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Comparison of the Response of Doubly Fed and Fixed-Speed Induction Generator Wind Turbines to Changes in Network FrequencyJanaka Ekanayake and Nick JenkinsAbstract—Synchronous andfixed-speed induction generators release the kinetic energy of their rotating mass when the power system frequency is reduced.In the case of doubly fed induction generator(DFIG)-based wind turbines,their control system operates to apply a restraining torque to the rotor according to a predetermined curve with respect to the rotor speed.This control system is not based on the power system frequency and there is negligible contribution to the inertia of the power system.A DFIG control system was modified to introduce inertia response to the DFIG wind turbine.Simulations were used to show that with the proposed control system,the DFIG wind turbine can supply con-siderably greater kinetic energy than afixed-speed wind turbine. Index Terms—Doubly fed,fixed speed,frequency response,in-ertia,wind turbines.I.I NTRODUCTIONI N recent years,there has been increasing interest indoubly fed induction generator(DFIG)-based wind tur-bines and many of the large wind turbines that are now commercially available are of this type.Operating a large number of DFIG-based wind turbines displaces conventional synchronous generators and reduces the system inertia.This is due to the fact that the DFIG control system decouples the mechanical and electrical systems,thus preventing the generator from responding to system frequency changes.This is undesirable when there are a large number of DFIG wind turbines operating,especially in periods of low load and on smaller power systems(e.g.,Great Britain and Ireland). The frequency of a power system with low inertia will,of course,change rapidly for abrupt variations in generation or load.In this case,additional frequency response ancillary services must be provided to ensure frequency limits are not exceeded[1].In this paper,a supplementary control loop is added to the DFIG controller to reintroduce inertia response.Simulations on an electromagnetic transient package(EMTDC/PDCAD) were carried out to compare the inertia response of a fixed-speed induction generator and a DFIG with the addi-tional control loop.It is shown that the greater variations in rotor speed available from the DFIG wind turbine lead to considerably more kinetic energy being released to support the power system frequency.Manuscript received December9,2003.Paper no.PESL-00146-2003.The authors are with the University of Peradeniya,Peradeniya20400,Sri Lanka;Manchester Center for Electrical Energy(MCEE),University of Man-chester Institute of Technology(UMIST),Manchester M601QD,U.K.;and also with the Tyndall Center for Climate Change Research,Norwich NR47TJ,U.K. Digital Object Identifier10.1109/TEC.2004.827712II.I NERTIA R ESPONSE F ROM F IXED-S PEED I NDUCTIONG ENERATOR-B ASED W IND T URBINESIn the case of synchronous or induction generators,energy is released automatically from the rotating mass for a drop in network frequency.For a change in frequencyfromto,assuming that the speed of the rotor changes proportionallyfromto,the kineticenergy released by themachine is givenby(1)(2)whereFrom(2)and substitutingfor,we canobtain(3) In order to investigate the response of afixed speed induction generator for system frequency changes,simulations were car-ried out using PSCAD for a2-MW,690-Vfixed-speed induction generator connected to an infinite busbar through a0.69/11-kV transformer.Fig.1shows the response obtained when the fre-quency of the infinite busbar was reduced exponentially from50 to49.75Hz.For simplicity and due to the lack of data,the gener-ator drive train was modeled as a lumped inertia of2MW/MV A with a mechanical damping of0.01p.u.A constant wind torque was applied.For a2-MW generator and a sudden frequency change from 50to49.75Hz,(2)indicates the kinetic energy of the rotating mass that will be released is40kW.This is confirmed by the area under the output power curve of Fig.1.III.DFIG C ONTROLA full description of a DFIG control system is given in[2]. Fig.2shows that part of the control system,which is used to extract maximum power from the wind.Any change in rotorspeed results in a change in the set torque or torquedemand.The torque demand is trans-lated into a rotor current demand and compared with the actual rotor current to obtain the rotor-injected voltage.This injected voltage acts upon the machine and changes the speed of the wind turbine.If another quantity varies(other than the wind torque),the control system acts to restore the speed back to its operating point.This control action is shown in Fig.3.As described0885-8969/04$20.00©2004IEEEFig.1.Response of a fixed-speed induction generator to a drop in network frequency (all quantities are in perunit).Fig.2.DFIG control for maximum powerextraction.Fig.3.Response of a DFIG wind turbine to a drop in network frequency (all quantities are in per unit).above,the DFIG control system acts against the system fre-quency change,thus responding with a negligible inertiaresponse.Fig.4.Supplementary control loop for inertia response.IV .DFIG C ONTROLLER M ODIFIED FOR I NERTIA R ESPONSE By taking the derivative of the kinetic energy available at anyspeed ,the power that can be extracted from a rotating mass can be obtainedas(4)The inertia constant H is de finedasin the usualnotation.Substitutingfor in (4)and (5)weget(5)Ifand are per-unit quantities of power and speed,then (5)can be writtenas(6)Therefore,the per-unit torque is givenby(7)The controller shown in Fig.2was modi fied by adding a signal corresponding to (7)to the set torque (torque demand).As the system frequency drops,the set point torque is increased slowing the rotor and extracting kinetic energy.In order to min-imize the impact of this supplementary control on mechanical drive train loads,the rate of change of power injection was mod-i fied by adding a first-order filter afterthe input.Delay on this filter was shown not only to reduce the rate of increase of the electromagnetic (EM)torque but also reduced the magnitude of the peak torque.The modi fied controller with the supplemen-tary control loop for inertia control is shown in Fig.4.V .DFIG I NERTIA R ESPONSEFig.5shows the power generated from the DFIG wind tur-bine,change in the rotor speed,and the EM torque of the gen-erator for an exponential change in the system frequency from 50to 49.75Hz.With the additional control loop as shown in Fig.4,kinetic energy of 700kW was released.Fig.5.Response of a DFIG with supplementary control loop to a drop-in network frequency(all quantities are in per unit).VI.C ONCLUSIONSynchronous generators andfixed-speed asynchronous generators release kinetic energy from their rotating mass for changes in the system frequency.This is not automatically true for converter-connected plant and converter-controlled plant such as DFIGs.However,during drops in network system frequency,the DFIG speed can be driven to a lower value using its controller,thus introducing an inertia response.A supple-mentary control loop was introduced to a DFIG controller to obtain inertia response.Estimates shows that with the given frequency change and with the natural choice of supplementary controller gain(2H),a DFIG can supply700kW of inertia while afixed-speed machine of similar rating can only provide 40kW.R EFERENCES[1]I.A.Erinmez,D.O.Bickers,G.F.Wood,and W.W.Hung,“NGC expe-rience with frequency control in England and Wales—Provision of fre-quency response by generators,”in Proc.IEEE Power Eng.Soc.Winter Meeting,New York,Jan.31–Feb.4,1999.[2]J.B.Ekanayake,L.Holdsworth,and N.Jenkins,“Control of doubly fedinduction generator(DFIG)wind turbine,”Proc.Inst.Elect.Eng.,Power Eng.,vol.17,no.1,pp.28–32,Feb.2003.。