An Integrated Control Strategy Adopting Droop Control with Virtual Inductance in Microgrid

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1 Engneerng, 013, 5, do:10.436/eng b008 Publshed Onlne January 013 (htt:// An Integrated Control Strategy Adotng Droo Control wth Vrtual Inductance n Mcrogrd Janjun Su 1, Jeyun Zheng, Demn Cu 1, Xaobo L 1, Zhjan Hu, Chengxue Zhang 1 Dezhou Power Suly Comany, Shandong Electrc Power Grou Co., Dezhou, Chna School of Electrcal Engneerng, Wuhan Unversty, Wuhan, Chna Emal: zhjan_hu@163.com Receved 013 ABSTRACT As there exsts sorts of dstrbuted generators n mcrogrd, an ntegrated control strategy contanng dfferent control methods aganst corresondng generators should be aled. The strategy n ths aer nvolves PQ control and droo control methods. The former ams at lettng generators lke PV outut maxmum ower. The latter stems from nverter arallel technque and ales to controllng generators whch can kee the network voltage steady to make the arallel system reach the mnmum crculaton ont. Due to the unworthness of droo control aled n low-voltage mcrogrd of whch the medance rato s rather hgh, the aer adots the droo control ntroducng vrtual generator and vrtual medance. Based on theoretcal analyss, smulaton n Matlab s also mlemented to verfy the feasblty of the strategy. Keywords: Mcrogrd; Integrated Control; PQ Control; Droo Control; Vrtual Imedance 1. Introducton Wth the demand for energy exandng and the concern about global warmng growng dee, more and more dstrbuted generators ncludng fuel cell, wnd ower and hotovoltac cell at al. get wdely used [1]. Thus we come to a defnton of mcrogrd, namely, a network grou consstng of DG unts whch are on the dstrbuton network sde and rovde energy for local areas, energy storage devces, and loads [,3]. Mcrogrd manly oerates n two modes: grd mode and sland mode. The control mode of mcrogrd s classfed nto two: rncle-subordnate control and eer-to-eer control. Accordng to ths thought, droo control whch s based on the droo characterstc of tradtonal grd s manly used [4,5].In the ractcal alcaton of mcrogrd, nevertheless, there s a dversty of DG unts rangng from PV to DG unts. Due to ths, dstnct control methods are desgned for each knd of DG unts, that s ntegrated control strategy [6]. Reference [5] ntroduces vrtual medance, however t doesn t gve a detaled descrton of the realzaton of ths mrovement. Ths aer not only ntroduces vrtual medance nto the ntegrated control of mcrogrd, but also secfes on ts detaled realzaton. Deart from analyzng ts feasblty, the aer verfes the strategy by smulaton.. Structure of Mcrogrd Controllers As can be seen from Fgure 1, the controller of DG s comosed of the controller whch comounds grd voltage reference and grd current reference and UI trackng controller. The aer wll secfy on the former. 3. Integrated Control Strategy of Mcrogrd When mcrogrd s connected nto ower dstrbuton network, every DG unt gets controlled wth PQ control method as the voltage and the frequency of the system have been adjusted by nfnte ower grd and t s the most mortant for DG unts to kee ower balance among each other. When mcrogrd s dsconnected from ower dstrbuton network, there s a need to mantan the amltude and frequency of the voltage of ower grd Fgure 1. Control dagram of droo control wth vrtual nductance n Mcrogrd system. Coyrght 013 ScRes.

2 J. J. SU ET AL. 45 As a result, droo control s adoted to rovde reference for grd voltage and ts frequency PQ Control Method PQ control method s based on the feed-forward decoulng of dq transformaton and realzes maxmum ower outut of DG by adjustng actve current and reactve current to track reference current [7]. The equaton of reference current s: dref qref Pref = U d Q = U Then by feed-forward decoulng of quadrature drect axs current, we get reference for outer loo voltage. That s to make the nverter reach the reference outut ower usng the classcal voltage and current dual-loo control. 3.. Droo Control Method As DG unts are connected to PCC wth solatng transformers, DC comonents of ther njecton currents are no more than 5 ercent of rated outut currents [8]. The sketch dagram of ower transmsson of a DG s shown as Fgure. Where, Pont A s the outut ont of a DG unt whch s conssted of a DG and ts flterng system. Pont B s the nut ont of ower dstrbuton network. The medance of the ntermedate transmsson lne s ZL = R + jx. Assume that the njected ower of ont A s S = P + jq, the outut ower of the DG can be exressed as follows. E1[ RE ( 1 Ecos δ) + EX sn δ] P = () R + X E1[ X( E1 Ecos δ) ER sn δ] Q = (3) R + X The feasblty of droo control s based on the lne beng nductve. The recondton s often satsfed by settng the arameters of the dual-loo controller wthout vrtual medance [9]. The equvalent outut medance of the nverter s ref d (1) Ls Zs () = (4) LCs + kk Cs + (1 + kk k ) s + kk k 3 wm wm wm where, k s the scale arameter of current control n dualloo control, k and k are the scale arameter and ntegral arameter of voltage control resectvely. k wm s the magnfcaton coeffcent of the nverter and we Vdc can take that k wm =. We can choose arameters whch make Z(s) nductve n the frequency range of 50Hz and the nductve range s not that wde Droo Control Method wth Vrtual Imedance As mcrogrd s mostly low voltage network, the medance rato of ts lnes s rather large. From (3), we know that wth large medance rato, the value of EX 1 ( E1 Ecos δ ) s relatvely small, whle the nfluence of EER 1 snδ whch s also affected by actve ower ncreases. In the arallel system of DG unts, when the outut actve ower dffers from each other, t may occur that some DG unts absorb reactve ower whle others release [10]. To solve ths roblem, we need to decoule P and Q. One soluton s to make use of coordnate transformaton [11], but t nvolves medance rato whch s dffcult to acqure sometmes. One soluton s to desgn arameters of the controller to turn outut medance nductve whch nevertheless s at the mercy of the arameter desgn of dual-loo control. Besdes those, the soluton of ntroducng vrtual medance should be the best. The advanced droo control embodes the thought of equvalent control. Accordng to t, a DG n mcrogrd s equvalent to a vrtual generator wth vrtual medance, whch s shown n Fgure 3. Reference [5] has secfed on the rncle and the feasblty of the advanced method. Ths aer wll tell ts detaled realzaton. A S=P+jQ B E1 δ ZL=R+jX E 0 Fgure. Power transmsson dagram. Fgure 3. Equvalent Mcrogrd system wth vrtual nductance. Coyrght 013 ScRes.

3 46 J. J. SU ET AL. 4. Realzaton of Advanced Droo Control 4.1. Structure of Droo Control wth Vrtual Imedance The control model of droo control s made of three arts: dq transformaton and reference ower comound, reference voltage and frequency comound, voltage and current dual-loo control. Accordng to equvalent thought, we have Q Qξ I Xξ = (5) The nstant reference voltage of orgnal DG s * ωc VDG = eξ Lξ ddg / dt s + ω ωc where, s the low-ass flter for restranng hghs + ωc frequency nose n the vrtual lne. 4.. Smulaton of Droo Control wth Vrtual Imedance In the ower comound module, ntroduce the outut current of DG unts. Comound outut reactve ower Q ξ of the vrtual generator wth the dq comonents of the outut voltage of DG unts. Meanwhle, cross multly the dq comonents of outut current to get multly by X ξ. On the bass of the logcal relaton n (6), we get reference reactve ower for DG unts. The actve ower comound module needn t modf- C (6) I and caton as P = P ξ. Accordng to (6), on the bass of orgnal dq comonents of reference voltage, mnus resectvely the voltage dro of outut currents down the vrtual medance and get new reference outut voltage. Where, the dro s acheved by dq transformng the outut current, gong through the dfferental art and a transfer functon of a flter wth a flterng caactor, fnally gong through a roortonal element of L ξ. Accordng to equvalent rncle, we have = + QX + PX + QX (7) ξ ξ ξ ξ E ( EDG ) ( ) EDG EDG EDG EDG Deendng on the thought of averagng, thus we have 1 Qmn Q [( max Eξ = EDG max + EDG mn ) + ( + ) Xξ ] (8) E E DG max DG mn Accordng to arameters n smulaton, assume L = 100 mh, E = 567 V. ξ ξ 5. Smulaton Analyss Establsh a model of a system havng 4 aralleled DG unts shown n Fgure 4. Fgure 4. Structure of smulaton model. PV resources are both controlled wth PQ control method. Energy storage devce (ESD) s both controlled wth droo control method and advanced droo control method. The advanced method sets the lne between DG and PCC nductve by desgnng the arameters of the controller, whle the orgnal realzes ower decoule by ntroducng vrtual medance. The voltage of two ESD s 800 V. Ther rated ower s 1 kw. Reference outut voltage s 380 V. Flterng nductance s 50 mh and caacty s 0 μf. Wth regard to lne medance, R = Ω / km, X = Ω / km L L and the length s 50 m. The PWM carrer frequency s 6000 Hz. The acton tme of swtches s: the llumnaton ntensty of PV decreases at s, mcrogrd becomes sland mode at 0.3 s, Load 4 s aled at 0.5s and cut at 0.8 s, mcrogrd s agan connected wth dstrbuton network at 5 1s. Smulaton ste: 5 10 s. Smulaton algorthm: ode3. Smulaton tme: s Smulaton Analyss of Integrated Control Accordng to PQ control rncle, for droo control, K = 1, K = 0.5, as to current control, K = 5. For PQ control, K = 155 and K = 1. Smulaton results are as follows. Fgure 5 and Fgure 5 are the outut voltage of ESD 1 and PV 1 resectvely. As can be seen, n grd mode, due to the effect of the voltage of dstrbuton network, the waveforms are steady and fluctuate lttle when llumnaton changes. At 0.3 s mcrogrd gets nto sland mode, the voltage deceases but becomes stable mmedately. The amltude of voltage changes but kees sne curve at 0.5 s and 0.8 s when Load 4 s aled and cut. Ths suggests that outut voltage of DG can be controlled nstantaneously to newly become steady wth ntegrated control strategy. The waveforms of mcrogrd gettng nto sland mode are shown n Fgure 6, takng ESD 1 and PV 1 for examle. Coyrght 013 ScRes.

4 J. J. SU ET AL. 47 The outut actve ower and reactve ower of each DG unt s shown n Fgure 7. As t can be seen, the ower curves of two energy storage devce are almost the same and mmedately reach a steady value whenever the swtches act, whch wll rovde reference voltage for controllng two PV unts. As PV s close to Load 4, ts waveform s a lttle dfferent from the one of PV1. But they all become steady swftly, whch suggests that PQ control s ft for PV. 5.. Smulaton Analyss of Integrated Control Adotng Advanced Droo Control Modfy the smulaton model accordng to 3.. Ths tme, K = 0.1, K = 0.008, as to current control, K = 5. As to PQ control, for PV1, K = 160 and K 1 =. For PV, K = 173 and K 1 =. Fgure 8 and Fgure 8 are the fltered outut voltage of ESD 1 and PV 1 resectvely. Fgure 5. Outut voltage of each dstrbuted generator. Fgure 6. Waveform of outut voltage. Fgure 7. Outut ower of each DG. Coyrght 013 ScRes.

5 48 J. J. SU ET AL. Fgure 8. Outut voltage of each dstrbuted generator. Fgure 9. Waveform of outut voltage. Fgure 10. Outut ower of each DG. The adjustng s swft, ther waveforms of voltage when swtches act are shown n Fgure 9. The outut actve ower and reactve ower of each DG unt s shown n Fgure 10. Wth vrtual medance, the ower of PV unts fluctuates lttle. Besdes, n comarson wth the reactve ower of orgnal droo control, the fluctuaton range of the energy storage devce greatly reduces, whch ndcates that outut ower s better controlled by ntegrated control strategy wth advanced droo control. 6. Concluson Uon arallel system of mcrogrd wth energy storage devce and PV, ths aer makes a study of ntegrated control strategy deendng on characterstcs of dverse DG unts. What s more, vrtual medance s ntroduced nto droo control to get rd of the restran of orgnal control on the lne medance. Besdes that, secfc realzaton s gven n the aer. The results of smulaton demonstrates that, the new ntegrated control strategy can kee outut voltage of DG unts steady n grd mode and Coyrght 013 ScRes.

6 J. J. SU ET AL. 49 sland mode, and realze swft transton between the two modes. Furthermore, wth vrtual medance, the strategy can better guarantee the stablty of outut reactve ower of each DG unts to realze the better decoulng control of actve ower and reactve ower. 7. Acknowledgements Ths work was fnancally suorted by the Ph.D. Programs Foundaton of Mnstry of Educaton of Chna ( ). REFERENCES [1] J. Huang, C. Jang, R. Xu. A revew on dstrbuted energy resources and mcrogrd, Renewable and Sustanable Energy Revews, Vol. 1, No. 9, [] W. Saad, Z. Han, H. V. Poor, T. Basar. Game-theoretc methods for the smart grd: an overvew of mcrogrd systems, demand-sde management, and smart grd communcatons. IEEE Sgnal Processng Magazne, Vol. 9, No. 5, 01, [3] E. Serban, H. Serban. A control strategy for a dstrbuted ower generaton mcrogrd alcaton wth voltage-and-current-controlled source converter. IEEE Transactons on Power Electroncs, Vol. 5, No. 1, 010, [4] W. Yang. Smulaton and research of grd connected hotovoltac generaton and mcrogrd oeraton control. Schuan: Southwest Jaotong Unversty, 007. [5] J. Cheng, S. L, J. Chen, Z. Wu. An analyss of decoulng mechansm of droo control usng vrtual reactance n mcrogrd. Automaton of Electrc Power System, Vol. 36, No, 7, 01, [6] C. Wang, Z. Xao, et al. Intergrated control and analyss of mcrogrd. Automaton of Electrc Power System, Vol. 3, No. 7, 008, [7] Z. Lu,A. Luo,F. Rong,L. Guo. Mcrogrd PQ control strategy analyss under utlty voltage mbalance. Power Electroncs, Vol. 44, No. 6, 010, [8] IEEE Standard for nterconnectng dstrbuted resources wth electrc ower systems [9] Y. L,C. Kao. An accurate ower control strategy for ower-electroncs-nterfaced dstrbuted generaton unts oeratng n a low-voltage multbus mcrogrd. IEEE Transactons on Power Electroncs, Vol. 4, No. 1, 009, [10] J. Km, J. M. Guerrero, P. Rodrguez, R. Teodorescu, K. Nam. Mode adatve droo control wth vrtual outut medances for an nverter-based flexble AC mcrogrd. IEEE Transactons on Power Electroncs, Vol. 6, No.3, 011, [11] X. Zhou,F. Rong,Z. Lv,S. Peng. V/f droo-control method wth vrtual ower adotng coordnate rotaton n low voltage mcrogrd. Automaton of Electrc Power Systems, Vol. 36, No. 7, 01, Coyrght 013 ScRes.