Frequency Response From Wind Turbines

Transcription

1 Frequency Repone Fro Wind Turbine by J.B. Ekanayake, N. Jenkin, G. Strbac REPRINTED FROM WIND ENGINEERING VOUME 32, NO. 6, 2008 MUTI-SCIENCE PUBISHING COMPANY 5 WATES WAY BRENTWOOD ESSEX CM15 9TB UK TE: +44(0) FAX: +44(0) E-MAI: cience@globalnet.co.uk WEB SITE:

2 WIND ENGINEERING VOUME 32, NO. 6, 2008 PP Frequency Repone Fro Wind Turbine J.B. Ekanayake 1,N.Jenkin 1,G.Strbac 2 1. Centre of Integrated Renewable Energy Generation and Supply, Cardiff Univerity, UK 2. Control and Power Group, Electrical and Electronic Engineering, Iperial College, UK ABSTRACT The frequency repone that can be provided by Doubly Fed Induction Generator (DFIG) and Full Power Converter (FPC) baed wind turbine during the firt 30 ec after a frequency event wa invetigated. A iplified odel, which can be ued for frequency tudie on a ingle bu equivalent of the power yte, wa developed. Studie were caied out on the preent and projected 2020 Great Britain power yte. The frequency deviation of the GB yte with and without the frequency repone of the wind turbine wa iulated. It wa hown that variable peed wind turbine with uitable control can ake an iportant contribution to frequency repone. NOMENCATURE T e v d, v, v dr, v i d, i R S, R r S, r, SS, H eq D S y f INTRODUCTION Electroagnetic torque d,q-axi tator voltage and d,q-axi rotor voltage d,q-axi tator cuent Stator and rotor reitance Stator, rotor leakage inductance and agnetizing inductance Stator, rotor elf inductance Equivalent inertia contant of the power yte Daping coefficient of the power yte Syte MVA rating Syte frequency Worldwide, electricity generation fro renewable energy i increaing rapidly with oe 100 GW of wind energy generation capacity now intalled. Renewable power ource contribute to reducing eiion of green-houe gae and to iniiing a country dependence on iported foil fuel. Hence a nuber of countrie have taken pecific policy initiative to encourage renewable power generation. For exaple, in the UK, 15% of all energy i to be upplied by renewable energy by Thi tranlate into 30-40% of electrical energy being generated fro renewable ource with wind power playing a doinant role, (perhap up to 40 GW of wind turbine generation on a Great Britain (GB) yte with a total of around 100 GW of generating plant). However due to the rather unuual architecture of wind turbine generator, particularly the ue of electronic converter, and the control philoophie adopted, a nuber of concern have been expreed over operation of the power yte with a very high penetration of wind power, in particular the proviion of frequency repone [1-3].

3 574 FREQUENCY RESPONSE FROM WIND TURBINES Hence, iplified odel that can be ued to repreent wind turbine generator uing Doubly Fed Induction Generator (DFIG) and Full Power Converter (FPC) in frequency tudie were derived. Electronically connected wind turbine, uch a DFIG and FPC, with their preent control yte will not contribute to the inertia of the power yte. Thi wa recognied and tudie were caied out to ae the ipact of a large penetration of wind turbine on the frequency of the GB power yte and to invetigate how their control yte could be odified to provide frequency repone. Frequency control in the UK electricity network The Security and Quality of Supply Standard tate that the yte frequency of the GB network under noral operating condition hould be aintained at 50 Hz within operational liit of ± 0.5 Hz [4]. The National Grid (the TSO) ipoe it own liit of ± 0.2 Hz. Thi i achieved by operating oe generator on a governor droop, norally 4% or le, and claified a continuou ervice. In an abnoral event (intantaneou lo of 1320 MW of generation) the axiu allowed yte frequency deviation i Hz. In the event of a udden failure in generation or connection of a large load, the yte frequency tart dropping (region 0X of Figure 1) at a rate ainly deterined by the total angular oentu of the yte (uation of the angular oentu of all generator and pinning load). For occaion that the frequency drop greater than -0.2 Hz, oe generating plant are contracted to provide frequency repone. The repone i claified a an occaional ervice and ha two part; priary repone and econdary repone. Priary and econdary repone are defined a the additional active power that can be delivered fro a generating unit that i available at 10 econd and 30 econd repectively after an event and that can be utained for a further 20 econd and 30 inute repectively [6]. Thi definition i baed on the repone of ynchronou generator baed power plant, where frequency repone within firt 30 ec i provided by two echani: a) Inertia repone: A the frequency drop (region 0X of Figure 1), the peed of the ynchronou generator alo reduce and oe of the kinetic energy tored in the rotating a i releaed a electrical energy. Thi i a fat repone and proportional to the rate of change of frequency. b) Governor action: The autoatic droop control loop of the governor act on the change in frequency and open the governor valve to increae the turbine output. Thi i a lower repone and depend on the dead band of the governor and tie lag of the prie over. Figure 1: Frequency control in England and Wale [6]

4 WIND ENGINEERING VOUME 32, NO. 6, Power electronically controlled and/or power electronically connected generator uch a DFIG and FPC wind turbine do not naturally provide inertia repone. However inertia repone can be eulated by adding a uppleentary control ignal proportional to the rate of change of frequency [7-11]. A coponent proportional to the change of frequency can alo be added to the uppleentary control ignal to enhance the inertia repone. Power plant in the future GB network can broadly be categoried a ynchronou generator (SG) baed power plant and electronically controlled and/or connected (EC) power plant. It i recognied that ot of the wind power plant will fall into the category of EC. By conidering that the hare of SG and EC power plant in the future GB network will be coparable, the frequency repone within the firt 30 ec ay be categoried a fat priary repone (the inertia repone of SC plant and the eulated inertia repone of EC plant) and lower priary repone (governor action). Siplified wind turbine odel for frequency tudie Detailed odel of wind turbine generator, baed on both DFIG and FPC, can be found in the literature [13-17]. Reference [8] and [11] deontrated how a uppleentary control loop a hown in Figure 2 can be incorporated into the detailed odel of DFIG and FRC wind turbine repectively. However thee detailed odel are not very uitable for frequency repone tudie, due to their coplexity and the difficulty of uing the in large power yte odel. Siplified odel to repreent DFIG and FPC baed on induction achine were developed and are given in Appendix A. They are uaried in Figure 2 and Table 1. Figure 2: Siplified odel for the DFIG and FPC wind turbine with uppleentary control loop for fat priary repone

5 576 FREQUENCY RESPONSE FROM WIND TURBINES Table 1: Paraeter for Figure 2 for different type of wind turbine Turbine type DFIG FPC (IG baed) For both odel, it wa aued that the converter loe are 5% and the generator output power i delivered to the yte via the converter without any tie delay. It ay be een fro Table 1, that the dynaic of DFIG and FPC wind turbine for a frequency event are iilar a SS and R S R r (ee Appendix B). The repone of the iplified FPC and DFIG odel with the uppleentary control loop paraeter of k 1 = 3, k 2 = 1, and T w = 1 to a yte frequency change were validated againt that obtained fro the detailed odel (defined by equation (A.1) to (A.9) [15] and with the ae uppleentary control loop paraeter hown in Figure 2) and are hown in Figure 3. The repone hown in the figure were achieved with a 0.06 pu change in rotational peed, thu howing iniu ipact on the wind turbine drive train. Subtantial agreeent between the two et of reult (full and iplified odel) wa obtained. The dicrepancie between thee reult were ainly due to eor caued by auption ade to derive the iplified odel. (a) Power yte frequency deviation (f)/hz

6 WIND ENGINEERING VOUME 32, NO. 6, (b) Fat Priary repone fro FPC wind turbine Figure 3: (c) Fat Priary repone fro DFIG wind turbine Reult obtained fro the iplified and detail odel of FPC and DFIG wind turbine Figure 4 how the different repone obtained by varying the value of k 2 and T w while fixing k 1 = 3. Fro iulation it wa found that k 1 et the initial repone and k 2 and T w et the repone after 2-3 econd. The eulated priary repone wa obtained by forcing the generator to drop it peed below the optiu peed, and taking out tored kinetic energy fro the rotating a. A the wind turbine i rotating off it optiu peed after the firt peak, it power output reduce. In order to iniie the ipact on frequency recovery after the frequency event, the value of k 2 and T w were elected to obtain the following condition: a) A high a poible poitive peak power and energy fro the wind turbine. b) A all a poible negative peak power after the initial peak. c) A long a poible delay fro the tie of the frequency event and the tie at which the negative power peak occur. Fro the different repone obtained (Figure 4) it i clear that k 2 = 1 and T w = 1 give the bet perforance. Output Power [p.u] Tie [] For the frequency deviation hown in Figure 3(a) where the frequency deviation i 0.25 Hz and the tie contant of the power yte frequency decay i 11 ec

7 578 FREQUENCY RESPONSE FROM WIND TURBINES Output Power [p.u] Tie [] (b) For a frequency deviation of 0.5 Hz where the tie contant of the power yte frequency decay i 5.75 ec Figure 4: Priary repone obtained with k 1 = 3 and for the different value of k 2 and T w Fat priary repone In order to invetigate fat priary repone under different cobination of SG and EC plant, a iple odel repreenting the inertia and daping of the GB yte without the contribution due to governor action of ynchronou generation wa ued (Figure 5). An aggregated wind turbine odel wa ued to repreent all the wind far. Figure 5: The odel ued to invetigate fat priary repone It wa aued that the preent peak load on the GB yte i 70 GW and it drop to 63.5 GW in 2020 due to energy aving eaure. Four cenario, one for the preent cae and three future cenario for 2020 were aued baed on the capacity value preented in reference [3]. The operating capacitie of conventional power plant were etiated including reerve requireent. For wind power generation the operating capacity wa aued to be 40% of the intalled capacity and for other renewable energy ource, the operating capacity wa aued to be 60% of the intalled capacity. The etiated operating capacity for each type of power plant i given in Table 2. Baed on the operating capacity of each power plant and auing the equivalent inertia contant for coal, ga and nuclear power plant a 4.5, 6.0 and 3.0 repectively, the equivalent inertia contant, Heq, on the yte bae wa calculated uing the following equation:

8 WIND ENGINEERING VOUME 32, NO. 6, H eq = S H i S i= coal, ga,... i y where H i and S i are the inertia contant and MVA rating of the different type of power plant. The equivalent inertia contant for each operational cae i given in Table 3. The yte daping wa aued a 1 [18]. For each cae given in Table 3, i.e. 2008, ow Wind , Mediu Wind and High Wind , the fat priary repone offered by SC and EC plant were iulated uing the odel hown in Figure 5. In thee iulation, it wa aued that the wind turbine are 2 MW FPC deign and the turbine were aggregated to obtain the total operating capacity. The paraeter of the generator of the wind turbine are given in Appendix B. For each wind turbine, the value of k 1, k 2 and T w (of Figure 2) were elected a 3, 1 and 1 repectively. Frequency deviation for a udden connection of a load of 1320 MW (the preent lo-ofpower deign liit for GB [4]) wa iulated with and without the fat priary repone of the wind power plant and hown in Figure 6. For 2008 yte For 2020 yte on low wind cenario For 2020 yte on ediu wind cenario For 2020 yte on high wind cenario Figure 6: Frequency drop of the GB yte with different wind generation capacitie, (no governor action fro ynchronou generation)

9 580 FREQUENCY RESPONSE FROM WIND TURBINES Table 2: Plant argin and operating capacitie Generator type Intalled 2008 ow Wind 2020 Mediu Wind High Wind Plant argin Operating Intalled Plant argin Operating Intalled Plant argin Operating Intalled New Coal Coal Ga Nuclear Interconnector Other Plant argin Operating Onhore wind Offhore wind Other Total Table 3: H eq on the yte bae (70 MVA for 2008 and 63.5 MVA for 2020) Generator type GW 2008 ow Mediu High GW GW GW New Coal Coal Ga Nuclear Interconnector Other Onhore wind Offhore wind Other Total

10 WIND ENGINEERING VOUME 32, NO. 6, Fro Figure 6, it ay be een that even though wind turbine do not contribute to the equivalent yte inertia, the fat priary repone offered by the liit the rate of drop of frequency over the firt 30 ec after a frequency event. Fat and low priary repone The odel hown in Figure 7 wa then ued to deontrate the cobined fat and low priary repone of plant connected to the power yte. The wind turbine were odelled a hown in Figure 2 where the value of k 1, k 2 and T w were again elected a 3, 1 and 1 repectively. The governor and turbine of the SG power plant were odelled uing iplified odel preented in [19]. The frequency deviation for a udden connection of an additional load of 1320 MW wa iulated with and without the priary frequency repone of the wind power plant and hown in Figure 8. Governor 1/Droop Turbine Deand/ Generation change + + ω = f D + 2H eq Wind turbine odel hown in Figure 2 Figure 7: Model to invetigate the low priary repone Figure 8: Frequency drop of the GB yte under different generating capacitie (Table 2)

11 582 FREQUENCY RESPONSE FROM WIND TURBINES Fro Figure 8, it can be clearly een that with the repone fro the wind turbine, a ubtantial reduction in the frequency deviation after a frequency event wa oberved. In addition to the four cae detailed in Table 2, a light load (total deand of 35 GW), high wind cenario wa alo conidered for Here it wa aued that the operating capacity of wind and other renewable i a pecified for High Wind cenario. The deand of 35 GW wa upplied by 5.4 GW of coal, 6.9 GW of ga, 19.4 GW of wind and 3.4 GW of other renewable. The frequency deviation for a udden connection of an additional load of 1320 MW with and without the priary frequency repone of the wind power plant are hown in Figure 9. Figure 9: Frequency drop of the GB yte under light load, high wind cenario Concluion Priary frequency upport that can be provided by wind turbine generator wa deontrated. In order to odel the wind turbine for frequency deviation, iplified odel of DFIG and FPC baed wind turbine were developed. The odel were ued with that of the GB network under five different cenario, one for 2008 with the peak deand, three for 2020 with the peak deand and one for light load high wind cae, to deontrate the priary repone that the wind turbine can provide during the firt 30 ec after a frequency event. The iulation reult confired that even though the electronically controlled or connected wind turbine do not naturally contribute to the inertia of the yte, their fat priary frequency upport provide ignificant perforance enhanceent during the firt 30 ec after a frequency event. However, the perforance deteriorate after the period of fat priary repone a wind turbine then operate off optial peed thu providing le active power upport. Fro the dynaic equation derived for the DFIG and FPC baed wind turbine, it wa hown that their dynaic repone to a frequency event are very iilar thu confiring the validity of the reult obtained in the paper for any ix of DFIG and FPC baed wind turbine. Even though the priary repone of a ingle wind turbine i ainly deterined by the uppleentary control loop ued, the repone provided by a wind far ay be haped by varying the repone of individual wind turbine.

12 WIND ENGINEERING VOUME 32, NO. 6, APPENDIX A a) Siplified odel for the FPC wind turbine baed on an induction generator Table A1 how the dynaic equation of an induction generator. Table A1: Voltage and flux equation of induction generator Voltage equation Flux equation 1 vd = Ri d d ψ + ω ψ d (A.1) ψ d = i d + i dr (A.5) 1 v = Ri + ψ d + ω d ψ (A.2) ψ = i + i (A.6) 1 vdr = Rr idr ψ + ω d ψ dr (A.3) ψ dr = i dr + i d (A.7) 1 v = Rr i + ψ dr + ω d ψ (A.4) ψ = i + i (A.8) Where SS = S + and = r + Torque equation T = ψ i ψ i = ψ i ψ i e d d dr dr (A.9) Manipulating equation (A.1) to (A.8), the following equation can be derived: d i d ω R ω ω = i + ω i + v + i d d di dr (A.10) d i ω R ω ω = i ω i + v i d dr di (A.11) For vector control of the FPC, the d-axi wa choen uch that it coincide with the axiu of the rotor flux, therefore ψ dr = 1pu and ψ dr = 0. In equation v dr = 0, ψ dr = 0, and dψ dr/, therefore i dr = 0. When a frequency event occur, the uppleentary loop act on i ref and change v. Therefore it wa aued that when FPC provide priary repone there i no change in i d. A the priary repone i provided through the tator via two back-to-back converter, it wa alo aued that there i no change in the rotor cuent (i dr and i ), thu di /. =0 Equation (A.10) can now be iplified and converted to the doain a: i i ω R ω = i + v 1 1 = R T v 1+ 2 (A.12)

13 584 FREQUENCY RESPONSE FROM WIND TURBINES where T = 2 ω R Subtituting ψ dr = 1 pu and ψ dr = 0 in equation (A.9) and then uing equation (A.8), the following equation can be obtained for the electroagnetic torque: T e = i = i (A.13) b) Siplified odel for the DFIG wind turbine Manipulating equation (A.1) to (A.8), the following equation can be derived: di ω R ω = i + ω i + v + ω dr dr dr r i di d (A.14) di ω R ω = i ω i + v ω dr r i d di (A.15) A hown in Figure 2, the inertia repone of the DFIG wa obtained by adding a uppleentary control loop to the torque control loop of the DFIG wind turbine [2]. When a frequency event occur, the uppleentary loop act on i ref and change v. Therefore it wa aued that when a DFIG provide priary repone there i no change in i dr or i d. The d-axi wa choen uch that it coincide with the axiu of the tator flux, therefore ψ d = 1 pu and ψ = 0. Subtituting in equation (A.6), the following equation can be obtained: i = i (A.16) Equation (A.15) can now be iplified and converted to doain a: 1 2 i ω R ω r = i + v (A.17) Defining : 0 = 2 i 1 1 = R T v r 1+ 1 (A.18) where T 1 = ω 0 R r

14 WIND ENGINEERING VOUME 32, NO. 6, Subtituting ψ d = 1 pu and ψ = 0 in equation (A9) and then uing equation (A.16), the following equation can be obtained for the electroagnetic torque: T e = i = i (A.19) APPENDIX B 2 MW induction wind turbine odel paraeter: Stator reitance (R ): pu, Stator reactance (X l ): pu, Magnetiing reactance (X ): pu, Rotor reitance (R r ): pu Rotor reactance (X lr ): 0.1 pu uped inertia contant (H): 4.5 Control odel paraeter: k p = 0.5 and k 1 = 0.5 Block ued in the GB Syte odel hown in Figure 7 [18]: Equivalent droop of the GB yte = 11 Governor = Turbine = REFERENCES 1. Strbac, G., Shakoor, A., Black, M., Pudjianto, D., Boppc, T., Ipact of wind generation on the operation and developent of the UK electricity yte, Electric Power Syte Reearch Vol. 77, 2007, pp Pearine, R., Song, Y.H., Chebbo, A., Influence of wind turbine behaviour on the priary frequency control, IET Renewable Power Generation, 2007, Vol. 1, Iue 2, pp Growth cenario for UK renewable generation and iplication for future developent and operation of electricity network, SKM - BERR Publication URN 08/1021, June Acce on 24/08/08: 4. GB Security and Quality of Supply Standard, Verion 1.0, Septeber 22, 2004; Acce on 29/08/2008: 5. Real tie operational data of the National Grid Copany PC. Acce on 24/08/08: 6. Erinez, I.A., Bicker, D.O., Wood, G.F., Hung, W.W., NGC Experience with frequency control in England and Wale - Proviion of frequency repone by generator, IEEE PES Winter Meeting, 31 January - 4 February 1999, New York, USA. 7. Ekanayake, J.B., Holdworth,., Jenkin, N., Control of doubly fed induction generator (DFIG) wind turbine, IEE Power Engineering, Vol. 17, Iue 1, February 2003, pp

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