power oscillation damping using wind turbines with energy storage systems

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Published in IET Renewable Power GenerationReceived on 23rd January 2012Revised on 15th November 2012Accepted on 4th March 2013doi: 10.1049/iet-rpg.2012.0019

T Renew. Power Gener., 2013, Vol. 7, Iss. 5, pp. 449–457oi: 10.1049/iet-rpg.2012.0019

ISSN 1752-1416

Power oscillation damping using wind turbineswith energy storage systemsGuoyi Xu1, Lie Xu2, John Morrow1

1School of Electronics, Electrical Engineering and Computer Science, Queen’s University of Belfast, UK2Department of Electronic and Electrical Engineering, University of Strathclyde, Glasgow, UK

E-mail: [email protected]

Abstract: Wind turbines are increasingly being expected to provide oscillation damping to the power system to which they areconnected. In this study, power oscillation damping control of variable speed wind turbines is studied. An energy storage devicewith a bidirectional DC/DC converter connected to the DC link of a fully rated converter-based wind turbine is proposed. Assystem oscillation is often induced by an AC fault, it is desirable for wind turbines to ride through the fault first and then providea damping effect. During the fault period, the energy storage system (ESS) is controlled to assist the fault ride through process,and the line side converter (LSC) is controlled to provide AC voltage support in accordance with the grid code. Methods basedon regulating the active power output of the ESS and modulation of reactive power output of the LSC are proposed so as to dampthe oscillations of the power system. Matlab/Simulink simulations based on a simplified Irish power system demonstrate theperformance of the ESS and LSC during fault periods and validate the damping effect of the proposed system.

1 Introduction

Wind power is considered as the most promising energysource capable of contributing to a solution of the currentenergy crisis. Many wind turbines (WTs) have beeninstalled all over the world. In order to ensure safeoperation of the power system with high penetration ofwind power, grid codes require wind farms to have somemandatory functions including active and reactive powercontrol, frequency control, terminal voltage control, faultride through (FRT) capability, and data exchange andcommunication requirement [1]. One particular aspect thathas attracted much attention is the influence of large windgeneration on power system stability. With high windpower penetration, system stability may be affected, andwind power is now expected to contribute to theimprovement of system stability.The dynamic behaviour of a power system is largely

determined by the behaviour and interaction of theconnected generators. In practice, there are three majortypes of WT generators installed in power system, that isfixed speed induction generators (FSIG), doubly fedinduction generators (DFIGs) and fully ratedconverter-based WT generators. Owing to the differenttechnologies of the WTs, their impacts on power systemstability are different. Much research has been carried out tostudy the dynamic behaviour of different WTs by means ofmodal analysis and time domain simulation. In reference[2], eigenvalue analysis was used to analyse power systemdamping with WTs. It is concluded that wind power tendsto increase the damping of inter-area oscillations,particularly for FSIG, whereas the impact on intra-area

oscillations is not significant. Reference [3] indicated thatthe general trend for DFIGs is to increase oscillationdamping, although the reactive power control scheme of theconverter may also reduce damping. Research on theNordic network has proven that FSIGs can contribute tonetwork damping, whereas the variable speed WTs decreasethe damping [4]. Studies on the New Zealand networkindicated that the damping performance is not essentiallyaffected by the high level of wind penetration [5]. Forvariable speed WTs using fully rated converter, thegenerators are fully decoupled from the network and thesystem behaviour is thus largely dependent on the convertercontrol strategy.Although the influence of WTs on different power network

is different, it is widely agreed that through auxiliary control,variable speed WTs can improve the damping effect [6–9]. Inprinciple, the damping effect could be improved by modulateboth active and reactive power output of WTs. The powersystem stabiliser (PSS) controller is usually designed bythe methods of root locus or phase compensation etc. Someresearchers are focused on the choice of input signals forthe PSS controllers, terminal voltage, active power androtor speed are typically used while others have made useof remote signals from phasor measurement units. Reactivepower modulation using a DFIG’s line side converter(LSC) for oscillation damping is investigated in [6, 7]. In[7, 8] propose a PSS with variable speed WT to damppower system oscillation where the auxiliary PSS loop isadded to the active power control loop and the kineticenergy of the WT serves as energy storage for the activepower modulation. The WT can deliver active dampingpower while delivering maximum available active power.

449& The Institution of Engineering and Technology 2013

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However, the amount of the inertial energy that can be usedneeds to be considered in order to have a satisfied dampingresult. Another method is to consider the active powerreserve of the WT in order to provide the extra activepower [9]. However, this may affect the operation andperformance of the WT and may not be economical. On theother hand, oscillations are often induced by grid faults andtherefore it is very important to study the response of theWTs during such a fault before they can be controlled toprovide a damping effect. One method of providing extraactive power is to embed an energy storage system (ESS)within the WTs [10–14]. However, most of this work hasconcentrated on improving power quality, enhancing FRTability and smoothing power fluctuation.The objective of this paper is to investigate a WT’s ability
to provide damping by incorporating an ESS on the DC linkof the variable speed fully rated converter-based WT. Suchsystem requires careful considerations as active power fromboth the ESS and wind generator have to pass through theLSC and therefore the action of the ESS and LSC must beclosely coordinated considering their respective powerlimits. The detailed responses of the ESS and LSC duringgrid faults also need careful examination as power systemoscillation can often be caused by such faults. This study isof particular interest for relatively weak and synchronouslyisolated power system such as the Irish power system,which has a large penetration of wind power generation,and is thus susceptible to power oscillation. The paper isorganised as follows. The system configuration and modelsare outlined first in Section 2. Section 3 describes thecontrol strategies for the proposed system to improve thedamping effect. The control of the LSC and ESS isinvestigated in Section 4. Simulation studies are carried outto verify the effectiveness of the proposed method inSection 5 and finally, Section 6 draws the conclusions.

2 System layout

In a fully rated converter-based WT, the generator isconnected to a network through a back-to-back converter.Thus, under normal operation the generator is completelydecoupled from the grid system although interaction maystill exist during large disturbances resulting in an abnormalDC-link voltage. The converter is usually composed of agenerator side AC/DC converter (GSC), a DC-linkcapacitor, and a line side DC/AC converter.In the proposed method, an ESS which comprises an

energy storage device and a bidirectional DC/DC converteris connected to the DC link of the converter as shown inFig. 1. Within the coupled ESS the energy storage device

Fig. 1 Layout of the proposed system


can be controlled to absorb or release energy through thecontrol of the bidirectional DC/DC converter. The powerflows in the DC link under normal operating conditions arealso shown in Fig. 1. By controlling PESS, the active poweroutput to the AC network, that is PGrid can be regulated tomeet the operation requirement of the network, for examplereduce power fluctuation, power system oscillation dampingand AC frequency support. The study carried out in thispaper primary focus on oscillation damping. For thegenerator, a direct-driven permanent magnet synchronousgenerator (PMSG) is considered in this paper.Currently, there are several kinds of energy storage devices

that are suitable for short-to-medium-term power exchange,such as a battery, electric double layer capacitor (EDLC),flywheel and superconducting magnetic energy storage. Ofthe various storage devices, the EDLC has attracted muchattention because of its long life cycle, low maintenance,fast charging/discharging speed, high energy and powerdensity etc. Some research has already been carried out toshow the effectiveness of using the EDLC storage systemwith wind power [13]. A typical EDLC cell is rated at 2.7V/3000 F, and the cells can be connected in serial and inparallel to form EDLC bank at different voltage andcapacity ratings. The price for the EDLC cells has droppedsignificantly in recent years and the cost for a typical 0.2MJ EDLC is less than $2 k with a volume of 0.015 m3

[15]. The typical cost for the full-sized converter system isof the order of $80–100 k/MW and the average WT cost is∼ $1100 k/MW [16]. Consequently, considering a 3-MJEDLC used for a 3MW WT, the extra cost of the EDLCcells and associated DC/DC converter is likely to add only∼ 10% of the converter cost and is thus only a smallfraction of the total cost of a WT. Thus, from an economicpoint of view, it is feasible to install an EDLC-basedstorage system with a WT. The total volume of the EDLCis ∼ 0.25 m3 so it can be easily accommodated alongsidethe converter in either the nacelle or the tower base of a WT.

3 Oscillation damping control

3.1 Generator motion analysis

Power system stability is mainly determined by the motioncharacteristics of the generators in the system. The motionequations of a synchronous generator are [17]

2H · s · Dvr = DTm − DTe (1)

DTe = KSDd+ KDDvr (2)

sd = v0Dvr (3)

where H is the inertia constant, s is the Laplace operator, andΔωr is the angular speed deviation. ΔTm and ΔTe are themechanical and electrical torque deviations, respectively. KS

and KD are the synchronising and damping torquecoefficients, respectively, and Δδ is the rotor angledeviation. It may be observed from (2) that the electrictorque is decomposed into synchronising torque anddamping torque. The synchronising torque is in proportionto the variation of power angle Δδ while the dampingtorque is in proportion to the variation of angular speedΔωr. According to the modal analysis in [17], increasing KD

increases the damping ratio, thus improves the dampingeffect. Therefore additional damping effect can be added byincreasing the coefficient KD.

IET Renew. Power Gener., 2013, Vol. 7, Iss. 5, pp. 449–457doi: 10.1049/iet-rpg.2012.0019

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3.2 Damping controller
Through modulating the field voltage of the exciter, a typicalPSS of a synchronous generator produces appropriatedamping torque and synchronising torque to minimisepower system oscillation [17]. For the proposed layout ofthe WT and ESS, the system emulate similar PSS functionof a synchronous generators by separately controlling theWT’s active and reactive power output without affecting theoperation of the generator/turbine. Active power modulationis provided by the combined control of the ESS and theLSC while reactive power modulation is provided solely bythe LSC.The PSS control loop which generates the reference signal

necessary for the ESS to provide damping control isschematically shown in Fig. 2a. Generally, WTs are locatedsome distance from conventional power plants and thus, itis not convenient for the WTs to have the information ofsynchronous generators’ rotor speed variation except forinstances where a wide area measurement system is in use.Thus, frequency variation which can be easily measuredlocally and is related to rotor speed variation, is typicallyused for the input to PSS controllers. As shown in Fig. 2a,the frequency error is first passed through a high-pass filterto eliminate the steady-state influence. A lead-lag regulatoris applied to provide the necessary filtering and phase shiftto the interested frequency range. The power order (PESS_P)is then send to the DC/DC converter to control the ESS toabsorb or release extra energy to the WT common DC link.The DC voltage is maintained by the LSC at a constantvalue to ensure active power input and output in the DClink is balanced. Thus, the overall active power output fromthe LSC which contains power from the generator and theESS is modulated. A PSS controller of the same structureshown in Fig. 2b is used for the LSC to modulate thereactive power output. Many studies have been carried outfor designing the PSS controller parameters to provideadequate system response and system stability [18, 19]. Thepurpose of this study is to validate the concept of theproposed WT and ESS considering the required converterrating and storage requirement. Thus, the detailedassessments on the PSS control parameters are not providedhere and they have been chosen based on a trial-and-errorapproach.It has been verified that active power modulation is more

effective when the source is located near the ends oftransmission line, while reactive power modulation is moreeffective when the injection point is close to the electricalmidpoint [19]. By combining active and reactive powercontrol effective damping can be obtained along thetransmission line no matter where the wind farm is located.

Fig. 2 Damping controller design

a Active power basedb Reactive power based


4 Control of WT and ESS

To investigate oscillation damping using WTs, it is importantto study the response of WTs during grid faults, as oscillationis often induced by such faults. The WTs must ride throughthe faults before they can contribute to power systemdamping. For the proposed WT configuration, control of theLSC and ESS during fault has been investigated in detail.

4.1 Grid requirement of FRT and voltage support

In the case of a WT with a fully rated converter, if the DC-linkvoltage can be controlled during the fault, the operation of thegenerator will not be affected. However, the drop of the ACvoltage is likely to prevent the LSC from transmitting allthe generated active power. In order to maintain theDC-link voltage, the excess power has to be dissipated orthe generator output power has to be reduced during the ACfault.Many grid codes not only require the wind farms to ride

through faults but also require the WTs to support gridvoltage recovery by generating reactive power duringnetwork faults. The Great Britain and Ireland grid codesrequire wind farms to produce their maximum reactivecurrent during a network fault [20, 21] whereas the E.ONgrid code has a more specific requirement of the reactivecurrent to be delivered during a voltage dip, as shown inFig. 3 [22, 23]. According to the E.ON grid code reactivecurrent on the low voltage side of the WT transformer mustbe provided at a rate of twice the voltage variation. Thus, areactive current output of 100% of the rated current isnecessary if the voltage drops 50% below the rated value.When considering voltage support using WTs, it is

necessary to investigate the power capacity of theconverters. In accordance with the E.ON grid code, agenerating unit should be able to operate at a power factorof 0.95 at 0.87 pu voltage without limiting the active poweroutput from the unit. For a WT with fully rated converter,this corresponds to a 20% increase of the LSC currentrating, for example, the maximum current capability of theLSC is 1.2 pu of the rated current. This 20% extra capacitycan bring great benefit to the system especially during faultconditions.According to the voltage support requirement, considering

the capacity of the LSC, the active and reactive current limitsand associated power of the LSC for different terminalvoltages are shown in Figs. 4a and b, respectively. TheLSC is usually controlled using vector control based on thed–q synchronous reference frame where the d-axis isaligned to the network voltage [24, 25]. Thus, the d-axiscurrent corresponds to active power whereas the q-axiscurrent refers to reactive power. Taking into account thereactive current requirement, the maximum active current

Fig. 3 Voltage support according to E.ON grid code [19]


Fig. 4 Active and reactive current and power of a WT duringvoltage drop

a Active and reactive currentb Active and reactive power

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that can be delivered by the LSC in per unit term is given by

id max =��1.22 − i∗2q

√(4)

where 1.2 is the maximum LSC current capability and id_max

is the maximum d-axis (active) current of the LSC. i∗q is theq-axis (reactive) current order for the converter.As shown in Fig. 4a if the voltage drops below 0.5 pu, a

reactive current equaling to 100% of the rated current isrequired. Considering the 1.2 pu converter capacity, amaximum active current of 0.66 pu can be used to transmita certain amount of active power from the DC link to thenetwork. As shown in Fig. 4b, the LSC mainly contributesreactive power for voltage support when the voltage is low.

4.2 Control of LSC

The controller for generating the active and reactive current ofthe LSC is shown in Fig. 5. As may be observed, the reactivepower request is composed of two parts, one from the voltagesupport unit and the other from the reactive power modulationof the PSS.Under normal operation, the LSC performs DC voltage

control to ensure balanced active power input and output inthe WT common DC link. There will be no active currentcurtailment considering the LSC’s capacity and reactive

Fig. 5 Current controller for the LSC


current requirement during normal AC voltage condition.During a grid fault, however, the AC voltage drops andreactive current is required to support the AC voltage. Themaximum active current and power transmitted by the LSCwill reduce as shown in Fig. 4b. Owing to the reduction ofthe power transmitted by the converter, the LSC may not beable to control the DC voltage under such conditions.

4.3 ESS control and charging management

With the proposed configuration, the ESS not only provides adamping effect when oscillation occurs, but also absorbsexcess power from the WT DC link during AC faults toassist the FRT process. During the operation, the ESS needsto absorb or supply energy from/to the WT DC link toprovide the necessary functions. Owing to the use of theEDLC which essentially is a capacitor, the resultantcharging and discharging inevitably cause the DC voltageof the EDLC to vary. It is also necessary that understeady-state the EDLC’s DC voltage is maintained at acertain level (i.e. initial charge) such that its DC voltage canrise (when the ESS is absorbing energy/charging) or drop(when the ESS is supplying energy/discharging) asrequired. The purpose of the ESS charging management isto maintain the steady-state ESS voltage at a pre-definedlevel but also allow it to vary during transients in order toprovide network support and FRT enhancement. This isaccomplished by adding an ESS DC voltage control loopwith a response time being a few times (say two to five)slower than a typical oscillation cycle of the AC system.Fig. 6a shows a typical design of the ESS DC voltagecontroller using a proportional integral (PI) regulator as

PESS dc = kp ESS dcDVESS + ki ESS dc

∫DVESS dt (5)

where ΔVESS = VESS−VESS* and the PI parameters can bedesign based on the damping factor ζ and the natural frequency ofthe DC voltage loop ωn as

kp ESS dc = 2zvn ki ESS dc = vn

( )2(6)

It should be noted that a positive power order for the ESSrefers to exporting active power to the common DC bus,

Fig. 6 ESS control loop design

a ESS DC voltage controller (ESS charging management)b Overall ESS control diagram


Fig. 7 Schematic diagram of the simulated system

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that is, discharging the ESS. Conversely, a negative valueindicates charging the ESS and absorbing active powerfrom the common DC bus. If a typical power systemoscillation is about 0.5–1 Hz, ωn can be two to five timessmaller than this, for example, around 0.628 rads/s (0.1 Hz).This would ensure the steady-state charging of the ESSremain at the optimal level (desired voltage) withoutaffecting the transient power exchange between the ESSand the WT common DC link.Thus, the total active power input to the common DC link
is given as

PIN = PGen + PESS P + PESS dc (7)

where PGen and PESS_P represent the output active power ofthe generator and the ESS controlled by the PSS controller.The maximum active power that can be transmitted by the

LSC to the grid may be approximated by [25]

PLSC max = 1.5Vac id max (8)

where Vac is the AC peak phase voltage.In order to balance the input and output power in the WT

common DC link and keep the DC-link voltage variationwithin a limited range, PIN should not exceed PLSC_max.Therefore it may be necessary to limit the PESS_P duringAC fault condition because of the reduced value ofPLSC_max. Taking this into account, Fig. 6b shows theoverall controller for producing the ESS’s power order.As seen if the AC voltage is high which results in high

id_max and PLSC_max being greater than PIN, thenP∗ESS = PESS P + PESS dc and the ESS working state is

thoroughly dependent on the PSS and its DC voltagecontrollers (charging management). Once a fault occurs andthe AC voltage drops, PLSC_max decreases because of thereduced AC voltage and active current capability (as thereactive current is increased shown in Fig. 4). In whichcase, PIN may exceed PLSC_max and if PIN were not limited,it would result in excess power in the WT common DC linkcausing Vdc over voltage. Thus, the power output from theESS has to be limited to P∗

ESS = PLSC max − PGen.During a severe AC voltage drop which results in very

small value of PLSC max, P∗ESS can become negative ensuring

the ESS absorbs the excess power in the DC linkautomatically, thus helping the FRT process. If, because ofthe limited power rating of the ESS, it is not able to absorball the excess power then DC overvoltage can still occur.After fault clearance, the AC voltage begins to recover andthe power transmitted by the LSC increases. Thus the ESSwill resume the PSS control mode to provide damping ofthe system oscillation.

4.4 Damping resistor

As stated earlier, if the ESS power capacity is not largeenough to absorb the excess power in the DC link during afault, other method should be adopted to ensure theDC-link voltage is limited. One method is to use the GSCto rapidly reduce the power out from the wind generator.The turbine speeds up because of the excessive energycaused by the imbalance between the mechanical input andelectrical output power. In this paper, in order to simplifythe control of GSC, a DC damping resistor connected to theDC link through a controllable switch is adopted. Thedamping resistor can be controlled by a hysteresiscontroller. During a fault, the operation of the WT is not


disturbed and it continues to produce maximum power fromthe available wind, the damping resistor dissipates excesspower, as necessary. Generally, the rating of the DCdamping circuit has to be the same rating as the WT.However, if an ESS is adopted, the size of the resistorcould be reduced.

5 Simulation results

To verify the effectiveness of the proposed configuration andcontrol methods for oscillation damping and operation duringAC faults, simulation analyses have been carried out usingMatlab/Simulink. The network developed in the simulationis based on a simplified Irish power system. The Irishpower system is a small and synchronously isolated system,consisting of networks operated by Northern IrelandElectricity (NIE) in the north and Electricity Supply Board(ESB) in the south. The NIE and ESB networks areconnected by a 275 kV double circuit interconnector. Thesystem has a peak load of ∼ 6300 MW in winter weekdays, and a low load of about 3000 MW during summernights. At present, the installed wind capacity of the systemis estimate to be 350 MW in the NIE system and 1095 MWin the ESB system but it is anticipated that significantadditional wind capacity, currently planned, will beconnected within the next few years. Inter-area oscillationhas been regularly observed on the NIE/ESB 275 kVinterconnector after system disturbances. With the increaseof wind generation and demand, the oscillation has becomemore sustained [26].The simplified model of the Irish network is shown in

Fig. 7. The NIE system is represented by the synchronousgenerator G1 rated at 2000 MW while the ESB system isrepresented by the 6000 MW synchronous generator G2.A 500 MW wind farm consists of 167 WTs each rated at3 MW is considered to represent the wind generation on theNIE system. The LSC current rating of each 3 MW WT is1.2 pu of the rated current based on previous analysis, thatis, the LSC is rated at 3.6 MVA when supplied with ratedAC voltage. For each WT, the DC/DC converter, atnominal ESS voltage, is rated at a third of the LSC powerrating and it could be implemented by adding a fourthconverter leg to the three-phase converter system. Duringthe simulation an aggregate model is used to represent the500MW wind farm with rated WT common DC-linkvoltage of 20 kV and an AC rms line-to-line voltage of 10kV. Thus, the LSC phase current limit is 49 kA (peak) forthe lumped model. The capacitance of the EDLC of theESS is 5 F and its maximum DC voltage is 13.5 kV with atotal storage energy of 455 MJ (2.7 MJ for each 3 MW


Fig. 8 Simulation results of minor voltage dip

a AC voltage at wind farm connecting pointb LSC output powerc LSC AC currentd ESS voltagee ESS currentf WT DC-link voltage

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WT). To ensure the ESS can absorb as well as supply energy,its nominal DC voltage (i.e. V ∗

ESS) is set at 10 kV for thecharging management unit (ESS DC voltage controller) andthe ESS DC current limit is set at 20 kA (200 MW at 10 kVnominal DC voltage). The WT is modelled using theexisting aerodynamic equations and is directly connected toa 44-pole PMSG. A detailed converter models including thepower electronics devices, switching and associatedcontroller are implemented. The switching frequencies forthe converters are 5 kHz. The NIE and ESB systems areconnected by a 275 kV interconnector. Wind powergeneration is ∼ 25% of the NIE system generation for thiscase, thus simulating a future scenario. The twosynchronous generators are each equipped with a PSS foroscillation damping. While simple in the form, the networkhas relevance to the actual Irish power system.For the case study, the WT operates in the maximum power

tracking mode, producing 350 MW of active power. Activepower transmission through the interconnector from NIE toESB is ∼ 100 MW. A 100 ms three-phase fault is appliedat the connection point A of the wind farm as shown inFig. 7. Considering the fast process of fault and oscillationdamping, the wind speed is assumed to be constant in thisperiod.

5.1 Simulation results during fault

During an AC voltage dip caused by an AC fault, the LSC’smaximum output power is dependent on the remaining AC


voltage (because of current limit), which also determinesthe maximum power the ESS needs absorb in order tomaintain the common DC-link voltage. To investigate theoperation of the LSC and ESS, two different faults whichresult in different voltage dips are considered. The first oneresults in a small voltage dip and the ESS power capacity islarge enough to absorb all the excess power, whereas thesecond fault is more severe resulting in a large voltage dropand activation of the WTs damping resistor because ofinsufficient ESS power capacity.Simulation results for the smaller voltage dip are shown in

Fig. 8. The effect of wind farm reactive power supportduring the fault on the AC voltage is shown in Fig. 8a.The reactive power output from the wind farm (LSC) is∼ 250 MVar and the active power transmit through theLSC is ∼ 220 MW as observed in Fig. 8b. The LSC’s ACcurrent reaches its limit of 49 kA during the fault period asobserved in Fig. 8c. Throughout the fault, the operation ofthe generator is not disturbed and it continues to generate350 MW. This results in 130 MW excess active power inthe DC link which is fully absorbed by the ESS resultingin an increased ESS voltage as shown in Fig. 8d. The ESScurrent is ∼ 13 kA as shown in Fig. 8e. During the faultperiod, the total energy absorbed by the ESS is ∼ 10 MJ,which corresponds to 60 kJ for each turbine in thisexample. As the ESS capacity is large enough to absorbthe excess active power, the DC-link voltage is controlledwithin the limited range as shown in Fig. 8f, and thedamping resistor is not activated. It is also observed in


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Fig. 8b, after fault clearance the LSC absorbs a certainamount of reactive power to limit the AC over-voltage.The operation of the ESS after fault clearance is largelybecause of the action of the damping control which will bediscussed in Section 5.2.Simulation results for a more severe fault are shown in
Fig. 9. As observed in Fig. 9a, the voltage drops to ∼0.2 pu without voltage support and to 0.4 pu without WTvoltage support. Owing to the reduced AC voltage, theLSC transmitted active power is reduced to 100 MW with200 MVar reactive power produced for voltage support, asshown in Fig. 9b. In this period, the LSC again reachesits current limit of 49 kA as observed in Fig. 9c. Duringthis fault, there is 250 MW excess power in the DC link.The ESS absorbs ∼ 200 MW with its charging currenthitting the limit of 20 kA and the ESS DC voltageincreases as can be observed in Figs. 9d and e. Thus,there exists 50 MW extra active power in the DC linkresulting in the increase of DC voltage and consequently,the damping resistor is activated to limit the DCover-voltage to within a preset range as observed inFig. 9f. During the fault period the total energy absorbedby the ESS is 20 MJ, which corresponds to 120 kJ foreach WT and the damping resistor dissipating 4 MJ ofenergy. Again the operation of the ESS after faultclearance is largely due to the action of the dampingcontrol which will be discussed in Section 5.2.

Fig. 9 Simulation results of severe voltage dip

a AC voltage at wind farm connecting pointb LSC output powerc LSC AC currentd ESS voltagee ESS currentf WT DC-link voltage


5.2 Simulation results of oscillation damping(after fault)

After AC fault clearance, the active and reactive power outputfrom the LSC is modulated to damp any possible oscillations.As the wind farm connection point is close to the synchronousgenerator G1, reactive power modulation for damping controlis found to be ineffective and the damping is mainly providedby the ESS’s active power control. In order to further illustratethe effect of active power modulation on system damping,two different ESS power (current) ratings are considered,one at 20 kA (as previously described) and the other at 10kA. Fig. 10 compares the simulation results withoutdamping control and with both reactive and active powermodulation. As shown in Fig. 10a, after the fault, thepower transmitted on the interconnector oscillates at afrequency of ∼ 0.4 Hz without active damping control fromthe WT. On the other hand, with the damping controllersoperational significant damping effect has been achieved.As may be observed in Figs. 10b and c, the poweroscillations of the two synchronous generators have alsobeen significantly damped. During this process, the LSC’sactive and reactive power outputs are modulated asobserved in Fig. 10d and the LSC AC current is wellcontrolled within its limit of 49 kA as shown in Fig. 10e.The ESS’s current, voltage and power are shown inFigs. 10f, g and h, respectively. For the ESS voltage, owing


Fig. 10 Simulation results of active and reactive power modulation

a Active power on interconnectorb Active power of G1c Active power of G2d LSC output powere LSC AC current (with 20 kA ESS)f ESS currentg ESS voltageh ESS power

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to the low natural frequency (slow response) of the ESS DCvoltage controller, the power exchange between the ESSand WT DC link is not affected whereas the steady-statevoltage tends to stay at the optimal level of 10 kV. For thedifferent ESS ratings, it can be observed that higher ESSpower rating results in better system damping but, at theexpense of larger ESS DC voltage variation due to theincreased power exchange between the ESS and the WTDC link. Owing to the extra 20% current capability of theLSC, even when the WT operates at rated power, there isstill additional capacity for the ESS to discharge throughthe LSC, thus providing damping to the network.

6 Conclusions

This study investigates how WTs can be used to improve thedamping of power system oscillation. An energy storage


device is connected to the DC link of a fully ratedconverter-based WT through a bidirectional DC/DCconverter. With this configuration the WT can be controlledto provide power oscillation damping through active powercontrol of the ESS and reactive power modulation of theLSC. To help the WT ride through AC faults, which are themost common cause of system oscillation, coordinate controlmethods for the LSC and ESS are proposed. The proposedconfiguration and control are validated by simulation studiesof a simplified Irish power network. One minor and onesevere AC fault has been applied to verify the controlmethods for the LSC and ESS. In the simulated system,since the wind farm is located near one of the synchronousgenerators, significant improvements of oscillation dampinghave been observed using ESS active power control withoutaffecting the WT operation. The required power and energyratings of the ESS for this purpose have also been found tobe relatively small compared to the WT rating. With the


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increasing development of wind power, the proposed WTlayout and control method can provide significant benefit toboth the wind farms and the transmission networks.
7 Acknowledgment

This work was supported in part by Science FoundationIreland (SFI) through the Charles Parsons Energy ResearchAward: 06/CP/E002.

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