Comparative Study on Electromagnetic and Electromechanical Transient Model for Grid-connected Photovoltaic Power System

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Energy and Power Engneerng, 13, 5, 47-5 do:1.436/epe.13.54b48 Publhed Onlne July 13 (http://www.crp.

1 Energy and Power Engneerng, 13, 5, 47-5 do:1.436/epe.13.54b48 Publhed Onlne July 13 ( Comparatve Study on and Tranent Model for Grd-connected Photovoltac Power Sytem Man Zhang 1, Hao Sun, Zhgang Chen, Xaorong Xe 1, Qrong Jang 1 1 State Key Lab. of Power Sytem, Department of Electrcal Engneerng, Tnghua Unverty, Bejng, Chna Guangdong Electrc Power Degn Inttute, Guangzhou, Chna Emal: zhangman8@gmal.com Receved February, 13 ABSTRT Wth the development of new energy technology, there are ncreang applcaton of grd-connected photovoltac power generaton ytem. However, there lttle reearch on development of electromechancal model of large cale photovoltac power taton. The computatonal peed wll be very low f electromagnetc tranent model ued for tablty tudy becaue of t complexty. Therefore, tudy on electromechancal tranent model of grd-connected photovoltac power generaton ytem of great meanng. In th paper, electromagnetc tranent model of photovoltac power generaton ytem ntroduced frt, and then a general electromechancal tranent model propoed. Thee two knd of mulaton model are et up n PSCAD. By comparng the mulaton reult of two model, the correctne and valdty of the electromechancal tranent model verfed. It provde reference model for effcent mulaton and modelng of grd-connected photovoltac power taton n large-cale power ytem. Keyword: Photovoltac Power; Tranent Model; Tranent Model; Smulaton Comparon 1. Introducton Wth the development of new energy technology, there are more and more applcaton of grd-connected photovoltac power generaton ytem [1,]. Tranent model of both accuracy and effcency needed n power ytem dynamc analy wth photovoltac power generaton. In the prevou tudy, two model uually ued a photovoltac power generaton model, of whch one the power flow model, and the other electro- magnetc tranent model. The former ue photovoltac ytem jut a a mple power ource wthout conderng t dynamc proce [3-5], whle the latter etablhed accordng to pecfc photovoltac ytem, and trctly reflect the maxmum power pont trackng(mppt) and the nverter control [6-9]. The latter very detaled and can meet the requrement of grd tranent proce analy, but t alo ha many problem, uch a: 1) the electromagnetc tranent model not unveral becaue the nternal tructure and control method of photovoltac ytem are dfferent of dfferent manufacture, o a lot of work needed f we want to etablh electromagnetc tranent model for dfferent manufacturer and type, ) the electromagnetc tranent model need propretary equpment nternal parameter, whch are dffcult to obtan, 3) the electromagnetc tranent model need mall compute tep becaue of t complexty, reultng n long computaton tme, and 4) at preent, large power grd analy often requre bune or engneerng mulaton oftware, o n order to mprove the automaton level and expand the cale of calculaton, unfed model needed for dfferent knd of power, ncludng photovoltac. In concluon, accordng to the demand of the electromechancal tranent mulaton of power ytem, a unveral modelng method needed whle analyzng the common feature of dfferent knd of photovoltac power generaton ytem. In th paper, electromagnetc tranent model of photovoltac power generaton ytem ntroduced frt accordng to the reference, and then a general electromechancal tranent model of grd-connected photovoltac power ytem propoed, and two mulaton model are etablhed n PSCAD/EMTDC. By comparng the mulaton reult of two model, the correctne and valdty of the electromechancal tranent model verfed, whch provde reference model for mulaton and modelng of large cale grd-connected photovoltac power taton. Copyrght 13 ScRe.

48 M. ZHANG ET AL.. The Tranent Model Grd-connected photovoltac ytem nclude photovoltac array, DC/DC, nverter, controller and MPPT control, a hown n Fgure 1.

Accordng to Fgure 1, we can etablh electromagnetc tranent model of photovoltac ytem, whch ntroduced n the followng text..1. The Photovoltac Cell Model There are manly two type of photovoltac cell mulaton model ued n related lterature [1]: the phycal model and the behavor model.

of the cell, and are hard to obtan, therefore, the behavor model often ued n tude [11].

. MPPT Control The I-U curve how that the nternal retant of photovoltac cell tme-varyng, and MPPT a proce of dynamc load matchng, whch uually acheved by the DC/DC crcut.

2 48 M. ZHANG ET AL.. The Tranent Model Grd-connected photovoltac ytem nclude photovoltac array, DC/DC, nverter, controller and MPPT control, a hown n Fgure 1. The photovoltac array change olar energy to DC electrcty, whch connected to the power grd through DC/DC and nverter. Accordng to Fgure 1, we can etablh electromagnetc tranent model of photovoltac ytem, whch ntroduced n the followng text..1. The Photovoltac Cell Model There are manly two type of photovoltac cell mulaton model ued n related lterature [1]: the phycal model and the behavor model. The phycal model baed on the phycal equvalent crcut of the cell, n whch ome emconductor parameter uch a photocurrent and PN coeffcent are needed [1], whch have no drect relatonhp wth the charactertc of the cell, and are hard to obtan, therefore, the behavor model often ued n tude [11]. In practce, photovoltac manufacture provde four parameter of the cell: I c, U oc, I m and U m under tandard envronment, accordng to whch we have the followng charactertc of the cell: (1) ().. MPPT Control The I-U curve how that the nternal retant of photovoltac cell tme-varyng, and MPPT a proce of dynamc load matchng, whch uually acheved by the DC/DC crcut. When the maxmum power pont change wth the envronment, the matched external retant can be obtaned by changng the duty cycle of the DC/DC crcut, thu when the external retant equal the nternal retant, the maxmum power of the cell can be obtaned. In practce ytem, the Boot crcut uually ued a the DC/DC crcut. The tructure of MPPT controller hown n Fgure 4. The MPPT controller gve the reference voltage of the cell though real-tme detecton of the actual cell voltage, then the dfference between the two voltage go through a PI regulator, and gve the carrer gnal, whch compare wth the trangle wave and then get the PWM gnal. Th proce a cloed loop control of the photovoltac cell voltage, through whch the actual voltage meet the maxmum power pont voltage quckly, and then the maxmum power of the photovoltac array obtaned. Fgure 1. Structure of photovoltac ytem. where, (3) And, S ref 1W/m, T ref 5 the tandard envronment, I c, U oc, I m and U m are the parameter under dfferent envronment, and the temperature compenaton coeffcent, and the llumnaton compenaton coeffcent. Take STP6-1/Sc for example, t I-U and P-U curve are hown n Fgure and Fgure 3 under dfferent llumnaton and the ame temperature 5, from whch we can ee that they are both non-lnear. The P-U curve ha a maxmum pont under the ame llumnaton and temperature, whch the maxmum power pont of the cell, and t change wth the llumnaton, temperature or load tate. In order to obtan the maxmum power, maxmum power pont trackng control mut be ued, called a MPPT. Fgure. I-U curve. Fgure 3. P-U curve. Fgure 4. The tructure of MPPT controller. Copyrght 13 ScRe.

3 M. ZHANG ET AL. 49 The core of the MPPT controller MPPT arthmetc [1,13], uch a perturbaton and obervaton method, ncremental conductance method and o on, whch not the focu of th paper, o we wll not elaborate t here..3. Grd-connected Inverter Control The tructure of three-phae grd-connected nverter hown n Fgure 5. The PQ decouplng control baed on ynchronou rotatng frame uually ued, whch nclude nner current loop control and outer power loop control [14-16]. Under tatc abc frame, three-phae nverter can be modeled a follow: a ua e ga d 1 1 b u b e gb d t L R. (4) + L+ R c u c egc where, a, b, c the output current of the nverter, ua, ub, u c the output voltage of the nverter, ega, egb, e gc the grd voltage, L the nductance and R the equvalent retant. Equaton (4) can be changed to ynchronou rotatng frame a: d d ω d 1 ud egd Rd dt q ω + (5) q L u q egq Rq where, ω the angular frequency of grd fundamental wave. If the grd voltage deal, the actve and reactve power can be decrbed a follow: P 3 e /, Q 3 e / (6) gd d gd q Equaton (6) how that PQ can be controlled ndependently. And equaton (5) the prncple of current control [14,16]. 3. The Tranent Model Now we have the electromagnetc tranent model of one photovoltac power ytem, whle there are many et of photovoltac workng together n an actual photovoltac power taton, whch need mulaton at the ame tme. Becaue the power flow model too mple to decrbe the dynamc proce, and the electromagnetc tranent model need mall mulaton tep and take a long computaton tme becaue of t complexty, o none of them utable for dynamc proce mulaton of large cale photovoltac ytem, therefore the tudy of an electromechancal tranent model of great meanng. The followng preent a general electromechancal tranent model utable for the mulaton of large-cale power ytem. Th model nclude the photovoltac array model, MPPT, DC/DC, the DC lnk, the nner and outer loop of nverter control. Fgure 6 how the rela- tonhp of gnal tranfer between them. The electromechancal tranent model baed on mathematcal calculaton, wth no electrc element and no hgh frequency wtchng devce. Compared wth the electromagnetc tranent model, MPPT control, DC/DC and the nverter are replaced by pure mathematcal model, whle the photovoltac cell model reman the ame. Thee module wll be dcued later The MPPT Model The man purpoe of MPPT module to acheve realtme trackng of the maxmum power pont voltage, through whch the photovoltac output voltage a frt order lag of the reference voltage. Although dfferent controller ha dfferent pure lag tme contant τ and frt order tme contant T, t effect can be decrbed by the followng equaton: e τ Vpv Vpvm + Vpvm (7) 1 + T where, V pv the real voltage of photovoltac, V pvm the reference voltage, and V the trackng error. 3.. DC/DC Module Take Boot crcut for example, DC/DC manly rae the voltage and tranmt the power. In electromagnetc tranent model, the Boot crcut help acheve the MPPT, whle not n electromechancal tranent model. DC/DC module can be decrbed by the followng equaton: Pout f1( Pn ) Pn η (8) Vout f ( Vn, D) Vn / (1 D) where, η the effcency of DC/DC, and D the duty cycle. PV Ua Ub Uc Fgure 5. Structure of three-phae grd-connected nverter. S T Other parameter Photovoltac array P PV1 V PV1 V PVm DC/DC MPPT V PVm1 V D P PV V D,ref pvm L The DC lnk V D Reactve power control The outer loop of nverter P De R ega egb egc The nner loop of nverter abc meaurem ent P, u e Qe abc I dq,ref I nve dq rter dq/abc abc Power Grd Fgure 6. Sgnal tranfer of electromechancal tranent model. Copyrght 13 ScRe.

4 5 M. ZHANG ET AL The DC Lnk The DC lnk connect the DC de and the de, and the DC bu voltage tablty a prerequte to enure the normal work of the nverter, whch need much attenton whle etablhng the electromechancal tranent model. The DC lnk module can be decrbed a follow: dec PPV PDe dt (9) E C 1CVD where, P PV the DC de nput power, P De the de nput power, C the capactance of DC lnk, V D the voltage of the DC lnk, and E C the energy of the capacty The Outer Loop of Inverter Control The outer loop of nverter control power control, realzng the PQ decouplng control. The dfference between the actual value and reference value of the DC lnk voltage, through a PI regulator, output the d ax current reference value, whch form the cloed loop control of DC bu voltage. The dfference between the actual value and reference value of the reactve power, through a PI regulator, output the q ax current reference value, whch form the cloed loop control of the grd reactve power. The tranfer functon of the outer loop a follow: B + B + B I V V Id,ref Sat ( Id,ref, Id,max, I d,mn ) 1 d d1 d d,ref D,ref D Ad Ad1 Ad ( ) (1) B 1 q + Bq 1+ Bq ' Iq,ref ( Qref Qe ) Aq + Aq 1+ Aq (11) 1 Iq,ref Sat ( Iq,ref, Iq,max, I q,mn ) where, 1) Sat ( xx, max, x mn ) aturaton functon, a follow: xmax x > xmax Sat ( xx, max, xmn ) xmn x< xmn x xmn x xmax ) I d,max, I d,mn, I q,max, Iq,mn can be decded by the model parameter, or deduced only by I max, a follow: Id,max Imax Iq,max Imax Id,ref. Id,mn Imax Iq,mn Iq,max 3) A d, A d1, A d, B d, B d1, B d, A q, A q1, A q, B q, B q1, B q are control parameter The Inner Loop of Inverter Control The nner loop of nverter control current control, through whch the actual current track the reference current, thu the actve and reactve power meet the demand. The followng gve the dervaton of t tranfer functon. Equaton (5) can be wrtten a follow: dd egd ud L + Rd L q dt + ω dq egq uq L + Rq Lωd dt Through the Laplace tranform, we have ( ) (1) egd ud L + R ωl d (13) e u gq q ωl ( L + R) q Through the PI regulator, the output voltage of nverter : K1 egd, ref ud + K1 p + ( d, ref d ) L q + ω (14) K egq uq + K p + ( q, ref q ) Lω d In matrx form, a follow: egd ud PI1() ωl d e u gq q ωl PI () q (15) PI1() d, ref + PI () q, ref Compare Equaton (13) and (15), we have: ( L + R) + PI1() d ( ) L + R + PI () q PI1() d, ref PI () q, ref That : PI1() d d, ref ( L + R) + PI1() (16) PI () q q, ref ( L + R) + PI () In a general form: bd + bd1+ bd d d, ref ad + ad1+ ad (17) bq + bq 1+ bq q q, ref aq + aq 1+ aq Equaton (17) the tranfer functon of the nner cur- Copyrght 13 ScRe.

5 M. ZHANG ET AL. 51 rent loop, whch nclude dq/abc tranformaton, the phae-lock loop and o on. The power can be obtaned by meaurement and dq/abc tranformaton and the phae-lock loop the ame a that n the electromagnetc tranent model. 4. Smulaton Reult The electromagnetc tranent model and electromechancal tranent model are etablhed n PSCAD/EMTDC. Take STP6-1/Sc poly-lcon for example, t parameter are a follow: U m 17.4 V, I m 3.56 A, U oc 1.8 V, I c 3.78 A, P m 6 W. In the mulaton model, we ue 4 cell n ere 3 cell n parallel n a module, and 1 module n ere 6 module n parallel n an array, thu the maxmum power of photovoltac array 44.6 kw n the tandard envronment. The MPPT control ue perturbaton and obervaton method. For both model, the mulaton tme 1 and tep 5 u. When ytem mulaton acheve a teady tate, rae the llumnaton ntenty from 8 W/m to 15 W/m, and oberve the actve power and ome other electrcal quantte. The followng are two condton accordng to dfferent reactve power reference. 1) The reactve power reference, whch mean the power factor of the grd 1. It take 13.5 to fnh the mulaton for electromagnetc model, whle jut 4.3 for electromechancal model. Fgure 7 to Fgure 1 how ome curve when the reactve power reference. Fgure 1 how that the current and the voltage ha the ame phae, whch mean that the power factor of the grd 1, meetng the control goal. From Fgure 7, Fgure 9, Fgure 1, we can ee that the photovoltac power, the actve and reactve power of the grd ncreae after the llumnaton denty ncreae. In addton, Fgure 7 to Fgure 11 how that the mulaton reult of two model are almot the ame, Actve Grd Power(MVA) Reactve of Grd Power(Mvar) Fgure 7. Actve power of grd Fgure 8. Reactve power of grd. Grd Current of Pahe A(kA) Photovoltac Power(MVA) DC Bu Voltage(kV) Voltage(kV) and Current(kA) Fgure 9. current of phae A Fgure 1. Photovoltac power Fgure 11. The DC bu voltage Fgure 1. voltage and current of phae A. current voltage whch mean that the electromechancal tranent model vald and reaonable. ) The reactve power reference.3 Mvar. It take 1.8 to fnh the mulaton for electromagnetc model, whle jut 4.1 for electromechancal model. The photovoltac power, the actve power of the grd and the DC bu voltage of condton ) are mlar to condton 1). Fgure 13 how the reactve power of the grd, and Fgure 14 how the current and voltage, between whch the phae not the ame but ha an angle, meanng that the grd power factor not Concluon In th paper, a general electromechancal tranent model of grd-connected photovoltac power generator propoed, and both electromagnetc and electromechancal tranent model are etablhed n PSCAD. By comparng mulaton reult of the grd actve and reactve power, the grd current, the photovoltac power and the DC bu voltage of two model, the correctne and valdty Copyrght 13 ScRe.

6 5 M. ZHANG ET AL. Reactve of Grd Power(Mvar) Voltage(kV) and Current(/kA) Fgure 13. reactve power of the grd. current voltage Fgure 14. voltage and current of phae A. of the electromechancal tranent model verfed, whch provde reference model for mulaton and modelng of large cale grd-connected photovoltac power taton. REFERENCES [1] Z. M. Zhao, J. Z. Lu, X. Y. Sun and L. Q. Yuan, Solar Photovoltac Power Generaton and It Applcaton, Bejng: Scence Pre, 6. [] Z. Zhang, H. Shen and R. X. Ca, Analy of the Development Trend of Solar Energy and the Cot, Power Sytem Technology, 8. [3] F. L, W. L, F. Xue, Y. J. Fang, T. Sh and L. Z. Zhu, Modelng and Smulaton of Large-cale Grd-connected Photovoltac Sytem, Internatonal Conference on Power Sytem Technology,1. [4],R. K. Varma, A. R. Shah, S. Vnay and V. Tm, Novel Control of a PV Solar Sytem a STATCOM (PV-STATCOM) for Preventng Intablty of Inducton Motor Load, 5th IEEE CCECE, 1. [5] R. K. Varma, V. Khadkkar and R. Seethapathy, Nghttme Applcaton of PV Solar Farm a STATCOM to Regulate Grd Voltage, IEEE TRANSTION on Energy Converon, Vol. 4, No. 4, 9. [6] X. Yang and F. Yang, Smulaton of three-phae PV Sytem baed on PSCAD/EMTDC, Journal of Shangha Unverty of Electrc Power, 11. [7] Z. Q. Yao and X. Zhang, Study and Smulaton of Three-phae PV Sytem Baed on PSCAD/EMTDC, Power Sytem Protecton and Control, 1. [8] W. Z. Yao and Y. T. Fu, Study of Three-phae Grd-connected Inverter, Power Electronc technology,11. [9] F. Lu and W. P. Xu, Study on Three-phae Grd-connected Control Sytem Baed on LCL Flter, Journal of Solar Energy, 8. [1] C. H. L and X. J. Zhu, Modelng and Performance Analy of Photovoltac/fuel Cell Hybrd Power Generaton Sytem, Power Sytem Technology, Vol. 33, No. 1, 9, pp [11] Y. Jao and Q. Song, Practcal Smulaton Model of Photovoltac Cell n Photovoltac Generaton Sytem and Smulaton, Power Sytem Technology, 1. [1] D. Q. Feng and X. F. L, Improved MPPT Algorthm Baed on Output Properte of PV cell, Computer Engneerng and Degn, 9. [13] C. Zhang, D. Zhao and X. N. He, Implementon of MPPT Baed on Power Equlbrum, Power Electronc, 1. [14] Q. H. Ru, S. W. Du, W. D. Jang and Q. Zhao, Current Regulaton for Three-phaed Grd-connected Inverter Baed on SVPWM Control, Power Electronc, 1. [15] F. Lu, X. M. Zha and S. X. Duan, Degn and Reearch on Parameter of LCL Flter n Three-Phae Grd-connected Inverter, Tranacton of Chna Electrotechncal Socety, 1. [16] L. Z. Y and H. M. Peng, Study on the Decouplng Control of Three-phae PV Grd-connected Inverter Baed on Space Vector, Journal of Solar Energy, 1. Copyrght 13 ScRe.

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