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Spring 2014 Instructor: Kai Sun

ECE 422/522 Power System Operations & Planning/

Power Systems Analysis II

1 – General Background

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Outline

• Structure of a power system • US Electric Industry (utilities, deregulation, energy

resources) • Overview of power system reliability and NERC guidelines • Introduction of power system stability (basic concepts,

definitions and examples) • Materials

– Part I (Chapters 1&2) of Kundur’s book – Glossary of Terms Used in NERC Reliability Standards, Dec 21, 2012 – IEEE/CIGRE Joint Task Force on Stability Terms and Definitions, “Definition and

Classification of Power System Stability,” IEEE Transactions on Power Systems, Vol. 19, No. 2., pp. 1387 – 1401, May 2004

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1st 100 Years of Electric Industry

• 1882: Pearl Street Station, the 1st DC system by Edison, operated in NYC • 1886: Commercially practical transformer and AC distribution system

developed by Stanley • 1888: Development of poly-phase AC by Tesla started AC vs. DC battle • 1889: 1st AC transmission line in the US (1-phase, 21km at 4kV in

Oregon) • 1893: 1st 3-phase line (2.3kV, 12 km by SCE) in North America; AC vs.

DC battle ended when AC was chosen at Niagara Falls. • 1912-1923: 1st 110kV and 220kV HVAC overhead lines • 1950s: 345kV-400kV EHV AC lines by USA, Germany and Sweden • 1954: 1st modern commercial HVDC transmission (96km submarine

cable) in Sweden. • 1960s: 735-765kV EHV AC in Russia, USA and Canada • 1972: 1st thyristor based HVDC Back-To-Back system between Quebec

and New Brunswick in Canada

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Structure of an AC Power System • Generation

– Low voltages <25kV due to insulation requirements

• Transmission system – Backbone system

interconnecting major power plants (11~35kV) and load center areas

– 161kV, 230kV, 345kV, 500kV, 765kV, etc.

• Sub-transmission system – Transmitting power to

distribution systems – Typically, 35/69kV-138kV

• Distribution system – Typically, 4kV-34.5kV

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Bulk Power System (Bulk Electric System) • NERC definition

– The bulk electric system is a term commonly applied to the portion of an electric utility system that integrates “the electrical generation resources, transmission lines, interconnections with neighboring systems, and associated equipment, generally operated at voltages of 100 kV or higher.”

– Radial transmission facilities serving only load with one transmission source are generally not included in this definition

• For short, a bulk electric system is the part of the transmission/sub-transmission system connecting – power plants, – major substations, and – HV transmission lines

• Most of power system reliability concerns are about bulk electric systems

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US Bulk Power Systems

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Energy Resources for US Electricity Generation

From “Electricity sector of the United States” at Wikipedia.org

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US Electric Industry Structure

Categories Examples

Investor-owned utilities 240+, 66.1% of electricity

AEP, American Transmission Co., ConEd, Dominion Power, Duke Energy, Entergy, Exelon, First Energy, HECO, MidAmerican, National Grid, Northeast Utilities, Oklahoma Gas & Electric, Oncor, Pacific Gas & Electric, SCE, Tampa Electric Co., We Energies, Xcel,

Publicly owned utilities 2000+, 10.7%

Nonprofit state and local government agencies, including Municipals, Public Power Districts, and Irrigation Districts, e.g. NYPA, LIPA,

Federally owned utilities ~10, 8.2%

Tennessee Valley Authority (TVA), Bonneville Power Administration (BPA), Western Area Power Administration (WAPA), etc.

Cooperatively owned utilities ~1000, 3.1%

Owned by rural farmers and communities

Non-utilities, 11.9% Generating power for own use and/or for sale in whole-sale power markets, e.g. Independent Power Providers (IPPs)

• 3,195 utilities in the US in 1996.

• Fewer than 1000 are engaged in power generation

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Deregulation: Competitive US Power Market Structure • The government sets down rules

and laws for market participants to comply with

• On electricity prices – Typically, determined by bid-

based, security-constrained, economic dispatch

– In a day-ahead market, the price is determined by matching offers from generators to bids from consumers at each node to develop a supply-demand equilibrium price, usually on an hourly interval.

– The price is calculated separately for sub-regions in which the system operator's power-flow model indicates that constraints will bind transmission imports.

Transmission Owner

Generation Owner

Generation Owner … Generation

Owner

Distribution Owner

Distribution Owner

Service Provider

Service Provider

… Distribution

Owner

Service Provider

Transmission Owner Transmission Owner

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• Compared to traditional economic dispatch, where the actual fuel cost function Ci is known by the dispatcher.

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1

5.3 0.008dC PdP

λ= + =

22

2

5.5 0.012dC PdP

λ= + =

33

3

5.8 0.018dC PdP

λ= + =

Equal incremental cost

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California was the 1st state to implement full

deregulation

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Reliability Concerns with Deregulation

Demand

Supply

“California Electricity Crisis” • Before passage of the deregulation law,

there was only one Stage-3 rolling blackout (intentional load shedding by utilities) declared.

• After passage, California had 38 Stage-3 rolling blackouts, mainly as a result of a poorly designed market system that was manipulated by traders and marketers.

• In order to sell electricity at a higher price, some trader intentionally encouraged suppliers to shut down plants (removing power from the market) for unnecessary maintenance.

(Source: Wikipedia.org and paper “Gaming and Price Spikes in Electrical Power Market,” by X. Guan, et al, on IEEE Trans. Power Systems, Vol. 16, No. 3, Aug 2001.)

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Power Blackouts in North America Date Area Impacts Duration

Nov 9, 1965 North America (NE) 20,000+MW, 30M people 13 hrs

Jul 13, 1977 North America (NY) 6,000MW, 9M people 26 hrs

Dec 22, 1982 North America (W) 12, 350 MW, 5M people

Jul 2-3, 1996 North America (W) 11,850 MW, 2M people 13 hrs

Aug 10, 1996 North America (W) 28,000+MW, 7.5M people 9 hrs

Jun 25, 1998 North America (N-C) 950 MW, 0.15MK people 19 hrs

Aug 14, 2003 North America (N-E) 61,800MW, 50M people 2+ days

Sep 8, 2011 US & Mexico (S-W) 4,300MW, 5M people 12hrs

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NERC (North American Electric Reliability Corporation) • As a non-government organization, formed by the electric utility industry

in 1968 to promote the reliability of bulk power systems in North America.

• Initially membership was voluntary and member systems followed the reliability criteria for planning and operating bulk power systems to prevent major system disturbances following severe contingencies

• As of June 2007, FERC (U.S. Federal Energy Regulatory Commission) granted NERC the legal authority to enforce reliability criteria with all users, owners, and operators of the bulk power systems in the U.S.

• NERC Membership is now mandatory. Member systems comply with NERC’s Reliability Standards (approved by FERC) to promote reliable operations and to avoid costly monetary penalties if caught non-compliant. Every system operator should read, understand and follow NERC’s Reliability Standards. (Visit http://www.nerc.com for more information on NERC.)

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NERC Functional Model Diagram

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Interconnections in North America

• Eight Regional Reliability Entities (RREs) assisting NERC – FRCC (Florida Reliability

Coordinating Council) – MRO (Midwest Reliability

Organization) – NPCC (Northeast Power

Coordinating Council) – RFC (Reliability First Corporation) – SERC (Southeastern Electric

Reliability Council) – SPP (Southwest Power Pool) – WECC (Western Electricity

Coordinating Council) – TRE (Texas Reliability Entity)

650GW

180GW

70GW

35GW

From EPRI tutorial (Peak loads are based on data in 2009)

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NERC Reliability Coordinators

Code Name ERCOT ERCOT ISO

FRCC Florida Power & Light

TE Hydro Quebec, TransEnergie

ISNE ISO New England Inc.

MISO Midwest ISO

NBSO New Brunswick System Operator

NYIS New York Independent System Operator

ONT Ontario - Independent Electricity System Operator

PJM PJM Interconnection

SPC SaskPower

SOCO Southern Company Services, Inc.

SPP Southwest Power Pool

TVA Tennessee Valley Authority

VACS VACAR-South

WECC WECC Reliability Coordinator

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NERC Balancing Authority • A Balancing Authority (BA) is a part of an interconnected power system

that is responsible for meeting its own load. • Each BA operates an Automatic Generation Control (AGC) system to

balance its generation resources to load requirements. – Generation resources: internal or purchased from other BAs and

transferred over tie-lines between BAs. – Load requirements: internal customer load, losses, or scheduled

sales to other BAs.

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NERC Balancing Authorities • EI has about 90

BAs, which range in load size up to 130GW peaks

• WI (WECC) has about 30 BAs.

• ERCOT and Hydro Quebec are each operated as single BAs.

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System Control Centers

(Source: bayjournal.com)

Duke Energy Control Center

(source: Patrick Schneider Photo.Com)

TVA Control Center

(source: TVA.com

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Reliable Electric Power Supply

• Requirements under both normal and emergency conditions – Voltage and frequency around normal values within

close tolerances

– Generators running synchronously with adequate capacity to meet the load demand

– The “integrity” of the bulk power network

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Reliability of Bulk Power Systems

• From both Planning and Operations perspectives: – Power systems should be built and operated to achieve a reliable

electric power supply

• Reliability is defined using two terms: – Adequacy (planning): The ability of the electric systems to

supply the aggregate electrical demand and energy requirements of their customers at all times, taking into account scheduled and reasonably expected unscheduled outages of system elements.

– Security (operation): The ability of the electric systems to withstand sudden disturbances, i.e. contingencies, such as electric short circuits or unanticipated loss of system elements

at the most economical cost

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Example of NERC’s Reliability Standards: Performance under Normal and Emergency Conditions

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Summary of NERC Contingencies

Category Description Stability Loss of load

A No contingencies Yes No

B N-1 (loss of 1 element) Yes No

C Loss of ≥2 elements (local events) Yes Planned or

controlled

D

Extreme events (loss of a transmission path, substation, power plant or major load, cascading outages, etc.)

Selecting contingencies for evaluation

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Contingencies to be studied

• Normal Design Contingencies (Categories A, B and C) – Have a significant probability of occurrence – Following any of these contingencies, the system is secure

(stability is maintained, and voltages and line and equipment loadings are within applicable limits. ) • All facilities are in service, or • A critical generator, transmission circuit, or transformer is out of

service, assuming that the area generation and power flows are adjusted between outages by use of a reserve.

• Extreme Contingencies (Category D) – After the analysis and assessment of selected extreme

contingencies, measures are developed to reduce the frequency of occurrence of such contingencies or to mitigate the consequences that are indicated by the simulations of such contingencies

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A B

Credible and Acceptable

C

D

NECR Contingencies

• Most utilities manually select NERC Category D contingencies to simulate: o Loss of a key substation o Loss of tie lines o Outages close to a

generation/load pocket

Frequency of Occurrence

Consequences

Needless to study

Not Existing in Well-designed Systems

Generator Outage

N-1 Line Outage

N-2 Line Outage

Extreme Events

Frequency may increase when system is stressed (e.g. Storm Approaching)

Unlikely but with Extreme Impacts

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How are reliability standards used?

• In Planning: –Reliability standards should never be violated

in designing the system. • In Operations:

–Reliability standards should never be intentionally violated

–Sometimes, violations occur due to mis-operations or delayed awareness of the real-time situation

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Related Terms

• Operating quantities: Physical quantities (measured or calculated) that can be used to describe the operating conditions of a power system, e.g. real, reactive and apparent powers, RMS values/phasors of alternating voltages and currents.

• Steady-state operating condition of a power system: An operating condition of a power system in which all the operating quantities that characterize it can be considered to be constant for the purpose of analysis.

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• In designing and operating an interconnected power system, its dynamic performance subjected to changes (i.e. contingencies, small or large) is considered

• It is important that when the changes are completed, the system settles to new operating conditions without violation of constraints.

• In other words, not only should the new operating conditions be acceptable (as revealed by steady-state analysis) but also the system must survive the transition to those new conditions. This requires dynamic analysis.

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Related Terms (cont’d)

• Disturbance: a sudden change or a sequence of changes in one or more parameters or operating quantities of the power system.

• Small and large disturbances – a small disturbance if the equations describing the

dynamics of the system may be linearized for the purpose of accurate analysis, e.g. a load change

– a large disturbance if the equations that describe the dynamics of the system cannot be linearized for the purpose of accurate analysis, e.g. a short circuit and loss of a generator or load.

δ

P

P(δ)

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Related Terms (cont’d) • Synchronous operation:

– A machine is in synchronous operation with another machine or a network to which it is connected if its average electrical speed (=ωr⋅P/2) is equal to the electric speed of the other machine or the angular frequency of the ac network.

– A power system is in synchronous operation if all its connected synchronous machines are in synchronous operation with the ac network and with each other.

• Asynchronous operation: loss of synchronism or out of step

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Stability of a Dynamical System

Assume origin x=0 is an equilibrium, i.e.

In other words, the system variable will stay in any given small region (ε) around the equilibrium point once becoming close enough (δ) to that point.

The equilibrium point x=0 is stable in the sense of Lyapunov such that

x

ε δ

Consider a nonlinear dynamical system

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Power System Stability

• Power system stability is the ability of a power system, for a given initial operating condition, to regain an acceptable state of operating equilibrium (i.e. the new condition) after being subjected to a disturbance

• Considering an interconnected power system as a whole – The stability problem with a multi-machine power system is mainly to

maintain synchronous operation of the machines (generators or motors)

• Considering parts of the system

– A particular generator or group of generators may lose stability (synchronism) without cascading instability of the main system.

– Motors in particular loads may lose stability (run down and stall) without cascading instability of the main system.

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Some Terms Related to System Dynamic Performance Secure (vs. Insecure)

Stable (vs. Unstable)

Oscillatory

Not violating given security criteria

A system is able to regain an equilibrium following a disturbance.

An operating quantity repetitively changes at some frequency around a central value (equilibrium).

(A stable power system may not be secure if the equilibrium or the transition to the equilibrium violates security criteria)

(When oscillation becomes uncontrollable to damage generators and other equipment, the system will become insecure and even unstable)

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Example: FIDVR (Fault-Induced Delayed Voltage Recovery)

NERC/WECC Planning standards require that following a Category B contingency,

• voltage dip should not exceed 25% at load buses or 30% at non-load buses, and should not exceed 20% for more than 20 cycles at load buses

• the post-transient voltage deviation not exceed 5% at any bus

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Stability Classification

• Power system stability is essentially a single problem; however, the various forms of instabilities that a power system may undergo cannot be properly understood and effectively dealt with by treating it as such.

• Because of high dimensionality and complexity of stability problems, it helps to make simplifying assumptions to analyze specific types of problems using an appropriate degree of detail of system representation and appropriate analytical techniques.

• Analysis of stability, including identifying key factors that contribute to instability and devising methods of improving stable operation, is greatly facilitated by classification of stability into appropriate categories

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Stability Classification • IEEE/CIGRE Joint Task Force on Stability Terms and Definitions, “Definition and

Classification of Power System Stability,” IEEE Trans. on Power Systems, Vol.19, No.2., pp. 1387-1401, May 2004.

• The classification of power system stability considers: – The physical nature of the resulting mode of instability as indicated by

the main system variable (angle, frequency or voltage) in which instability can be observed.

– The size of the disturbance (small or large disturbance) considered, which influences the method of calculation and prediction of stability.

– The devices, processes and time span that must be taken into consideration in order to assess stability. Typical ranges of time periods • Transient or short-term: 0-10s • Mid-term: 10s to several minutes • Long-term: several to tens of minutes

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Stability Classification

Physical nature

Disturbance size

Time span

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Homework #1 • Learn the IEEE paper “Definition and Classification of Power System Stability” • Select 1 journal/conference paper published by IEEE since 2010 that introduces

or addresses some stability problems on bulk power systems – Source: http://ieeexplore.ieee.org or http://scholar.google.com – Keywords: e.g. “power system” + “stability”

• Write a 1-2 pages essay (not Q&A’s): – Title, authors, source of the paper – Background:

• What stability problem is concerned? (Which IEEE categories?) • Why is the problem significant? (Any real-world stories?) • In which aspect(s) was the problem not addressed well in earlier literature?

– Approach • What new approach is proposed? (Outline of the procedure or steps) • Any key techniques are applied by the approach? • How does the new approach perform?

– Remark • Any conclusions from the work, or any room for further work

• Give a 3-5 minutes talk on your chosen paper and hand in your essay in the class of Jan 23 (Thursday). Please email me the paper title by Jan 22 (Wed.) 5pm

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Rotor Angle Stability

• Rotor Angle Stability refers to the ability of synchronous machines of an interconnected power system to remain in synchronism after being subjected to a disturbance.

• Phenomenon: increasing angular swings of some generators leading to their loss of synchronism with others.

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Rotor Angle Stability (cont’d)

• Rotor angle stability depends on the ability to maintain/restore equilibrium between electromagnetic torque (TE) and mechanical torque (TM) of each synchronous machine in the system.

• A fundamental factor in this problem is the manner in which the power outputs of synchronous machines vary as their rotor angles change (Power vs. Rotor angle)

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Rotor Angle Stability (cont’d)

E∠δ V∠0

3 3 sins

E VP

Xφ δ=

max(3 ) 3s

E VP

Xφ =

3 3 sines

P E VT

Xφ δω ω

= =

,max 3es

E VT

Xω=

Te (P3φ)

Tm Steady-state limit:

Ta=Tm-Te>0 (accelerates)

Ta=Tm-Te<0 (decelerates)

δ0

Unstable

Large disturbance

Small disturbance

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Rotor Angle Stability (cont’d)

For a simple power system consisting of a generator tied to a load bus, only when both sides have rotating mass, rotor angle stability can be a concern

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Rotor Angle Stability (cont’d)

• Interconnected power system with multiple generators

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Small signal stability

• Small-disturbance angle stability or small signal stability is the ability of a power system to maintain synchronism under small disturbances. – The disturbances are considered to be sufficiently

small that linearization of system equations is permissible for purposes of analysis

– Small signal stability depends on the initial operating state of the system (eigenvalues of the linearized system at the state).

– In today’s power systems, the small-signal stability problem is usually associated with insufficient damping of oscillations

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Small signal stability (cont’d)

• Small signal stability problems may be either local or global in nature. – Local plant mode oscillations (at 0.7~2.0Hz): oscillations of a small

part of the power system (typically, a single power plant) against the rest of the system

– Inter-area mode oscillations (at 0.1~0.7Hz): oscillations of a group of generators against the rest of the system

• The time frame of interest is 10 to 20 seconds following a

disturbance. However, oscillations may last several minutes

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• 1.2 Hz local plant mode oscillations lasting 4 minutes

Source: slides of Gary Kobet (TVA)

Small-signal unstable

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Transient Stability • Large-disturbance angle stability or transient stability is

concerned with the ability of the power system to maintain synchronism when subjected to a severe disturbance, e.g. a short circuit on a transmission line.

– The resulting system response involves large excursions

of generator rotor angles and is influenced by the nonlinear power-angle relationship.

– Transient stability depends on both the initial operating state of the system and the severity of the disturbance.

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Transient Stability (cont’d)

• Transient instability is usually in the form of aperiodic angular separation, which is often referred to as first swing instability.

• However, in large power systems, transient instability may occur after multiple swings as a result of, e.g., superposition of multiple oscillation modes.

• The time frame of interest in transient stability studies is usually 3 to 5 seconds following the disturbance. It may extend to 10-20 seconds (to observe a number of swings) for very large systems with dominant inter-area oscillations.

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“Dynamic Stability”

• The term “dynamic stability” also appears in the literature as a class of rotor angle stability. – In the North American literature, it has been used

mostly to denote small signal stability. – In the European literature, it has been used to denote

transient stability.

• Both CIGRE and IEEE have recommended that it not be used.

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Voltage Stability

• Voltage stability refers to the ability of a power system to maintain steady voltages at all buses in the system after being subjected to a disturbance from a given initial operating condition. – It depends on the ability to maintain/restore equilibrium between

load demand and supply – In order words, it depends on the ability to maintain bus voltages

so that when the system nominal load at a bus is increased, the real power transferred to that load will increase.

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Voltage Stability (cont’d)

• The term voltage collapse is also often used. It is the process by which the sequence of events accompanying voltage instability leads to a blackout or abnormally low voltages in a significant part of the power system.

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Voltage Stability (cont’d)

• Small-disturbance voltage stability – ability to maintain steady voltages when subjected to small

perturbations such as incremental changes in system load. – studies using linearized models for sensitivity analysis

• Large-disturbance voltage stability

– ability to maintain steady voltages following large disturbances such as system faults, loss of generation, or circuit contingencies.

– studied using nonlinear models on involved devices, e.g. motors, transformer tap changers, generator field-current limiters, etc.

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Voltage Stability (cont’d) • Short-term voltage stability

– involves dynamics of fast acting load components, e.g. induction motors, electronically controlled loads and HVDC convertors.

– The study period of interest is in the order of several seconds – requires solution of appropriate system differential equations

• Long-term voltage stability

– involves slower acting equipment, e.g. tap-changing transformers, thermostatically controlled loads, and generator current limiters.

– the study period of interest may extend to several or many minutes – requires long-term simulations

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B. Gao, et al, “Towards the development of a systematic approach for voltage stability assessment of large-scale power systems, IEEE Trans. Power Systems, Vol. 11 No. 3 Aug. 1996

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Relationship between rotor angle instability and voltage instability

• Typical systems vulnerable to two stability problems

– Rotor angle stability

– Voltage stability

• However, two problems often occur together – For example, as rotor angles between two groups of generators approach

180o, the loss of synchronism causes rapid drop in voltages at intermediate points in the network.

– Loss of synchronism of some generators may result from the outages caused by voltage collapse or from operating conditions that violate generator field current limits

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System Operation

• Establish most economical operating conditions under “normal” circumstances

• Operate the system such that if an unscheduled event occurs, it does not result in uncontrolled (or cascading) outages

• Establish “Safe Operating Limits” for all situations • Meet reliability criteria

– Voltage limits – Line and component loading limits (thermal limits) – Stability – Dynamic performance

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Normal Secure with sufficient

margin; able to withstand a contingency

Alert Secure with insufficient

margin; Contingency may cause overloading

Emergency Insecure; system is

still intact

Restorative

Preventive control

Corrective control

Emergency control

Restorative control

Transition due to control action

Transition due to disturbance

Extreme Power outages;

system separates

Cascading events

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Design and Operating Criteria for Stability

Design and operating criteria play an essential role in preventing major system disturbances following severe contingencies. • The use of criteria ensures that, for all frequently

occurring contingencies (i.e. credible contingencies, e.g. Categories B and C), the system will, at worst, transit from the normal state to the alert state, rather than to a more severe state such as the emergency state or the extreme state.

• When the system enters the alert state following a contingency, operators can take actions to return the system to the normal state.

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System Stability Studies Types Approach Purposes

Small signal stability

• Using linear system analysis tools to study the modal system response to a small disturbance.

• Details on the disturbance may not be important

• Obtain safe operating limits and guidelines • Identify poorly damped modes of

oscillation • Setting of controls (e.g., exciters, power

system stabilizers) Transient stability

• Using nonlinear system analysis tools to study the system response to a large disturbance.

• Traditionally using time-domain simulation to “track” the evolution of system states and parameters during the transient period.

• Every study is for a completely specified disturbance scenario including the pre-disturbance system condition and disturbance sequence (any change requires a new study)

• New generation studies (to meet reliability criteria at the least cost)

• Transmission planning studies (to analyze plans for future transmission expansion, and to meet reliability criteria)

• Operations planning studies (to check if a given system configuration or operations schedule meets reliability criteria)

• Special control to maintain stability (e.g., generation tripping, load shedding, etc.)

• Severe disturbance (extreme contingency) studies

• Special purpose studies (e.g., system blackstart or restoration plan, etc.)

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Trends in North American Interconnections • Fewer HV transmission lines built due to cost and environmental

concerns • Heavier use of some power plants away from load centers due to

conservation of oil and natural gas • Heavier loading of HV transmission due to growing electricity markets

under the “open transmission access” environment • Generation trends have become more stability-conscious

– Lower inertia – Higher short circuit ratio – More dependence on controls (e.g. excitation control) – Large concentration of generation – More power electronics based resources, e.g. renewables (intermittent)

may alter the basic inertial response • Effects of HVDC systems and solid state electronic devices (e.g.

flexible ac transmission systems, or FACTS)

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Structure of a Power System and Associated Controls