Control of Voltage Source Converters in Wind Farm Based Multi-Terminal HVDC Transmission Systems

Transcription

1 Control of Voltage Source Converters n Wn Farm Base Mult-Termnal HVDC Transmsson Systems Mohamma Khenar Babol (Noshrvan) Unversty of Technology, Babol, Iran m.majmehrasa@gmal.com Ers Pouresmael Centre for Energy Informatcs, Unv. of Southern Denmark, Oense, Denmark, an INESC-ID, IST, Unv. Lsbon, Lsbon, Portugal ep@t.su.k João P. S. Catalão Unv. Bera Interor, Covlhã, Portugal, an INESC-ID, IST, Unv. Lsbon, Lsbon, Portugal catalao@ub.pt Abstract Ths paper escrbes a control technue for control of voltage source converters (VSCs) n the mult-termnal hgh voltage rect current (HVDC) transmsson systems. The propose control technue s base on a mult-loop current an voltage control scheme for trackng the reference value of the DC lnk voltage n the rectfer se to acheve a proper performance urng the rregular crcumstances of wn farm operaton. In aton, the propose control scheme s able to transmt the maxmum power to the consumpton sectors. Moreover, the propose control metho on the gr se converter guarantees least amount of current harmoncs njecton nto the AC power gr. The MATLAB smulaton results are presente to confrm the effectveness of the propose control technue for varaton n AC voltage ampltue an freuency of wn turbnes output voltage. Inex Terms Wn farms; mult-termnal; hgh voltage rect current; power ualty. I. INTRODUCTION Energy consumpton s ever ncreasng an over the past ecaes, the ncrement n energy eman has been hghly balance by capacty evelopment of conventonal power sources. But, a further electrcty generaton to balance energy consumpton s consere by unsustanable energy sources, especally ue to lmte source of ther prmary energes an ue to negatve mpacts they ntrouce nto the envronment. In orer to supply the future electrcty eman as well as to replace ageng exstng generatons, a number of new generaton technologes base on the renewable energy sources e.g. wn an solar have been evelope. In European countres, evelopment of wn power plants s growng fast for electrcty generaton; but stll a small percentage of the energy eman s supple by contrbuton of wn power sources. Wth the purpose of ntegratng the future far offshore wn farm power plants nto the power gr an, regarng the capacty of these plants, long transmsson lnes wth hgher capacty woul be essental. But, varable wn spees an lnke generate power woul result n a low capacty factor of the transmsson an conseuently relatvely hgh transmsson cost per amount of energy elvere. Ths capacty factor wll be ncrease by connecton of mult offshore wn power plants nto the transmsson lnes. Furthermore, f the transmsson s extene more, t can be use to smooth power trang between fferent socetes as well as evacuate power from the wn power plant. If we conser these solutons, multple offshore wn farms woul be ntegrate to multple onshore power grs an conseuently t woul lea to the evelopment of a transnatonal offshore network. Hgh-voltage rect-current (HVDC) transmsson technology can be consere as a sutable alternatve for such a mult-termnal offshore network, where huge amount of power can be transmtte over a long stance. Furthermore, because the offshore network can act as a power pool where power can be njecte to an extracte from the network at fferent noes, control of recton of power urng mantanng voltage n the network s neee. Implementaton of voltage source converter HVDC (VSC- HVDC) technology s a constructve soluton for changng the recton of power whle mantanng voltage n the DC network. In aton, an nepenent control for actve an reactve power can be performe. A number of control strateges have been propose n the lterature for operaton of a mult-termnal VSC- HVDC network, e.g., Combne an coornate control [7], current margn control [8], voltage margn control [9], an AC-se voltage control [10]. However, the weakness an lack of strong regulatory system has been sense to guarantee the generaton of safe an stable DC voltage n the se of HVDC transmsson lnes. A control technue s presente n ths paper for trackng the reference value of the DC lnk voltage n orer to acheve a stable DC voltage for connectng to the long transmsson system base on mult-termnal HVDC system. By applcaton of the propose control technue, maxmum avalable power wll be njecte through an nterface VSC- HVDC to the power gr. The rest of the paper s organze nto three sectons. Followng the ntroucton, the general schematc agram of a mult-termnal DC system wth three termnals as a case stuy, an control of wn farm se an gr se converters uner fferent contons wll be ntrouce n secton III. Moreover, smulaton results have been performe to emonstrate the effcency an applcablty of the evelope control strategy n Secton IV. Fnally, conclusons are rawn n Secton V. The reset of the paper s organze as follows. Secton II escrbes fferent parameters of a three termnal HVDC system an control of wn farm se an gr se converters uner fferent contons. MATLAB smulaton results are presente n Secton III for fferent contons of wn farm applcatons an gr contons. Fnally, conclusons are rawn n Secton IV.. II. HVDC STRUCTURE AND CONTROL OF CONVERTERS Fg. 1 shows the block agram of a three termnal HVDC system base on VSCs, whch are connecte to the AC gr. Ths system nclues two wn farm termnals an a termnal connecte to the power gr.

2 Fg. 1. Three termnal HVDC moel. Snce the varable wn power s the nput rvng force of turbnes, the output voltage of wn turbnes s n the form of unregulate AC wth varatons n both ampltue an freuency. A control system must have the capablty to regulate the output voltage n a preetermne reference urng mentone varatons. As the termnals n HVDC structure are locate n fferent places an they use varous amount of wn energy, the control system shoul help each termnal to elver constant DC voltage; then, ths voltage woul be converte from DC to AC n the gr se converter to prouce regular an stable AC voltage. A. DC lnk voltage regulaton As shown n Fg.2, the prncple of ths control metho s base on fferent control loops. Each nner loop nclues a seres of reference values, whch are specfe by the esgner or other loops. The output proucton of the outer control loop s the reference voltage to generate the swtchng pattern for the power electronc converters. DC lnk voltage s compare wth ts reference value an regulate va PI controller to generate reference value of. Ths current obtans the nputs for the current controller to generate reference voltages for the converter. From Fg.2 an neglectng resstance of lne mpeance, AC voltage of the converter can be calculate as: V = L + u (1) t Rewrtng (1) n the Laplace oman leas to: V = sl + u (2) where v s the voltage at the common bus, L s the leakage nuctance of the phase reactor, s the current flowng at the AC se of the converter, u s the voltage generate by the converter an s s the Laplace operator. Transformng (2) to the components results n: V = sl ωl + u (3) V = sl + ωl + u (4) where, L an L leas to a cross couplng between an axs uanttes whch makes the nepenent control of actve power more complex. In other wors, when s regulate to control the reactve power, V wll be altere, an thus, the actve power wll also change. In orer to elmnate the cross couplng, V an L are fe forwar on the -axs controller whle V an L are fe forwar on the -axs controller. The voltage reference values are then transforme to the abc uanttes an V -ref an V -ref are fe as nputs to the controlle voltage source. B. Control of the HVDC converter n the consumpton se After DC lnk voltage regulaton, the control metho shoul be able to transmt the maxmum nput actve power to the gr. Also, t shoul nject the least amount of current harmoncs n to the consumers an gr. In orer to be able to control the actve an reactve power nepenently, the control scheme mplemente on the gr se VSC s evelope base on the vector control metho. The actve an reactve power exchange at the common bus can be calculate as: P = V a a+ V b b+ V c c (5) 1 Q = ( Vabc + Vbca + Vcab ) (6) 3 where, Va, Vb, an Vc are three phase phase voltage at the common bus an a, b, c are three phase currents flowng at the ac se of the converter Transformng (5) an (6) to the 0 components an wrtng the results n pu of the converter rate capacty leas to: P = V + V + 2V (7) 0 0 Q = V V (8) Fg. 2. Dagram of wn farm se controller loops.

3 C. Connecton to the stable gr. For a balance three phase system, the 0 components are eual to zero. Moreover, by conserng V algne n phase wth V a, V s zero. Therefore, (7) an (8) can be rewrtten as: P = V (9) Q = V (10) It can be seen that the actve an reactve powers can be controlle nepenent of each other by regulatng an. The gr freuency shoul be sample by the PLL block. Three-phase gr currents shoul be transforme from abc to 0 n ths freuency accorng to the euaton (11). Then, these currents are compare wth ther references, whch can be calculate from (9) an (10). The error values are regulate through PI controllers an then, transforme to the abc frame n orer to prouce voltage references for PWM block. Schematc agram of ths control s epcte n Fg.3. 2π 2π cos( θ) cos θ cos θ a 2 2π 2π = sn ( θ) sn θ sn θ. b (11) 0 c D. Gr connecton an unbalance loa compensaton. Fg. 4 shows a mult-termnal HVDC system connecte to the AC gr n whch an unbalance nonlnear loa s connecte to the pont of common couplng (PCC). Ths loa may causes nstablty n the gr se current by proucng an unbalance nonlnear current. After connecton of mult-termnal HVDC to the PCC, t can supply the gr an loa power as well as mprovng the power ualty. It means that, n aton to perform as an nepenent DG, t also acts as an actve power flter (APF) to mprove the power factor an total harmonc storton (THD). In ths partcular case, the control metho shoul work as the followng way to comply the reure performance of mult-termnal HVDC system. Accorng to Fg.4, harmonc njecton an actve power njecton blocks are use to prepare reure current references for power converter. A hysteress ban current control (HBCC) s use to obtan swtchng pulses of the converter. Accorng to (9) an (10),t can be conclue that current reference of gr se converter have to be consere as - component of loa current n orer to compensate loa reactve power. Therefore: * = (12) l Actve power transfer shoul also be one by HVDC system base on the followng euaton for the -component of reference current. * Pref h1 = (13) V Ths euaton generates reference current at funamental freuency. P ref s the actve reference power for HVDC system an V s the -component of the PCC voltage. Accorng to Fg.4, the reference current for HBCC block s a combnaton of harmonc currents an funamental currents as: = + (14) * ~ tot h1 h In orer to have harmonc compensaton capablty, ~ h have to be etermne by harmonc njecton block. The loa current shoul be transforme from abc to αβ reference frame by (15). As shown n Fg. 5, ths current entere nto the self-tune flter (STF) to extract the esre harmonc freuency an s efne n ts preetermne set pont freuency [11, 12] α a β = 0. (15) b c where n s the set pont freuency of the esre harmoncs an K s the gan coeffcent of the STF whch have to be smaller to ncrease the accuracy of harmonc extracton. By settng the n n the funamental freuency (50 Hz) an subtractng the output of STF from ts nput (^αβ), remanng harmoncs can be acheve. These harmoncs have to be njecte nto the gr by VSC to act as an APF. By enterng these amounts to the HBCC an compare them wth the actual values, the eal swtchng pattern woul be generate for correct operaton of converter. Fg. 6 shows the man prncple of HBCC n whch the actual current s compare wth reference current. Upper an lower bans are consere aroun the current. When current passes upper an lower bans, converter leg swtches changes n such a way to brng the current back between ban by changng the leg voltage polarty[13]-[15]. III. SIMULATION ANALYSIS Table 1 shows smulaton parameters for a three termnal HVDC of Fg.1 n whch two wn farms are connecte to a DC lne an the power s transferre to AC gr an an unbalance nonlnear nustral loa. Two followng scenaros are consere for the smulaton analyss n ths stuy. A. Control of WFSVSC At frst, varatons of the wn farm for both voltage magntue an freuency s examne. Fg. 7 shows reacton of the propose control metho to the varatons of wn farm uner four worst case scenaros for freuency an magntue changes. These scenaros may seems to be unrealstc but performance of the WFSVSC an ts control strategy to regulate the voltage uner fferent crcumstances can be verfe n sever contons. As epcte n fgures, V abc1 an V abc2 are the output voltage of wn turbnes 1 an 2 respectvely. V c s the DC voltage n the output of rectfer an V c-ref s the DC voltage n the nput of converter after passng from DC lne. The objectve s to mantan the DC voltage n the 700 V set pont voltage level. Fg. 3. Schematc Dagram of gr se control.

4 Fg. 4. Schematc agram of gr se control urng connecton of the unbalance nonlnear loa. Fg. 5. Dagram of the Self-Tune Flter (STF). Fg. 6 (a). Reference current, upper an lower bans n HBCC an generate voltage, (b) HBCC wth 3 separate controllers for each converter leg. The frst type of changes n Fg. 7 s the ramp type, whch s the common type of changes n wn farm turbnes. Changes are apple between t 1 an t 2 that are shown wth ash lnes. Results show that for ramp changes of WT 1 an WT 2 voltages n Fg. 7, the DC lnk voltage tracks ts reference values. Fg. 8 shows that the DC lnk voltage s change after freuency varatons but transents are ampe after a short tme. B. Control of GSVSC Control scheme of GSVSC shoul be able to transmt the maxmum nput power to the gr n fferent stuatons. Gr connecton on the consumpton se s moele n two forms: HVDC system connecte to (1) the stable gr, an (2) the gr an an unbalance nonlnear loa at PCC. Fg. 9 shows DC lnk voltages of rectfer output voltage, DC lnk voltage of nverter nput an network current an voltage. The control scheme eases synchronous connecton of the HVDC system to the gr wth njecton of least amount of current harmoncs (PF=1 an current THD less than 3%). A three phase unbalance nonlnear loa s connecte to the PCC an HVDC. In ths case, the propose control technue shoul connect the HVDC lne to the AC gr. Fg. 10 shows HVDC, gr, an loa currents before an after connecton of HVDC lne to AC system. As can be seen, after connecton of HVDC system to AC system, the maxmum power s njecte to the gr, an the gr voltage an current are n phase an gr current s free of harmonc freuences (PF=1 an THD=1.9%). TABLE I SIMULATION PARAMETERS Parameters Value Input voltage source 3 phase, 600 V, 60 Hz Transformer Yg/D, 600/240 V Interfacng resstance (R c) 0.1mΩ/phase Interfacng nuctance (L c) 2mH/phase DC Capactors (C) 37.5mF Each se Lne moel 75 km moel,π AC Gr 380 V 50 Hz Swtchng/Samplng freuency (f sw) 3000 Hz DC-lnk voltage set pont (V c-ref ) 700 V Gr resstance (R g) 0.1mΩ Per phase Gr nuctance (L g) 5 mh Per phase Three phase oe converter loa1 R=30Ω an l= 20mH Sngle phase oe converter loa2 R=20Ω an l= 20mH Fg. 7. Ramp type varaton of the wn turbnes voltages.

5 Dfferent types of varaton were smulate. The results confrme the ablty of the propose control strategy to regulate the DC lnk voltage. The propose control strategy was also nvestgate for controllng nterface converters between HVDC lne an AC systems urng the connecton of unbalance nonlnear loas nto the AC gr. Smulaton results confrme that by usng the propose control technue, the nterface converter can compensate unbalance an nonlnear current components, thus guaranteeng a balance three phase currents for utlty gr. Fg. 8. Step type freuency varaton by 10 Hz ncrement (ecrement) for WT 1 (WT 2). Fg. 9. Connecton to the stable gr. Fg. 10. Connecton to the gr an unbalance nonlnear loa. IV. CONCLUSIONS Ths paper presente a control metho for controllng voltage source converters n wn farms base mult-termnal HVDC systems. Due to nherent varatons of output voltage n wn turbne generators n terms of ampltue an freuency, the propose control metho ha the ablty to control the nterface converters to prove a constant DC lnk voltage. REFERENCES [1] M. Baraar an M. R. Hesamzaeh, "A stochastc SOCP optmal power flow wth wn power uncertanty," n PES General Meetng Conference & Exposton, 2014 IEEE, 2014, pp [2] T. Hammons, D. Woofor, J. Loughtan, M. Chama, J. Donahoe, D. Povh, et al., "Role of HVDC transmsson n future energy evelopment," IEEE Power Engneerng Revew, vol. 20, pp , [3] M. P. Bahrman an B. K. Johnson, "The ABCs of HVDC transmsson technologes," Power an Energy Magazne, IEEE, vol. 5, pp , [4] W. Lu an B.-T. Oo, "Optmal acuston an aggregaton of offshore wn power by multtermnal voltage-source HVDC," Power Delvery, IEEE Transactons on, vol. 18, pp , [5] N. Flourentzou, V. G. Agels, an G. D. Demetraes, "VSC-base HVDC power transmsson systems: An overvew," Power Electroncs, IEEE Transactons on, vol. 24, pp , [6] S. Johansson, L. Carlsson, an G. Russberg, "Explore the power of HVDC lght -a web base system nteracton tutoral," n Power Systems Conference an Exposton, IEEE PES, 2004, pp [7] J. Reeve an M. Sultan, "Gan scheulng aaptve control strateges for HVDC systems to accommoate large sturbances," Power Systems, IEEE Transactons on, vol. 9, pp , [8] J. Answorth, "The phase-locke oscllator-a new control system for controlle statc convertors," Power Apparatus an Systems, IEEE Transactons on, pp , [9] J. Lang, O. Goms-Bellmunt, J. Ekanayake, N. Jenkns, an W. An, "A mult-termnal HVDC transmsson system for offshore wn farms wth nucton generators," Internatonal Journal of Electrcal Power & Energy Systems, vol. 43, pp , [10] F. Karleck-Maer, "A new close loop control metho for HVDC transmsson," IEEE transactons on power elvery, vol. 11, pp , [11] H. Akag, "Actve an hybr flters for power contonng," n Inustral Electroncs, ISIE Proceengs of the 2000 IEEE Internatonal Symposum on, 2000, pp. TU26-TU36 vol. 1. [12] H.-S. Song, H.-g. Park, an K. Nam, "An nstantaneous phase angle etecton algorthm uner unbalance lne voltage conton," n Power Electroncs Specalsts Conference, PESC th Annual IEEE, 1999, pp [13] E. Pouresmael, M. F. Akoree, D. Montesnos-Mracle, O. Goms- Bellmunt, an J. C. T. Caballero, "Hysteress current control technue of VSI for compensaton of gr-connecte unbalance loas," Electrcal Engneerng, vol. 96, pp , [14] E. Pouresmael, M. Mehrasa, an J. P. Catalão, "A Multfuncton Control Strategy for the Stable Operaton of DG Unts n Smart Grs," Smart Gr, IEEE Transactons on, vol. 6, pp , [15] E. Pouresmael, C. Mguel-Espnar, M. Massot-Campos, D. Montesnos-Mracle, an O. Goms-Bellmunt, "A control technue for ntegraton of DG unts to the electrcal networks," Inustral Electroncs, IEEE Transactons on, vol. 60, pp , 2013.