THE application of distributed generation (DG) has been

Transcription

1 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 6, JUNE An Improved Droop Control Strategy for Reactve Power Sharng n Islanded Mcrogrd Hua Han, Yao Lu, Yao Sun, Member, IEEE, Me Su, and Josep M. Guerrero, Senor Member, IEEE Abstract For mcrogrd n slanded operaton, due to the effects of msmatched lne mpedance, the reactve power could not be shared accurately wth the conventonal droop method. To mprove the reactve power sharng accuracy, ths paper proposes an mproved droop control method. The proposed method manly ncludes two mportant operatons: error reducton operaton and voltage recovery operaton. The sharng accuracy s mproved by the sharng error reducton operaton, whch s actvated by the low-bandwdth synchronzaton sgnals. However, the error reducton operaton wll result n a decrease n output voltage ampltude. Therefore, the voltage recovery operaton s proposed to compensate the decrease. The needed communcaton n ths method s very smple, and the plug-and-play s reserved. Smulatons and expermental results show that the mproved droop controller can share load actve and reactve power, enhance the power qualty of the mcrogrd, and also have good dynamc performance. Index Terms Droop control, low-bandwdth synchronzaton sgnals, mcrogrd, reactve power sharng, voltage recovery operaton. I. INTRODUCTION THE applcaton of dstrbuted generaton (DG) has been ncreasng rapdly n the past decades. Compared to the conventonal centralzed power generaton, DG unts have advantages of less polluton, hgher effcency of energy utlzaton, more flexble nstallaton locaton, and less power transmsson losses. Most of the DG unts are connected to the grd va power electronc converters, whch ntroduces system resonance, protecton nterference, etc. To overcome these problems, the mcrogrd concept was frst proposed n the US by the Consortum for Electrcal Relablty Technology Solutons [1]. Compared to use a sngle DG unt, mcrogrd could offer superor power management wthn the dstrbuton networks. Moreover, the mcrogrd can operate n grd-connected mode or slanded mode and beneft both the utlty and customers n economy [2] [7]. In an slanded mode, the load power n the mcrogrd should be properly shared by multple DG unts. Usually, the droop control method whch mmcs the behavor of a synchronous generator n tradtonal power system s adopted, whch does Manuscrpt receved February 9, 2014; revsed May 3, 2014; accepted June 6, Date of publcaton June 20, 2014; date of current verson January 16, Ths work was supported by the Natonal Natural Scence Foundaton of Chna under Grant and Hunan Provncal Natural Scence Foundaton of Chna under Grant 14JJ5035. Recommended for publcaton by Assocate Edtor Q.-C. Zhong. (Correspondng author: Yao Sun.) H. Han, Y. Lu, Y. Sun, and M. Su are wth the School of Informaton Scence and Engneerng, Central South Unversty, Changsha , Chna (e-mal: hua_han@126.com; yaolu@csu.edu.cn; yaosuncsu@gmal.com; sumecsu@mal.csu.edu.cn). J. M. Guerrero s wth the Department of Energy Technology, Aalborg Unversty, 9220 Aalborg East, Denmark (e-mal: joz@et.aau.dk). Dgtal Object Identfer /TPEL not need the use of crtcal communcatons [8] [14], [21], [22]. The actve power sharng s always acheved by the droop control method easly. However, due to effects of msmatched feeder mpedance between the DGs and loads, the reactve power wll not be shared accurately. In extreme stuatons, t can even result n severe crculatng reactve power and stablty problems [11]. To overcome the reactve power sharng ssue, a few mproved methods have been proposed. Specfcally, there are manly three approaches to address the effect of the nterconnectng lne mpedance on droop-based control. The frst approach s to ntroduce the vrtual output mpedance by modfyng the output voltage reference based on output current feedback [11], [13], [14], [23]. Ths method can reduce the reactve power sharng error by reducng the relatve error of the output mpedances. However, the ntroducton of the vrtual mpedance may lead to degradaton of the system voltage qualty. The second approach s based on a sgnal njecton technque. In [15], a certan harmonc sgnal contanng reactve power nformaton s njected nto the output voltage reference of each DG unt, and the output reactve power s regulated to mprove the accuracy of the reactve power sharng accordng to the harmonc power. However, ths method results n output voltage dstorton. In [16], n order to reduce the reactve power sharng errors, the method njects some small dsturbance sgnals contanng reactve power nformaton nto the frequency reference of each DG unt. By usng the actve power error before and after the njectng sgnal, ths method can elmnate the reactve power sharng error. However, ths method s a classc event-trggered control and ts stablty s not easy to be guaranteed. Addtonally, the thrd approach s usually based on constructed and compensated method. In [17], the method constructs an ntegral control concernng the common bus voltage to ensure the reactve power sharng. However, n practcal stuaton, the common bus voltage nformaton s dffcult to get. In ths paper, a new reactve power sharng method s proposed. The method mproves the reactve power sharng by changng the voltage bas on the bass of the conventonal droop control, whch s actvated by a sequence of synchronzaton events through the low-bandwdth communcaton network. It s a cost-effectve and practcal approach snce only a lowbandwdth communcaton network s requred. Smulaton and expermental results are provded to verfy the effectveness and feasblty of the proposed reactve power sharng method. Ths paper s organzed as follows: Secton II gves the system confguraton and the reactve power sharng error analyss wth conventonal droop control. Secton III proposes an mproved reactve power sharng control strategy, and the convergence and IEEE. 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2 3134 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 6, JUNE 2015 Fg. 1. Illustraton of the ac mcrogrd confguraton. common ac bus voltage. Compared wth the nductance of the LCL flter, the lne resstance can be gnored. Then, the mpedance between nverter and the common bus can be descrbed as ( = ωl ). Accordng to the equvalent crcut n Fg. 2, the nverter output apparent power s S, and t can be gven by S = P + jq = E V pcc sn δ + j [ E V pcc cos δ V 2 pcc (1) From (1), the output actve and reactve power of the DG unts are shown as P = E V pcc sn δ Q = E V pcc cos δ V 2 pcc (2) Usually, the phase-shft angle δ s small. Therefore, the real power P and reactve power Q of each DG can be regulated by δ and the output voltage ampltude E, respectvely [24]. Then, the conventonal droop control s gven by { ω = ω m P (3) E = E n Q where ω and E are the nomnal values of DG angular frequency and DG output voltage ampltude, m and n are the actve and reactve droop slopes, respectvely. P and Q are the measured averaged real and reactve power values through a low-pass flter, respectvely. ] Fg. 2. Model of a DG unt. robustness s analyzed. Smulaton and expermental results are gven n Secton IV. Secton V gves the concluson. II. ANALYSIS OF CONVENTIONAL DROOP CONTROL METHOD A. Confguraton and Operaton of AC Mcrogrd A classc confguraton of a mcrogrd that conssts of multple DG unts and dspersed loads s shown n Fg. 1. The mcrogrd s connected to the utlty through a statc transfer swtch at the pont of common couplng (PCC). Each DG unt s connected to the mcrogrd through power electronc converter and ts respectve feeder. Ths paper ams to solve the fundamental actve and reactve power sharng n slanded mode, and the power sharng ssue on harmonc currents s out of the scope of ths paper. B. Conventonal Droop Control Fg. 2 shows the equvalent model of a DG unt, whch s nterfaced to the common bus of the ac mcrogrd through a power nverter wth an output LCL flter. As shown n Fg. 2, E δ s the voltage across the flter capactor, and V pcc 0 s the C. Reactve Power Sharng Errors Analyss For smplcty, a smplfed mcrogrd wth two DG unts s consdered n ths secton. Accordng to (2) and (3), the reactve power of the th DG unt s obtaned Q = V pcc(e cos δ V pcc ) (4) + V pcc n cos δ Assume the th and jth DG unt are workng n parallel wth the same nomnal capacty and droop slope. Note that shft angle δ s usually very small (snδ δ, cosδ 1), then the reactve power sharng relatve error wth respect to Q can be expressed as follows: ΔQ err = Q Q j Q X j X j + V pcc n j (5) It shows that the reactve power sharng relatve error s related to some factors, whch nclude the mpedance X j,the mpedance dfference (X j ), the voltage ampltude V pcc of the PCC, and the droop slope n j. Accordng to (5), there are two man approaches to mprove the reactve power sharng accuracy: ncreasng mpedance X j and the droop gan n j.usually, ncreasng mpedance s acheved by the vrtual mpedance [11], [13], [14], whch requres a hgh-bandwdth control for nverters. Increasng the droop gan n j s a smpler way to reduce the sharng error. However, t may degrade the qualty of the mcrogrd bus voltage, and even affects the stablty of the mcrogrd system [18] [20].

3 HAN et al.: IMPROVED DROOP CONTROL STRATEGY FOR REACTIVE POWER SHARING IN ISLANDED MICROGRID 3135 III. PROPOSED REACTIVE POWER SHARING ERROR COMPENSATION METHOD A. Proposed Droop Controller The proposed droop control method s gven as follows: ω = ω m P (6) k 1 k E (t) =E n Q (t) K Q n + G n ΔE (7) where k denotes the tme of synchronzaton event untl tme t. Accordng to (7), the control s a hybrd system wth contnuous and dscrete trats. In the dgtal mplementaton of the proposed method, the contnuous varables E (t) and Q (t) are dscretzed wth samplng perod T s, and T s s greatly less than the tme nterval between two consecutve synchronzaton events. Therefore, the droop (7) at the kth synchronzaton nterval could be expressed as E k k 1 = E n Q k K Q n + k G n ΔE (8) where ω and E are the values of DG angular frequency and output voltage ampltude at no-load condton; m and n are the droop gans of frequency and voltage of DG- unt; G n s the voltage recovery operaton sgnal at the nth synchronzaton nterval, G n has two possble values: 1 or 0. If G n =1, t means the voltage recovery operaton s performed. Q n represents the output reactve power of DG- unt at the nth synchronzaton nterval. K s a compensaton coeffcent for the DG- unt, ΔE s a constant value for voltage recovery. For smplcty of descrpton, the thrd term of (8) s referred to the sharng error reducton operaton, and the last term s called the voltage recovery operaton. For smplcty, the output voltage for the DG- unt n (8) s wrtten as follows n teratve method: E k = E k 1 n (Q k Q k 1 ) K Q k 1 + G k ΔE (9) Therefore, for ts mplementaton, only E k 1 and Q k 1 should be stored n DSP. To better understand the proposed method, a specfc example s gven. If there are two DG unts wth the same capacty workng n parallel, and the conventonal droop s only used. There wll exst some reactve power sharng error due to some factors. If the sharng error reducton operaton for each DG unt s performed at the tme, the resultng reactve power sharng error wll decrease. The prncple behnd the sharng error reducton operaton can be understood wth the ad of Fg. 3. If the aforementoned operaton s repeated wth tme, the reactve power sharng error wll converge. However, the assocated operatons wll result n a decrease n PCC voltage. To cope wth the problem, the voltage recovery operaton wll be performed. That s to say f the output voltage of one DG unt s less than ts allowed low lmt, then the DG unt wll trgger the voltage recovery operaton untl ts output voltage s restored to ratng value. The output voltage of all the DG unts wll be added an dentcal value ΔE to ncrease the PCC voltage. The dea for the voltage recovery operaton can be comprehended wth the ad of Fg. 4. Fg. 3. Fg. 4. Schematc dagram of the sharng error reducton operaton. Schematc dagram of voltage recovery mechansm. B. Communcaton Setup A DG unt can communcate wth other DG unts by RS232 seral communcaton. Each DG unt has the opportunty to trgger a synchronzaton event on the condton that the tme nterval between two consecutve synchronzaton events s greater than a permssble mnmum value and the output voltage of each DG unt s n the reasonable range. If the output voltage of one DG unt s less than ts allowed low lmt, t wll ask for havng the prorty to trgger a synchronzaton event at once. Untl the constrant that the nterval between two consecutve synchronzaton events s greater than a permssble mnmum value s satsfed, the DG unt wth the prorty wll trgger a synchronzaton event, and n ths event, the command for voltage recovery operaton wll be sent to other DG unts. If the communcaton fals (the tme nterval between two consecutve synchronzaton events s greater than a permssble maxmum value), all the error reducton operatons and voltage recovery operatons should be dsabled and the proposed control method reverts back to the conventonal one. Accordng to the aforesad analyss, such a mcrogrd system only needs a low-bandwdth communcaton. And t s robust to

4 3136 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 6, JUNE 2015 TABLE I ASSOCIATED PARAMETERS FOR POWER STAGE AND CONTROL OF THE DG UNIT Parameters Values Parameters Values Fg. 5. Control tmng dagram of one DG wth the two consecutve synchronzaton events. u rate (V) 220 k pu 0.05 L f (H) 1.5e-3 k p 50 r f (Ω) 0.25 K p 0.2 C f (μf) 20 w c (rad/s) 31.4 L Lne1 (H) 0.6e-3 m(rad/sec.w) 5e-5 L Lne2 (H) 0.3e-3 n(v/var) 5e-3 f s (khz) 12.8 Ke(v/var) f rate(hz) 50 ΔE 0 (V ) 5 T s (s) 1/12.8e3 T syn (mn)(s) 0.1 the delay of communcaton. To llustrate ths pont, the control tmng dagram s shown n Fg. 5. The sharng error operaton and the voltage recovery operaton are performed n update nterval. Samplng operaton occurs n samplng nterval. There s a tme nterval τ, whch s long enough to guarantee the system beng n steady state. It s obvous that proposed method s robust to the tme delay because all the necessary operatons only need to be completed n an nterval, not a crtcal pont. C. Convergence Analyss In ths secton, the convergence of the proposed method wll be proved. Wthout loss of generalty, the sharng reactve power error between DG- and DG-j wth the same capacty wll be analyzed. Accordng to (8), the reactve droop equaton for DG-j can be expressed as E k j k 1 = E n j Q k j K j Q n j + k G n ΔE (10) Subtractng (10) from (8), then k 1 ΔEj= k nδq k j KΔQ n j (11) where n = n j = n, K = K j = K, and ΔEj k s the voltage magntude dervaton of DG and j n the kth control perod; ΔQ k j s the reactve power sharng error. Smlarly, we can get (11) n the (k +1)th nterval k ΔE j = nδq j KΔQ n j (12) Combnng (11) and (12), t yelds ΔE j ΔEj k = nδq j + nδq k j KΔQ k j (13) Accordng to the feeder characterstc, as shown n (2), the followng expressons can be obtaned: ΔE j = 1 ( Q Vpcc Q ) j X j (14) ΔEj k = 1 ( Q k Vpcc Q k ) j X j (15) Assume the PCC voltage value satsfy the followng: V pcc V k pcc V 1 (16) Subtractng (13) from (14), t yelds ΔE j ΔE k j= X j V ( ΔQ j ΔQ k j where ΔX = X j. Combnng the expresson (13) and (17), then ΔQ j = rδq k j ) ΔX ( + Q Q k ) V (17) ΔX [ Q Q k ] V (n + X j /V ) (18) where r = n+x j/v K n+ X < 1. Accordng to the contracton mappng theorem, f r < 1 and ΔX =0, then reactve power shar- j/v ng error wll converge to zero. Generally, ΔX 0, we should also consder the effect of the second term of (18). Accordng to the feeder characterstc, as shown n (1), we have Q Q k = ( E ) E k V (19) Because of the voltage recovery mechansm, we can ensure E mn E k E max for all k Q Q k (Emax E mn ) V (20) Therefore, the second term of (18) s bounded. Accordng to aforesad analyss, t can be concluded that the reactve power sharng error s also bounded. IV. SIMULATION AND EXPERIMENTS RESULTS A. Proposed Droop Controller The proposed mproved reactve power sharng strategy s verfed wth MATLAB/Smulnk and experment. In the smulatons and experments, a mcrogrd wth two DG systems, as shown n Fg. 1, s employed. The assocated parameters for power stage and control of the DG unt are lsted n Table I. Also, n order to facltate the observaton of the reactve power sharng, the two DG unts are desgned wth same power ratng and dfferent lne mpedances. The detaled confguraton of the sngle DG unt s depcted n Fg. 6, where a LCL flter s placed between the nsulated-gate bpolar transstor brdge output and the DG feeder. The DG lne current and flter capactor voltage are measured to calculate the real and reactve powers.

5 HAN et al.: IMPROVED DROOP CONTROL STRATEGY FOR REACTIVE POWER SHARING IN ISLANDED MICROGRID 3137 Fg. 9. Output actve powers of two nverters wth the mproved droop control. Fg. 6. Confguraton of one sngle-phase DG unt. Fg. 10. Output reactve powers of the two nverters when 0.02-s tme delay occurs n synchronzaton sgnal of DG1 unt. Fg. 7. control. Output reactve powers of two nverters wth the mproved droop Fg. 8. Output voltage ampltude of two nverters wth the mproved droop control. In addton, the commonly used double closed-loop control s employed to track the reference voltage [5], [7], [12]. 1) Case 1: Power Sharng Accuracy Improvement: Two dentcal DG unts operate n parallel wth the proposed voltage droop control. Fg. 7 llustrates the reactve power sharng performance of the two DGs. Before t =0.5 s, the sharng error reducton operaton and voltage recovery operaton are dsabled, whch s equvalent to the conventonal droop control beng n effect. There exsts an obvous reactve power sharng error due to the unequal voltage drops on the feeders. After t =0.5 s, the reactve power sharng error reducton operaton s performed, and t s clear that the reactve power sharng error converges to zero gradually. After t =1s, the voltage recovery operaton s performed. It can be observed that the output reactve power ncreases but the reactve power sharng performance does not degrade. Fg. 8 shows the correspondng output voltages. It can be observed that the output voltages decrease durng the sharng error reducton operaton, whle the voltage recovery operaton ensures that DG output voltage ampltudes can restore back nearby to the rated value. The whole process of adjustment can be done steadly n a relatvely short perod of tme. Fg. 9 llustrates actve power sharng performance of the two DG unts. It s obvous that the proposed mproved reactve power sharng strategy does not affect actve power sharng performance. 2) Case 2: Effect of the Communcaton Delay: To test the senstvty of the proposed mproved droop control to the synchronzed sgnal, a 0.02-s delay s ntentonally added to the sgnal receved by DG1 unt at t =0.5 s as shown n Fg. 11, and the smulaton results are shown n Fgs Compared to the case 1 n Fgs. 7 and 9, a small dsturbance appears n both the reactve and actve power, whle the voltage recovery operatons are stll able to ensure that the DG unt can delver the expected reactve power. After t = 2.0 s, the actve and reactve power sharng errors are almost zero. Therefore, the proposed reactve power sharng strategy s not senstve to the communcaton delay. Then, t s llustrated that t s robust to some small communcaton delays. 3) Case 3: Effect of Load Change: In order to test the effect of load change wth the proposed method, the actve load ncreases about 1.6 kw and the reactve load ncreases about 0.4 kvar at t =2.5 s, and at t =4.5 s the actve load decreases about 3.0 kw and the reactve load decreases about 0.8 kvar. The correspondng smulaton results are shown n Fgs. 13 and 14. As can be seen, a large reactve power sharng devaton appears at t =2.5 s and t =4.5 s. However, the devaton becomes almost

6 3138 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 6, JUNE 2015 Fg. 11. DG output voltage of the nverters when 0.02-s tme delay occurs n synchronzaton sgnal of DG1 unt. DG output voltage of the mproved droop control (wth load chang- Fg. 15. ng). Fg. 12. Output actve powers of the two nverters when 0.02-s tme delay occurs n synchronzaton sgnal of DG1 unt. Fg. 16. Prototype of parallel nverters system setup. Fg. 13. Reactve power sharng performance of the mproved droop control (wth load changng). Fg. 17. control. Steady-state expermental waveforms wth the conventonal droop Fg. 14. Actve power sharng performance of the mproved droop control (wth load changng). zero after a whle. Fg. 15 llustrates the correspondng output voltage waveforms. It can be found that there exst obvous output voltage decrease and output voltage ncrease processes durng each reactve power sharng error reducton process. B. Expermental Results A mcrogrd prototype s bult n lab as shown n Fg. 16. The mcrogrd conssts of two mcrosources based on the snglephase nverter. The parameters for output flter are the same as those n smulaton. The load conssts of a resstor of 16 Ω and an nductor of 3 mh. The sample frequency s 12.8 khz. A permssble mnmum tme nterval between two consecutve synchronzaton events s 0.5 s. The permssble mnmum output voltage does not less than the rated voltage by 90%. Fgs. 17 and 18 show the measured waveforms wth the conventonal and mproved droop control methods, respectvely. The waveforms from top to down are crcular current ( 0H = ), the output current of nverter 1 ( 01 ), the output current ( 02 ) of nverter 2 and PCC voltage (U L ),

7 HAN et al.: IMPROVED DROOP CONTROL STRATEGY FOR REACTIVE POWER SHARING IN ISLANDED MICROGRID 3139 Fg. 18. control. Steady-state expermental waveforms wth the mproved droop Fg. 20. Crculatng current and PCC voltage waveforms of DGs wth only sharng error reducton operaton performed. Fg. 19. Steady-state actve power and reactve power: (a) wth the conventonal droop; (b) wth the mproved droop control. respectvely. As can be seen from Fg. 17, there s a qute large phase dfference between two output currents when the conventonal droop control s appled. As a result, the crcular current s pretty hgh and the peak value of crcular current s up to 1.80 A. The man reason for t s the mpedance dfference of DG feeders. Compared wth the crcular current n Fg. 17, the crcular current n Fg. 18 s very small, whch ndcates that the mproved method s effcent n reducng the crcular current manly caused by the output reactve power dfference between the nverters. Fg. 19 shows the steady-state output actve and reactve power of each nverter wth the conventonal and the mproved droop control. Fg. 19(a) shows the results wth the conventonal droop. The steady-state output actve powers of the nverters are 31.4 and 30 W, and the output reactve powers are 21.2 and 10.4 Var. When usng conventonal P-f droop control, no actve power dvergence appears snce frequency s a global varable,.e., same frequency can be measured along the mcrogrd; however, voltage may drop along the mcrogrd power lnes, whch produce the well-known reactve power dvergence. Fg. 19(b) shows the results wth the mproved droop. As can be seen, the output actve powers of the nverters are 30.6 and 31.1 W, and the reactve powers are 3.9 and 4.4 Var. These results ndcate that the proposed mproved droop control has no effect on the actve power sharng performance, but makes reactve power be shared precsely. To verfy the effectveness of the sharng error reducton operaton and voltage recovery operaton of the proposed method, Fg. 21. Crculatng current and PCC voltage waveforms of DGs wth only voltage recovery operaton performed. Fg. 22. Crculatng current and PCC voltage waveforms of DGs wth the mproved droop. the experments wth only one operaton beng contnuously used are performed. As can be seen from Fg. 20, the crcular current converges to a small value gradually when only the reactve power sharng error reducton operaton s performed. In the meanwhle, a contnuous decrease n PCC voltage could be found. Fg. 21 shows the results when only the voltage recovery operaton s performed. It can be seen that the PCC voltage ncreases lnearly durng ths tme, and the crcular current s always small and almost kept constant. Fg. 22 shows the

8 3140 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 6, JUNE 2015 Fg. 23. Output current and crculatng current waveforms when 0.2-s tme delay occurs n synchronzaton sgnal of DG1 unt. V. CONCLUSION In ths paper, a new reactve power control for mprovng the reactve sharng s proposed for power electroncs nterfaced DG unts n ac mcrogrds. The proposed control strategy s realzed through the followng two operatons: sharng error reducton operaton and voltage recovery operaton. The frst operaton changes the voltage bas of the conventonal droop characterstc curve perodcally, whch s actvated by the low-bandwdth synchronzaton sgnals. The second operaton s performed to restore the output voltage to ts rated value. The mproved power sharng can be acheved wth very smple communcatons among DG unts. Furthermore, the plug-and-play feature of each DG unt wll not be affected. Both smulatons and expermental results are provded to verfy the effectveness of the proposed control strategy. REFERENCES Fg. 24. Output current and crculatng current waveforms when the synchronzaton sgnal s lost n DG1 unt. results when the two operatons are combned,.e., the proposed method s appled. The crcular current s controlled to be a small value, and the qualty of the PCC voltage s guaranteed successfully. To test the senstvty of the proposed method to synchronzaton sgnal, a 0.2-s delay s ntentonally added to the synchronzaton sgnal receved by DG1 unt every tme. The assocated expermental results are shown n Fg. 23. Compared to the normal case, there s no obvous dfference between the two cases, and the reactve power sharng error can stll reduce to a small value. Therefore, the proposed method s robust to the communcaton delay because all the necessary operatons only need to be completed n an nterval, not a crtcal pont. Fg. 24 shows the expermental results when the synchronzaton sgnal of DG1 unt fals, whch s equvalent to the tme delay s nfnty. It s obvous that, before t = t 1, the crculatng current s kept to be a small value because the mproved droop control s n effect. After t = t 1, the sharng error reducton operaton and voltage recovery operaton are dsabled due to the lost of the synchronzaton sgnal of DG1 unt. As a result, the peak value of the crculatng current ncreases to about 2.8 A from a small value. In concluson, the results n Fgs. 23 and 24 ndcate that the proposed method only needs a low-bandwdth requrement, and t s robust to a small tme delay of communcaton. However, once communcaton fals completely, the reactve power sharng accuracy performance may be worse. [1] W. Deng, W. Pe, and Z. Q, Impact and mprovement of dstrbuted generaton on voltage qualty n mcro-grd, n Proc. 3rd Int. Conf. Electrc Utlty Deregulaton Restruct. Power Technol., Apr. 2008, pp [2] R. H. Lasseter, Mcrogrds, n Proc. IEEE Power Eng. Soc. Wnter Meetng, New York, NY, USA, 2002, pp [3] J. A. Peas Lopes, C. L. Morera, and A. G. Madurera, Defnng control strateges for mcrogrds slanded operaton, IEEE Trans. Power Syst., vol. 21, no. 2, pp , May [4] Y. W. L, D. M. Vlathgamuwa, and P. C. Loh, Desgn, analyss, and realtme testng of a controller for multbus mcrogrd system, IEEE Trans. Power Electron., vol. 19, no. 5, pp , Sep [5] Y. Mohamed and E. F. El-Saadany, Adaptve decentralzed droop controller to preserve power sharng stablty of paralleled nverters ndstrbuted generaton mcrogrds, IEEE Trans. Power Electron., vol. 23, no. 6, pp , Nov [6] J. M. Guerrero, J. C. Vásquez, J. Matas, M. Castlla, and L. G. Vcuña, Control strategy for fexble mcrogrd based on parallel lnenteractve UPS systems, IEEE Trans. Ind. Electron., vol. 56, no. 3, pp , Mar [7] J. M. Guerrero, J. C. Vásquez, J. Matas, L. Garca de Vcuña, and M. Castlla, Herarchcal control of droop-controlled AC and DC mcrogrds A general approach towards standardzaton, IEEE Trans. Ind. Electron., vol. 58, no. 1, pp , Jan [8] M. C. Chandorkar, D. M. Dvan, and R. Adapa, Control of parallel connected nverters n standalone AC supply systems, IEEE Trans. Ind. Appl., vol. 29, no. 1, pp , Jan./Feb [9] J. M. Guerrero, J. Matas, L. G. de Vcuña, M. Castlla, and J. Mret, Wreless-control strategy for parallel operaton of dstrbuted generaton nverters, IEEE Trans. Ind. Electron., vol. 53, no.5, pp ,Oct [10] K. De Brabandere, B. Bolsens, J. Van Den Keybus, A. Woyte, J. Dresen, and R. Belmans, A voltage and frequency droop control method for parallel nverters, IEEE Trans. Power Electron., vol.22,no.4,pp , Jul [11] J. M. Guerrero, L. G. Vcuña, J. Matas, M. Castlla, and J. Mret, Output mpedance desgn of parallel-connected UPS nverters wth wreless loadsharng control, IEEE Trans. Ind. Electron., vol. 52,no.4, pp , Aug [12] Y. W. L and C.-N. Kao, An accurate power control strategy for power-electroncs-nterfaced dstrbuted generaton unts operatng n a low-voltage multbus mcrogrd, IEEE Trans. Power Electron., vol. 24, no. 12, pp , Dec [13] J. M. Guerrero, J. Matas, L. Garca de Vcuna, M. Castlla, and J. Mret, Decentralzed control for parallel operaton of dstrbuted generaton nverters usng resstve output mpedance, IEEE Trans. Ind. Electron., vol. 54, no. 2, pp , Apr [14] J. M. Guerrero, L. G. Vcuña, J. Matas, M. Castlla, and J. Mret, A wreless controller to enhance dynamc performance of parallel nverters n dstrbuted generaton systems, IEEE Trans. Power Electron., vol. 19, no. 5, pp , Sep [15] A. Tuladhar, H. Jn, T. Unger, and K. Mauch, Control of parallel nverters n dstrbuted AC power systems wth consderaton of lne mpedance

9 HAN et al.: IMPROVED DROOP CONTROL STRATEGY FOR REACTIVE POWER SHARING IN ISLANDED MICROGRID 3141 effect, IEEE Trans. Ind. Appl., vol. 36, no. 1, pp , Jan./Feb [16] J. He and Y. W. L, An enhanced mcrogrd load demand sharng strategy, IEEE Trans. Power Electron., vol. 27, no. 9, pp , Sep [17] C. K. Sao and P. W. Lehn, Autonomous load sharng of voltage source converters, IEEE Trans. Power Del., vol.20,no.2,pp ,Aug [18] A. Moawwad, V. Khadkkar, J. L. Krtley, A new P-Q-V droop control method for an nterlne photovoltac (I-PV) power system, IEEE Trans. Power Del., vol. 28, no. 2, pp , Apr [19] E. A. A. Coelho, P. C. Cortzo, and P. F. D. Garca, Small-sgnal stablty for parallel-connected nverters n stand-alone AC supply systems, IEEE Trans. Ind. Appl., vol. 38, no. 2, pp , Mar./Apr [20] N. Pogaku, M. Prodanovć, and T. C. Green, Modelng, analyss and testng of autonomous operaton of an nverter-based mcrogrd, IEEE Trans. Power Electron., vol. 22, no. 2, pp , Mar [21] H. Mahmood, D. Mchaelson, and J. Jang, Accurate reactve power sharng n an slanded mcrogrd usng adaptve vrtual mpedances, IEEE Trans. Power Electron., vol. PP, no. 99, pp. 1, 1, Apr [22] Q.-C. Zhong, Robust droop controller for accurate proportonal load sharng among nverters operated n parallel, IEEE Trans. Ind. Electron., vol. 60, no. 4, pp , Apr [23] C. N. Rowe, T. J. Summers, R. E. Betz, D. J. Cornforth, and T. G. Moore, Arctan power frequency droop for mproved mcrogrd stablty, IEEE Trans. Power Electron., vol. 28, no. 8, pp , Aug [24] A. Luna, F. Blaabjerg, and P. Rodríguez, Control of power converters n AC mcrogrds, IEEE Trans. Power Electron., vol. 27, no. 11, pp , Nov Yao Sun (M 13) was born n Hunan, chna, n He receved the B.S., M.S., and Ph.D. degrees from the School of Informaton Scence and Engneerng, Central South Unversty, Changsha, Chna, n 2004, 2007, and 2010, respectvely. Hs research nterests nclude matrx converters, mcrogrds, and wnd energy converson systems. He has been an Assocate Professor wth the School of Informaton Scence and Engneerng, Central South Unversty. Me Su was born n Hunan, Chna, n She receved the B.S., M.S. and Ph.D. degrees from the School of Informaton Scence and Engneerng, Central South Unversty, Changsha, Chna, n 1989, 1992, and 2005, respectvely. Her research nterests nclude matrx converters, adjustable speed drves, and wnd energy converson systems. Snce 2006, she has been a Professor wth the School of Informaton Scence and Engneerng, Central South Unversty. Hua Han was born n Hunan, Chna, n She receved the M.S. and Ph.D. degrees from the School of Informaton Scence and Engneerng, Central South Unversty, Changsha, Chna, n 1998 and 2008, respectvely. Her research nterests nclude mcrogrds, renewable energy power generaton systems, and power electronc equpment. She was a vstng scholar at the Unversty of Central Florda, Orlando, FL, USA, from Aprl 2011 to Aprl She s currently an Assocate Professor wth the School of Informaton Scence and Engneerng, Central South Unversty. Yao Lu was born n Hunan, Chna, n He receved the B.S. and M.S. degrees from the Central South Unversty, Changsha, Chna, n 2011 and 2014, respectvely. Hs research nterests nclude renewable energy systems, dstrbuted generaton, and mcrogrd. Josep M. Guerrero (S 01 M 04 SM 08) receved the B.S. degree n telecommuncatons engneerng, the M.S. degree n electroncs engneerng, and the Ph.D. degree n power electroncs from the Techncal Unversty of Catalona, Barcelona, Span, n 1997, 2000 and 2003, respectvely. Snce 2011, he has been a Full Professor wth the Department of Energy Technology, Aalborg Unversty, Aalborg, Denmark, where he s responsble for the Mcrogrd Research Program. Snce 2012, he has been a Guest Professor at the Chnese Academy of Scence and the Nanjng Unversty of Aeronautcs and Astronautcs, Nanjng, Chna; and snce 2014 he has been a Char Professor n Shandong Unversty, Jnan, Chna. Hs research nterests s orented to dfferent mcrogrd aspects, ncludng power electroncs, dstrbuted energy-storage systems, herarchcal and cooperatve control, energy management systems, and optmzaton of mcrogrds and slanded mngrds. Dr. Guerrero s an Assocate Edtor of the IEEE TRANSACTIONS ON POWER ELECTRONICS, the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS,andthe IEEE INDUSTRIAL ELECTRONICS MAGAZINE, and an Edtor of the IEEE TRANS- ACTIONS ON SMART GRID. He has been a Guest Edtor of the IEEE TRANS- ACTIONS ON POWER ELECTRONICS SPECIAL ISSUES: POWER ELECTRONICS FOR WIND ENERGY CONVERSION AND POWER ELECTRONICS FOR MICROGRIDS; the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS SPECIAL SECTIONS: UN- INTERRUPTIBLE POWER SUPPLIES SYSTEMS, RENEWABLE ENERGY SYSTEMS, DISTRIBUTED GENERATION AND MICROGRIDS, AND INDUSTRIAL APPLICATIONS AND IMPLEMENTATION ISSUES OF THE KALMAN FILTER;andtheIEEETRANSAC- TIONS ON SMART GRID SPECIAL ISSUE ON SMART DC DISTRIBUTION SYSTEMS. He was the Char of the Renewable Energy Systems Techncal Commttee of the IEEE Industral Electroncs Socety. In 2014, he was receved as ISI Hghly Cted Researcher.