On some aspects of power system stability and grid code requirements relevant for large scale wind power integration

On some aspects of power system stability and grid code requirements relevant for large scale wind power integration (Summary Report of Vindforsk project V-369) Elforsk rapport 13:04 Seon Gu Kim, Math Bollen, STRI January 2013

On some aspects of power system stability and grid code requirements relevant for large scale wind power integration (Summary Report of Vindforsk project V-369) Elforsk rapport 13:04 Seon Gu Kim, Math Bollen, STRI January 2013

Preface Sweden and other Nordic countries have ambitious renewable energy source (RES) integration target. This will represent a significant share of wind power in the future generation mix of Nordic countries. From a power system point of view, total understanding of technical impacts of this new generation source on the existing power system is vital to ensure a secure and reliable operation of the power system. In a higher wind power penetration scenario, wind power plants will need to contribute to system voltage and frequency control support, which is quite obvious and logical. In order to identify the possible impact of large scale wind power integration and to recommend on possible approaches to manage the impact the project described in this report was carried out with the research program Vindforsk III as project V-369 PosStaWind. The project consists of three parts focusing on different aspects of impact of wind power on the angular, frequency and voltage stability of a power system. This report consist the summary report of the whole project. The whole project is reported also in Elforsk reports 13:01, 13:02 and 13:03. The project is financed by Vindforsk III with substantial initial funding from the power system operators in Finland, Norway and Sweden, Fingrid, Statnett and Swedish National Grid. Vindforsk-III is funded by ABB, Arise windpower, AQ System, E.ON Elnät, E.ON Vind Sverige, Energi Norge, Falkenberg Energi, Fortum, Fred. Olsen Renewables, Gothia Vind, Göteborg Energi, HS Kraft, Jämtkraft, Karlstads Energi, Luleå Energi, Mälarenergi, o2 Vindkompaniet, Rabbalshede Kraft, Skellefteå Kraft, Statkraft, Sena Renewable, Svenska kraftnät, Tekniska Verken i Linköping, Triventus, Wallenstam, Varberg Energi, Vattenfall Vindkraft, Vestas Northern Europe, Öresundskraft and the Swedish Energy Agency. The work has been carried out by STRI with Nayeem Ullah and later with Seon Gu Kim as a project leader. Several people at STRI have contributed to the work.

Comments on the work and the final report have been given by a reference group with the following members: Tuomas Rauhala, Nikkilä Antti-Juhani Terje Gjengedal, Katherine Elkington, Johan G. Persson, Staffan Mared, Kjell Gustafsson, Fingrid Fingrid Statnett Svenska Kraftnät (National Swedish Grid) E.on Vattenfall Statkraft Stockholm January 2013 Anders Björck Programme manager Vindforsk-III Electricity and heat production, Elforsk AB

Sammanfattning Denna rapport sammanfattar de metoder och studieresultat från tre olika aspekter av integrering av vindkraft i ett transmissionsnät: effektpendlingar i kraftnätet, syntetisk tröghet och transient reaktiv effekt. Studien utfördes i ett projektpaket innehållande tre olika delprojekt som var och en avhandlar en av dessa tre aspekter. Del 1: Effektpendlingar i kraftnätet Detta delprojekt analyserar vilken påverkan en vindkraftsparks spänningsreglering har på effektpendlingar i kraftnätet. I grundstudien har man använt IEEE:s Two-Area System, tillsammans med en utökad version av Nordic-32 modellen för att representera det sammanlänkade nordiska kraftsystemet. Studien baseras på 0,35 Hz moden på grund av att det faktum att generatorerna i Finland svänger mot generatorerna i södra Sverige och Norge med denna frekvens. En grundläggande slutsats av studien är att påverkan från en vindkraftpark på effektpendlingen mellan olika områden i nätet är starkt beroende av anslutningspunkten. En viss vindkraftspark med en viss regulator ger en liten förbättring i en anslutningspunkt, men en kraftig försämring i annan. Studiefallet med vindkraftsparker lokaliserade i Finland visar att 0,35 Hz moden påverkas positivt. Men vid hög effektproduktion (och med ökad export till Sverige), blir svängningsmoden negativt dämpad. Detta beror troligen på det ökade effektflödet i förbindelsen och inte på vindkraftsparkens spänningsregulator. Studiefallet med vindkraftsparker i tre länder (Sverige, Norge och Finland), visar att dämpningen förbättras i jämförelse med fallet utan vindkraft ansluten. Denna förbättring kan dock bero på det förändrade lastflödet och produktionssituationen. Sammanfattningsvis kan det sägas att vindkraftsparker i Finland bidrar till ökad dämpning i fallet med lokal kraftkonsumtion, och kraftexport till Sverige via de norra delarna av nätet. Men fallen med vindkraftsproduktion fördelat i Sverige, Norge och Finland har bara en mindre påverkan på effektpendlingen mellan olika områden i nätet. Del 2: Syntetisk tröghet från vindkraftsparker Målet med denna del är att studera frekvensstabiliteten i elkraftsystemet när vindkraftsparker deltar i frekvensregleringen via den s.k. syntetisk tröghet (eng. synthetic inertia). Vindkraftsparker med generatorer av typen DFIG eller FPWTC, bidrar inte till den totala trögheten i kraftsystemet. I enlighet med detta innebär en ersättning av konventionell produktion med vindkraftproduktion en minskning av den totala trögheten i kraftsystemet, med följd att kvaliteten på frekvensen försämras. Ett sätt att förhindra denna försämring är att installera vindkraftsturbiner med syntetisk tröghet. Ett antal studier har utförts i en utökad Nordic-32 modell, med syfte att studera inverkan av syntetisk tröghet på frekvensstabiliteten efter bortkoppling av en stor produktionsenhet. En bortkoppling av 4,3% av

produktionen, ger en lägsta frekvens på 49,45 Hz med syntetisk tröghet, i jämförelse med en 49,30 Hz utan. Vindkraftsparker med syntetisk tröghet ger således upphov till en förhöjd lägstafrekvens och motverkar lastbortkoppling till följd av låg frekvens. Oavsett detta bidrag, kan dock den syntetiska trögheten fördröja återhämtningen och ställa högre krav de primära reserverna. De studier som utförts för att hitta optimal inställning för regulatorn visar att de standardvärden som tillhandahålls av tillverkaren anses som de mest optimala. Att försöka hitta den mest optimala regulatorinställningen innebär att man får kompromissa mellan att kunna få ett maximalt bidrag under de första sekunderna efter en förlust av en produktionsenhet och behovet av ytterligare kraft som då resulterar i en fördröjd återhämtning av nätfrekvensen. Del 3: Transient reaktiv effekt Målet med denna del är att studera spänningsåterhämtningen under de första hundratals millisekunderna efter bortkoppling av ett fel (eng. transient voltage recovery). Den reaktiva effekten som krävs vid en transient spänningsåterhämtning (vid bortkoppling av ett fel), säkras i de flesta länder via en vindkraftsparks förmåga att inte kopplas bort från nätet vid låg spänning (eng. LVRT=Low Voltage Ride Through). I takt med att andelen vindkraft i nätet ökar, har många länder börjat införa omfattande krav på kontroll av reaktiv ströminjektion för vindkraftsparker i syfte att kontrollera den reaktiva effekten under en nätstörning. Simuleringar av spännings återhämtning efter 130-kV och 400-kV fel nära stora vindkraftverk i Nordic-32 modellen visar att den använda spännings regulatorn möjliggör en stabil återhämtning även för ett svagt system. Simuleringar har utförts för vindkraft produktion av 10 %, 15 % och 20 % av den totala produktionen. Systemet visade sig vara stabil för kortslutningsförhållande (short-circuit ratio) ner till 2,06. Spänning visade en överspänning upp till ca 120 % av den nominella spänningen omedelbart efter bortkoppling av ett fel. Dock inga negativa effekter från denna överspänning är att vänta. Ett effektivt sätt att minska överspänningen är genom att minska den övre gränsen av styrkommandot som kommer från spänningsregulator. Detta kan dock också begränsa möjligheten för vindkraftsparken att reglera spänningen under normal drift. Simuleringarna visar också svängningar i spänningens amplitud ca 400 ms efter bortkoppling av ett fel. Dessa svängningar är inte ett bekymmer i sig, men de kunde peka på potentiell instabilitet från växelverkningen mellan spänningsregleringen av vindkraftparken och de konventionella produktionsenheterna. Det visas att dessa svängningar kan dämpas genom att ändra värdena hos vissa reglerparametrar eller genom att sänka den tröskel vid vilken överspänningsstyrning av omvandlaren aktiveras.

Summary This report summarizes the methods and results of a study after three specific aspects of wind-power integration in the transmission system: inter-area power oscillation; synthetic inertia; and transient reactive power. This study was conducted in a project package consisting of three sub-projects each covering one of these aspects. Part 1: Inter-area power oscillation This part deals with the effect of wind farm voltage controllers on inter-area power oscillations. The IEEE two-area test system is used for a fundamental investigation, whereas a modified Nordic-32 test system is used to represent the Nordic interconnected system. Particularly, taking into account that the generators in Finland oscillate against generators in south of Sweden and Norway at around 0.35 Hz in the interconnected Nordic system, the studies are based on the 0.35 Hz mode. The fundamental investigation shows that the impact of a wind farm on the inter-area oscillation is strongly location dependent. The same wind farm with the same controller shows a slight improvement at one location, but a huge deterioration at another location. The study with wind farms only located in Finland shows that the impact on the 0.35 Hz mode is in general positive. However, when the production from wind farm is high, increasing the export into Sweden, the mode becomes negatively damped. This is most likely a ramification of the increased tie line flow and not due to the voltage controller of the wind farm. The study with wind farms located in three countries (Sweden, Norway, and Finland) shows that the damping of the inter-area mode is improved compared to the case without wind power. However, this improvement could be due to the changed load flow and generation dispatch conditions considered. In conclusion, the presence of wind farms i Finland could contribute to increase the damping for local consumption as well as exporting the produced power to Sweden through the north. Whereas, the consideration of wind power throughout Sweden, Norway and Finland has only minor impact on the inter-area oscillations for the cases studied. Part 2: Synthetic inertia from wind farms The goal of this part is to study power system frequency stability when wind farms contribute to system frequency control through the so called synthetic inertia. Wind farms with DFIG (double fed induction generator) machines and those using a full-power converter, do not contribute to the total moment of inertia in the system. Accordingly, replacing conventional production units with wind farms results in a reduction of the total moment of inertia, eventually deteriorating the frequency quality. Installing wind turbines with synthetic inertia is a way of preventing this deterioration. A number of studies have been performed to study the impact

of synthetic inertia on frequency excursion in a few seconds after the loss of a large production unit by means of augmented Nordic-32 model. For a 4.3% loss of production, the synthetic inertia results in a minimum frequency of 49.45 Hz, versus 49.30 Hz without synthetic inertia. Accordingly, wind farms with synthetic inertia have the ability to increase the minimum frequency and prevent under-frequency load shedding. Regardless of this contribution, synthetic inertia may delay the frequency recovery and put higher demand on the primary reserves. The default parameter values provided by the manufacturer are deemed as optimal parameters. However the selection of optimal parameter might be a trade-off between the contribution during the initial seconds after the production loss and the need for additional power resulting in a delayed frequency recovery. Part 3: Transient reactive power The goal of this part is to study the voltage recovery during the first few hundred milliseconds after clearing a fault (the transient voltage recovery ). Reactive power requirement for transient voltage recovery after clearing a fault is secured via low voltage ride through capability with wind power integration in most countries. As the scale of grid connected wind power is increased, many countries have started to impose advanced requirements of reactive current injection control on wind farms so as to control reactive power during network disturbances. Simulations of transient voltage recovery after 130-kV and 400-kV faults close to large wind-power installations in the Nordic-32 model show that the used voltage controller enables a stable recovery even for a weak system. Simulations have been performed for wind power production of 10%, 15% and 20% of the total production. The system was shown to remain stable for short-circuit ratios down to 2.06. The rms (root mean square) voltage showed an overvoltage up to about 120% of the nominal voltage immediately after fault clearing. However, no adverse impacts from this overvoltage are to be expected. An effective way of reducing the overvoltage is by reducing the upper limit of the excitation command from the voltage controller. This might however also limit the ability of the wind farm to control the voltage during normal operation. The simulations also show oscillations in rms voltage about 400 ms after fault clearing. These oscillations are not a concern by themselves, but they could point to potential instabilities from the interaction between the voltage controllers of the wind farm and those of the conventional units. It is shown that these oscillations can be damped by changing the values of certain controller parameters or by lowering the threshold at which the overvoltage management of the converter is activated.

Contents 1 Motivation and Objectives 1 1.1 Background... 1 1.2 Motivations and Objectives... 1 2 Modelling and Study Approaches 3 2.1 Main Models... 3 2.2 Study Approaches... 3 2.3 Software... 4 3 Achievements 5 3.1 Part 1: Inter-area power oscillations... 5 3.2 Part 2: Synthetic inertia from wind farms... 6 3.3 Part 3: Transient reactive power... 7 4 Conclusions and Future Works 8 4.1 Conclusions... 8 4.2 Future Works... 9 5 References 10

1 Motivation and Objectives 1.1 Background The Nordic countries have ambitious renewable energy source integration targets. This will result in a significant share of wind power in the future generation mix of Nordic countries. In a higher wind power penetration scenario, wind farms will need to contribute to system voltage and frequency control support. However, this needs to be done in a systematic way through detailed system level studies. There is limited published system level operational and control related experience with large amount of wind power contributing to the voltage and frequency stability of large transmission networks. 1.2 Motivations and Objectives The purpose of this project package is to carry out system stability studies to be able to identify the impact of large scale wind power integration on interarea oscillation, frequency stability and transient voltage recovery of a power system. This project package consists of three sub-projects, covering these three aspects of system stability. Part 1: Inter-area power oscillation Among the power system stability phenomena, poorly damped inter-area oscillations in the range of 0.1 Hz to 0.8 Hz are a concern for a reliable operation of modern large interconnected power systems. The poorly damped oscillations might be sensed by the voltage/reactive power controllers within wind farm and could be reflected on the output of the wind farm in the form of varying reactive power. In a certain worst case, the wind farm controller output may exhibit oscillatory behaviour at a certain frequency and the resulting oscillatory reactive power injection from wind farms might degrade the damping of the existing inter-area electromechanical oscillations depending on the wind power penetration level. The main goal of the Part 1 is the investigation of the effect of wind farm voltage controller on inter-area power oscillations in the Nordic interconnected power system. Part 2: Synthetic inertia from wind farms The incoming wind power integration scenario into the existing power system may vary considerably. The impact of wind power integration on the system frequency control function will also depend on the integration scenario. In this regard, one of the concerns is reduced system inertia in the presence of large amount of wind power which will result in higher rate of change of frequency and higher frequency drops after a generation disconnection scenario, which may lead to frequency instability. The background for this concern is the fact that power electronic interfaces in variable speed wind turbines will normally hide generator inertia from contribution to the system. Thus the goal of this 1

Part 2 is to study power system frequency stability when large amount of wind power plants contribute to system frequency through synthetic inertia. Part 3: Transient reactive power Connecting large wind farms to the transmission system makes that transmission system operators (TSOs) have to consider the impact of these wind farms on the transmission grid. The main impacts are related to the replacement of conventional power stations by wind farms during periods with high levels of production from wind power. The impact is complex and covers a range of transient stability phenomena, steady-state phenomena as well as phenomena related to faults in the grid. Thus the goal of this Part 3 is to study one of impacts from the high level production of wind farm. The studied impact is transient voltage recovery, which is related to the voltage recovery during the first few hundred milliseconds after clearing a fault. 2

2 Modelling and Study Approaches 2.1 Main Models Wind farm model A manufacturer released model of a multi-mw commercial variable speed wind turbine (GE 3.6 MW) is used in this work which is adopted from [4]. This model is also included in the model library of PSS/E as user written models, the description of which can be found in [5]. Network models The original CIGRE Nordic-32 system, which is intended to represent the Nordic interconnected power system for studying voltage instability phenomena [6], is modified in order to study the different phenomena within Part 1, Part 2 and Part 3. 2.2 Study Approaches Part 1: Inter-area power oscillation Firstly the selected General Electric (GE) wind turbine voltage controller is used to investigate the wind farm s voltage controller performance under various grid conditions. Secondly the fundamental study of inter-area power oscillation is performed by connecting a single wind farm at different buses (sending end, receiving end, on tie-line) in the IEEE two-area system. Finally, the planned wind farm within Nordic region is considered for inter-area power oscillation. Planned wind farms only located in Finland is firstly considered, and then wind farms located in three countries (Sweden, Norway and Finland); in both cases the wind-farm models are included in the Nordic-32 system simulations. Part 2: Synthetic inertia from wind farms Basic approach in this part of the project is to compare frequency change caused by the loss of production units between with and without wind farm in the modified Nordic-32 network model. Two operational scenarios are used in the studies. Firstly Nordic-32 system is assumed to operate condition with high nuclear/low hydro production and the power production of wind farm is 22% of total active power production. Secondly Nordic-32 system is assumed to operate with moderate nuclear/high hydro production and the power production of wind farm is 23% of total active power production. Finally parameter tuning studies of wind inertial controller model is performed to propose the optimal control parameters by alternating control parameter values. 3

Part 3: Transient reactive power Firstly three capacity scenarios (10%, 15% and 20%) are used to consider the impact of power production level from wind farm. Secondly the locational impact of wind power integration is included in wind power capacity scenario so that two areas (Northern and Central area) are equally taken into account as wind power integration region. 2.3 Software PSS/E function modules (Load Flow and LSYSAN) have been used in simulation studies. In addition, Matlab is used for some scientific calculations. 4

3 Achievements 3.1 Part 1: Inter-area power oscillations Disturbance rejection capability of voltage controller The purpose of disturbance rejection capability is to investigate the mechanism by which control parameter will influence the existing inter-area oscillation at the grid. Particularly, taking into account that the generators in Finland oscillate against generators in south of Sweden and Norway at around 0.35 Hz, disturbance rejection capability is reviewed around this frequency. The simulations in his project have shown that slowly varying disturbance is effectively rejected by the voltage controller up to 0.1 Hz. The lead angle becomes around 160 degrees and voltage controller exhibits a high gain at 0.35 Hz, showing that disturbance rejection capability could be impacted. Fundamental study on inter-area oscillation by wind farm voltage controller The IEEE two-area system is studied to verify the impact of wind turbine voltage controller on power system oscillation. The fundamental studies show that the impact is strongly location dependent. The same wind farm with the same controller shows a slight improvement at one location but a huge deterioration at another location. Nordic-32 system study on inter-area oscillation by wind farm voltage controller The study of wind farms from only Finland shows that the sole impact of the planned wind farm s voltage controllers is in general positive on the 0.35 Hz mode. However, when the production from wind farm is high, increasing the tie line flow to Sweden, the mode becomes negatively damped. The negative damping of the 0.35 Hz mode due to the planned wind farms cannot be directly attributed to the wind farm voltage controllers, instead it could be a ramification of the changed system power flow (increased tie line flow) caused by the introduction of new production in Finland. The study of wind farms from three countries (Sweden, Norway, and Finland) shows that the damping of the inter-area mode is improved compared to the case without wind power. However, as mentioned above, the improvement of the damping could be due to the changed system load flow and generation dispatch conditions considered. Thus if power system operational condition is changed, that might result in a different damping mode. Accordingly, it might be inferred that the presence of wind farms within Finland could contribute to increase the damping for local consumption as well as exporting the produced power to Sweden through the north. Considering wind power throughout 5

Sweden, Norway and Finland has only minor impact on the inter-area oscillations for the cases studied. 3.2 Part 2: Synthetic inertia from wind farms Impact of wind turbines on Nordic system inertia Wind farms based on double fed induction generator do not contribute to the moment of inertia of the system. The same holds for wind turbines connected to the grid by a full-power converter. When conventional synchronous generators are replaced by wind power without synthetic inertia, the total moment of inertia of the system will become less and the frequency stability will deteriorate. Thus it might be inferred that as the scale of wind is increased, the frequency stability might be more deteriorated without any prevention measures. Effects of Synthetic Inertia for improving Nordic system inertia Given that the power production of wind farm is assumed to have 22% of total active power production in the operation condition with high nuclear/low hydro production as well as the amount of power production loss of synchronous generators is assumed by 4.3% of total consumption, the minimum primary frequency is investigated by means of two options (without and with synthetic inertia), showing the minimum frequency as below. Loss of production Without synthetic inertia With synthetic inertia 4.3% 49.30 Hz 49.45 Hz It is shown that wind farms are able to contribute to the frequency stability during the first few seconds after a loss of production by extracting the stored kinetic energy through the synthetic inertia of the wind turbine. As a result, it is possible to increase the minimum frequency and to prevent underfrequency load shedding. However the contribution from the synthetic inertia might not be sufficient to prevent a large frequency drop with a severe loss of production. Optimal tuning of synthetic inertia for Nordic system inertia Optimal tuning is performed to find the best control parameter within synthetic inertia for improving frequency stability support by the wind turbine. Two control parameters, i.e. wind inertia gain and wash-out time constant are investigated to find the optimal values. The criterion for selecting the most optimized value for a parameter is to obtain best frequency support considering both rate of change and minimum value. The default parameter values provided by manufacturer are deemed as optimal parameters. However the selection of optimal parameter needs a trade-off between the contribution during the initial seconds after the production loss and the need for additional power resulting in a delayed frequency recovery. 6

3.3 Part 3: Transient reactive power Literature review on transient reactive power Reactive power requirement for transient voltage recovery after clearing a fault is secured via LVRT (low voltage ride through) capability with wind power integration in the most countries. As the scale of grid connected wind power is increased, many countries have started to impose advanced requirements of reactive current injection control on wind farms so as to control reactive power during network disturbance. Technical review shows that the grid strength could determine the voltage behaviour of wind power connection point in DFIG-connected power system. Also, voltage collapse point might be changed by replacing conventional synchronous machine with wind power, deteriorating voltage stability. Thus, the maximum amount of wind power integration should be carefully reviewed from the transient reactive power perspective. Impact of large scale wind farm on transient reactive power Reactive power requirements for transient voltage recovery after clearing a fault are secured via the low voltage ride through (LVRT) capability with wind power integration in the most countries. As the scale of grid-connected wind power is increased, many countries have started to impose advanced requirements of reactive current injection control on wind farms so as to control reactive power during a network disturbance. Simulations of transient voltage recovery after 130-kV and 400-kV faults close to large wind-power installations in the Nordic-32 model show that the used voltage controller enables a stable recovery even for a weak system. Simulations have been performed for wind power production of 10%, 15% and 20% of the total production. The system was shown to remain stable for short-circuit ratios down to 2.06. The rms (root mean square) voltage showed an overvoltage up to about 120% of the nominal voltage immediately after fault clearing. However, no adverse impacts from this overvoltage are to be expected. An effective way of reducing the overvoltage is by reducing the upper limit of the excitation command from the voltage controller. This might however also limit the ability of the wind farm to control the voltage during normal operation. The simulations also show oscillations in rms voltage about 400 ms after fault clearing. These oscillations are not a concern by themselves, but they could point to potential instabilities from the interaction between the voltage controllers of the wind farm and those of the conventional units. It is shown that these oscillations can be damped by changing the values of certain controller parameters or by lowering the threshold at which the overvoltage management of the converter is activated. 7

4 Conclusions and Future Works 4.1 Conclusions Part 1: Inter-area power oscillation The case study with wind farms from only Finland shows that the impact of wind farm s voltage controllers is in general positive on the 0.35 Hz mode. However, when the production from wind farm is highest and the surplus production is exported into Sweden, increasing the tie line flow, the mode becomes negatively damped. The case study with wind farms from three countries (Sweden, Norway, and Finland) shows that the damping of the inter-area mode is improved compared to the case without wind power. The improvement of the damping could be due to the changed system load flow and generation dispatch conditions considered. However, taking into account that the impact of wind farm voltage controller characteristics depended on operational conditions or wind integration scenarios of Nordic power system, the extent of the case studies may not allow us to draw general conclusions. From the perspective of Nordic transmission network, those issues need to be further analysed in a more comprehensive and systematic manner. Part 2: Synthetic inertia from wind farms Wind farms based on double fed induction generator do not contribute to the moment of inertia of the system. However, the synthetic inertia within wind turbines can support the frequency in the first few seconds after a loss of production by extracting the stored kinetic energy through the synthetic inertia within wind turbine. As a result, it is possible to increase the minimum frequency and to prevent under-frequency load shedding. Part 3: Transient reactive power As the scale of grid-connected wind power is increased, many countries have started to impose advanced requirements of reactive current injection control on wind farms so as to control reactive power during a network disturbance. Simulations of transient voltage recovery after 130-kV and 400-kV faults close to large wind-power installations in the Nordic-32 model show that the used voltage controller enables a stable recovery even for a weak system. The system was shown to remain stable for short-circuit ratios down to 2.06. The rms voltage showed an overvoltage up to about 120% of the nominal voltage immediately after fault clearing. An effective way of reducing the overvoltage is by reducing the upper limit of the excitation command from the 8

voltage controller. This might however also limit the ability of the wind farm to control the voltage during normal operation. 4.2 Future Works Based on observed conclusions of the respective reports, some recommendations are given to operate power systems with large scale wind power integration. Firstly, the results from the IEEE model should be analysed in more depth to find out if there are locational advantage for reducing damping in Nordic grid. In addition, further study is needed to quantify the relation between the disturbance-rejection capability of the controller and the reduction or increase of the damping of the inter-area oscillations. Secondly, further work is needed in the development of control algorithms for synthetic inertia, where the main challenge is to be able to contribute to the inertia during the first few seconds without delaying the frequency recovery unnecessary and without putting extra requirements on the primary reserve. A fundamental study is needed to find out under which circumstances (e.g. low consumption in combination with high amounts of wind power) the activation of synthetic inertia or a similar measure is needed and how often such circumstances occur. A related study is to do find out when and how often synthetic inertia in all or a selected number of wind turbines is insufficient. Finally, a potential oscillation is observed and damped by changing certain control value of voltage controller with large scale wind power integration. Thus, further work is needed in deciding the hosting-capacity, which is not included in this project, with large scale wind farm connection from a transient stability perspective, and analysing the interaction between the voltage controllers from wind farms and those from conventional production units. 9

5 References [1] Nayeem R. Ullah, The effect of voltage control response characteristics of wind plants on damping of inter-area power oscillations, Vindfork V-369 Report (Part 1), September, 2012. [2] Mohammad Seyedi, The utilization of synthetic inertia from large number of wind farms and its impact on existing speed governors and system performance, Vindfork V-369 Report (part 2), September, 2012. [3] Seon Gu Kim, Towards the development of a set of grid code requirements for wind farms: Transient reactive power requirements, Vindfork V-369 Report (Part 3), September, 2012. [4] K. Clark, N. W. Miller, J. J. Sanchez-Gasca, Modeling of GE wind turbine generators for grid studies, GE power system energy consulting, U.S.A., technical report version 4.3, April 2009. [5] Y. Kazachkov, PSSE wind modelling package for GE 1.5/3.6/2.5MW wind turbines user guide, Siemens energy, Inc., issue 5.1.0, June 2009. [6] K. Walve, Nordic32-A Cigre test system for simulation of transient stability and long term dynamics, Svenska Kraftnät, Sweden, technical report, 1993. [7] J. Ekanayake and N. Jenkins, Comparison of the response of doubly fed and fixed speed induction generator wind turbines to changes in network frequency, IEEE Trans. Energy Conv., vol. 19, no. 4, pp. 800-802, Dec. 2004. [8] A. Mullane and M. O Malley, The inertial response of induction machine based wind turbines, IEEE Trans. Power Syst., vol. 20, no. 3, pp. 1496-1503, Aug. 2005. [9] K. Clark et el., Modeling of GE wind turbine-generators for grid studies, GE International, Inc., USA, Tech. Rep., v 4.3, April 2009. [10] M. Bollen and F. Hassan, Integration of distributed generation in the power system, Wiley, 2011. 10

11