Review of Active and Reactive Power Sharing Strategies in Hierarchical Controlled Microgrids

Transcription

1 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc Revew of Actve and Reactve Power Sharng Stratege n Herarchcal Controlled Mcrogrd Yang Han, Member, I, Hong L, Pan Shen, rnane A.A. Coelho and Joep M. Guerrero, Fellow, I Abtract Mcrogrd cont of multple parallel-connected dtrbuted generaton (DG) unt wth coordnated control tratege, whch are able to operate n both grd-connected and landed mode. Mcrogrd are attractng more and more attenton nce they can allevate the tre of man tranmon ytem, reduce feeder loe, and mprove ytem power qualty. When the landed mcrogrd are concerned, t mportant to mantan ytem tablty and acheve load power harng among the multple parallel-connected DG unt. However, the poor actve and reactve power harng problem due to the nfluence of mpedance mmatch of the DG feeder and the dfferent ratng of the DG unt are nevtable when the conventonal droop control cheme adopted. Therefore, the adaptve/mproved droop control, network-baed control method and cot-baed droop cheme are compared and ummarzed n th paper for actve power harng. Moreover, nonlnear and unbalanced load could further affect the reactve power harng when regulatng the actve power, and t dffcult to hare the reactve power accurately only by ung the enhanced vrtual mpedance method. Therefore, the herarchcal control tratege are utlzed a upplement of the conventonal droop control and vrtual mpedance method. The mproved herarchcal control approache uch a the algorthm baed on graph theory, mult-agent ytem, the gan chedulng method and predctve control have been propoed to acheve proper reactve power harng for landed mcrogrd and elmnate the effect of the communcaton delay on herarchcal control. Fnally, the future reearch trend on landed mcrogrd are alo dcued n th paper. Index Term Actve power harng, reactve power harng, mcrogrd, graph theory, conenu control, herarchcal control, droop control, nonlnear and unbalanced load, communcaton delay. Abbrevaton CC CVS CCM CCVSI NOMNCLATUR Central control Controllable voltage ource Current control mode Current controlled voltage ource nverter Manucrpt receved Feb 0, 06; reved Aprl 0, 06; accepted May, 06. Date of current veron ; date of current veron. Th work wa upported n part by the Natonal Natural Scence Foundaton of Chna under Grant , and n part by the State Key Laboratory of Power Tranmon qupment & Sytem Securty and New Technology under Grant 007DA , and n part by the Open Reearch Subject of Schuan Provnce Key Laboratory of Power lectronc nergy-savng Technologe & qupment under Grant zjj05-067, and n part by the Open Reearch Subject of Artfcal Intellgence Key Laboratory of Schuan Provnce under Grant 05RZJ0, and n part by the Fundamental Reearch Fund of Central Unverte of Chna under Grant ZYGX05J087. Paper no. TPL-Reg Y. Han, H. L and P. Shen are wth the Department of Power lectronc, School of Mechatronc ngneerng, Unverty of lectronc Scence and Technology of Chna, No.006, Xyuan Avenue, Wet H-Tech Zone, Chengdu 673, Chna (e-mal: hanyang@uetc.edu.cn; L_Hong0@6.com; panhen0@6.com). rnane A.A. Coelho wth the Unverdade Federal de Uberlanda, Uberlanda , Brazl (ernane@ufu.br). J. M. Guerrero wth Department of nergy Technology, Aalborg Unverty, 90 Aalborg, Denmark (e-mal: joz@et.aau.dk). Color veron of one or more of the fgure n th paper are avalable onlne at Dgtal Object Identfer /TPL. DG MS LBC MAS MG MGCC MPPT PCC PI PLL PR PRPS PV RS SP VCM VSG Varable P Q V MG Δf ΔP ΔQ ΔV m n m Q n P ω f MG β ω β pω β,k Parameter ω f ref P max Q max R v S N T V ref V mn Dtrbuted generaton nergy management ytem Low bandwdth communcaton Mult-agent ytem Mcrogrd Mcrogrd central controller Maxmum power pont trackng Pont of common couplng Proportonal ntegral Phae-locked loop Proportonal reonant Proportonal reactve power harng Photovoltac Renewable energy ource Smth predctor Voltage control mode Vrtual ynchronou generator Output voltage ampltude of the mcrogrd Output voltage ampltude of the th DG Meaured averaged actve power through a low-pa flter Meaured averaged reactve power through a low-pa flter Voltage ampltude of the mcrogrd Frequency devaton Actve power devaton Reactve power devaton Voltage devaton Actve droop coeffcent n P-f droop control Reactve droop coeffcent n Q-V droop control Reactve droop coeffcent n Q-f droop control Actve droop coeffcent n P-V droop control Output angular frequency of the th DG Frequency of the mcrogrd Changeable ntegral gan cheduler Changeable proportonal gan cheduler Gan coeffcent Nomnal value of the DG output voltage ampltude Nomnal value of the DG angular frequency Frequency reference Maxmum value of the actve power Maxmum value of reactve power Retve vrtual mpedance Nomnal apparent power Samplng tme Voltage reference Mnmum value of voltage ampltude (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

2 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc W I. INTRODUCTION ITH the ncreaed penetraton of dtrbuted generaton (DG) unt on the electrcal grd ytem, the renewable energy ource (RS) ncludng mcro-turbne, fuel cell, photovoltac (PV) ytem and wnd energy ytem have been wdely ued n the dtrbuted power ytem n the pat decade [], []. The DG unt play an mportant role n reducng polluton, decreang power tranmon loe and mprovng local utlzaton of RS, whch become a trong upport for the large-cale power grd [3]. However, DG unt may alo brng challenge to the dtrbuton network uch a nvere power flow, voltage devaton and voltage fluctuaton. When a number of DG unt are clutered together, they can form a mcrogrd (MG) that olve the problem caued by hgh penetraton of DG unt uccefully and make the large-cale applcaton of DG ytem poble [4]. Fg. how the bac archtecture of an AC MG ytem. The PV ytem and energy torage ytem (SS) are connected to the AC bu through the DC/DC/AC converter and wnd turbne are ted to the AC bu through the AC/DC/AC converter. In the cae of landng operaton, RS manly provde AC power to the load through the local control. In the grd-connected mode, the AC MG connected to the uptream grd through a te lne at the pont of common couplng (PCC) and there power flow between MG ytem and the grd [5-9]. In order to enure tablty and economcal operaton of MG, the actve and reactve power of the DG unt hould be hared multaneouly. The droop control are the well developed control method wthout communcaton lne and can be ued to acheve the power harng by mtatng teady tate charactertc of the ynchronou generator (SG) n landed MG [0-3]. A new control method called vrtual ynchronou generator (VSG) ha been propoed to mmc the teady-tate and tranent charactertc by ung the wng equaton. Although the nerta of the DG unt can be enhanced compared to the droop control, the output actve power of VSG ocllatory and dynamc power harng among the DG unt would be luggh due to the vrtual nerta, epecally under weak mcrogrd condton [4-0]. Therefore, the utlzaton of the mproved droop control method tll popular to hare the actve and reactve power among the DG unt n landed MG [-5]. To enure the tate optmzaton of a complex MG, the accuracy and dynamc tablty of actve power harng hould be condered. A tatc droop compenator utlzed for actve power harng n [6]. An enhanced droop control featurng a tranent droop performance propoed n [7]. To mprove the actve and reactve power decouplng performance, mproved droop controller wth vrtual output mpedance are reported [8]. However, the low-frequency dynamc of the nverter due to the tme-cale eparaton between the power, voltage, and current dynamc are not mproved n [4-8]. Therefore, an optmzed droop control preented n [9] to mprove the dynamc tablty of the actve power harng and an adaptve decentralzed droop-baed power harng control cheme preented n [30] to adjut the dynamc performance of the power harng wthout affectng the tatc droop gan. Moreover, n order to hare actve power under complex load condton, an algorthm-baed actve power regulaton trategy propoed n [3], and a herarchcal actve power management trategy preented n [3]. Although the actve power harng acheved and the dynamc repone of the mcrogrd enured, the complex feeder mpedance and generaton cot of the mcrogrd are not condered. In [33], a hgh dturbance rejecton performance agant voltage dturbance acheved when harng the actve power and ome mproved P-V and Q-f droop control method are preented n [34] to hare the actve power under retve/unknown feeder mpedance condton. A an economc problem ntroduced n herarchcal control, the crteron for actve power hould be baed on generaton cot of the mcrogrd ntead of a mple proportonal or equvalent relaton baed on the generator ratng. A nonlnear cot baed cheme whch propoed n [35] and a lnear cot prortzed droop cheme preented n [36] both can optmze actve power harng and multaneouly mnmze the total cot of generaton. Communcatn Flow Power Flow Hgher Control Level Secondary Control Tertary Control PV Array DC DC DC AC Flter Actve and Reactve Power Current/ Voltage/ Frequency Wnd Turbne AC DC DC AC Local Control Level Flter Actve and Reactve Power Current/ Voltage/ Frequency Load nergy ecurty when mcrogrd wtche to landed mode nergy Supply Grd Dtrbuted nergy Storage Fg.. Archtecture of the AC mcrogrd [5]. Power Converton Sytem Common AC Bu PCC (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

3 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc When all DG unt operate at the ame frequency n the teady-tate condton, the actve power can be regulated well n mproved droop control cheme, but reactve power harng tll poor and harmonc power wll appear n DG unt under unequal feeder mpedance and nonlnear load condton [37]. Under extreme tuaton, the poor reactve power harng may reult n evere crculatng reactve power among the DG unt and may caue ytem ntablty [38]. To hare the reactve power, varou droop control method have been propoed, whch nclude three man categore: the mproved prmary droop control method [8], [0], [39-4], [70], [7], the mproved vrtual mpedance method [4-5] and the mproved herarchcal control tratege [63-66], [7]-[79]. An adaptve voltage droop control preented n [39] to hare the reactve power. The effect of the mmatched feeder mpedance compenated by the adaptve droop coeffcent and a reactve power harng can be hared. The method mmune to the communcaton delay, but the nonlnear and unbalanced load are not condered. An enhanced control trategy preented n [40] to hare the reactve power accurately, where the actve power dturbance adopted to dentfy the error of reactve power harng and t elmnated by ung a low ntegral term. Unfortunately, the gnal njecton method deterorate the power qualty and affect the ytem tablty. To regulate the unbalance power and the reactve power, an adaptve nvere control wth the mproved droop control algorthm propoed to adjut the weght coeffcent of dgtal flter n real tme [4]. However, the reactve power harng of landed MG mght be poor under unbalanced and nonlnear load condton. It dffcult to hare the reactve power accurately under the mmatched feeder mpedance, and nonlnear and unbalanced load condton by the mproved droop control. A a upplement of the mproved droop control, the method baed on the vrtual mpedance or mproved vrtual mpedance, have been propoed to hare the actve and reactve power [4-49]. Although the nductve vrtual mpedance can enhance the capacty of the reactve power harng under the mmatched feeder mpedance condton, the reactve power cannot be hared accurately when the load are nonlnear and unbalanced n landed MG. The energy management ytem (MS) utlzed n [50] to allocate the reactve power to DG unt accordng to ther own capacte, the total demand of load and the adaptve coeffcent, but the adaptve coeffcent are dffcult to be obtaned. In [5], an enhanced vrtual mpedance control trategy ha been propoed to hare the reactve power n the landed MG wth the mmatched feeder mpedance, where vrtual mpedance are ued at the fundamental and harmonc frequence. However, a poor reactve power harng may occur n a three-phae converter wth nonlnear load. In [5], the control trategy baed on vrtual retance preented to hare the reactve power under mmatched feeder mpedance, and nonlnear and unbalanced load condton. However, f the feeder or load dconnected, the MG ytem would be untable, whch lmt t practcal applcaton. Snce t hard to acheve the reactve power harng by mproved vrtual mpedance method n complex MG ytem, the herarchcal control trategy ha been propoed to guarantee the teady-tate and dynamc performance of the current harng. Moreover, the redundancy of MG mproved, and the mcrogrd frequency and voltage ampltude can be retored to the rated value whle harng the actve and reactve power. Specfcally, the herarchcal control tructure of MG dvded nto three layer a ndcated n Fg. [53], [54]. ) Prmary Level: The prmary control focue on the tablty of voltage and frequency. The droop control appled n th level to acheve the actve and reactve power harng wthout ung communcaton channel. ) Secondary Level: The econdary control perform the functon to elmnate the frequency and voltage devaton caued by the droop control, whch nclude power flow control of the nterconnecton lne. 3)Tertary Level: The tertary control deal wth economc dpatchng, operaton chedulng, and power flow between the MG and grd by regulatng the voltage and frequency of the grd-connected MG, and adjutng the power generaton n real tme. The energy torage and energy management ytem are alo requred for the MG to enure a mooth tranton between landed and grd-connected mode [55], [56]. To realze a mooth tranton between grd-connected and landed mode, ome lterature avod the tertary control n ther control tratege [57-6]. A flexble control method for landed/grd-connected MG wth enhanced tablty preented n [60], where only local nformaton ued n controller to yeld better relablty of the MG and make the ytem table over a wde range of operaton condton wth mnmum tranent. Conderng the cae of the unplanned landed mcrogrd, a mult-mater control method wth econdary frequency control preented n [6] to modfy the local generaton profle of the MG to reduce the mbalance between local load and power generaton, and reduce the dconnecton tranent [6]. However, the herarchcal control tratege are often ued to realze the eamle mode tranfer n complex mcrogrd, and the detal of uch control method are out of th paper nce th paper manly focue on the actve and reactve power harng n landed mcrogrd. To hare the reactve power by the adaptve droop control and retore voltage ampltude and frequency to the rated value by the econdary control, a elf-adjutng trategy baed on herarchcal control preented [63]. Moreover, a control method whch combne the mcrogrd central controller (MGCC) and droop control preented n [64] to hare the reactve power. The MGCC utlzed to calculate the averaged reactve power and regulate reactve power reference to the correpondng DG unt. Actually, the phycal mode of the MG are complex and the reactve power can be erouly affected by the communcaton delay. To hare the actve and reactve power, the dtrbuted trategy whch ntegrate the current control mode (CCM) and voltage control mode (VCM) unt preented n [65]. The droop and revere droop control are added to the CCM and VCM controller to regulate the reactve power adaptvely. Moreover, the dynamc control method preented n [66] to enure the reactve power harng and prevent the voltage well/collape ahead of tme. Mot of the extng work dcu the control and power management for landed MG whle the power harng problem wth the mmatched feeder mpedance and nonlnear load are eldom fully condered [67-69]. In [70], the potve-equence power ued to generate the voltage reference and the negatve-equence reactve power ued for the voltage unbalance compenaton, whch realze the load power harng. An enhanced power harng method propoed n [7] to hare the reactve power of the landed MG, where the frequency droop utlzed to compenate reactve, unbalance and harmonc power harng error. Wth the nteracton between the frequency droop control and the varable vrtual mpedance n the MG, the unknown feeder mpedance can be compenated and an accurate reactve power harng acheved n the teady tate. Wth the further reearch on MG, the mmatch of the DG feeder mpedance and nonlnear and unbalanced load uppled by MG and communcaton delay n the (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

4 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc Control tratege for the reactve power harng Improved prmary droop control Improved vrtual mpedance method Improved herarchcal control TABL I. Advantage and Dadvantage of the Dfferent Control Stratege for the Reactve Power Sharng of Ilanded Mcrogrd Major technologe Advantage Dadvantage Optmzed droop equaton [8], [0], [39-4], [70], [7] Adaptve/nhanced vrtual mpedance [4-5]. Optmzed econdary control [63-66], [75]. Algorthm baed on graph theory [7-74]. Mult-agent ytem [76] low bandwdth communcaton (LBC) lne how that the control tratege for the accurate reactve power harng tll need mprovement. Recently, t popular to mtate the phycal tructure of MG by the graph theory and then optmze the control tratege ung the algebrac algorthm [7]. An optmzed algorthm baed on graph theory preented n [73] to acheve the reactve power harng under the mmatched feeder mpedance condton. In [74], the programmng algorthm preented to enure the afety of the equpment and acheve a prece reactve power harng multaneouly. The tochatc reactve power management trategy preented n [75] and the uncertan actve power njecton are utlzed to obtan an onlne control method for the reactve power. Note that th trategy fully dtrbuted and only the data of actve power njecton are requred. Conderng that the uncontrollable RS entve to the outde envronment, an agent-baed method preented n [76] to tablze the actve and reactve power. The advantage and dadvantage of dfferent control tratege for the reactve power harng are ummarzed n Table I. Snce the communcaton delay alway ext n herarchcal control, the output correcton gnal ent to prmary control need a tme delay owng to the communcaton lne, whch wll caue damage to mcrogrd ytem. To acheve a better actve and reactve power harng, the communcaton delay caued by the low bandwdth communcaton lne need to be condered. A gan cheduler method n [77] utlzed to adjut the reference gnal from the econdary control and decreae the nfluence on low bandwdth communcaton delay. In [78], th nfluence mnmzed by ung the predctve control cheme a well. Moreover, a cooperatve dtrbuted econdary/prmary control paradgm ued to realze the reactve power harng by conderng the communcaton delay for the MG [79]. The ret of the paper organzed a follow: Secton II analyze the hortcomng of the conventonal droop control cheme for actve power harng, and ummarze the varou actve power harng tratege conderng the effect caued by feeder mpedance, generaton cot of MG. In addton, the drawback of the conventonal econdary control method and the necete for harng the reactve power are analyzed n Secton III, and conventonal herarchcal reactve power harng tratege n landed MG are preented. Secton IV preent the varou method for reactve power harng under the mmatched feeder mpedance and changeable envronmental condton, whch nclude the algorthm baed on graph theory, programmng and mult-agent ytem. Bede, method for reactve power harng under the mmatched feeder mpedance, nonlnear and unbalanced load condton are revewed n Secton V. Secton VI preent No communcaton lne Hgh relablty Hgh redundancy Good performance for the reactve power harng Sutable for nonlnear and unbalanced load No hgh bandwdth requrement Retore the voltage and frequency to rated value Smplfy complex model of MG Share the reactve power wth mmatch feeder mpedance Need complex algorthm Not utable for complex load Not utable for complex MG The adaptve coeffcent dffcult to be obtaned It not eay to degn a hgh effcency algorthm Communcaton delay n low bandwdth lne Poor reactve power harng under nonlnear/ unbalanced load condton The algorthm are complex P Q P Q (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton. I R +jx Z R jx ZL RL jx L L L L feeder lne feeder lne + V PCC Load DG DG I R +jx Fg.. The equvalent chematc of two parallel-dg n an landed mcrogrd [50]. predctve control and cooperatve dtrbuted control to decreae the effect of LBC delay. The future trend of MG are ummarzed n Secton VII. Fnally, th paper concluded n Secton VIII. II. CONTROL STRATGIS FOR ACTIV POWR SHARING PROBLM It mportant to mprove the tablty of DG unt and acheve the load power harng n landed MG. The actve power uually condered to be hared n a decentralzed manner when the droop coeffcent adopted reaonably. However, there are tll ome hortcomng for actve power harng n the conventonal droop control tratege [80-85]. A. Problem of the Actve Power Sharng n the Droop Control Generally, for a large/medum ytem, the mpedance approxmately nductve and the power-frequency (P-f) and reactve power-voltage (Q-V) droop control are alway ued [-3]. The P-f and Q-V droop control can be determned a [-3], [8], [4]: mp, nq () where ndex repreentng each converter, ω and are rated angular frequency and voltage ampltude of converter, repectvely. P and Q are meaured average actve and reactve power value through a low-pa flter, repectvely. m and n are actve and reactve droop coeffcent, repectvely. The equvalent crcut of two parallel-dg unt hown n Fg.. Z L and Z L are feeder mpedance of lne and lne, repectvely. X (X, X L, X and X L) and R (R, R L, R and R L) are the reactance and retance value of feeder mpedance, repectvely. δ repreent the voltage of DG, and δ the phae angle dfference between and V PCC ( repreent the th DG). The output actve power and reactve power for DG can be obtaned a [8], [4], [80], [8], [86]:

5 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc P Q ( X X L )( V PCC co VPCC ) ( R RL ) V PCC n ( X X L ) ( R RL ) ( X X L )( V PCC co VPCC ) ( R RL ) V PCC n ( X X L ) ( R RL ). (). (3) In addton, the power angle δ mall and t can be aumed that nδ =δ, coδ =. Moreover, when the reactance much larger than the retance of the feeder mpedance, () and (3) can be mplfed: P V PCC, X X L VPCC ( VPCC ) Q. (4) X X When the feeder mpedance approxmately nductve (retance neglgble), the actve power can be hared when the droop coeffcent adopted reaonably, but ome hortcomng for actve power harng are nevtable n the conventonal droop control [53-56], [80-85], [0-03]. ) For the lmted range of frequency devaton, the droop coeffcent ha to be mall, whch volet harng actve power. Although a larger droop coeffcent can mprove actve power harng performance, t would reult n a hgher voltage devaton from the nomnal value [33-36]. ) Only the equvalent actve power harng can be guaranteed n the conventonal droop control under nductve feeder mpedance cenaro. However, actve power harng accuracy may be compromed, and actve and reactve power couplng may ext n the retve network. Bede, the proportonal actve power harng cannot be acheved [0-03]. 3) A dfferent type of DG may ext, the conventonal droop control cannot reduce the generaton cot for the condered MG. Furthermore, the tranton between a grd-connected and an landed mcrogrd mode yeld a large-gnal dturbance and the dynamc tablty of the actve power harng affected [35], [36]. Therefore, the droop control for actve power harng hould be further mproved to get an accurate and robut actve power harng for MG, and the detal and charactertc of varou control method wll be dcued heren. B. quvalent Actve Power Sharng under Inductve Feeder Impedance Condton In order to get hgh dturbance rejecton performance of the actve power harng controller agant voltage dturbance and elmnate voltage and frequency devaton, an adaptve droop control preented n [33] wth the followng droop functon: dp dq ˆ m P m, ˆ d nq n (5) d dt dt where and nˆd are adaptve gan. In th adaptve droop control, the dynamc performance of the actve power harng can be adjuted wthout affectng the teady-tate regulaton requrement. The adaptve droop control hown n (5) can enhance the relablty of mcrogrd, but the dynamc tablty of the actve power harng under dfferent mcrogrd operatng condton are not condered. An optmzed actve power harng trategy baed on performance functon preented n [9] to mprove the dynamc tablty of actve power harng under dfferent mcrogrd topologe. A quadratc performance ndex J condered to fnd the optmum tranent droop parameter m d and maxmze the mcrogrd tablty under dfferent operatng condton wth the followng expreon: ˆ d m n l [ ( )] (6) k J kt k where ω(k) repreent the frequency error at the tme k for DG, T the amplng tme, l the total number of ample, and n the L total number of DG unt n an landed mcrogrd. In (6), the frequency error weghted by the repectve tme k, whch enure optmzed gan tunng under dfferent operatng condton. Combnng the partcle-warm optmzaton technque n [87], the robut and flexble mcrogrd operaton wth eamle tranfer n the tranton mode can be obtaned wth optmzed dynamc power harng performance. C. Improved P-V/Q-f Droop Control under Retve Feeder Impedance Condton The actve power harng accuracy may be compromed by the conventonal P-f and Q-V droop control under retve network. Before ung the conventonal P-f and Q-V droop cheme wth retve network, the decouplng technque uch a performng lnear tranformaton and nertng vrtual mpedance are preented to olve th problem [88-90]. Moreover, P-V and Q-f droop control tratege are often ued to acheve equvalent actve power harng under retve feeder mpedance condton [9], [9], and the tranfer functon of droop equaton are denoted a: m Q, n P (7) Q P where n P and m Q are the actve and reactve droop coeffcent n P-V and Q-f droop control, repectvely. However, many problem cannot be olved by ung the conventonal P-V and Q-f droop control, uch a lne mpedance dependency, naccurate actve power or reactve power regulaton and low tranent repone [93], [94]. In [46], the mproved P-V and Q-f droop control wrtten: m ( P Q ), n ( P Q ). (8) quaton (8) how that the mproved P/V and Q/f droop control can mplfy the coupled actve and reactve power relatonhp, and a good dynamc performance can be acheved n cae of retve network. Moreover, except for ntroducng dervatve control nto the droop method [33], [95], [96], an enhanced retve droop method (RDM) propoed to guarantee the voltage regulaton and enhance power harng performance [97], whch can be obtaned a: ( m md) P, n np nd Q (9) where n P another reactve power droop gan, and m D and n D are the actve and reactve dervatve droop coeffcent, repectvely. The enhanced RDM adopted to elmnate the nherent contradcton between voltage regulaton and power harng performance, and the tablty of mcrogrd can be mproved under retve feeder mpedance condton. D. Actve Power Sharng Stratege under Unknown Feeder Impedance Condton In many extng lterature, the networked-baed actve power harng tratege are propoed [98], [99]. However, there are two major drawback: ) The frequency drop cannot be elmnated due to the preence of the frequency and voltage droop loop. ) The communcaton delay would ncreae ytem entvty under parameter uncertante [99]. An mproved networked-baed power harng trategy preented n [34] to hare actve power under unknown mpedance condton and the control functon n tme-doman can be obtaned: m DG () t nlt P Ptot P KP (0) where ω nl the frequency when DG operate at no load condton, γ P the dered hare of the actve power generated by the (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton. DG. P the total average actve power. tot K the addtonal actve power harng controller gan, and δ DG the command angle P

6 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc of DG. The dtrbuted power regulator are located at each DG unt to obtan the delay-free local power meaurement. Note that the mproved control trategy can acheve the equvalent actve power harng whle mantanng the teady-tate frequency contant. Bede, th method mprove dynamc performance of MG and mnmze actve power harng error under unknown lne mpedance, and the hgh relablty and robutne of the MG ytem can be acheved agant network falure.. Proportonal Actve Power Sharng Stratege Baed on Nonlnear Cot Functon A common varable-baed proportonal actve power harng trategy propoed n [00] for nverter wth retve output mpedance, whch modfed a: Ke Vcom Kq P dt () where K e and K q are ntegral gan and V com the common voltage. Although th control trategy can acheve proportonal load harng and be robut to the ytem parameter varaton, t need the load voltage nformaton and the common voltage may not ext n complex mcrogrd. Bede, the crteron for power harng hould no longer be a mple proportonal relaton baed on the generator ratng when economcal dpatchng ntroduced n herarchcal control [47], [53], [98]. Several nonlnear cot-baed droop cheme have been preented by ung a ngle econd-order reference cot functon for formulatng frequency and voltage offet added to the conventonal droop equaton [0], and a cot-baed droop cheme preented n [0] to realze actve power harng conderng reducng the generaton cot of the mcrogrd. An optmal power harng trategy preented n [35] to guarantee the proportonal power harng and ncreae actve power generaton of DG unt, and decreae the generaton cot of the mcrogrd. The generaton cot for the DG can be approxmated a: C ( P ) P P () where α, β and γ are the gan parameter. Combnng the auxlary controller, mp P j j mp P can be obtaned n fnte tme and the mnmal total cot of generaton can be acheved whle atfyng Actve power harng method quvalent actve power harng wth nductve feeder mpedance P-V/Q-f droop control wth retve feeder mpedance Actve power harng tratege under unknown mpedance condton Proportonal actve power harng tratege TABL II. Advantage and Dadvantage of Dfferent Actve Power Sharng Stratege ytem actve power balance requrement. Note that the whole ytem fully dtrbuted and the dynamc performance of the econdary controller can be guaranteed. F. Proportonal Actve Power Sharng Baed on Lnear Cot Functon Compared wth the extng nonlnear cot-baed cheme, the control cheme wth lnear droop functon can be ued to optmze the total generaton cot. When hgh-cot of load ext n DG unt, a lnear cot-prortzed droop cheme preented n [03] to reduce actve power harng. In addton, an mproved lnear power harng cot-baed cheme for DG unt are preented n [36] to reduce the total generaton cot of the autonomou mcrogrd. The cot avng realzed by tunng the DG droop gradent n accordance to ther repectve maxmum generaton cot, and the actve power harng mplemented eaer wth reduced cot. A lnear cot-baed droop cheme gven n (3) and (4). fmax f mn, fref, fmax P (3) P max, fmax fmn fmn, fmax (4) max( C, C, C,, C ) C max, max, max,3 max, max, where f mn, repreent the mnmum frequency of DG, f mn and f max repreent the maxmum and mnmum frequency, repectvely. C max, repreent the maxmum cot ncurred by DG, and max(c max,) a functon that return the maxmum cot among all DG unt n the mcrogrd. The prncple of the dervaton of maxmum cot-baed lnear droop cheme utlze the DG maxmum generaton cot to dfferentate them on the droop plot o that the leat cotly DG wll have hgher power generaton. Therefore, the actve power harng can be acheved whle reducng the total generaton cot of MG autonomouly wthout compromng the flexblty of a lnear droop mplementaton. The advantage and dadvantage of the varou method for actve power harng n MG ytem are ummarzed n Table II. Major technologe Advantage Dadvantage Adaptve droop control [33]. Optmzed droop control [9], [87] Decouplng technque [88-90]. Improved P-V and Q-f droop control [46], [9], [9]. nhanced RDM [95-97] Networked-baed actve power harng cheme [34], [98], [99] A common varable-baed actve power harng trategy [00] Nonlnear cot-baed droop cheme [35], [0], [0] Lnear cot functon [36], [03] Acheve equvalent actve power harng lmnate voltage and frequency devaton Hgh dturbance rejecton performance Improve the dynamc tablty of actve power harng Improve tranent repone Improve nherent contradcton between voltage and power harng Improve the tablty of mcrogrd Improve dynamc performance of mcrogrd Improved actve power harng under unknown lne mpedance Hgh robutne on communcaton delay Acheve proportonal load harng lmnate voltage and frequency devaton Robut to the ytem parameter varaton Share actve power lmnate voltage and frequency devaton Mnmze total cot of generaton Share actve power Reduce the total generaton cot of MG eaer and autonomouly lmnate voltage and frequency devaton Not utable for multple DG unt Not conderng total cot of generaton Proportonal actve power harng not acheved Not utable for complex feeder mpedance Not utable for complex MG Not conderng total cot of generaton Proportonal actve power harng not acheved Not conderng total cot of generaton Proportonal actve power harng not acheved Sentve to communcaton delay Not utable for complex MG Not conderng total cot of generaton Cot functon dffcult to be computed Not utable for complex feeder mpedance condton Not utable for complex MG Be entve to communcaton delay Not utable for complex MG Not utable for complex feeder mpedance (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

7 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc III. BACKGROUND OF TH RACTIV POWR SHARING IN ISLANDD MICROGRIDS A dcued n ecton II, P-V and Q-f droop control are uually appled n a mall ytem where the feeder mpedance more retve, whle P-f and Q-V droop control are ued n a medum or large ytem where the feeder mpedance approxmately nductve [47]. In th paper, the droop control (P/f, Q/V) for landed MG are dcued to evaluate the performance of reactve power harng. A. Problem of the Reactve Power Sharng n the Droop Control In the conventonal droop control, by combnng () and (4), Q can be obtaned a: VPCC ( VPCC ) Q (5) X X nv L PCC where the reactve power of the DG related to the feeder mpedance, PCC voltage and reactve droop coeffcent. It can be deduced from (5), although the two DG unt (n Fg. ) have the ame capacty and reactve power droop coeffcent, the reactve power of the DG can alo be maller than DG under a mmatched feeder mpedance condton (X >X ) [8]. Fg. 3 how the voltage devaton problem of the reactve power harng n conventonal droop control method. reference voltage and larger than n Fg. 3. When the reactve droop coeffcent n, DG and DG operate at, whle DG and DG operate at when the reactve droop coeffcent n. A and B ndcate that the reactve power of DG Q (droop coeffcent n ) and Q (droop coeffcent n ), repectvely. C and D ndcate that the reactve power of DG Q (droop coeffcent n ) and Q (droop coeffcent n ), repectvely. The reactve power dfference of DG and DG ΔQ (Q - Q ) when they operate at, and the dfference ΔQ (Q - Q ) when they operate at. Although Δ maller than Δ, ΔQ larger than ΔQ (when n >n ). Therefore, the reactve power devaton can be reduced by ncreang the droop coeffcent, but t wll caue a large voltage devaton n the teady tate [8], [84], [04]. B. Problem of Reactve Power Sharng n the Secondary Control In order to olve the problem caued by the conventonal droop control, a econdary control ued to elmnate the frequency and voltage devaton [05], [06]. Fg. 4 how the clacal econdary control cheme for the two parallel-dg n landed MG. The output frequency (f MG) and voltage (V MG) of the MG are compared wth the frequency and voltage reference, repectvely. The frequency/voltage devaton (Δf/ΔV) then adjuted through proportonal-ntegral (PI) controller. The adjuted frequency and voltage of the MGCC are ent to the prmary and nner control loop through a communcaton lne to regulate the ntal voltage and frequency reference [05]. The reactve power harng poor when the voltage regulated by ung the conventonal econdary control. When two dentcal DG are connected to a common dtrbuton bu, a hown n Fg. 4, the two feeder reactance are dfferent (X > X ). Bede, a phae-locked loop (PLL) needed to calculate V MG and f MG from the meaured voltage at PCC. The -Q droop charactertc wth and wthout a conventonal econdary control are depcted n Fg. 5 [73]. The blue/green dahed lne the econdary control curve for DG /DG and the black old lne the conventonal droop control curve. In Fg. 5(a), A (Q, ) and C (Q, ) repreent the output voltage of DG wth the njecton of reactve power Q and the voltage of DG wth Q n the conventonal droop control, repectvely. B (Q, ) and D (Q, ) repreent the output reactve power of DG Q and DG Q when the voltage retored to the rated value n the conventonal econdary control. However, the reactve power devaton between DG and DG ncreae (Q <Q <Q <Q ). The tuaton n Fg. 5(b) can be obtaned by one of the cheme preented n next ecton. A hown n Fg. 5(b), when the reactve power regulated a Q =Q =Q (a pecal tuaton of proportonal reactve power harng) n the conventonal econdary control, B (Q, ) and D (Q, ) are the output voltage of DG ( ) and DG ( ), repectvely. However, the voltage of DG and DG cannot be retored to the rated value and the voltage dfference larger compared to the prmary control ( < < < ). Therefore, the conventonal econdary control cannot regulate the voltage accurately whle harng the reactve power equally or proportonally [73] (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton. Q Q droop coeffcent: n droop coeffcent: n Q Q C D B Q Q A DG DG DG DG Fg. 3. Charactertc curve of the reactve power droop control wth two DG [04]. f MG G pf Prmary control I L Inner control Loop V dc f f V C f ref Gf MGCC V ref X X G pv G Communcaton Lnk C C PCC v V V MG V Prmary control Inner control Loop V PCC V PCC L I I lne lne L V C I L Q Vdc Fg. 4. The conventonal econdary control for two parallel-dg n the MG operatng n landed mode [05].

8 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc B Prmary Control A D Conventonal Droop(DG & DG ) Secondary Droop (DG ) Secondary Droop (DG ) Secondary Control DG DC Source LBC LCL flter LBC Z 4 LCL flter Load Load LBC Z34 DG 4 DC Source Q Q C Q (a) Q Q DG DC Source LCL flter LCL flter DG 3 Z DC Source LBC Z 3 A Q Prmary Control B D C Q Q Conventonal Droop(DG & DG ) Secondary Droop (DG ) Secondary Droop (DG ) Secondary Control (b) Fg. 5. Dagram of the econdary control for the landed mcrogrd. (a) -Q droop curve when the voltage retored to the rated value. (b) -Q droop curve when the reactve power harng acheved (X >X ) [73]. C. Conventonal Reactve Power Sharng Stratege n the Secondary Control It well-known that the reactve power harng poor under the mmatched feeder mpedance and complex load condton n the conventonal droop control. To olve the problem of reactve power harng n the econdary control, ome control tratege have been uggeted by many lterature [6], [63], [7], [07-]. A decentralzed elf-adjutng control trategy for reactve power management preented n [63] to prevent reactve power crculaton among the DG unt under unequal feeder mpedance condton. A vrtual nductve mpedance loop utlzed to enhance the teady-tate precon and tranent repone for reactve power harng. Although the complex load condton are not condered, t ha preented an dea to hare the reactve power by combnng adaptve control trategy and vrtual mpedance. In [07], a robut nonlnear dtrbuted controller preented to mantan the tablty of the actve and reactve power, and enure fater repone when MG operate on dfferent condton (three-phae hort-crcut fault, load change, etc.). Combnng wth the genetc algorthm, an mproved vrtual mpedance controller utlzed to mnmze the global reactve power harng error [08], and gve a good drecton to degn algorthm by utlzng the knowledge on computer cence, mathematcal cence, etc. The reactve power harng and voltage retoraton method preented n [09], whch employ both conenu control and adaptve vrtual mpedance control for landed MG under mmatched feeder mpedance. Moreover, a conenu-baed dtrbuted voltage control for reactve power harng preented n [7] to guarantee the dered reactve power dtrbuton n the teady tate, and how that the dtrbuted regulaton le entve to the falure of communcaton lnk. Therefore, when a pare communcaton tructure ued, mproved tablty and relablty of complex MG ytem can be enured [7], [09]. In ummary, frequency and voltage tablty, and accurate reactve power harng are all mportant performance crtera n MG Q x x x 3 x (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton LBC (a) A (b) (c) Fg. 6. The phycal model and adjacency matrx for four DG unt baed landed MG. (a) The phycal model of the mcrogrd wth four DG unt. (b) The graph repreentaton of the phycal model. (c) The adjacency matrx of the graph [74]. ytem [59], [0-]. Therefore, the conventonal herarchcal control tratege need to be mproved, n order to hare the actve and reactve power n complex MG, and the cheme to realze the reactve power harng n the mproved herarchcal control tratege are analyzed n detal n the forthcomng ecton. IV. RACTIV POWR SHARING WITH MISMATCHD FDR IMPDANC In the conventonal econdary control, the reactve power harng cannot be acheved when the voltage ampltude and frequency are retored to the rated value n landed MG wth mmatched feeder mpedance. Currently, the mot popular method to olve thee problem can be ummarzed nto three man categore: optmzed the econdary control equaton [7], [3-7], programmng algorthm [9], [74], [-3] and mult-agent ytem (MAS) algorthm [7], [76], [3], [33]. A. Control Method Baed on Graph Theory ). Graph Theory Conderng the theory of weghted graph n dcrete mathematc, a mult-agent cyber-phycal ytem can be expreed wth a graph from the perpectve of control method, where agent (ource) are modeled a node of the graph and communcaton lnk are mapped to edge connectng node [74], [79]. The model of MG hown n Fg. 6 (a) and t equvalent weghted graph n Fg. 6 (b). The DG repreented by node x n Fg. 6 (b) and the edge lne (double arrow n Fg. 6 (b)) ndcate that communcaton lnk among the DG unt. Fg. 6(c) how that the adjacency matrx Α NN j R the n n weghted adjacency matrx of the graph wth element α j=α j 0. The weght of edge denoted by (υ j, υ ), and α j=0 when there no communcaton between DG and DG j. Note that the adjacency matrx A uually a pare matrx and only low bandwdth communcaton needed n the herarchcal control algorthm [3], [4].

9 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc NN In addton, the n-degree matrx D dag R defned a a dagonal matrx wth d jn j and the correpondng Laplacan matrx defned a L=D A. A graph called to have a pannng tree, when all node have a drected path to a root node. Moreover, a graph carre the mnmum redundancy f t contan uffcent redundant lnk, and the graph wll reman connected and preent a balanced Laplacan matrx, where any ndvdual lnk fal [5], [6]. Snce complex model of MG analyzed by the graph theory, the control algorthm can be mproved effectvely by ung A, D, L and other reaonable parameter. Note that an equaton and two theorem are alway ued n the control algorthm baed on the graph theory: x u aj ( x j x ) (6) jn where any calar x mut atfy the prncple of dtrbuted averagng (conenu) n the contnuou tme. In addton, the two mportant theorem n the graph theory are decrbed a [7]: Theorem I: If there a pannng tree n the communcaton graph, conenu control can be reached and the Laplacan matrx L ha a mple zero egenvalue and all the other egenvalue have potve real part. Theorem II: If there a pannng tree n the communcaton graph and a root node atfyng, all agent tate wll converge to the external control gnal v. Theorem I and II are often ued n the conenu control to analyze the tablty of MG ytem [7]. Moreover, almot all the algorthm baed on graph theory atfy (6) to enure the elected varable to be equal to a known parameter n the teady tate. B Accordng to (6), t aumed that x j equal to ω and x equal to ω, and the angular frequency of MG equal to the nomnal angular frequency (ω =ω ) n the teady tate. ). Reactve Power Sharng wth Mmatched Feeder Impedance Baed on Graph Theory. The dtrbuted average proportonal ntegral (DAPI) control method baed on the graph theory preented n [73] to hare the reactve power. The optmzed econdary control equaton can be decrbed by (7)-(9), where the Ω /e only an ntegral term n the conventonal econdary control method. mp, n Q e (7) d k (8) n ( ) j ( dt j j ) de Q Q k b (9) n j ( ) j ( ) dt j Q Qj where gan coeffcent β and k are all potve, and defne an n-order matrx B whch compoed of b j (b j>0). In the DAPI control cheme, the conventonal current and voltage control loop, actve and reactve power droop control loop and vrtual mpedance loop are all appled n the prmary control [8], [85], [8-0]. Accordng to (6), Conenu-Baed Prmary Controller de ( k ) 0 dt can be acheved to hare the reactve power n the teady tate. If β = 0 and B 0, then: Q Q j. (0) Q Q j V PCC Q Prmary Communcaton Network MGCC Q Q () Neghbour Q ( k) PI Q n Q V V Secondary Communcaton Network V () Neghbour ( k ) () Neghbour ( k ) PI PI m P V PWM Converter Mcrogrd Network Conenu-Baed Secondary Controller DG Fg. 7. The control tructure of the conenu algorthm baed on the herarchcal control [7]. quaton (0) how that the equvalent reactve power harng acheved. Moreover, = can be acheved when β 0 and B= (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

10 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc Therefore, the reactve power harng can be realzed and the voltage can be retored to the rated value when β 0 and B 0. Although the DAPI controller can be tuned to enforce ether voltage regulaton, reactve power harng, or a comprome between the two, the proportonal reactve power harng and accurate voltage regulaton cannot be acheved. Moreover, the reactve power capacty and LBC delay are not condered [73].. A conenu-baed dtrbuted control method preented n [7] to acheve the proportonal reactve power harng by ntegratng the droop and econdary control, where only a pare network requred. The conenu-baed prmary and econdary control hown n Fg. 7. The voltage reference obtaned by elmnatng the reactve power and voltage ampltude mmatch among DG unt through PI controller. In addton, the reference frequency can be regulated by conenu-baed econdary controller. Note that the dentcal Q /n can be acheved by ung the protocol n the conenu-baed prmary controller. Accordng to (6), x replaced by a controllable varable v : Qj Q v ( ) () jn n n where v 0 and Q /n = Q j/n j n the teady tate. Accordng to (6), the control gnal ent to a et of networked agent wth only pare communcaton lnk, and the reactve power harng acheved by ung the conenu control protocol n the conenu-baed econdary controller: ( j ) b ( ). () jn Accordng to Theorem II, all local δ wll converge to δ n the teady tate. Note that the communcaton lne ext n DG and DG j when b =, or there no communcaton lne between them when b =0. The conenu control parameter track the reference gnal δ through the MGCC, and the econdary control gnal δ wll be obtaned under the dfferent rated capacte of DG unt and mmatched feeder mpedance condton. Furthermore, the reactve power harng can be acheved and the voltage ampltude can be retored to the rated value by combnng the conenu-baed droop and econdary control algorthm. Snce the voltage regulaton and reactve power harng are realzed by ung a pare communcaton wth gnfcantly lower bandwdth requrement, the communcaton cot can be greatly reduced. Note that the proportonal reactve power harng and voltage regulaton can be acheved multaneouly n the conenu-baed prmary and econdary control. However, mlar to the DAPI control method, the reactve power capacty and LBC delay are not condered n [7]. B. Reactve Power Sharng Conderng Charactertc of the RS The actve power of the RS fluctuate greatly, the poor actve and reactve power harng are nevtable when the envronment change. If the actve power extracted from the maxmum power pont trackng (MPPT) algorthm, the tablty of MG ytem may be affected and overload may occur n the ytem [74]. Therefore, the algorthm for harng actve and reactve power mentoned n the prevew ecton may not be applcable, whch may affect the tablty of MG. ).Programmng Algorthm for the Reactve Power Management n the MG wth RS The core of the programmng algorthm procedural, and the degn of uch algorthm manly rele on the three charactertc j v [-3]: ()Fntene: The algorthm can top after a fnte number of tep. ()Defntene: very tep of the algorthm hould be clearly defned. Moreover, reult of each tep can be effectvely mplemented. (3)Sequentalty: From the ntal tep, every tep the prerequte for the next tep. In addton, teratve method, dynamc programmng method, branch/bound method and neted loop algorthm can all acheve the control for one DG to n DG unt (e.g. the reactve power harng between two DG unt extended to n DG unt) [9], [74], [-3]. A reaonable algorthm can mprove the relablty of the ytem and multaneouly reduce the cot of the hardware nvetment [4-9]. Conderng the capacty lmtaton of ndvdual RS, a control method baed on programmng-algorthm preented n [74] to hare the reactve power and lmt the apparent power of each converter to be lower than t rated value. The relatonhp between the maxmum apparent power Q max and reactve power : Q S P (3) max N where Q max the maxmum reactve power of the converter. S N the nomnal apparent power of the converter. The contrant condton of the reactve power harng can be obtaned a: P Q S S, Q S P (4) N max N where S and S N are the apparent power and the nomnal power of the th converter. When the converter are operatng wth apparent power hgher than the nomnal power, the reactve power of thee converter need to be lmted. But the capacty of reactve power of the unlmted converter are free, whoe apparent power are lower than nomnal value. Specfcally, the proportonal reactve power harng (PRPS) algorthm can be mplfed to the followng tep. The frt tep to calculate the total actve power P L, reactve power Q L and the maxmum of poble reactve power of converter Q max, and P L, Q L are derved a: PL P, QL Q. (5) The econd tep to analyze whether the reactve power of each DG exceed the reactve power lmt and regulate Q of lmted converter. Moreover, an ntermedate varable utlzed to ave all the reactve power of unlmted converter to make them equal to Q. The thrd tep to obtan the adjuted reactve power (Q u) for converter by the preet algorthm [74]. The PRPS algorthm can realze the proportonal reactve power harng among n DG unt. However, t tll dffcult to degn a programmng algorthm that can accelerate the computng and proceng peed, and reduce the communcaton cot n the extng lterature. ). The Reactve Power Management n a Mult-Agent Baed RS Sytem The MAS a group or organzaton of autonomou computatonal entte (agent) that communcate n a peer-to-peer fahon, whch ha the capacty to perceve t envronment and olve control problem effectvely n complex ytem [30], [3]. The agent have varyng degree of ntellgence baed on ther role and the archtecture, whch can be categorzed nto pave agent, actve agent and complex agent [3]. Bede, the agent purue global vew for the ytem and accomplh goal by ung of knowledge and optmzng the certan performance wthn a pecal envronment [33], [34]. In addton, agent have the capablty to nteract wth other agent, whch nvolve negotaton or cooperaton (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

11 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc TABL III. Advantage and Dadvantage of the Improved Herarchcal Control Stratege for Reactve Power Sharng Conderng the Mmatched Feeder Impedance or Changeable Producton and load Major technque Reactve power harng problem Advantage Dadvantage Optmzed the econdary control equaton [7], [3-7] Programmng algorthm [9], [74], [-3] Mult agent ytem [76], [33] The reactve power affected by unequal feeder mpedance qupment may be damaged when RS operate ung MPPT method due to overloadng The output actve and reactve power are affected by envronment Be utable for a complex MG Actve power harng acheved Frequency devaton elmnated Reactve power harng realzed wthout hgh bandwdth communcaton The control law can be mplfed by graph theory Proportonal reactve power harng can be acheved The equpment afety enured Good performance for expanblty Be ued to control complex DG unt The table actve and reactve power harng can be acheved The control law can be mplfed by graph theory Organze nformaton autonomouly computatonal entte Be benefcal to exchange nformaton Proportonal reactve power harng dffcult to be acheved Communcaton delay ext n the LBC Control equaton need to be further optmzed n the MG wth complex load The programmng algorthm dffcult to be degned n a complex MG Delay/data drop n algorthm need to be condered Delay n algorthm need to be condered Communcaton delay n LBC lne Good protocol n agent dffcult to be degned The actve and reactve power harng are poor when data drop ext n the preet algorthm In a complex ytem, agent are requred to exchange, analyze, proce, accept and reject the nformaton from other agent to reach a common goal under a changeable envronment condton [35]. The RS entve to the envronment and t would affect the actve and reactve power harng of landed MG. However, the nfluence can be elmnated by the cooperaton among agent. The decentralzed econdary control method baed on the MAS preented n [76] to regulate the actve and reactve power. A et of control law for agent n any gven network can be derved by the econdary control and only local nformaton needed. A hown n Fg. 8, the control cheme of the MG depcted and the bottom layer compoed of DG unt wth local controller, and the top layer encompaed by the agent and the communcaton network. The agent connected to an uncontrollable or partally controllable DG (repreented by crcle) are called controllable and partally controllable agent, repectvely. The other agent are called controllable agent (repreented by damond). When the output power of the RS tart to change, uncontrollable agent receve nformaton from ther correpondng uncontrollable DG and then end the nformaton to adjacent controllable agent accordng to the preet control method. Thee controllable agent adjut the correpondng controllable DG to get the dered actve and reactve power. The actve power producton of DG depend on envronment (e.g. PV panel depend on unlght) and t wll be larger once the ntenty of unlght ncreae. Snce the controllable DG adjacent to DG, Agent and Agent 3 wll end the nformaton to Agent (neghbour Agent and Agent 3), repectvely. And the output power of the controllable DG regulated by Agent, makng the total actve power to the dered value. In the communcaton network, agent exchange nformaton and make decon accordng to the preet control law, whch nclude the envronment, the load demand and the delay, etc. Although the power ratng of a RS uually degned along wth the actve power generaton, the reactve power of the ytem can be utlzed to upply the apparent power to reduce the actve power requrement. When the nformaton ent to the controllable DG unt by correpondng agent, a MAS can be ued to regulate the actve and reactve power properly after the power at the next tme ntant are etmated. Therefore, the output power of DG unt can be regulated by local control under the changeable envronmental condton to acheve a balance between the producton and conumpton of the energe n the MG. Conderng the capacty of the reactve power and changeable producton and load, a well-degned MAS algorthm can acheve equvalent or proportonal reactve power harng. However, the communcaton delay alo nevtable n the MAS. The advantage and dadvantage of the varou method for the reactve power harng under the mmatched feeder mpedance and changeable envronmental condton are ummarzed n Table III. xchange Informaton Top Layer Communcaton Network Compoed of Agent P,Q Bottom Layer MG Compoed of DG wth LC Agent local controller DG DG 3 DG Load Load Load 3 Load 4 local controller Agent Agent 4 local controller DG 4 local controller Fg. 8. Control cheme of the mult-agent baed landed MG ytem [76]. Communcaton Delay: G d() Power calculaton Q 4 Q 3 Q Q Q Reactve Power Reference Caculaton P n m mp Mcroource Inverter Q 4 Q 3 Q Q Secondary Control PI Communcaton Delay: G d() nq MGCC Prmary control Agent 3 AC bu V PWM Fg. 9. Control cheme of a DG wth mmatched reactve power compenatng [4] (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

12 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc V. RACTIV POWR SHARING WITH MISMATCHD FDR IMPDANC It well-known that output power of MG wth RS are affected by the feeder mpedance and the loadng/producton of power. Moreover, the reactve power harng would be poor due to the nonlnear and unbalanced load n the landed mode [64], [84], [36-40]. A. Drect Control Method for the Reactve Power Sharng The man tak of drect control method for the reactve power harng to get the requred value n each DG, do the um of thee value, and then agn thee adjuted power to each DG on average (weghted average) [4]. In [4], each converter reponble for provdng the nformaton of the requred reactve power to the MG va low bandwdth communcaton lnk. In addton, the MGCC determne the way that how the reactve power upple for each converter. Fg. 9 how a dtrbuted control cheme for elmnatng reactve power mmatched n an landed MG, where the reactve power Q obtaned from the droop control ent to the econdary control to make a um, and the Q obtaned from the econdary control ent to the prmary control through PI controller. Moreover, the compenaton of the droop control adjuted by Δ. The reactve power demand Q for each converter can be calculated by: Q Q (6) k n( ) n where ΣQ the total reactve power uppled by all the converter. Q the reactve power demand uppled to the th converter and n the droop gan of the th converter. The MGCC reponble for regulatng the reactve power accordng to the reactve power reference of each converter, and the communcaton delay G d () defned a: Gd () T d. (7) The control method hown n Fg. 9 can olve the problem of the equvalent or proportonal reactve power harng wth a certan communcaton delay. The value of the reactve power are extracted drectly and not affected by load mpedance, thereby th control method utable for both the lnear and nonlnear load condton. However, the communcaton delay alway uncertan and t may reult n a poor reactve power harng. B. Indrect Method for the Reactve Power Sharng Compared to the drect method for harng the reactve power, the ndrect method are more comprehenve. Note that the default varable controlled to hare the reactve power ndrectly n the ndrect control cheme, and more factor that may affect the reactve power harng are condered [73] [7]. A two-layer cooperatve method that control the voltage/ frequency a well a the actve/reactve power preented n [4], a hown n Fg. 0. The voltage controlled voltage ource nverter (VCVSI) are ued to elmnate the voltage and frequency devaton n the frt layer, and the current controlled voltage ource nverter (CCVSI) are reponble for harng the actve and reactve power n the econd layer. In ummary, the control objectve of the VCVSI are to regulate voltage magntude and frequency, and CCVSI are ued to control output actve and reactve power of each DG n the two-layer dtrbutve cooperatve control. Note that a pare network need to be contructed by the graph theory, and each DG only requre t neghbour and own nformaton on the LBC network [3-6]. ) Frequency Control of VCVSI The control of the voltage and frequency utlzed to ynchronze all voltage and frequence of the VCVSI to nomnal voltage and frequency, repectvely. The aumpton that make the frequence retored to the rated value baed on (8)-(3) a: m P m P (8) f (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton. v m P (9) v dt v c ( a ( ) g ( ) a ( m P m P )) f f j j ref j j j jn jn f (30) (3) where v f an auxlary varable, whch choen baed on each VCVSI own and the neghbour nformaton n the communcaton graph. ω the nput angular frequency controlled by v f, and c f the control gan and g 0. It aumed that VCVSI can communcate wth each other through the communcaton graph to acheve the ynchronzaton. Accordng to (6), t can be obtaned that every DG can operate at the ame frequency n the teady tate. ) Reactve Power Control of CCVSI The reactve power of CCVSI et baed on ther reactve power ratng a: Q Q Qref Q Q. (3) Accordng to each CCVSI and t adjacent nformaton, the auxlary control v Q can be choen a: Q Q j Q vq cq ( aj ( ) g ( )) Qref Q Q Q (33) where Qref jn j repreent the pre-pecfed reactve power rato reference, and c Q and g are the control gan. Conderng the conenu prncple of (6), the Q /Q can be ynchronzed to a reference value, and equvalent or proportonal reactve power harng can be acheved. Although the algorthm n two-layer control complex, t not affected by the feeder mpedance and t utable for harng the reactve power n landed MG wth unbalanced and nonlnear load. C. The Reactve Power Sharng n the Herarchcal Control Stratege wth Harmonc Compenaton A frequency droop control wth addtonal dturbance utlzed to produce ome actve power harng varaton to regulate the DG vrtual mpedance at the fundamental potve equence, fundamental negatve equence, and harmonc frequence under the unknown feeder mpedance, and acheve the reactve power harng by combnng varable vrtual mpedance wth the frequency droop control n [43]. In [70], the potve- and negatve-equence component of the voltage and current are appled to calculate potve-equence actve and reactve power, repectvely. The output voltage and phae angle reference are obtaned by the potve-equence power, and then the voltage unbalance can be compenated by negatve-equence reactve power. Fnally, the voltage well-regulated and the reactve power harng realzed n the MG wth nonlnear load. Fg. how a clacal approach to hare the reactve power. The load voltage harmonc are elmnated by the harmonc compenaton algorthm, and the devaton of voltage ampltude and frequency are compenated by the econdary control. In addton, the actve and reactve power can be hared by combnng the vrtual mpedance and econdary control [44]. ach converter requred to tranmt the reference reactve power to the MG (Q and Q ) by ung the MGCC, and the MGCC determne how the total reactve power allocated to each converter (Q and Q ).

13 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc Communcaton lnk Layer VCVSI Frequency and Voltage Control VCVSI CCVSI, PQ,, Prmary Control P, Q Communcaton lnk CCVSI Mcrogrd 4, 4 P4, Q4 VCVSI 4 CCVSI 4 3, P 3 3, Q3 VCVSI 3 CCVSI 3 Fg. 0. Scheme of the two-layer dtrbuted control n the landed mcrogrd [4] VSI L L L L VSI v pcc Layer Actve and Reactve Power Control v v dc dc L C r Nonlnear load r C L Prmary Control Inner loop V PWM Harmonc xtracton V PWM Inner loop Prmary Control Droop Control v PCC vref k pq k Q Q vh Harmonc Reference Generator Q v 3rd v5th v7th v9th K 3rd K 5th K 7th K 9th vc3rd vc5th v v c7th c 9 th Harmonc Compenaton n Q total Zd () v h Harmonc Reference Generator n Q Q v ref k pq k Q Droop Control vpcc k p k Secondary Control Q Q Q total nn n n Reactve Power Compenaton wth Voltage Retoraton Fg.. Block dagram of the reactve power harng of landed MG ncludng harmonc compenaton loop [44]. TABL IV. Advantage and Dadvantage of the Dfferent Control Stratege for Reactve Power Sharng under Nonlnear or Unbalanced Load Condton Control method Advantage Dadvantage Integraton of the MGCC and the prmary control [64], [4] The herarchcal control baed on graph theory [73], [7], [4] Integraton of econdary control and vrtual mpedance loop [70], [43], [44] The equvalent/proportonal reactve power harng realzed Method eay to expand Frt-order communcaton delay condered Be utable for lnear or nonlnear load condton The two-layer control fully dtrbuted and wll not affect each other. The proportonal reactve power harng can be acheved The model utable for a complex MG The control method utable for landed MG wth nonlnear load condton Suppre harmonc voltage effectvely The tablty of the ytem enhanced The equvalent/proportonal reactve power harng realzed Influence on nonlnear load condered Cannot hare the reactve power n landed mcrogrd wth more complex load Wthout conderng communcaton delay Wthout conderng data drop n communcaton lne Total generaton cot of MG not condered The algorthm complex Wthout conderng LBC delay The performance of controller affected by data drop The nfluence of feeder mpedance not condered Not utable for large cale MG Poor expandablty of the control trategy The reactve power harng nfluenced by LBC delay (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

14 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc MGCC 0 Secondary Control k p k SF P ref Partcpaton Factor P P P SF ref 4 SF ref 3 SF ref P SF ref, T Communcaton Channel Tme Stamp T DC P ref SF P ref Droop Control P Gan Scheduler, PLL k pp k p p Vabc P SF ref, T dref d Tme Stamp T Fg.. Scheme of gan-chedulng method for compenatng the communcaton delay [77]. k p k v d Local Control v L d q V PWM GPS The formula of computng the reactve power demand (Q ) the ame a (6) and the reactve power harng compenator for any converter can be expreed a: k pqs ( Q Q ) kqs ( Q Q ) dt (34) where k pqs and k QS are the control gan and Δ the addtonal voltage devaton whch added to the droop control loop. The harmonc compenaton loop ued to mprove the power qualty and tablty of the MG. Specfcally, the harmonc caued by the nonlnear load are compenated by harmonc controller, and a retve vrtual mpedance R V ued to mprove the tablty of the mcrogrd. The tranfer functon of vrtual mpedance can be obtaned a: cnkcn Zd() RV n3,5,7,9 cn (35) n where Z d() the vrtual mpedance tranfer functon. k cn are the harmonc reonant gan, ω cn are the harmonc reonant bandwdth and ω n the n th harmonc frequency. The voltage acro the capactor of the output flter can be expreed a: V ( ) V ( ) ( ) Z ( ) (36) h ref k d where V ref () the reference voltage that determned by the outer droop control loop. V h() the compenated nput to the nner loop and k() the output current of the k th nverter. The harmonc compenaton ued to damp the voltage harmonc at the PCC. In addton, a econdary voltage harmonc compenaton loop can be appled to further reduce the voltage harmonc at the PCC. Overall, the mproved reactve power harng trategy wth harmonc compenaton controller acheve the reactve power harng whle elmnate the voltage and frequency devaton. The advantage and dadvantage of the varou control method for reactve power harng n landed MG wth nonlnear and unbalanced load condton are ummarzed n Table IV. VI. PROBLMS OF COMMUNICATION DLAY IN TH HIRARCHICAL CONTROL The communcaton delay alway ext n both the econdary and mult-agent control. The frequency and voltage ampltude are retored to the rated value n the herarchcal control, but the output correcton gnal ent to prmary control need a tme delay owng to the communcaton lne. Therefore, thee gnal may be dfferent to the theoretcal value, whch wll caue damage to MG. However, the communcaton delay problem have not yet been condered n many tude. A. Reduce the Impact of Delay through a Gan Scheduler The delay n the communcaton lne between local control and MGCC ha been mentoned n [77], the charactertc of the delay can be contant, bounded, or random n term of the network tructure. The wahout flter-baed power harng trategy wth no communcaton lne preented n [45] to replace the econdary control and elmnate the mpact on delay, but the nonlnear and unbalanced load are not condered. Uually, when econdary control ued n actve and reactve power harng tratege to elmnate the frequency and voltage devaton, the communcaton delay cannot be gnored. Many lterature have preented varou gan chedulng method to deal wth the problem brought by tme-delay n a complex MG ytem [46-49]. The gan chedulng approach preented n [77] to compenate the effect of the communcaton delay on the econdary frequency control to guarantee the actve power harng and table operaton of the MG. A hown n Fg., there are conventonal droop control and current loop n the local control, whch are ued to regulate the output actve and reactve power and output current. Bede, the local control equpped wth a gan cheduler to counteract the communcaton delay and the PLL utlzed to meaure the MG frequency. The actve power reference for each DG are calculated by the econdary frequency controller to retore the frequence to the nomnal value and ent to local converter. Thee reference are obtaned by P SF' ref,t, where T the tme tamp (ent by the MGCC). The reference are marked a P SF' ref,t, where T another tme tamp (receved by local control). The communcaton delay τ =T T calculated by comparng the two gnal P SF' ref,t and P SF' ref,t. To counteract the effect of communcaton delay, a gan chedulng approach ued n econdary controller wth the followng tranfer functon: Gf Gf, Gpf p Gpf (37) where G f /G pf are fxed ntegral/proportonal coeffcent n the econdary frequency controller. β ω/β pω denote an changeable ntegral/proportonal gan cheduler, and G f and G pf are the equvalent gan of the econdary frequency controller after gan cheduler equpped n each local control. Conderng (37), the equalzed load frequency controller ha the followng form: G SF f SF Gf Pref Gpf ( ), Pref Gpf ( ) (38) (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

15 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc VSI L L Feeder mpedance L L VSI v u lne C lne u C v dc dc Inner Controller V PWM e Power Calculaton Secondary Voltage Control r P C m Droop control cf Z L cf m C P r Power Calculaton V PWM e Droop control Secondary Voltage Control Inner Controller H PLL Gˆ Gd e G p = L d e Ĥ Gˆ P u PI n G p = MPC Gd e PLL H PLL Smth predctor baed econdary control F Model predctve controller baed econdary control Fg. 3. Control cheme of the frequency econdary control wth the predctve controller [78]. where P SF ref the upplementary power et pont of the th DG agned by the econdary frequency controller. quaton (38) can be utlzed to nvetgate the root locu of the tme-delay mall-gnal model to fnd optmal β ω and β pω and the table operaton of the mcrogrd ytem could be guaranteed under dfferent LBC delay condton. In addton, the cot functon J bult to fnd the relatonhp between the gan-cheduler varable and the ytem performance of the MG, whch defned a: T J, d ( t) ( t) T (39) t 0 where the frequency of the DG when the mcrogrd operate wth and wthout communcaton delay are repreented by ω,d(t) and ω,d(t), repectvely. Conderng relatonhp between the tme delay τ and t correpondng feable gan β τ ω/β τ pω, a proper cot ndex needed to be bult to obtan the gan value of dfferent MG. After a certan delay margn obtaned by takng nto account the relatonhp between the cot functon and the gan varable, the mpact of communcaton delay on the LBC lne can be compenated whle the performance of actve power harng guaranteed by the gan chedulng method. And the communcaton delay on management of reactve power can alo be elmnated when the gan chedulng method are further mproved. B. Reducng the Impact of Delay by ung Predctve Control A known n [77], [78] and [50-5], econdary frequency controller affected by the LBC delay and then the poor actve power harng nevtable n the herarchcal control. Generally, predctve control ued to deal wth the LBC delay problem. In [50] and [5], the predctve control method preented to compenate for the mpact on the communcaton delay and data lo. In addton, the model predctve controller (MPC) and mth predctor (SP) are preented to olve the problem brought by communcaton delay [78], [5]. A hown n Fg. 3, the output current of the converter paed through an LCL flter to reject the hgh-frequency wtchng noe. In addton, the capactor voltage and output current of each converter are ent to the droop controller to calculate the actve and reactve power. Droop controller et the voltage and frequency reference baed on the generated actve and reactve power. Moreover, the output frequency and voltage are adjuted by SP or MPC. When the control ytem are decoupled, the charactertc equaton of the econdary control ytem (SCS) obtaned a [78]: e G G H 0 (40) (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton. p where H the PLL tranfer functon. e -τ the tranfer functon of the communcaton delay. G c the delay tranfer functon of PI controller and G p the delay tranfer functon of the ytem devce to be controlled. It can be nferred that the accurate etmaton of the delay tranfer functon n a typcal operatng pont requred when a SP mplemented n the econdary frequency controller, whch hown n Fg. 3. For the MPC, a et of future control acton need to be calculated by optmzng a cot functon wth contrant on the manpulated and controlled varable. In ummary, the MPC and SP controller can be ued to tet the unknown communcaton delay n a MG. Specfcally, n the MPCbaed SCS, the future behavor predcted by optmzng a cot functon wth contrant on the controlled varable n the MG. The dynamc performance of the MPC lower than the SP, but the MPC more robut to tme delay and preferred to operate n ytem wth unknown communcaton delay. In addton, the reactve power harng performance of a well-degned algorthm can be further mproved, and the ytem can be mmune to the communcaton delay by ung the MPC and SP controller. C. The Reactve Power Sharng Scheme Conderng Feeder Impedance, Complex Load and Communcaton Delay Conderng the communcaton delay, the econdary control trategy for the reactve power harng facng challenge and t dffcult to olve th problem effectvely. The dtrbuted cooperatve control method for large-cale DG wth tme-varyng delay preented n [53] to acheve the reactve power harng. A more comprehenve method preented n [79] to combne the advantage of prmary and econdary control. A weghted graph ued to replace the actual nformaton ytem and the control algorthm further optmzed to acheve the reactve power harng. Specfcally, th method baed on the voltage, reactve power and actve power regulator module n the MG. A hown n Fg. 4, each nverter condered a an agent of a mult-agent ytem to exchange data wth a few other neghbor nverter, and proce the nformaton to update t local voltage et pont and ynchronze ther normalzed power and frequence. Moreover, global voltage regulaton, frequency ynchronzaton and c

16 Data Format: =[, ] Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc proportonal load harng can be acheved by the cooperaton among voltage, reactve power and actve power regulator effectvely n a fully dtrbuted control trategy, and the tablty and robutne of MG can be mproved. Specfcally, the node receve the nformaton Ψ j from t neghbor node j, and regulate the neghbor and local data Ψ to update t voltage and frequency reference ( and ω ). The voltage reference obtaned by two voltage correcton term (δ and δ ) from the voltage and reactve power regulator, and the reactve power and frequency can be regulated by elmnatng the reactve power and frequency devaton among the neghbor through PI controller. Then, the reference voltage of DG can be obtaned a: ( t) ( ) ( ) t t (4) mq b ( ) j Qj Q, c j ( Pj P ) (4) jn jn where the rated voltage magntude of the MG. The voltage regulator at node compared wth the rated voltage, where the dfference fed to a PI controller (G ()) to generate the frt voltage j Q j Voltage regulator Voltage tmator G () Reactve power regulator Control Layer b j ( Qj Q ) H () jn correcton term δ, b and c are gan coeffcent. The neghborhood reactve loadng mmatch m Q, whch meaure the dfference between the normalzed reactve power of the ource and the average value of t neghbor, and the mmatch n (4) then fed to a PI controller (H ()) to adjut the econd voltage correcton term δ. The frequency correcton term δω repreent the nformaton of neghborhood actve loadng mmatch. Due to the performance of the PI regulator, all reactve power wll be ynchronzed to the ame value and reactve power harng acheved. The actve power regulator module keep the frequency at the rated value, and precely tune the phae angle reference δω to reroute the actve power acro the MG and mtgate the neghborhood actve power mmatch. In order to reduce the mpact of LBC delay, the cooperatve dtrbuted control trategy teted to fnd a delay margn to enure controller parameter mmunty and tablty of the MG. In addton, the equvalent actve and reactve power can be hared when the LBC delay contraned wthn the delay margn [79], [54]. V PWM nergy Source Inverter (three phae) Phycal Layer LCL Flter Power/Voltage Meaurement Mcrogrd Dtrbuton Bu PCC Actve power regulator Data Format : Neghbor Data P j jn c P P j ( j ) Cooperatve Secondary/Prmary Control at Node Data Format: j [ j, Qj, Qj ] j N Data Format: [, ] Cyber layer (Communcaton Network) [, Q, P ] Tertary Control Unt Fg. 4. Control cheme for the cooperatve dtrbuted control n landed mcrogrd [79]. TABL V. Advantage and Dadvantage of Dfferent Herarchcal Control Stratege Conderng the ffect of Communcaton Delay Control method Major technologe Advantage Dadvantage Gan chedulng method [77] [46-49] Predctve control [78], [50-5] Cooperatve dtrbuted control [79], [53] The data from the MGCC adjuted by the gan cheduler Predct the unknown delay by the SP or MPC A pare network needed and hare the actve and reactve power by the dtrbuted control Provde a general model Reduce the cot by degnng the reaonable cot functon The ytem can guarantee a good power harng n the delay margn Good robutne to the contant communcaton delay Provde a general model The ytem can guarantee a good power harng n the delay margn The equvalent actve and reactve power harng can be acheved under complex load condton Good plug-and-play capablty Have relency to a ngle communcaton lnk falure Good robutne to the contant communcaton delay Gan coeffcent are not eay to elect Communcaton delay n reactve power controller not condered Data drop not condered The algorthm complex Poor expandablty Cannot deal wth the problem brought by random delay Data drop not condered Cannot deal wth the problem brought by random delay The proportonal reactve power can not be realzed (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

17 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc In the cooperatve dtrbuted control trategy, MG can run at the rated voltage and angular frequency, combnng the actve power regulator, the voltage and reactve power regulator. The cooperatve dtrbuted controller can realze the equvalent reactve power harng under the mmatched feeder mpedance and nonlnear load condton when the LBC delay wthn the delay margn, but proportonal actve and reactve power harng cannot be acheved. Dfferent herarchcal control tratege for elmnatng the LBC delay n MG are ummarzed n Table V. revenue for the aggregator. A genetc algorthm ued n [6] to reveal the economc beneft of both dtrbuton network and mcrogrd. A drectly operatng chedule for a whole day preented n [63], whch allocate the power to the load n an optmal trategy by contructng the cot functon reaonably. A hort-tme predctve control preented n [64] to regulate the actve and reactve power n the mcrogrd, and coordnate the optmal operaton of dpatchable reource and the daly cot of the energy mported from the grd. VII. DISCUSSION ON FUTUR TRNDS From the prevou dcuon, t can be een that each of thee control technque ha t own charactertc, advantage and dadvantage. Mcrogrd can be better utlzed when the problem of load actve and reactve power harng are effectvely olved. The future trend on MG reearch and applcaton can be ummarzed a follow [55-69]. A. Advanced Dtrbuted Control n Mcrogrd Wth a hgh penetraton level of the DG unt, the reearch on how to realze accurate actve and reactve power harng among multple DG unt, mprove the robutne and relablty of the ytem and multaneouly optmze/elmnate the energy flow ung the graph theory/predctve control/mult-agent ytem ha been a mantream trend [55], [56]. A mcrogrd model can be mplfed by graph theory, and the complexty of an algorthmc degn can be further reduced. The mult-agent cooperatve control method for coordnatng power allocaton between the ultra-capactor and battere dtrbuted throughout the mcrogrd preented n [55]. Bede, a predctve control degned n [56] to acheve the actve and reactve power harng wth nonlnear load, whch ndcate that the future predctve control need to be mproved n order to deal wth the effect of harmonc and unbalanced load. B. Control for Mcrogrd wth Complex Load The method for actve and reactve power harng n mcrogrd wth lnear load are well developed, but t tll dffcult to enure the reactve power harng when mcrogrd upply complex load uch a dynamc load, nducton motor, the puled load and the electrc vehcle, etc. The realzaton of the reactve power harng n uch varable load one of the mportant drecton n the future reearch [57-60]. The tuaton of dfferent nonlnear load analyzed n [57], and the reult from t experment ndcate that the actual operaton of the mcrogrd nfluenced by dfferent load condton. The mcrogrd wth entve load dcued n [58], and a new method to upply energy for the load by ung the fuel cell a energy torage equpment propoed. A hybrd DC power ytem degned n order to upply a puled load [59]. Moreover, a plug and play method preented n [60] when the mcrogrd upple the entve and unbalanced load, whch enhance the power qualty. C. Cot-Prortzed Control Scheme It mportant to mnmze the operaton cot and coordnate upportng ervce, meanwhle maxmzng the relablty and controllablty of mcrogrd. Therefore, optmzaton of the MG cot functon one of the trend n the future reearch [6-64]. Conderng realtc value for the bd, actual market prce, typcal load profle and renewable producton, the economc evaluaton of a mcrogrd partcpatng n a real-tme market obtaned n [6], whch how that the economcal mcrogrd operaton can reduce energy prce for the conumer and ncreae D. Reduce the Impact of Communcaton Delay The control method are alway nvolved wth the tranmon of data when multple DG unt are connected, but the delay nevtable n both low and hgh bandwdth communcaton lne. Therefore, t mportant to conder the load power harng problem n the contant, bounded, or random delay. Furthermore, t eental to develop tablty analy tool for practcal cae n the future. The open communcaton nfratructure ncludng thernet, Internet, worldwde nteroperablty for mcrowave acce (WMax), and wrele fdelty (WF) are ncreangly mplemented for mart grd communcaton [65-69]. However, the delay or data lo may occur durng ther tranmon. Therefore, the oluton to decreae the cot and ncreae the delay margn one of mportant reearch drecton of mcrogrd n the future. VIII. CONCLUSION Th paper preent an overvew of the dfferent actve and reactve power harng method. Owng to the lmtaton of the conventonal droop and econdary control, the poor actve and reactve power harng of the DG unt are nevtable. A comprehenve analy and comparon of the mproved control method to hare the actve and reactve power have been preented. In a complex MG, the dynamc tablty of actve power harng need to be enhanced and ome mproved droop control method are analyzed n th paper to acheve the optmzed actve power harng. Conderng that the actve power may be affected by feeder mpedance, th paper preent ome mproved P-V and Q-f droop control cheme to hare the actve power under retve/unknown feeder mpedance condton. Moreover, a economc problem condered n herarchcal control, the crteron for actve power need to be etablhed on generaton cot of the mcrogrd, and varou lnear/nonlnear cot baed cheme are analyzed n th paper to optmze actve power harng and mnmze the total cot of generaton multaneouly. Mot of the extng method only conder the reactve power harng under lnear load condton wth mmatched feeder mpedance. However, a poor reactve power harng may ext when MG operate on mmatched feeder mpedance, nonlnear and unbalanced load condton. Therefore, the algorthm baed on graph theory, mult-agent ytem, predctve control and cooperatve dtrbuted control have been dcued n detal to hare the reactve power under thee complex crcumtance. In addton, owng to the low bandwdth communcaton lne n the herarchcal control, the tranmon peed of the upper control much lower than the repone rate n the prmary control. Therefore, th paper analyze the problem on LBC delay n the herarchcal control and preent ome oluton to olve thee problem, uch a gan chedulng cheme and predctve control method. Fnally, the future trend of the control technologe n MG are dcued. The way to mplfy the complcated control algorthm and decreae the low bandwdth communcaton by graph theory, (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

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Huang, and B. Yu, Robut trackng control of networked control ytem: applcaton to a networked DC motor, I Tran. Ind. lectron., vol. 60, no., pp , Dec. 03. [69] Y. Chompoobutrgool and L. Vanfrett, Analy of tme delay effect for wde-area dampng control degn ung domnant path gnal, n proc. I PS General Meet. 04, pp. -5. Yang Han (S 08-M 0) receved the B.S. degree n lectrcal ngneerng from Unverty of lectronc Scence and Technology of Chna (USTC), Chengdu, Chna, n 004, and receved the Ph.D. n lectrcal ngneerng from Shangha Jaotong Unverty (SJTU), Shangha, Chna, n 00. He joned the Department of Power lectronc, School of Mechatronc ngneerng, Unverty of lectronc Scence and Technology of Chna (USTC) n 00, and ha been promoted to an Aocate Profeor nce 03. From March 04 to March 05, he wa a vtng cholar (guet potdoc) n the area of renewable energy and mcrogrd at the Department of nergy Technology, Aalborg Unverty, Aalborg, Denmark. H reearch nteret nclude AC/DC mcrogrd, grd-connected converter for renewable energy ytem and DG, phae-locked loop (PLL), power qualty, actve power flter and tatc ynchronou compenator (STATCOM). He ha authored more than 0 ISI-ndexed journal paper and one book chapter n the area of power electronc, power qualty condtoner, and mart grd. He receved Bet Paper Award from 03 Annual Conference of HVDC and Power lectronc Commttee of Chnee Socety of lectrcal ngneer (CS) n Chongqng, Chna, and the 4th Internatonal Conference on Power Qualty n 008, n Yangzhou, Chna. He ha x ued and ten pendng patent. Currently, he the upervor for nne mater tudent, one of whch ha been nomnated a provncal outtandng graduate tudent. He an actve revewer for I Tranacton on Power lectronc, I Tranacton on Smart Grd, I Tranacton on Indutral lectronc, I Tranacton on Sutanable nergy and I Tranacton on nergy Converon. Hong L receved the B.S. degree n lectrcal ngneerng and Automaton from Unverty of lectronc Scence and Technology of Chna (USTC), Chengdu, Chna, n 05. He currently workng toward the M.S. degree n Power lectronc and lectrc Drve at USTC, Chengdu, Chna. H current reearch nteret nclude the optmzaton of ac mcrogrd, power management, herarchcal and cooperatve control, and grd-ntegraton of renewable energy reource. Pan Shen receved h B.S. n lectrcal ngneerng and Automaton from Anhu Agrcultural Unverty, Hefe, Chna, n 03. He currently workng toward the M.S. degree n Power lectronc and lectrc Drve at the Unverty of lectronc Scence and Technology of Chna (USTC), Chengdu, Chna. H current reearch nteret nclude ac/dc mcrogrd, power qualty, power converter, power ytem automaton, and actve power flter (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.

22 Th artcle ha been accepted for publcaton n a future ue of th journal, but ha not been fully edted. Content may change pror to fnal publcaton. Ctaton nformaton: DOI 0.09/TPL , I Tranacton on Power lectronc rnane Antôno Alve Coelho receved the B.S. degree n electrcal engneerng from the Federal Unverty of Mna Gera, Belo Horzonte, Brazl, the M.S. degree from the Federal Unverty of Santa Catarna, Floranopol, Brazl, and the Ph.D. degree from the Federal Unverty of Mna Gera n 987, 989, and 000, repectvely. In 989, he joned the lectrcal ngneerng Faculty at Federal Unverty of Uberlanda, where he currently a Full Profeor. H reearch nteret are Power-factor Correcton, PV and Fuel Cell Sytem, Mcrogrd Modellng and Dgtal Control by mcrocontroller and DSP. Joep M. Guerrero (S 0-M 04-SM 08-FM 5) receved the B.S. degree n telecommuncaton engneerng, the M.S. degree n electronc engneerng, and the Ph.D. degree n power electronc from the Techncal Unverty of Catalona, Barcelona, n 997, 000 and 003, repectvely. Snce 0, he ha been a Full Profeor wth the Department of nergy Technology, Aalborg Unverty, Denmark, where he reponble for the Mcrogrd Reearch Program. From 0 he a guet Profeor at the Chnee Academy of Scence and the Nanjng Unverty of Aeronautc and Atronautc; from 04 he char Profeor n Shandong Unverty; from 05 he a dtnguhed guet Profeor n Hunan Unverty; and from 06 he a vtng profeor fellow at Aton Unverty, UK. H reearch nteret orented to dfferent mcrogrd apect, ncludng power electronc, dtrbuted energy-torage ytem, herarchcal and cooperatve control, energy management ytem, mart meterng and the nternet of thng for AC/DC mcrogrd cluter and landed mngrd; recently pecally focued on martme mcrogrd for electrcal hp, veel, ferre and eaport. Prof. Guerrero an Aocate dtor for the I TRANSACTIONS ON POWR LCTRONICS, the I TRANSACTIONS ON INDUSTRIAL LCTRONICS, and the I Indutral lectronc Magazne, and an dtor for the I TRANSACTIONS on SMART GRID and I TRANSACTIONS on NRGY CONVRSION. He ha been Guet dtor of the I TRANSACTIONS ON POWR LCTRONICS Specal Iue: Power lectronc for Wnd nergy Converon and Power lectronc for Mcrogrd; the I TRANSACTIONS ON INDUSTRIAL LCTRONICS Specal Secton: Unnterruptble Power Supple ytem, Renewable nergy Sytem, Dtrbuted Generaton and Mcrogrd, and Indutral Applcaton and Implementaton Iue of the Kalman Flter; the I TRANSACTIONS on SMART GRID Specal Iue: Smart DC Dtrbuton Sytem and Power Qualty n Smart Grd; the I TRANSACTIONS on NRGY CONVRSION Specal Iue on nergy Converon n Next-generaton lectrc Shp. He wa the char of the Renewable nergy Sytem Techncal Commttee of the I Indutral lectronc Socety. He receved the bet paper award of the I Tranacton on nergy Converon for the perod In 04 and 05 he wa awarded by Thomon Reuter a Hghly Cted Reearcher, and n 05 he wa elevated a I Fellow for h contrbuton on dtrbuted power ytem and mcrogrd (c) 06 I. Peronal ue permtted, but republcaton/redtrbuton requre I permon. See for more nformaton.