling Type 4 WTGs (i.e., full-converter resources) also apply to the other types of inverter-based resources. BESS Short Circuit ModelingIn general, inverters are voltage-dependent current sources. This means that the amount of active and reactive current injection by any inverter-based resource during a fault condition is dependent on the terminal voltage. Inverter control logic is what dictates the voltage dependency, and typically is non-linear. Particularly for BESSs, the amount of fault current injection is also dependent on the state-of-charge (SOC) prior to the fault as well as whether the BESS is in charging or discharging mode. For example, if the BESS is charging when the fault happens, and if the inverter control requires the BESS to continue charging (absorbing active power) during the fault, the BESS may have limited capability to provide dynamic reactive fault current.

The IEEE PSRCC WG C24 report recommends that for IBRs, the fault current injection information is collected in a tabular form from the resource owner. An example is shown in Table 1. The report acknowledges that considering unbalanced faults, information may be provided in sequence domain (positive, negative and zero sequences) or in phase domain (phase A, B, and C), although data in sequence domain is preferred. The report also acknowledges that current contribution during faults is driven by the inverter’s control system and it may take some time for a control system to drive current to next steady-state value. Hence, it recommends that the data is provided for various time instants after initiation of a fault. For example, provide data in Table 1 at time T = 1, 3 and 5 cycles after initiation of a fault.

The example in Table 4.1 provides a sample data for a steady-state positive sequence fault current contribution. For example, if a three-phase fault somewhere on a transmission network results in an inverter terminal voltage to drop to 50%, then it is expected to inject 120% of rated current at a power factor angle of -45 degrees. The negative power factor angle means the reactive current is injected into the network, i.e., injected current lags the terminal voltage. Assuming that the inverter is not designed to inject unbalanced current during unbalanced faults, the inverter would inject the same current if a L-L fault on the network results in an inverter terminal positive sequence voltage of 50%. However, if the inverter can inject an unbalanced current, then a similar table representing negative sequence quantities is expected to be provided by the resource owner.

55 The capability curve is almost symmetrical because when the BESS is operated in the second and third quadrant (consuming active power), a rise in dc voltage could limit the amount of power generation where reactive power also has to be consumed.56 https://www.nrel.gov/docs/fy19osti/74426.pdf



Make it crystal clear that the table is an example. Not de facto response.



Table 4.1: Example of Steady-state fault current contribution from BESS Time since initiation of a fault, T (cycles): Steady State

Fault Type: Balanced Three-Phase

Positive Sequence Voltage V1 (per unit)

Positive Sequence Current I1 (per unit) Angle between V1 and I1 (degrees)

Active Current

Reactive Current

Total Current

0.9 1.00 0.17 1.01 -9.70.8 1.00 0.34 1.06 -18.80.7 1.00 0.51 1.12 -27.00.6 0.80 0.68 1.20 -34.50.5 0.85 0.85 1.20 -45.00.4 0.63 1.02 1.20 -58.30.3 0.15 1.19 1.20 -82.90.2 0.0 1.20 1.20 -90.00.1 0.0 1.20 1.20 -90.0

At the time of writing of this guideline, the major commercial short circuit program developers [2-3] have accommodated this new way of representing voltage-dependent current sources into their respective programs.

For BESS, although the charging/discharging mode as well as the state-of-charge (SOC) prior to the fault affect the magnitude of fault current injection, the Table 4.1 data provided for BESS in discharging mode and at maximum state-of-charge (SOC) is considered an adequate and reasonable BESS short-circuit model. If for some reason the reduced fault current injection due to the BESS in charging mode and/or the actual (lower) state-of-charge (SOC) has to be determined, then additional tables would be needed.

Hybrid Short Circuit ModelingAs with the steady-state and dynamics modeling recommendations described in Chapter 2 and Chapter 3, respectively, short-circuit modeling recommendations depend on whether the plant is ac-coupled or dc-coupled:

DC-Coupled Hybrid Plant: As noted earlier, the fault current contribution is from inverter that couples the AC side with multiple resources on DC side. The behavior of an inverter does not change if there are multiple energy sources behind it. For the purpose of short circuit modeling, inverter should be modeled same as noted above. In other words, DC-coupled plants are modeled just like any other IBR plant.

AC-Coupled Hybrid Plant: An ac-coupled hybrid power plant couples each form of generation or storage at a common collection bus after it has been converted from dc to ac at each individual inverter. In this case, inverters used for each form of generation could be different make, manufacturer, vintage etc. As such, it is recommended that inverters for each generating resource is modeled separately.

References: 1. IEEE PES Technical Report TR78: Modification of Commercial Fault Calculation Programs for Wind Turbine

Generators available at https://resourcecenter.ieee-pes.org/technical-publications/technical-reports/PES_TP_TR78_PSRC_FAULT_062320.html [resourcecenter.ieee-pes.org]

2. Siemens Technical Bulletin - Inverter-Based Generator Models with Controlled Power and Current – 2019 PSS CAPE User Group Meeting

3. ASPEN Technical Bulletin – Modeling Type-4 Wind Plants and Solar Plants



Need something describing HOW the SOC affects the fault current injection. Need discussion/reference/study.Evangelos to review.Prashant – SOC would matter only if you are at the end of the charge/discharge (where you don’t operate the battery). So SOC and fault current are relatively independent. K factor is important for fault current injection, similar to other IBRs. Reactive has no impact on DC side. Evangelos to describe how pre-fault conditions affect fault current injection (and how to model that).Hari – this discussion on fault current contribution and SOC should go in Chapter 1, not in Chapter 4. Evangelos – need table(s) for charging and table(s) for discharging for different fault current characteristics. Or a clear note from GO/OEM that they are the same. Deepak – what about closed loop current injection for faults? Sid – GE gives options (open vs. closed loop), but most are open loop configuration. Hassan – consider negative sequence. TOs should clearly define the tables and quantities required at the time of interconnection (pre-fault conditions, SOC conditions/sensitivity, etc.).Action: Evangelos, Deepak, Hari, Manish, Normann, Prashant, Sid to fine-tune this section and provide redline edits back for team review.Key points to make: Sensitivities to SOC, pre-fault conditions, charge vs. discharge setup, etc.

https://urldefense.proofpoint.com/v2/url?u=https-3A__resourcecenter.ieee-2Dpes.org_technical-2Dpublications_technical-2Dreports_PES-5FTP-5FTR78-5FPSRC-5FFAULT-5F062320.html&d=DwMFAw&c=AgWC6Nl7Slwpc9jE7UoQH1_Cvyci3SsTNfdLP4V1RCg&r=9NXkq3VhTL24-H5OSqihfd7o0X2x3LbjkZAfeVaku1M&m=z85l36BJkGw_p_dr2xO74BGLoXyhGALdaLBiHqGCPWA&s=embEexZAsOIWKpaw3eCMkSRi1Ew3aoLLzYQzdE4KMjY&e=

https://urldefense.proofpoint.com/v2/url?u=https-3A__resourcecenter.ieee-2Dpes.org_technical-2Dpublications_technical-2Dreports_PES-5FTP-5FTR78-5FPSRC-5FFAULT-5F062320.html&d=DwMFAw&c=AgWC6Nl7Slwpc9jE7UoQH1_Cvyci3SsTNfdLP4V1RCg&r=9NXkq3VhTL24-H5OSqihfd7o0X2x3LbjkZAfeVaku1M&m=z85l36BJkGw_p_dr2xO74BGLoXyhGALdaLBiHqGCPWA&s=embEexZAsOIWKpaw3eCMkSRi1Ew3aoLLzYQzdE4KMjY&e=

Chapter 5: Studies for BESS and Hybrid Plants

As BESS and hybrid plants become more prevalent, it will become increasingly important to accurately reflect these resources in simulations of BPS reliability, including studies during the interconnection process as well as operational planning and annual planning assessments. When considering study assumptions, the primary difference between BESS (including hybrid plants with BESS), when compared to other resources, revolves around the assumptions regarding charging and discharging operating points under various system conditions. This chapter describes considerations to be accounted for in these studies modeling the various dispatches and studying the reliability impacts of these resources.

Interconnection StudiesInterconnection studies for new or modified BESS and hybrid plants include the same types of studies performed for any other IBR, including steady-state, short circuit, and stability analyses. These studies should be designed to consider all reasonable charging and discharging scenarios the plant may be expected to experience and that may be expected to stress the system and the plant under study. Consideration should be given to the characteristics of the system where the plant is interconnecting, including other resource types in the area.

Table 5.1 provides a list of example scenarios possibly studied during the interconnection process and considerations for each. This list is not exhaustive, nor should it be thought that every interconnection study should cover all of these scenarios. In general, it should be assumed that the owners of BESS and hybrid plants will follow system and market incentives when deciding when to charge and discharge batteries. For example, in a market, when the price of power is high, the battery will typically be discharging, and when the price is low, the battery will typically be charging. Also, usually the price of power will be high during peak load and low during low load or high renewable generation output conditions. The below table was constructed with these assumptions in mind with exceptions noted.

Table 5.1: Potential BESS and Hybrid Plant Study ScenariosSystem Conditions

Plant Type Plant Dispatch Considerations

Peak load

BESS

Fully discharging This is a feasible scenario.

Fully charging

Depending on market mechanisms and system rules, this scenario may not be feasible. However, there may be situations where this is a feasible scenario. For example, in a system that has a lot of wind generation, if there is high wind output at peak load a BESS may be charging to prepare for a time later in the day when the wind is expected to die down. Another feasible scenario would be when a BESS is charging right before peak load, when the system is “near” peak.

HybridMaximum plant output

This is a feasible scenario. This scenario could be achieved by a combination of maximum renewable generation output and/or maximum battery output to achieve the maximum facility rating as limited by the power plant controller.

Maximum renewable generation output with battery fully charging

This may be a feasible scenario. Though it is unlikely to stress the system, this scenario could stress the plant and may need to be studied in transient simulations.



From Thomas Schmidt Grau:For planning studies there might not be a concern to use Generic Models for high level mass studies. However for interconnection studies I wont recommend to use Generic Models. A lot of details in how the plant will respond will not be captured and it will be very hard to ensure Grid Compliance using the models. Dispatch strategies, Communication, Ramping, Rise Times, POD, etc. We can already see today that Generic Models are not adequate to represent actual turbine performance, this discrepancy will be much bigger when combining different types of generation. Furthermore there will not be a map between what is being studied and what is going to be commissioned. We also need to consider the challenges at ERCOT and the direction they are moving, which I think is good from both a Planning and Operational perspective.




No or low renewable generation output with battery fully discharging

This is a feasible scenario where the battery injects power output at its maximum capability with some or no contributions from the renewable generation.

No or low renewable generation output with battery fully charging from the grid

This may or may not be a feasible scenario depending on market mechanisms and rules, interconnection agreement limitations, and plant design. Some tax rules may incent hybrids to not charge from the grid during the first five years of operation, but it may be feasible starting in year six.

Low load

BESSFully discharging

This is an unlikely scenario, but it is possible an area could have a high price, due to nearby constraints, so it could need to be studied.

Fully charging This is a feasible scenario.

Hybrid

Maximum plant output

This is a feasible scenario. This scenario could be achieved by a combination of maximum renewable generation output and/or maximum battery output to achieve the maximum facility rating as limited by the power plant controller.

Maximum renewable generation output with maximum battery charging


No or low renewable generation output with battery fully discharging

This may be a feasible scenario.

No or low renewable generation output with battery fully charging from the grid

This may be a feasible scenario, depending on market mechanisms and rules, interconnection agreement limitations, and plant design. Some tax rules may incent hybrids to not charge from the grid during the first five years of operation, but it may be feasible starting in year six.

High system-wide renewable generation output

BESSFully discharging This is an unlikely, but possible scenario.

Fully charging This is a feasible scenario.

Hybrid

Maximum plant output This is a feasible scenario.

Maximum renewable generation output with maximum battery charging






Congestion Management

BESS Fully discharging or charging

This may be a feasible scenario. BESS may be dispatched to manage a local congestion independent of a system load. Although it is unlikely to stress the overall system, this scenario may be considered to study nearby line-overloads.

Changes in operating mode

BESS Variable

BESS can operate in different operating modes that may change over time. Examples include: Frequency controlling, peak shaving, market, etc. Transmission Planners should consider the impact of each operating mode on the system.

Changes in dispatch BESS Variable

Many BESS change dispatch frequently and quickly. BESS at the subtransmission level have been observed to change dispatch from full load to full gen in the ‘seconds’ timeframe, repeating this behavior tens to hundreds of times per day. Transmission Planners should analyze the impact that this change in power dispatch can have the system. Potential concerns include power quality, flicker, voltage deviation, and successive operations of voltage control devices to maintain normal system voltage. Some voltage control devices such as transformer LTCs or fixed capacitors are limited in the number of operations that are allowed in a given timeframe.

Transmission Planning Assessment StudiesTraditionally, system-assessment steady-state and stability studies tend to focus on peak-load and off-peak study conditions. These studies typically have a single set of base generator dispatch assumptions with perhaps an alternate generator dispatch sensitivity. However, with the growth of variable energy resources, combined with an increase in BESS and hybrid resources, operational planning and long-term planning studies need to evolve to analyze more scenarios, as there may be critical and stressed conditions outside of those traditionally studied. Transmission Planners and Planning Coordinators should work with appropriate stakeholders to develop a set of study conditions reasonably expected to stress the system for their region. Planner may consider the impact of available State-of-Charge of the batteries to ensure the BESS provide migrations for entirety of the event. Table 5.1 above provides a good reference to begin these conversations. Additionally, Appendix X provides examples of scenarios studied by some planners.

Another good approach to get the charge-discharge dispatch scenarios correct is to employ Production-Cost models (like PROMOD or PLEXOS) to output the dispatch patterns of the BESS (stand-alone or hybrid) for all the hours of the year. From this, the stressed conditions can be determined through engineering judgement. This method can produce realistic dispatch patterns.

A tool able to analyze every hour of dispatch and load (like PSS SINCAL) could also be used for the power-flow and dynamics analyses to avoid guessing at the most stressed conditions.

Other StudiesFor model quality testing purposes, it is recommended to perform numerous scenarios and analysis during the interconnection process to ensure appropriate model behavior. These tests should be at least as rigorous as those


Jeff Billo, 07/24/20,

Add examples from planners already doing this (e.g. CAISO, etc.) in the Appendix.

Nihal Mohan, 08/14/20,

BESS can be used for providing migrations, however, planners should consider the minimum energy BESS should carry and NOT run out of it before automatic / operators actions can take place.

Warren Hess, 08/12/20,

Deleting this word helps the document be applicable many years into the futue.


Not sure if this is true. generally PCs have differet rules for dispatch assumptions. Recommend removing this line as subsequent line can still be used to make the case.

Hung-Ming Chou (Services - 6), 08/15/20,

Addition by David Piper


Addition by David Piper


Batteries could provide congestion mangement irrspectve of the load period. However not sure if this language belongs to interconnection study.


currently required by regional entities with some additional scenarios to address the complexity of projects with a hybrid composition including a BESS.

Generic models (such as the generic models developed under the aegis of the WECC MVWG) of the hybrid plant may be used in planning analysis. However, it should be noted that while conducting a detailed analysis during the interconnection process, the use of generic models for a hybrid plant may have the potential to misrepresent the actual control capabilities and limitation of the hybrid project. An example of this misrepresentation is provided below:

In order to ensure that the most accurate representation of the hybrid plant is studied during the interconnection process, once the OEM of the hybrid plant is known, it is recommended that first the manufacturer user-written models be used for a detailed interconnection study of the plant. This will ensure a more accurate representation of the hybrid resource is studied and configured. Using manufacturer user-written models for the additional studies to be performed during the interconnection process will also make it easier to confirm that the appropriate settings are installed in the field and implemented in various other model representations needed by regional entities. Once a manufacturer user-written model representation for the BESS and the renewable resource is configured, a generic controller model can be tuned to match the response of the user-written model as closely as possible under typical grid disturbances. This will ensure that, if used for interconnection wide planning, the generic representation is as representative as possible of the user-written model and by extension the hybrid resources field performance.

Many regional entities currently require resources to provide a series of model quality tests in order to demonstrate appropriate performance of the resource model before moving forward in the interconnection process. Below is a summarized list of recommended tests which reflect current regional guidelines:

High Voltage Ride Through Analysis and Low Voltage Ride Through Analysis.

Perform with the hybrid resource under both a leading and lagging scenario.

Perform under full charging and full discharging conditions.

Perform under some more typical operating conditions (i.e. full renewable discharge with BESS partially charging).

Small Voltage Disturbance Analysis.

Perform under both charging and discharging conditions.

o Perform under some more typical operating conditions (i.e. full renewable discharge with BESS partially charging).

Small Frequency Disturbance Analysis,

o Perform under full charging and full discharging conditions.

– This will demonstrate the models respect resource capabilities in both charge states.

o Perform under slightly de-rated charging and discharging conditions.

– This will demonstrate active power response for both charge states.

o Perform with the resource near 0 active power output.

– This will demonstrate the resources capability to transition from charging to discharging and vice versa during a frequency disturbance.

Grid Strength Analysis.

Perform a series of POI faults at differing grid SCR.



Alex, could you kindly provide this example?


Added by Alexander Shattuck


Perform at both full charging and discharging scenarios.

If a BESS or hybrid plant is to be used as part of a black start plan, additional studies need to be conducted to ensure proper transition to and from islanding mode, operation in islanding mode, and to test the ability to black start. It is assumed that the plant will have grid forming capability, such that it can operate in islanding mode without any support from synchronous machines. For these studies, BESS and hybrid plants need to have the available energy and be dispatched suitably for the corresponding studies (i.e., not at the current limit). Some examples and considerations for these studies are summarized below:

Transitioning to and from islanding mode: The objective is to ensure stable transition of BESS operation between grid connected mode and islanding mode. An example of such study is to consider loss of the last synchronous machine in the network that results in the BESS or hybrid plant (possibly along with other IBRs) being the only sources of energy to serve load. Following the transition, and for any subsequent events within the island (example a fault or load change), the BESS or hybrid plant (and other IBR) controls should be able to bring voltage and frequency back close to their nominal values while meeting existing reliability and system security metrics. The same stable transition should be delivered when returning to a grid connected mode.

Operating in an islanding mode: the objective is to ensure proper control coordination (both voltage and frequency) between other IBRs and synchronous machines. Simulation tests to be performed may include load step up/down, ringdown, voltage ride-through and frequency ride-through tests.

Blackstart: if the BESS or hybrid plant is expected to be used as a black start resource, then the objective is to ensure the black start capability can be met whether the BESS or hybrid plant is the sole resource or is deployed to start another unit. A typical example of a black start study can be conducted as follows: energize main power transformer from project side, connect the project to a load, and then apply a bus fault at the POI, and clearing the fault, and demonstrate XXX.

For all the above examples, voltage and frequency should be stable and settle back close to their nominal values after the disturbance.

Hybrid Additions – Needed StudiesExample: 100 MW solar plant, want to offer a “surplus service”, I want to offer 50 MW to a battery developer, and they can use 50 MW of the capacity that I am allocated. The total amount will never exceed the agreed upon 100 MW, but may come from either the solar/wind/gen and the BESS (or other gen). Question: What types of studies should be required/needed given this situation? Normally do interconnection studies: voltage, powerflow (thermal overload), dynamics, short circuit, etc. But what types of studies would need to be executed for this new type of interconnection? How does this relate to FAC-002-2? Provide clarity on this.

Songzhe:Typically come from same owner – want to add BESS to existing facility. Use “material modification process”. Don’t necessarily perform powerflow since total capacity remains the same. If charging from grid, then this may require. Short circuit duty – definitely, dynamics – definitely. After approving addition, it is seen as any other hybrid facility in terms of how it is studied in the planning realm.

Venkat:Material modification was not allowed for changing any dc-coupled. That required a new queue request due to the full topology change. But in general, this particular thing is different between entities; at large, ac-coupled would use the material modification.



Introduced by Doug Bowman - SPP


Added by Warren Hess. His comment: We need to add the reason and results of this study.

Alexander Shattuck, 08/12/20,

Synchronous machines will likely be part of any black start plan and would likely be one of the first resources back online during a black start so we would need to at least include interactions with synchronous machines.


We just defined above that an island is formed when the last synchronous machine is disconnected. Consequently, should synchronous machines be deleted from here?


Added by Yahyaie


Do we assume grid-forming BESS only exist for the case where it could operate in islanding mode? Is it possible that BESS has grid-forming control even if it only operates in grid connected mode?


How are we going to define an island? Just any standalone portion of the network which has no synchronous machines?


Deleted by Alex S.


Would it be possible to just write a small statement here to the effect: Similar to model quality tests conducted for other IBR resources, model quality tests should also be conducted for hybrid plants both in the charging mode and discharging mode.The reason is, if all these test are similar to what other resources are expected to do, why not just put a reference?


Supriya:NYISO – separate interconnection request for the new battery addition (new changes coming for hybrid projects). This is true even if the total capacity of the facility does not change. Reason is because no rules at NYISO that could enable this or prevent unexpected changes. So these require new interconnection requests at this time.

- Doug – Same situation at SPP, they just call it a “surplus interconnection request”

(TO BE DELETED)

- Types of models (UDM and generic?)- Dispatch considerations for BESS/hybrid under test

o Hybrid: CAISO allows customer to choose their choice (choose their operation mode). Studies are

then set up accordingly. Need to consider regulatory aspects Can charge from the grid – need to talk about who is the “source” and the “sink” of the

transactions for charging. – some different regulationso Brad:

Focus on modeling and studies recommendations, not on the individual market requirements/economics.

- Dispatch considerations for other BESS and hybrid plants- Annual planning assessments AND interconnection process – any differences? - Charging vs. discharging modes – what is different in study?

o Which “types” of studies, or study processes, are performed?o Charging mode – generation areas could become load pockets in some cases… - what requirements

exist when operation in this mode (i.e., voltage control)?- Voltage control capability – hybrids able to act as battery when no solar/wind power output – can provide

voltage support to system.- Ride-through studies.

Andrew:- The battery wouldn’t look like a conventional IBR anymore. It would look completely different.- BESS really need to be treated as loads in the studies (not just generators).

o BESS switching to loads can “wreak havoc” in studies, particularly if not controlling voltage. Power flow issues (thermal violations during contingency events) Steady-state voltage collapse issues Transition from full load to full generation can be problematic (particularly for larger sized

projects) Power quality issues (David Piper)

David: - I’d like to have a discussion on how BESS can be analyzed in the operations planning horizon. If a plant is in

frequency control mode and changes dispatch from full load to full gen and back, Operations Engineers must account for the facility being in both conditions.



- I’d like to also have a discussion on the concept/practice of offsetting transmission upgrades by installing BESS. If we’re going to offset a transmission upgrade, there needs to be some level of assurance that the resource will be available in the condition that it is deemed necessary for, and some course of action should be considered/developed for a case where the resource is not available in the real-world.

Deepak:- Transition of BESS from generation to load should be studied in a transient time frame if the situation need

arises.- In the power flow realm, hosting capacity studies should be carried out for BESS both in the generation

mode and in the load mode and this should be coordinated with system loading levels.- This transition should also consider the state of charge dynamics in a quasi-steady state realm.

Fabio:- Lots of requests for hybrid plants- What do you have on interconnection requirements?

o Can only charge from PV?o Can charge from the system?o What are the barriers that need to be broken down to enable grid charging? (Study as a load)

Figure 1.1: Do not Use this Text in The Appendix

Example text. See Table 2.1.

Table 2.1: Report Template Formatting

Font Size Typeface Space Before Space After Color

Cover Page

Title Tahoma 48 Bold 0 0 dark blue

Subtitle Tahoma 24 Default 0 0 dark blue

Internal Pages

Chapter Title Tahoma 16 Bold 0 0 dark blue

Section Heading Tahoma 14 Bold 0 0 black

Sub-Heading 1 Tahoma 11 Bold 0 0 black



Added by David Piper


Sub-Heading 2 Tahoma 11 Bold, Italic 0 0 black

Body Text Calibri 11 Default, Justified 0 0 black

Header/Footer Calibri 9 Bold, Centered 0 0 dark blue

Footnotes Calibri 9 Default 0 0 black


Appendix A: Relevant FERC Orders to BESSs and Hybrids

The Federal Energy Regulatory Commission (FERC) recently issued Orders pertaining to electric storage resources, relevant to the guidance contained in this Reliability Guideline. FERC defined an electric storage resource as:

Electric Storage Resource (FERC Definition):57 a resource capable of receiving electric energy from the grid and storing it for later injection of electric energy back to the grid.”

FERC’s determinations in Order No. 841, Order No. 842, and Order No. 845 are leading to new wholesale market participation models, updates to interconnection studies processes, and new operating practices.

FERC Order No. 841In Order No. 841 (February 15, 2018), FERC required Regional Transmission Organizations (RTOs) and Independent System Operators (ISOs) under its jurisdiction to establish participation models that recognize the physical and operational characteristics of electric storage resources. Each participation model, per the Order, must “ensure that a resource using the participation model for electric storage resources is eligible to provide all capacity, energy, and ancillary services that it is technically capable of providing in the RTO/ISO markets” and “account for the physical and operational characteristics of electric storage resources through bidding parameters or other means.” 58 These ancillary services may include blackstart service, primary frequency response service, reactive power service, frequency regulation, or any other services defined by the RTO/ISO.59

The Commission gave flexibility to both transmission providers, in determining telemetry requirements, as well as to electric storage resources, in managing state of charge.60 To the extent that electric storage resources are providing ancillary services, such as frequency regulation, an electric storage resource managing its state of charge is required to follow dispatch signals. For ease of reference, the Commission provided a chart of “physical and operational characteristics of electric storage resources for which each RTO’s and ISO’s participation model for electric storage resources must account”, as shown in Table I.XXX.61 How these characteristics are accounted for in participation models may vary between RTOs and ISOs. Note that these definitions are not endorsed by IRPTF; rather, they are provided here only as a reference.

Table I.XXX: FERC Participation Model ParametersPhysical or Operational Characteristic Definition

State of Charge (SOC) The amount of energy stored in proportion to the limit on the amount of energy that can be stored, typically expressed as a percentage. It represents the forecasted starting State of Charge for the market interval being offered into.

Maximum State of Charge (SOCmax)

A State of Charge value that should not be exceeded (i.e., gone above) when a resource using the participation model for electric storage resources is receiving electric energy from the grid (e.g., 95% State of Charge).

57 https://www.nerc.com/comm/PC/InverterBased%20Resource%20Performance%20Task%20Force%20IRPT/Fast_Frequency_Response_Concepts_and_BPS_Reliability_Needs_White_Paper.pdf58 https://www.nrel.gov/docs/fy19osti/74426.pdf59 https://www.nrel.gov/docs/fy19osti/74426.pdf60 https://www.nerc.com/comm/PC_Reliability_Guidelines_DL/Inverter-Based_Resource_Performance_Guideline.pdf61 In addition to any requirements imposed by the TO or BA regarding acceptable charging behavior, the structure of investment tax credits may also contribute to the charging characteristic. For example, currently a hybrid plant may need charge the BESS by renewable energy for more than 75% of the time for the first five years of commercial operation, and the tax credit value for the storage component is derated in proportion to the amount of grid charging between 0% and 25%.


Ryan Quint [2], 03/10/20,

From Bill Quaintance:Is the base defined? One could define a theoretical maximum and then limit it to 95%, or one could define the practical maximum as the base, in which case the maximum would be 100% of that.

Ropp, Michael Eugene [2], 03/04/20,

We’re presently having a very active and interesting discussion around the definition of “state of charge” in the IEEE P147.9 WG. The trouble is in defining “the limit on the amount of energy that can be stored” in a way that all stakeholders agree and understand. I’d recommend that we stay in communication on this issue.






Appendix B: BESS Dynamic Model Parameterization

Table I.XXX: FERC Participation Model ParametersPhysical or Operational Characteristic Definition

Minimum State of Charge

A State of Charge value that should not be exceeded (i.e., gone below) when a resource using the participation model for electric storage resources is injecting electric energy to the grid (e.g., 5% State of Charge).

Maximum Charge Limit The maximum MW quantity of electric energy that a resource using the participation model for electric storage resources can receive from the grid.

Maximum Discharge Limit

The maximum MW quantity that a resource using the participation model for electric storage resources can inject to the grid.

Minimum Charge Time The shortest duration that a resource using the participation model for electric storage resources is able to be dispatched by the RTO/ISO to receive electric energy from the grid (e.g., one hour).

Maximum Charge Time The maximum duration that a resource using the participation model for electric storage resources is able to be dispatched by the RTO/ISO to receive electric energy from the grid (e.g., four hours).

Minimum Run* Time The minimum amount of time that a resource using the participation model for electric storage resources is able to inject electric energy to the grid (e.g., one hour).

Maximum Run Time The maximum amount of time that a resource using the participation model for electric storage resources is able to inject electric energy to the grid (e.g., four hours).

Minimum Discharge Limit

The minimum MW output level that a resource using the participation model for electric storage resources can inject onto the grid.

Minimum Charge Limit The minimum MW level that a resource using the participation model for electric storage resources can receive from the grid.

Discharge Ramp Rate The speed at which a resource using the participation model for electric storage resources can move from zero output to its Maximum Discharge Limit.

Charge Ramp Rate The speed at which a resource using the participation model for electric storage resources can move from zero output to its Maximum Charge Limit.

* Note that the definitions here interchange “run” and “discharge”. The preferred term is “discharge”.

FERC Order No. 842In Order No. 842 (February 15, 2018), the Commission determined that electric storage resources under its jurisdiction are only required to provide primary frequency response (PFR) when they are “online and are dispatched to inject electricity to the grid and/or dispatched to receive electricity from the grid.” 62 This excludes situations when an electric storage resource is not dispatched to inject or receive electricity. 63 The Commission required electric storage resources and transmission providers to specify an “operating range for the basis of the provision of primary frequency response.”64 The operating range, the Commission explained, represents the minimum and maximum states of charge between which an electric storage resource must provide PFR. The operating range for each electric storage resource must:

62 Some BESSs may have more than one substation transformer, and each should be explicitly modeled.63 During the interconnection study process.64 The repc_b module in PSLF is equivalent to the combined PLNTBU1 and REAX4BU1/REAX3BU1 in PSS®E.


Ropp, Michael Eugene [2], 03/04/20,

I agree with Bill here—if it’s possible, we really should fix this. “MW of energy” is definitely not terminology we want to have appearing in our documentation. If we’re forced to keep this because it’s FERC language, could we add a footnote discussing the unit disagreement?

Quaintance, William Harford, 03/03/20,

A shame that it says “MW of energy”. This is really a “rate”. I understand if we are quoting FERC then we can’t change these.


be agreed to by the interconnection customer and the transmission provider, in consultation with the balancing authority

consider the system needs for primary frequency response consider the physical limitations of the electric storage resource as identified by the developer and any

relevant manufacturer specifications be established in Appendix C of the LGIA or Attachment 5 of the SGIA65

The Commission noted that this suite of requirements “effectively allows electric storage resources to identify a minimum and maximum set point below and above which they will not be obligated to provide primary frequency response comparable to synchronous generation.”66 In sum, the Commission provided electric storage resource interconnection customers with the ability to propose an operating range and the transmission provider or BA the ability to consider system needs for primary frequency response before determining final operating ranges.

Given that “system conditions and contingency planning can change” and that “capabilities of electric storage resources to provide primary frequency response may change due to degradation, repowering, or changes in service obligations,” the Commission determined that the ultimate operating ranges may be dynamic values.67 If a dynamic range is implemented, then transmission providers must also determine the periodicity of reevaluation and the factors that will be considered during reevaluation of the operating ranges. The Commission provided electric storage resources specific exemptions from PFR provision for a “physical energy limitation”:

“the circumstance when a resource would not have the physical ability, due to insufficient remaining charge for an electric storage resource or insufficient remaining fuel for a generating facility to satisfy its timely and sustained primary frequency response service obligation, as dictated by the magnitude of the frequency deviation and the droop parameter of the governor or equivalent controls.”68

The Commission also clarified that MW droop response is derived from nameplate capacity. 69 If dispatched to charge during an abnormal frequency deviation, the Commission required electric storage resources to meet PFR requirements by increasing (for overfrequency) or decreasing (for underfrequency) the “rate at which they are charging according to the droop parameter.”70 To illustrate, the Commission gave an example of an electric storage resource charging at two MW with a calculated response per the droop parameter to increase real-power output by one MW. According to the Commission, during an underfrequency deviation the electric storage resource could “satisfy its obligation by reducing its consumption by one MW (instead of completely reducing its consumption by the full two MW and then discharging at one MW, which would result in a net of three MW provided as primary frequency response).”71 Electric storage resources are not required to change from charging to discharging, or vice versa, if technically incapable of doing so during the event when PFR is needed.

The Commission also noted that requirements adopted in Order No. 842 are minimum requirements. An electric storage resource may elect, in coordination with its transmission provider and BA, “to operate in a more responsive mode by using lower droop or tighter deadband settings.”72

65 FERC Order No. 841, paragraph 4.66 FERC Order No. 842, paragraph 183.67 Ibid.68 FERC Order No. 842, paragraph 187.69 FERC Order No. 845, paragraph 279.70 Ibid.71 https://www.cpuc.ca.gov/General.aspx?id=3462 . 72 https://www.cpuc.ca.gov/General.aspx?id=5935




As with all frequency-responsive resources connected to the BPS, speed of response has a significant impact on frequency performance during large disturbances, particularly in low inertia systems with high ROCOF. FERC Order No. 842 does not prescribe any speed of response characteristics for electric storage resources. See Chapter 1 for more details on how the performance of BESSs and hybrid plants can be configured to support BPS frequency response needs.

FERC Order No. 845In Order No. 845 (April 19, 2018), the Commission clarified that “in certain situations, electric storage resources can function as a generating facility, a transmission asset, or both.”73 The Commission made clear that electric storage resources under its jurisdiction greater than 20 MW had the option to interconnect pursuant to the Large Generator Interconnection Procedures (LGIP) and Large Generator Interconnection Agreement (LGIA), “so long as they meet the threshold requirements as stated in those documents.”74 In the event the LGIA does not accommodate for the load characteristics of electric storage resources, transmission providers may enter into non-conforming LGIAs.75

Further, in Order No. 845, the Commission declined to move forward with “any requirements for modeling electric storage resources”:

“…given the limited experience interconnecting electric storage resources and the abundant desire for regional flexibility, we are not imposing any standard requirements at this time and instead continue to allow transmission providers to model electric storage resources in ways that are most appropriate in their respective regions.” 76

Instead, the Commission encouraged transmission providers to continue to consider modeling approaches that will “save costs and improve the efficiency of the interconnection process.”77

73 There are other types of reliability studies such as harmonics and electromagnetic transient (EMT) simulations that are often performed during the interconnection study process; however, those studies are outside the scope of this guideline.74 https://energystorage.org/why-energy-storage/technologies/hydrogen-energy-storage/75 In ERCOT, a BESS was added to a quick-start combustion turbine for participation in ERCOT’s Responsive Reserve Service. The combustion turbine is normally offline, and if frequency falls outside of a pre-defined deadband, the BESS will provide fast frequency response until the combustion turbine is turned on to sustain the provided response. 76 The capability curve is almost symmetrical because when the BESS is operated in the second and third quadrant (consuming active power), a rise in dc voltage could limit the amount of power generation where reactive power also has to be consumed.77 https://www.nerc.com/comm/PC/InverterBased%20Resource%20Performance%20Task%20Force%20IRPT/Fast_Frequency_Response_Concepts_and_BPS_Reliability_Needs_White_Paper.pdf






A BPS-connected BESS can be modeled using the second-generation renewable energy system generic library models with the following models:

regc_a: BESS converter model

reec_c: BESS electrical controls model

repc_a: BESS plant controller model

Figures XXX-XXX show the associated block diagrams for the regc_a, reec_d, and repc_a models, respectively.

Figure B.1: REGC_A Dynamic Model [Source: PSS®E]



Should this be _c or _d?


Figure B.2: REEC_C Dynamic Model [Source: PSS®E]

Figure B.3: REPC_A Dynamic Model [Source: PSS®E]


Appendix C: Hybrid Plant Dynamic Model Parameterization

A BPS-connected hybrid plant uses the same types of dynamic models as a solar PV or BESS plant, with some modifications. Namely, the following second-generation renewable energy system generic library models can be used:

regc_a: BESS converter model

reec_c: BESS electrical controls model

repc_a: BESS plant controller model

Tables XXX-XXX provide the list of model parameters for each model as well as a default parameter value. Note that many of these model parameters are tunable for specific installations, and any model of an actual installed resource or expected future resource should use a dynamic model representative of that specific installation. These parameter values are highlighted in red below.

Table XXX.XXX: Hybrid Plant Generator/Converter ModelREGC_A Parameter Value

mva

lvplsw

rrpwr

brkpt

zerox

lvpll

vtmax

lvpnt1

lvpnt0

qmin

accel

tg

tfltr

iqrmax

iqrmin

Xe

Table XXX.XXX: Hybrid Plant Electrical Control Model ParametersREEC_C Parameter Value

mvab

vdip

vup

trv

dbd1




dbd2

kqv

iqh1

iql1

vref0

SOCini

SOCmax

SOCmin

T

tp

qmax

qmin

vmax

vmin

kqp

kqi

kvp

kvi

tiq

dpmax

dpmin

pmax

pmin

imax

tpord

pfflag

vflag

qflag

pqflag

vq1

iq1

vq2

iq2

vq3

iq3

vq4

iq4

vp1




ip1

vp2

ip2

vp3

ip3

vp4

ip4

Table XXX.XXX: Hybrid Plant Plant Controller Model ParametersREPC_A Parameter Value

mvab

tfltr

kp

ki

tft

tfv

refflg

vfrz

rc

xc

kc

vcmpflg

emax

emin

dbd

qmax

qmin

kpg

kig

tp

fdbd1

fdbd2

femax

femin

pmax

pmin

tlag

ddn



Table XXX.XXX: Hybrid Plant Plant Controller Model ParametersREPC_A Parameter Value

dup

frqflg

outflag

puflag


Appendix D: References

[X] IRPTF Performance Guideline

[X] IRPTF Interconnection Requirements Guideline

[X] WECC Modeling Guidelines

[X] EPRI Modeling Guidelines

[X] REGC_B Model? (and _C?)

[X] P. Pourbeik, J.K. Petter, “Modeling and validation of battery energy storage systems using simple generic models for power system stability studies,” CIGRE Science and Engineering, no. 9, October 2017.

[X] BESS Modeling Spec? (ask Songzhe)

[X] Interconnection Queues (?)

https://www.ferc.gov/industries/electric/indus-act/gi/stnd-gen/LGIA-agreement.pdf

http://www.caiso.com/planning/Pages/GeneratorInterconnection/InterconnectionRequest/Default.aspx

http://www.caiso.com/InitiativeDocuments/RevisedStrawProposal-HybridResources.pdfhttp://www.caiso.com/Documents/IssuePaper-HybridResources.pdf

https://rimspub.caiso.com/rims5/logon.dohttp://www.oasis.oati.com/NEVP/


http://www.oasis.oati.com/NEVP/

https://rimspub.caiso.com/rims5/logon.do

http://www.caiso.com/Documents/IssuePaper-HybridResources.pdf

http://www.caiso.com/InitiativeDocuments/RevisedStrawProposal-HybridResources.pdf

http://www.caiso.com/planning/Pages/GeneratorInterconnection/InterconnectionRequest/Default.aspx

https://www.ferc.gov/industries/electric/indus-act/gi/stnd-gen/LGIA-agreement.pdf

Appendix E: Example Hybrid Plant Configurations

Here is an example of an off-shore wind unit that had batteries associated with it. Each G was 220 MW for a total of 880MW.

Figure X.XXX: Text [Source: ISO-NE]


Appendix F: Modeling Different Hybrid Plant Operations

From JP:What about instances in ERCOT where entities are trying to site 50 MW battery to supplant 50 MW peaking facility such that they don’t need to spin the plant to play into their frequency response program? Thinking on it, it’s a little different than Brad’s example because the controls between the BESS and the synchronous generator are supposed to be coordinated such that the output is as if the generation unit was online, but they aren’t actually burning any “fuel” to spin it. Should we at least have a small section regarding that instance? Or push it towards the dynamic modeling as for steady state purposes the generator is likely to be off as it is a peaking plant?


Contributors

NERC gratefully acknowledges the invaluable contributions and assistance of the following industry experts in the preparation of this guideline. NERC also would like to acknowledge all the contributions of the NERC IRPTF.

Name EntityMark Ahlstrom NextEra EnergyHassan Baklou San Diego Gas and ElectricLeo Bernier AESJeff Billo (IRPTF Vice Chair) Electric Reliability Council of TexasRajni Burra REPlantSolutionsKevin Collins First SolarRansome Egunjobi Lower Colorado River AuthorityEvangelos Farantatos Electric Power Research InstituteRoberto Favela El Paso Electric CompanyLou Fonte California ISOVahan Gevorgian National Renewable Energy LaboratoryIrina Green California ISOAndy Hoke National Renewable Energy LaboratoryWarren Hess Midcontinent ISOKaitlyn Howling InvenergyFred Huang Electric Reliability Council of TexasHenry Huang Pacific Northwest National LaboratoryAndrew Isaacs ElectranixHimanshu Jain National Renewable Energy LaboratoryDan Kopin Utility ServicesSergey Kynev SiemensChester Li Hydro OneAndrew Lopez Southern California EdisonAdam Manty American Transmission CompanyBrad Marszalkowski ISO New EnglandJulia Matevosyan Electric Reliability Council of TexasNihal Mohan Midcontinent ISODanny Musher Key Capture EnergyDavid Narang National Renewable Energy LaboratoryOm Nayak NAYAK CorporationSid Pant General ElectricDavid Piper Southern California EdisonBill Quaintance Duke Energy ProgressDeepak Ramasubramanian Electric Power Research InstituteMatthew Richwine Telos EnergyMark Robinson AESFabio Rodriguez Duke FloridaMichael Ropp Sandia National LaboratoryThomas Schmidt Grau VestasAl Schriver (IRPTF Chair) NextEra EnergyJay Senthil Siemens PTIAlexander Shattuck VestasLakshmi Srinivasan Lockheed MartinSirisha Tanneeru Xcel Energy



Songzhe Zhu California ISORich Bauer (IRPTF Coordinator) North American Electric Reliability CorporationStephen Coterillo North American Electric Reliability CorporationHongtao Ma North American Electric Reliability CorporationRyan Quint (IRPTF Coordinator) North American Electric Reliability Corporation
