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Infrastructure Access Report

Infrastructure: SINTEF Renewable Energy Lab -‐ SmartGrids

User-‐Project: SYNERTIA-‐PLUS

Synthetic inertia from wind generation

Olimpo Anaya-‐Lara, University of Strathclyde

Marine Renewables Infrastructure Network

Status: Interim Report (3rd visit – one week) Version: 01 Date: 02-‐Aug-‐2014

EC FP7 “Capacities” Specific Programme

Research Infrastructure Action

Status: Report 3rd visit (two weeks) Version: 01 Date: 15-‐Sep-‐2014

Infrastructure Access Report: SYNERTIA-‐PLUS

Rev. 01, 15-‐Sep-‐2014 of 19

ABOUT MARINET MARINET (Marine Renewables Infrastructure Network for emerging Energy Technologies) is an EC-‐funded network of research centres and organisations that are working together to accelerate the development of marine renewable energy -‐ wave, tidal & offshore-‐wind. The initiative is funded through the EC's Seventh Framework Programme (FP7) and runs for four years until 2015. The network of 29 partners with 42 specialist marine research facilities is spread across 11 EU countries and 1 International Cooperation Partner Country (Brazil). MARINET offers periods of free-‐of-‐charge access to test facilities at a range of world-‐class research centres. Companies and research groups can avail of this Transnational Access (TA) to test devices at any scale in areas such as wave energy, tidal energy, offshore-‐wind energy and environmental data or to conduct tests on cross-‐cutting areas such as power take-‐off systems, grid integration, materials or moorings. In total, over 700 weeks of access is available to an estimated 300 projects and 800 external users, with at least four calls for access applications over the 4-‐year initiative. MARINET partners are also working to implement common standards for testing in order to streamline the development process, conducting research to improve testing capabilities across the network, providing training at various facilities in the network in order to enhance personnel expertise and organising industry networking events in order to facilitate partnerships and knowledge exchange. The aim of the initiative is to streamline the capabilities of test infrastructures in order to enhance their impact and accelerate the commercialisation of marine renewable energy. See www.fp7-‐marinet.eu for more details. Partners

Ireland University College Cork, HMRC (UCC_HMRC)

Coordinator

Sustainable Energy Authority of Ireland (SEAI_OEDU)

Denmark Aalborg Universitet (AAU)

Danmarks Tekniske Universitet (RISOE)

France Ecole Centrale de Nantes (ECN)

Institut Français de Recherche Pour l'Exploitation de la Mer (IFREMER)

United Kingdom National Renewable Energy Centre Ltd. (NAREC)

The University of Exeter (UNEXE)

European Marine Energy Centre Ltd. (EMEC)

University of Strathclyde (UNI_STRATH)

The University of Edinburgh (UEDIN)

Queen’s University Belfast (QUB)

Plymouth University(PU)

Spain Ente Vasco de la Energía (EVE)

Tecnalia Research & Innovation Foundation (TECNALIA)

Belgium 1-‐Tech (1_TECH)

Netherlands Stichting Tidal Testing Centre (TTC)

Stichting Energieonderzoek Centrum Nederland (ECNeth)

Germany Fraunhofer-‐Gesellschaft Zur Foerderung Der Angewandten Forschung E.V (Fh_IWES)

Gottfried Wilhelm Leibniz Universität Hannover (LUH)

Universitaet Stuttgart (USTUTT)

Portugal Wave Energy Centre – Centro de Energia das Ondas (WavEC)

Italy Università degli Studi di Firenze (UNIFI-‐CRIACIV)

Università degli Studi di Firenze (UNIFI-‐PIN)

Università degli Studi della Tuscia (UNI_TUS)

Consiglio Nazionale delle Ricerche (CNR-‐INSEAN)

Brazil Instituto de Pesquisas Tecnológicas do Estado de São Paulo S.A. (IPT)

Norway Sintef Energi AS (SINTEF)

Norges Teknisk-‐Naturvitenskapelige Universitet (NTNU)


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DOCUMENT INFORMATION Title Synthetic inertia from wind generation Distribution Public Document Reference MARINET-‐TA3-‐SYNERTIA-‐PLUS User-‐Group Leader, Lead Author

Olimpo Anaya-‐Lara University of Strathclyde 204 George Street, Glasgow, G1 1XW, United Kingdom

User-‐Group Members, Contributing Authors

Olimpo Anaya-‐Lara University of Strathclyde

Infrastructure Accessed: SINTEF Renewable Energy Lab -‐ Smartgrids Infrastructure Manager (or Main Contact)

Ole Christian Spro

REVISION HISTORY Rev. Date Description Prepared by

(Name) Approved By Infrastructure

Manager

Status (Draft/Final)

01 19/6/13 Interim report – first 2-‐week visit O. Anaya-‐Lara A. Endegnanew Final 01 19/8/13 Interim report – second 2-‐week visit O. Anaya-‐Lara A. Endegnanew Final 01 15/9/14 Third and Fourth 1-‐week visit O. Anaya-‐Lara O. C. Spro Final


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ABOUT THIS REPORT One of the requirements of the EC in enabling a user group to benefit from free-‐of-‐charge access to an infrastructure is that the user group must be entitled to disseminate the foreground (information and results) that they have generated under the project in order to progress the state-‐of-‐the-‐art of the sector. Notwithstanding this, the EC also state that dissemination activities shall be compatible with the protection of intellectual property rights, confidentiality obligations and the legitimate interests of the owner(s) of the foreground. The aim of this report is therefore to meet the first requirement of publicly disseminating the knowledge generated through this MARINET infrastructure access project in an accessible format in order to:

• progress the state-‐of-‐the-‐art • publicise resulting progress made for the technology/industry • provide evidence of progress made along the Structured Development Plan • provide due diligence material for potential future investment and financing • share lessons learned • avoid potential future replication by others • provide opportunities for future collaboration • etc.

In some cases, the user group may wish to protect some of this information which they deem commercially sensitive, and so may choose to present results in a normalised (non-‐dimensional) format or withhold certain design data – this is acceptable and allowed for in the second requirement outlined above.

ACKNOWLEDGEMENT The work described in this publication has received support from MARINET, a European Community -‐ Research Infrastructure Action under the FP7 “Capacities” Specific Programme.

LEGAL DISCLAIMER The views expressed, and responsibility for the content of this publication, lie solely with the authors. The European Commission is not liable for any use that may be made of the information contained herein. This work may rely on data from sources external to the MARINET project Consortium. Members of the Consortium do not accept liability for loss or damage suffered by any third party as a result of errors or inaccuracies in such data. The information in this document is provided “as is” and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and neither the European Commission nor any member of the MARINET Consortium is liable for any use that may be made of the information.


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EXECUTIVE SUMMARY Extensive research has been conducted on the provision of frequency support from variable-‐speed wind turbines employing different methodologies. However, in addition to the impacts on network operation, provision of short-‐term frequency support has implications on the turbines themselves. In essence, the control implementation to deliver the ‘synthetic inertia’ response required for the power system will introduce additional and possibly significant torque demands on the turbine. In this research context, the main goal of this project was to observe and assess experimentally the impact of control loops for provision of synthetic inertia in the Doubly-‐fed Induction Generator (DFIG), and Fully-‐Rated Converter (FRC) power electronic converters. In addition, preliminary experimental research has been conducted to assess implications of providing damping of oscillations on FRC power electronic converters. The work conducted in this project has been organised over 6 weeks of effective work in the lab at SINTEF. This Report covers the activities performed and results achieved during the third and fourth visits, each of which consisted of one week (under the SYNERTIA-‐PLUS component of the project). The main work during the first two-‐week visit focused on setting up the hardware in the laboratory and configuration of the DFIG wind turbine topology. In addition, the structure of a frequency control loop for the DFIG has been amended to make it compatible and to facilitate the implementation in the Opal-‐RT. The DFIG and network models, which enable to represent the inertia response of the DFIG were significantly improved and made compatible with the Opal-‐RT tested on the real machine. During the second two-‐week visit, a small-‐scale power system, which was able to demonstrate typical load change-‐frequency variation characteristic was set up as the test rig for DFIG frequency support controller. During the SYNERTIA-‐PLUS visit the work focused on the Fully-‐Rated Converter (FRC) wind turbine, and experiments were conducted in order to assess the FRC capabilities to provide synthetic inertia and to identify issues associated with controller´s settings and time delays associated to different elements in the experimental set up. In addition, initial investigations were conducted in order to explore the capabilities of FRC to provide damping of power system oscillations. Preparation and results obtained in these investigations are also presented in this report.

Acknowledgements Dr. Olimpo Anaya-‐Lara wishes to thank deeply MARINET for the financial suport provided. Also great thanks are given to SINTEF Energy Research for hosting this research and to Atle Rygg Årdal and Kjell Ljøkelsøy for providing all the support required for the lab experiments.


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CONTENTS

1 INTRODUCTION & BACKGROUND ...................................................................................................................... 7 1.1 INTRODUCTION .................................................................................................................................................... 7 1.2 BACKGROUND ..................................................................................................................................................... 7 1.2.1 Frequency control in power systems ........................................................................................................... 7 1.2.2 Definition of inertia constant H .................................................................................................................. 8 1.2.3 Inertial response from wind turbines ........................................................................................................ 10 1.2.4 Summary ................................................................................................................................................... 11

2 DEVELOPMENTS ACHIEVED ............................................................................................................................. 12 2.1 STAGE GATE PROGRESS ....................................................................................................................................... 12 2.2 THE LABORATORY SET-‐UP .................................................................................................................................... 13 2.3 THE WIND EMULATOR ......................................................................................................................................... 13 2.4 THE VOLTAGE SOURCE CONVERTER ....................................................................................................................... 14 2.5 THE INERTIA EMULATION CONTROLLER ................................................................................................................... 14

3 CASE STUDIES AND EXPERIMENTAL RESULTS ................................................................................................... 15 3.1 VERIFICATION OF THE SYNTHETIC INERTIA CONTROL LOOP .......................................................................................... 15 3.2 VARYING WIND SPEED CONDITIONS ....................................................................................................................... 16

4 CONCLUSIONS ................................................................................................................................................. 18 5 REFERENCES .................................................................................................................................................... 19


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1 INTRODUCTION & BACKGROUND

1.1 INTRODUCTION Extensive research has been conducted on the provision of frequency support from variable-‐speed wind turbines employing different methodologies [1][2]. However, in addition to the impacts on network operation, provision of short-‐term frequency support has implications on the turbines themselves. In essence, the control implementation to deliver the ‘synthetic inertia’ response required for the power system will introduce additional and possibly significant torque demands on the turbine. It is therefore necessary to conduct experimental tests that shed light and provide understanding of the impact that different control strategies have on sensitive components of the turbines such as the power electronics. The capability tests have consisted in triggering a kinetic energy release from the generators under different input torque conditions (emulating different wind speed inputs), and measuring the generated power output, its magnitude and behaviour, for both the DFIG and the FRC. The impact of the sudden release of kinetic energy in the form of active power from the generators has be assessed for the partial-‐power back-‐to-‐back converter of the DFIG and the full-‐scale back-‐to-‐back converter of the FRC. Questions to be answered by this project include: How the proposed synthetic inertia controllers work in real conditions? Which wind generator technology is better suited for providing synthetic inertia? Which generator technology imposes less stress to their associated power electronics during the kinetic energy release? Do the back-‐to-‐back converters of the two wind generator technologies under test require being over-‐dimensioned? The following subsection elaborates on the issues associated with frequency stability and control in power systems in order to give the reader a better undertanding into the work conducted in this SYNERTIA (and SYNERTIA-‐PLUS) project.

1.2 BACKGROUND With the increasing wind penetration into power systems, frequency stability has become a particular focus of attention and concern for Transmission System Operators (TSOs). Power system inertia can be defined as the total amount of kinetic energy stored in all rotational generators and motors that are synchronously connected to the power network [3] . During a power system frequency disturbance, e.g. a sudden loss in generation or increase in load, a mismatch between generation and supply will occur. The power system frequency will consequently change at a rate initially determined by the total power system inertia. Kinetic energy will be taken from or released into power systems when those turbo-‐generator units which are synchronously connected with the power network accelerate or decelerate. This is referred to as inertial response. However, many of the renewable generator technologies employ power electronic converters. This is the case for variable-‐speed wind turbines which are thus decoupled from the power system frequency and do not intrinsically contribute to power system inertia.

1.2.1 Frequency control in power systems For the secure operation of a power system, the frequency should remain nearly constant and it is dependent on active power balance. The initial rate of change of frequency is determined by the initial mistmatch and power system inertia. As frequency is a common factor throughout the system, a change in active power demand at one point is reflected throughout the system by a change in frequency.


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The kinetic energy stored in the rotating masses of generators and loads, i.e. the power system inertia, determines the sensitivity of the change in system frequnecy. The higher the power system inertia, the lower the rate-‐of-‐change of frequency in case of an imbalance between generation and demand. In the event of a suddent failure in generation or connection of a large load the system frequency starts dropping (region OX in Figure 1) at a rate mainly determined by the total angular momentum of the system (addition of the angular momentum of all generators and spinning loads connected to the system) [4]. For the ocassions when the frequency drops greater than 0.2 Hz, generation plants are contracted to provide additional frequency response duties. These response duties are classified as occasional services and have two parts namely, primary response and secondary response. In the UK, the primary and secondary response are defined as the additional active power that can be delivered from a generating unit that is available at 10 seconds and 30 seconds respectively. Primary response is provided by an automatic droop control loop and generators increase their output depending on the dead band of their governor and time lag of their prime mover (e.g. that of the boiler drum in steam units). Secondary response is the restoration of the frequency back to its nominal value using a slow supplementary control loop.

Figure 1. Frequency control in England and Wales [4] However, this is not the case for modern wind turbine technology. The extracted power from variable-‐speed wind turbines is controlled by power electronic converters and there is no inherent relation between frequency of the system and the rotational speed of the tubrine. Hence, modern wind turbines cannot naturally provide an instantaneous power boost in response to a system frequency fall and thus contribute to power system inertia. As wind generation is gradually replacing conventional power plant, an increasing wind share in instantaenous generation may make the power system more vulnerable.

1.2.2 Definition of inertia constant H The inertial power contribution available from a synchronous generator can be determined by taking the derivative of the kinetic energy stored in the rotating rotor and any coupled rotating components (such as the turbo-‐machinery). The standard derivation of the simple mathematical relation that results from this can be found in [3]:

P = dEk

dt= J ⋅ω ⋅ dω

dt

(1) (1)(1)


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where J is the effective inertia of the rotating components (taking account of gear ratios where they might exist), and ω is the rotor speed. The inertial constant H is defined as the ratio of kinetic energy stored in the rotor system at nominal speed to the nominal power of the electrical machine. It has dimensions of time and represents the time duration of releasing stored kinetic energy when providing nominal power, given by, [3]:

H =12Jω0

2

S

(1) (1(2)

where ω0 is the rated rotor speed, and S is the rated power of the electrical machine. From (1) and (2):

PS= 2H ⋅

ωω0

⋅d ω ω0( )

dt

(1) (1(3)

Let P and ω denote the power and rotor speed in per unit, then power and torque are given by:

P = 2H ⋅ω ⋅dωdt

(1) (1(4)

T = 2H ⋅dωdt

(1) (1(5)

Aside from transient conditions where the load angle is varying, the speed of a synchronous machine is locked to the network frequency. Consequently (4) and (5) can be written in terms of f and df dt in per unit rather than ω and dω dt :

P = 2H ⋅ω ⋅dfdt

(1) (1(6)

T = 2H ⋅dfdt

(1) (1(7)

The relationship between the power mismatch on the system and the frequency can be given as:

2Hf0

dfdt= Pm −Pe = ΔP (1) (1(8)

where f0 is the nominal frequency of the system in Hz, Pm is the mechanical power in pu, and Pe is the electrical power in pu, ΔP is the power mismatch in pu. The total system inertia Ht comprises the combined inertia of all rotational generators and motors that are synchronously connected to the power network (or near synchrnously connected in the case of induction generators and motors). It can be calculated by:


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Ht =Hi ⋅Si

i=1

n

∑

Sii=1

n

∑

(1) (1(9)

where Hi is the effective inertia of the electrical machine, Si is the rated power.

1.2.3 Inertial response from wind turbines The provision of inertial response from variable speed wind turbines can be obtained by controlling the power output in response to frequency changes thereby making these turbines appear more like conventional generators with synchronously connected inertia. There have been various approaches proposed in the open literature which generally involve modification of the turbine controller. 1.2.3.1 Torque control approach The principle of this approach is to modify the demanded torque in response to a change in system frequency by adding a supplementary torque control loop as shown in Figure 2. The modified demanded torque is given by:

Td = Tref +Tinertia (1) (1(10)

where Td is the modified demanded torque, Tref is the demanded torque in normal turbine operation, Tinertia is the added torque corresponding to the change in system frequency.

Figure 2. Inertia controller schematic

It was shown in [5] that variable-‐speed wind turbine controllers can be modified to give a response similar to inertia in response to changes in network frequency. The turbine controller is modified by adding a supplementary control loop which is independent of normal wind turbine operation (as shown in Figure 3), and responds to system frequency changes using the derivative of system frequency, dω dt . Results presented in [5] suggest that the proposed inertia response method is a ‘one-‐shot’ scheme that probably responds in proportion to the rate-‐of-‐change of frequency (ROCOF). The same inertial control strategy was presented in [6].


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Figure 3. Supplementary control loop for inertial response from a DFIG, taken from [5].

The impact of inertia response on power system frequency from fixed-‐speed and DFIGs has been examined in [6]. Figure 4 shows the supplementary control loop proposed in this reference. Here the supplementary control torque TSC is added to the reference torque Tref (in normal turbine operation) to provide the modified demanded torque as

shown in Figure 4. The addition of the supplementary controller can give a similar response to the inertial response of the conventional generator.

Figure 4. Supplementary control loop for dfig controller, taken from [6]

The size of the response will be dependent on the value of the control constant K. Under normal operation, the controllers of variable-‐speed wind turbines will keep the turbines at its optimal speed in order to produce maximum power. The controller gives a torque set point that is based on measured speed and power. The torque set point is an input for the converter control that realizes the torque by controlling the generator currents. The additional torque term will adapt the torque set point as a function of the rate-‐of-‐change of the grid frequency df dt .

1.2.4 Summary Based on this brief review of synthetic inertia control strategies for wind turbines both in academic papers and brochures from industry, it can be seen that wind turbines have the capability to provide inertial response support in response to frequency changes in the system. However, a number of potential issues require further analysis in order to fully understand potential benefits and drawbacks of the concept.

• In contrast to synchronous generators, frequency measurements have to be first detected by wind turbine controllers. How the frequency filtering should be implemented still requires further investigation and discussion with turbine manufacturers.

• Depending on the control strategies used, e.g. torque approach or contrast power approach, the potential active power reduction will vary because these approaches have different impacts on rotor aerodynamics efficiency during the provision of inertial support. As a result, it also will lead to different recovery process for the wind turbine. How these factors may interact with power system recovery and system stability needs to be carefully examined.

• The inertia support from wind turbines may result in potential revenue loss for wind plant operators and its impact on turbine lifetime remains unclear.

• The approach followed for the experiments presented in this report very much follow that presented in Figure 3.


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2 DEVELOPMENTS ACHIEVED During the third (two-‐week) visit the work has concentrated on testing the FRC capabilities to provide synthetic inertia. The right set up was constructed in the laboratory and various scenarios were addressed to identified capabilities and impact on power electronic converters as highlighted in the Stage Gate Progress table below.

2.1 STAGE GATE PROGRESS Previously completed: ü Planned for this project: Ü

STAGE GATE CRITERIA Status Stage 1 – Simulations • DFIG model in Matlab/Simulink ü • Grid model to represent inertia response characteristic ü • 3 types of DFIG controller to increase the inertia support from DFIG in Simulink ü • Validation of the effectiveness of the proposed controller ü • Modification of the Simulink model to be applied to real machine ü Stage 2 – Hardware set-‐up dFIG set up and testinf • Induction machine with speed control ü • Rotor and grid side converter for DFIG set-‐up ü • Applying DFIG controller on the induction machine and converters. ü • Inertia support of DFIG based on current mode control ü Stage 3 – FRC Hardware set-‐up and testing • Inertia support of DFIG with fully rated converter ü


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2.2 THE LABORATORY SET-‐UP This section describes the laboratory model implemented in the SINTEF Renewable Energy Lab – SmartGrids as shown in Figure 5. The model consists of the following components:

• Wind farm equivalent o A 55 kVA motor-‐generator set o Motor-‐drive for the 55 kVA motor

• Weak grid equivalent o A transmission line equivalent including a transformer o A 17 kVA synchronous generator-‐induction motor set o A 22 kW frequency converter for controlling the induction motor driving the 17 kVA synchronous

generator o A controllable resistor load

• Remote control program in Labview

Figure 5. Connexion of the FRC generator to the laboratory electrical network

The induction motor-‐generator set is used as wind farm equivalent and the motor drive is used to emulate wind power profiles. The synchronous generator-‐induction motor set is used to represent a stand alone weak AC grid. This weak AC grid also contains a transmission line equivalent including a transformer and a controllable resistor bank. The converters and the motor-‐drive are equipped with a CAN-‐bus interface which enables receiving status messages and measurements and sending control actions [7].

2.3 THE WIND EMULATOR The drive used to control the motor of the induction motor-‐generator set is an ABB ACS600 unit with a rated power of 75 kW and input voltage of 400 V AC. The drive can be controlled and monitored remotely with Labview through a National Instruments DAQ-‐unit. It can be turned on/off, and the torque-‐reference can be dynamically changed. Speed and torque measurements can also be logged.


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2.4 THE VOLTAGE SOURCE CONVERTER The data for the converters used in the experiment are presented in Table 1. The control system for the 60 kVA laboratory converters is developed by SINTEF. It runs on a Xilink Virtex5 FPGA-‐based processor system. The control system is configured with a wide range of parameter settings and possible operation modes. Depending on the application different control objectives can be chosen, e.g. C-‐voltage, active/reactive current. Frequency (grid-‐connected mode), torque, speed, flux (motor-‐drive mode).

Table 1. Laboratory Voltage Source Converter data

Parameter Value Main supply voltage 0-‐400 V rms (AC) DC voltage 550-‐750 V Rated power 60 kVA Rated current 100 A RMS (AC) Switching frequency Maximum 7 kHz LCL filter 500µH, 50 µF, 200 µH DC filter capacitance

2.5 THE INERTIA EMULATION CONTROLLER The block diagram of the control loop implemented in the laboratory to enable the FRC wind turbine to provide synthetic inertia is illustrated in figure 6. It can be observed that the control concept is simple, and works on modifying the torque set point similarly to that presented in Figure 3.

Figure 6. FRC control loop to enable inertia emulation


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3 CASE STUDIES AND EXPERIMENTAL RESULTS This section presents a selection of results obtained in the laboratory experiments which allow confirming the capability of the FRC wind turbine technology to provide short-‐term frequency support (synthetic inertia), and to understand what is the impact if any, on the power electronic converters. The approach adopted was to first test the correct operation of the additional control loop to provide synthetic inertia using small and large load increases, and then introducing the operating condition of the turbine, where experiments which represented different wind conditions were conducted. This latter case also allowed understanding the potential FRC contribution to network damping whilst supporting the inertia response.

3.1 VERIFICATION OF THE SYNTHETIC INERTIA CONTROL LOOP The correct operation of the synthetic inertia loop was verified under small and large load increase. How small or large was somewhat arbitrary as long as a condition could be produced in the lab which allowed observing the effect of the controller on the grid frequency. The responses presented may not be found in practice but they are clearly artificially generated for control design and verification purposes. The results for both small and large load increase are presented in Figure 7 and Figure 8, respectively.

Figure 7. Results for a small load increase without and with frequency support (the x-‐axis represents time).

Figure 8. Results for a large load increase without and with frequency support


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The results presented in Figure 7 and Figure 8 demonstrate the synthetic inertia contribution of the FRC wind turbine. In can be clearly observed that the frequency drop is reduce significantly in both cases when the synthetic inertia control loops is enabled. It is very important to comment that no drastic variations were observed in the currents or dc voltage in the power electronics. However, further tests are necessary to confirm fully that this is the case (although not for every operating condition). Nevertheless, it is safe to note that fault conditions create more demanding requirements from the turbine. Of importance when considering the provision of synthetic inertia may not be in the sense of magnitudes but duration of the service provision.

3.2 VARYING WIND SPEED CONDITIONS The results discussed in this section aim to illustrate the FRC capabilities to provide synthetic inertia support at different power output levels (due to different prevailing wind speeds). The scenarios cover the range from low, medium and near-‐rated wind speed. This has been achieved by manupulation of the wind turbine generator speed. A base case scenario was conducted with no additional control loop. This is used for comparison with case with the contoller enabled using different control gains, Kcontrol. The frequency response against time for these scenarios is presented in Figure 9, Figure 10, and Figure 11. It can be observed that in every case, the frequency response is significantly improved when the additonal control loop is enabled as shown in the plots with Kcontrol = 0.2. It can be seen that the frequency drop becomes less as the wind speed increases. Another aspect which is quite noticeable is the contribution to system damping. It has to be mentioned that this additional featured was achieved without introducing an additional control loop but by the introduction of a filter as shown in Figure 6. This feature needs to be further explored by the implementation of a multi-‐machine grid in the lab. Low wind speed. In this scenario, the speed of the wind generator was set 300 rpm. The results are illustrated below in Figure 9.

Figure 9. Results for a low wind speed scenario


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Figure 9. Results for a low wind speed scenario (continued) Medium wind speed. In this scenario, the speed of the wind generator was set 600 rpm. The results are illustrated below in Figure 10.

Figure 10. Results for a medium wind speed scenario


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Near-‐rated wind speed: In this scenario, the speed of the wind generator was set 1000 rpm. The results are illustrated below in Figure 11.

Figure 11. Results for a near-‐rated wind speed scenario

4 CONCLUSIONS The experiments carried out under the SYNERTIA-‐PLUS visit focused on assessing the FRC capabilities to provide synthetic inertia and to identify issues associated with controller´s settings and time delays associated to different elements in the experimental set up. Initial investigations were conducted in order to explore the capabilities of FRC to provide damping of power system oscillations. As it was mentioned before no drastic variations were observed in the currents or dc voltage in the power electronics. However, further tests are necessary to confirm fully that this is the case (although not for every operating condition). Nevertheless, it is safe to note that fault conditions create more demanding requirements from the turbine. Of importance when considering the provision of synthetic inertia may no be in the sense of magnitudes but duration of the service provision. The natural follow-‐up step to the work conducted is to confirm the results using a multi-‐machine system. A recommendation for future work is in the direction of frequency measurements and commnication to the wind power plant controller when it is interfaced through an HVDC link.


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5 REFERENCES [1] Anaya-‐Lara, O., Hughes, F. M., Jenkins, N., and Strbac, G.: “Contribution of DFIG-‐based wind farms to power

system short-‐term frequency regulation,” IEE Proceedings Generation, Transmission and Distribution, Vol. 153, No. 2, pp. 164-‐170, March 2006.

[2] Hughes, F. M., Anaya-‐Lara, O., Jenkins, N., and Strbac, G.: “Control of DFIG-‐based wind generation for power network support,” IEEE Transactions on Power Systems, Vol. 20, No. 4, pp. 1958-‐1966, November 2005.

[3] Kundur, P. (1994): “Power system stability and control”, McGraw-‐Hill, ISBN: 0-‐07-‐035958-‐X. [4] Erinmez, I.A., Bickers, D.O., Wood, G.F., Hung, W. W. (1999): “NGC Experience with frequency control in England

and Wales – Provision of frequency response by generator”, IEEE PES Winter Meeting. [5] Ekanayake, J. and Jenkins, N., "Comparison of the response of doubly fed and fixed-‐speed induction generator

wind turbines to changes in network frequency," Energy Conversion, IEEE Transactions on, vol. 19, pp. 800-‐802, 2004.

[6] Lalor, G., Mullane, A. and O'Malley, M., "Frequency control and wind turbine technologies," Power Systems, IEEE Transactions on, vol. 20, pp. 1905-‐1913, 2005.

[7] Stoeylen, H., Laboratory Demonstration of Provision of Primary Frequency Control Services from HVDC-‐VSC Connected Wind, Farms, MSc Thesis, NTNU, May 2014.

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