Aalborg Universitet. Published in: Proceeding of the 5th Nordic Wind Power Conference. Publication date: 2009

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1 Aalborg Unerstet Modellng and Smulaton of VSC-HVDC Connecton for Wnd Power Plants Chaudhary, Sanjay K.; Teodorescu, Remus; Rodrguez, Pedro; Kjær, P.C.; Chrstensen, P. W. Publshed n: Proceedng of the 5th Nordc Wnd Power Conference Publcaton date: 2009 Document Verson Publsher's PDF, also known as Verson of record Lnk to publcaton from Aalborg Unersty Ctaton for publshed erson (APA): Chaudhary, S. K., Teodorescu, R., Rodrguez, P., Kjær, P. C., & Chrstensen, P. W. (2009). Modellng and Smulaton of VSC-HVDC Connecton for Wnd Power Plants. In Proceedng of the 5th Nordc Wnd Power Conference: Power System Integraton and Electrcal Systems of Wnd Turbnes and Wnd Farms General rghts Copyrght and moral rghts for the publcatons made accessble n the publc portal are retaned by the authors and/or other copyrght owners and t s a condton of accessng publcatons that users recognse and abde by the legal requrements assocated wth these rghts.? Users may download and prnt one copy of any publcaton from the publc portal for the purpose of prate study or research.? You may not further dstrbute the materal or use t for any proft-makng actty or commercal gan? You may freely dstrbute the URL dentfyng the publcaton n the publc portal? Take down polcy If you belee that ths document breaches copyrght please contact us at bn@aub.aau.dk prodng detals, and we wll remoe access to the work mmedately and nestgate your clam. Downloaded from bn.aau.dk on: oktober 9, 208

2 Modellng and Smulaton of VSC-HVDC Connecton for Wnd Power Plants S. K. Chaudhary #, R. Teodorescu #2, P. Rodrguez 3, P. C. Kjær 4, P. W. Chrstensen 5 # Department of Energy Technology, Aalborg Unersty Pontoppdanstraede 0, Aalborg 9220, Denmark skc@et.aau.dk 2 ret@et.aau.dk Department of Electrcal Engneerng Techncal Unersty of Catalona, Span 3 prodrguez@ee.upc.edu Vestas Wnd Systems A/S, Denmark. 4 pck@estas.dk 5 pewch@estas.dk Abstract Ths paper descrbes the modellng and smulaton of offshore wnd power plants (WPP) connected to the onshore power system grd by VSC based HVDC transmsson. Offshore wnd power plant s modelled wth seeral wnd turbne generators connected to two separate collector buses wth ther own plant step-up transformers. VSC s are modelled usng deal IGBT swtches suppled by the gate pulses from ther respecte controllers. A sequence of operaton has been smulated from the startng up of the VSC-HVDC transmsson, energzng the offshore grd and subsequent synchronzaton of the nddual wnd turbnes. Smulaton of power rampng up and down as well as steady state operaton has been demonstrated. As an addtonal case, the prmary resere control logc has been mplemented and smulated n PSCAD model. I. INTRODUCTION Wnd power plants (WPP) hae come a long way from solated wnd turbnes to clusters of large wnd turbnes of a few MW power szes. Now WPP s regarded as a able and compette source of renewable energy. By the end of 2008, total nstalled capacty of WPP reached aboe 20.8 GW n the world; out of ths nearly 66 GW has been nstalled n Europe, mostly onshore WPP s[]. Due to scarcty of onshore stes, deelopments n offshore technologes and aalablty of a better aerodynamc profle, the trend n Europe s to deelop large offshore WPP. By 2030, Europe expects to hae 300GW of wnd energy, out of whch 20GW s expected from Offshore WPP[2]. A WPP comprses of a large number of wnd turbne generators (WTG s) connected together at the collector bus. Each WTG has a wnd turbne wth dre tran assembly for drng the generator. The generator may be drectly connected squrrel cage nducton machne runnng at a fxed speed, or a doubly fed nducton generator whch can allow 25-30% speed araton or a synchronous machne wth full scale conerters. Whle arable speed nducton generators prode a flexble couplng to the grd, synchronous machne has a stff couplng to the grd as t can run only at synchronous speed. Such a stff couplng between the generator and the grd s undesrable n wnd turbne generator as the transent torques produced n the shaft causes sgnfcant mechancal stress on the gears [3]. Full scale conerters can decouple the synchronous machne speed from the grd frequency and thus enable flexble operaton oer a wde range of speed. The present paper assumes synchronous generators wth full conerters, though t s equally applcable for any other generator usng full power rated conerters. VSC-HVDC transmsson s a faorable transmsson technology n ew of ts numerous adantages lke use of extruded polymer nsulated DC submarne cables, fast control of acte and reacte power, experence wth offshore nstallatons, ablty to connect to weak and passe grds etc.[4]. BorWn offshore wnd-farm n the North Sea wll soon be connected to the grd by VSC-HVDC transmsson [5]. Ths paper descrbes the modellng and smulaton of a WPP wth a large number of wnd turbne generators and connected to the grd by VSC-HVDC transmsson. Secton II presents the system outlne and modellng detals. In Secton III, smulaton of a sequence of operatons s descrbed. Secton IV presents a method of relayng onshore grd frequency to the offshore grd through VSC-HVDC lnk. Fnally the paper s concluded n secton V.

3 3 Aggregated WTG groups kv Vcb Vwppa Xt Xph Vc Vdc Xf OFFSHORE kv, 400A, 50km DC Cable Chopper Resstor Xf ONSHORE Vdc2 Vc2 Xph2 Xt2 400 kv Vg P+jQ Vwpp Vf VSC Xf Xf VSC2 Vf2 VSC based HVDC Connecton 2 Aggregated WTG groups 2 33 kv Vcb Vwppb 2x200MW WPP 6 x 6MW FCWTG unts connected n a strng. Remanng aggregated WTG model VSC-HVDC 400MW, ±50kV DC and 50kV 3ph. ac AC rms d = 0 q dm qm d q dm qm R X dm qm ph dm qm R+ X qm dm ph Fg. 2 Block Dagram of Offshore VSC Controller Q V rms V rms, m V DC Q m + - Q + - V R X Fg. Sngle lne dagram of the system abc dm qm ph V DCm dm dm + - d - PI + - PI - + DQ ABC (Lmts abc q Imposed) PI + - PI qm qm R+ X Fg. 3 Block Dagram of Onshore VSC Controller P Q abc a a c b Q = P b 2 b b a 3 abc c c a c where, abc = ( a + b + c ) qm dm ph a b c abc Fg. 4 Determnaton Voltage Source model for WTG wth FSC I. SYSTEM DESCRIPTION AND MODELING Fg shows a sngle lne dagram of the system beng studed. A smple Theenn s equalent oltage source a b c behnd mpedance s used to represent the onshore grd, whle the HVDC has been modelled n detal wth swtched conerters and ther controller, conerter transformers, phase reactors, flters and DC capactors, HVDC cable, DC lne reactors. The assumed system data s gen n the Appendx. Modellng of the VSC controllers and the WTG s descrbed n the followng sub-sectons. A. Offshore VSC Controller The offshore VSC controller (VSCC) controls the offshore grd oltage and frequency. The desred oltage reference s set to the d-axs oltage control loop whle the q- axs oltage reference s set to 0. The rotatng reference frame s selected such that the oltage phasor s algned wth the d- axs. Current references are generated from these AC oltage controllers n the outer loop. The nner loops produce the reference oltages whch are augmented by the feed-forward of the oltage at the flter bus and compensaton for the drop n the phase reactor [6]. Pulse wdth modulaton (PWM) sgnals are generated usng sne-trangle comparson as shown n Fg. 2. B. Onshore VSC Controller The onshore VSC controller (VSCC2) regulates the HVDC oltage and the reacte power (or termnal oltage) exchanged wth the onshore grd as shown n Fg 3. The HVDC oltage regulaton loop sets the d-axs current reference whle the reacte power s controlled by settng the q-axs current reference. These are controlled by the nner current loop controls as descrbed n the preous secton [6]. The reacte power control loop may be swtched n the oltage (at PCC) control mode. Then the senstty of the PCC oltage wth respect to the reacte power njecton s used to determne the q-axs current reference.

4 C. WTG wth FSC model Eery WTG s assumed to be equpped wth ts own FSC. In ths smulaton study the WTG wth ts FSC s modeled as a current controlled oltage source. As shown n Fg. 4, poste sequence component of the termnal oltage s estmated and then for a specfed acte and reacte power output, the phase currents to be njected nto the offshore grd are computed. Second order generalzed ntegrator (SOGI) based controllers then set the oltage source references to achee the current njectons. In ths study the 400MW WPP s dded nto two groups of 200MW each. In one group, a strng of 6x6MW WTG s modeled and the remanng unts of 64MW are lumped together nto two equalent unts. In the other group, 2x00MW lumped models hae been used. D. Frequency dependent power controller Lke conentonal power plants WPP are expected to cater to the prmary and secondary frequency control. Though ths s subjected to the wnd aalablty, WPP can be estmated to hold certan resere capacty by not operatng on the maxmum power cure. The cure tself may be a functon of estmated wnd speed, so that the amount of spnnng resere s predctable. Snce VSC-HVDC decouples the offshore grd frequency from the onshore grd frequency, there has to be some mechansm to relay the onshore frequency aratons. VSC.HVDC controllers controllng the HVDC oltage and offshore grd frequency can be used for the purpose.[7]. II. SIMULATION OF OPERATIONAL MODES A sequence of processes has to be followed so as to energze the VSC-HVDC lnk, and the offshore grd and synchronzng the WTG before power generaton can be ramped up. The operaton range can be dded nto the followng sequence of operatons. Chargng the DC capactors and energzng VSC- HVDC. Energzng the Offshore grd. Synchronzaton of offshore WTG and power control. Steady state operaton at maxmum P and Q output. Fg. 5 shows the power and VSC-HVDC oltage waeforms durng the whole sequence of operatons. E. Chargng of VSC-HVDC In the begnnng, the VSC-HVDC as well as the offshore WPP grd s not energzed. When the crcut breaker s closed to connect the conerter to the grd through the conerter transformer, the ant-parallel dodes n VSC2 does the rectfyng acton and a large nrush current flows n to charge the HVDC capactors and the HVDC lne to the DC oltage leel of uncontrolled rectfers, gen by, Howeer, snce the offshore VSC s blocked, the dode rectfer acton of the onshore VSC sees a capacte mpedance of the cable and the DC capactors. Hence the DC capactors and the cables get charged to the peak lne to ground oltage leels,.e. ±22.5kV (.e. pu) or 245kV polepole oltage. In Fg. 6, the oltage has rsen to 255kV by the rectfer and boostng actons of the dodes and phase reactors. The ntal magntude of nrush current s lmted by the mpedance of the grd, conerter transformers and the phase reactors. In ths smulaton, pre-nserton resstors of kω hae been used for a perod of 70ms. After 200 ms, VSC2 controller s de-blocked and gate pulses are appled to the IGBT s. The HVDC capactors and the lnes then get charged to the operatng oltage of the VSC- HVDC n a controlled manner. The power requred for the chargng and energzaton s drawn from the grd. F. Energzng the Offshore grd After the VSC-HVDC lne oltage s stablzed, then the offshore VSC (VSC) s de-blocked. Its controller ramps up the reference oltage and the offshore oltage bulds up gradually. After nomnal oltage leel s attaned n the offshore-grd, the WTG cables are connected to the collector bus one-by-one to aod oscllatons. At ths pont of tme the WPP s fully energzed, and the WTG s are ready for synchronzaton. Intal chargng of the VSC-HVDC and energzng of the offshore grd s shown n Fg. 6 G. Synchronzaton of offshore WTG and power control The WTG s are runnng at rated oltage but wth no load generaton. The PLL n the FSC-nerter detects the magntude and phase of the WPP grd oltage. The FSCnerter output oltage s matched wth the grd oltage at no load and then t s ready for synchronzaton. The breaker s closed. After that the power can be ramped up or down as per the requrement. Both acte and reacte power command can be gen to the FSC. Whle the acte power comes from the WTG, VSC s capable of generatng or absorbng reacte power as long as ts maxmum current ratng s not exceeded. H. Steady State operaton Fg 7 and 8 show the current and oltage waeforms when the WTGs are operatng n steady state at no generaton and at maxmum power generaton respectely. In Fg 7, the WPP currents are laggng the WPP oltage by approxmately 90 mplyng that the WPP has capacte ar generaton whch flows towards the offshore VSC. The capacte ar generaton can be attrbuted to the cable capactances and the L-C-L flters. The no load ac current waeform of the onshore VSC shows the flow of 0-sequence component. 3 3 = () π V dc 0 V m

5 Fg. 5 WPP Power generaton and njecton nto the onshore grd Fg. 6 Chargng VSC-HVDC and energzng the offshore grd Fg. 7 Steady state operaton at no load

6 Fg. 8 Steady State operaton at full 400MW WPP generaton Fg. 9 WPP response to onshore grd frequency araton Fg. 0 Dfference n oltages and frequences measured onshore and offshore

7 In Fg 8, the onshore VSC AC currents hae a fundamental component wth supermposed swtchng rpples. The onshore current waeform shows the current enterng the onshore VSC. Hence t appears to be n phase opposton mplyng that the power s flowng out of the VSC towards the grd. III. PRIMARY RESERVE CONTROL AND FREQUENCY SUPPORT Large WPP s are expected to partcpate n the frequency support acttes through prmary resere control. A case has been smulated for ths. In the smulaton, the onshore grd frequency s frst rased to 5 Hz and then decreased to 49 Hz and the response of the wnd power plant s obsered. In ths study, the HVDC oltage s used to relay the onshore grd frequency to the offshore [7]. Once the grd frequency state s known to the offshore WPP, the FCWTG can be controlled to prode the prmary frequency response. The control algorthm can be summarzed nto the followng steps. Measure onshore grd frequency deaton from and modfy HVDC reference oltage for the onshore VSC conerter as follows, f f mod m n V = V HVDC ref HVDC ref + Sf fn (2) where, f m and f n are the measured and nomnal frequences respectely. S f s the frequency to oltage senstty settng of the onshore VSC controller. In the present smulaton, S f =5 has been used to achee 5% change n HVDC oltage reference per unt percentage change n onshore grd frequency. V HVDC_ref s the orgnal HVDC reference oltage at onshore termnal. Deadbands may be ncluded f requred.. y V V V Δ Vpu _ HVDC _ off = V + m _ HVDC _ off cable HVDC _ ref cable HVDC _ ref. Modfy the offshore grd frequency n proporton to the pu deaton n offshore HVDC oltage. ( ) f = f +ΔV S (4) off _ ref off _ n pu _ HVDC _ off f where, S f s the oltage to frequency senstty settng of the offshore VSC controller. If S f s set as the recprocal of S f, then the onshore grd frequency can be emulated n the offshore grd.. The WTG controllers can then be controlled to prode frequency support to the grd n response to the frequency obsered at ther termnals. In the smulaton, a WPP generaton ncreases by 2.5% per unt percentage (.e. 0.5 Hz) change n frequency. (3) Fg 9 shows the oltage, frequency and power cures for ths smulaton. At the nstant of 7 sec, the onshore grd frequency s rased by 2% to 5 Hz. Correspondngly, the HVDC oltage rses to. pu (330kV) and the power generaton falls to 326MW from the preous alue of 342MW. When the onshore grd frequency s decreased to 49Hz (.e. 0.98pu), the HVDC oltage falls to 276kV and the offshore generaton ncreases to357mw. Fg 0 shows the small dfference between the onshore and offshore frequences. IV. DISCUSSION A model of WPP wth VSC-HVDC connecton to the onshore grd has been deeloped and the dfferent operatng condtons hae been smulated. Operatonal sequence of startng up the VSC-HVDC, and energzng the offshore grd, sequental synchronzaton and connecton of a number of aggregated WTG followed by power generaton rampng up to full power leel and then rampng down to 0 generaton leels hae been demonstrated. The smulaton ges an oerew of the oerall system and ts operaton. If the HVDC oltage s permtted to ary, then VSC- HVDC can effcently relay the onshore grd frequency to the offshore grd. In smulaton study t was found that the frequency nformaton was relayed wthn a perod of 0 ms. ACKNOWLEDGMENT Ths research s a part of Vestas Power Program at Department of Energy Technology and Department of Energy Technology, Aalborg Unersty. It s jontly supported by Vestas Wnd Systems A/S and Aalborg Unersty. APPENDIX TABLE LIST OF PARAMETERS USED IN SIMULATION Onshore Grd Base MVA 500 MVA 2 Base oltage (rms, lne-lne) 400 kv 3 Short Crcut Capacty 25 pu 4 Grd Impedance Angle 80 degree Conerter Transformers (Onshore) Sze 500MVA 2 Voltage Rato (for onshore) 400//kV 3 Voltage Rato (for offshore) 50/33 kv/kv 4 Leakage Reactance 0.2 pu 5 Cu-loss 0.0 pu 6 Fe-loss 0.0 pu Phase reactors Inductance 0.05 pu 2 resstance 2.E-04 pu HVDC System Pole to pole DC boltage 300 kv DC 2 Power ratng 400 MW HVDC Cable Cable length 200 km 2 resstance Ω 3 Inductance 22.3 mh 4 Shunt Capactance (at ends) 28.5 uf DC capactors (at VSC termnal) 35.5 uf REFERENCES [] Global wnd report 2008, Global Wnd Energy Councl (GWEC), Aalable: Report_2008/Global_Wnd_2008_Report.pdf [2] S. K. Chaudhary, R. Teodorescu and P. Rodrguez, "Wnd Farm Grd Integraton Usng VSC Based HVDC Transmsson - An Oerew," Energy 2030 Conference, ENERGY IEEE, pp. -7, 2008.

8 [3] J. Machowsk, J. W. Balek and J. R. Bumby, Power System Dynamcs : Stablty and Control.,2nd ed.chchester, U.K.: Wley, 2008, pp [4] K. Erksson, C. Lljegren and K. Sobrnk, "HVDC lght experences applcable for power transmsson from offshore wnd power parks," n 42nd AIAA Aerospace Scences Meetng and Exhbt, Reno, Neada. 2004, [5] Grd connecton of offshore wnd farms - NordE.ON, Aalable at [6] L. Xu, B. W. Wllams and L. Yao, "Mult-termnal DC transmsson systems for connectng large offshore wnd farms," Power and Energy Socety General Meetng - Conerson and Delery of Electrcal Energy n the 2st Century, 2008 IEEE, pp. -7, [7] S. Jensen and F. W. Fuchs. Load-frequency control of synchronous areas usng a wnd farm connected a HVDC-VSC. Presented at Presented at 2nd EPE Wnd Energy Semnar, KTH, Stockholm, Sweden, Aprl 2009.

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