Abstract. Subject Areas. Keywords. 1. Introduction. Aphrodis Nduwamungu

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1 Open Access Lbrary Journal 27, olume 4, e379 ISSN Onlne: ISSN rnt: Revew on the Coordnaton and Energy Management of Mcrogrds Broad Based on Controller and Droop Control Some Useful Informaton Is Gven n Ths aper Aphrods Nduwamungu School of Electrcal Engneerng and Automaton, Tanjn olytechnc Unversty, Tanjn, Chna How to cte ths paper: Nduwamungu, A (27) Revew on the Coordnaton and Energy Management of Mcrogrds Broad Based on Controller and Droop Control Some Useful Informaton Is Gven n Ths aper Open Access Lbrary Journal, 4: e379 Receved: June 7, 27 Accepted: July 7, 27 ublshed: July 2, 27 Copyrght 27 by author and Open Access Lbrary Inc Ths work s lcensed under the Creatve Commons Attrbuton Internatonal Lcense (CC BY 4) Open Access Abstract The ntegraton of dstrbuted Energy Resources (DER) s challengng electrcal network qualty whch s the man vtal and ubqutous all over the world Coordnaton and energy management of mcrogrds n grd connected and n slanded mode s one of solutons of dstrbuton power qualty wth best relablty, and stablty to ensure power system reslence In ths revew, the coordnaton and energy management of mcrogrds broad based on controller and droop control some useful nformaton s gven n detals and analyzed Adonally, possble structures, optons and control methods of DER unts are presented whch s followed by the descrptons of system controls and energy management strateges Eventually, future trends of mcrogrds are dscuss, pont out how ths concept can be a key to select a best method of coordnaton and managng mcrogrds for future researchers n order to tackle some challenges based on power qualty Subject Areas Electrc Engneerng Keywords Control Strateges of Mcrogrd, Droop Control, Controller, Energy Management System Introducton A mcrogrd s made up by an nterconnected loads and dstrbuted energy re- DOI: 4236/oalb379 July 2, 27

2 sources wthn clearly defned electrcal boundares that acted as a sngle controllable entty wth respect to the grd A mcrogrd can connect and dsconnect from the grd to enable t to operate n both grd-connected or sland-mode to provde a customzed level of hgh relablty and reslence to the grd dsturbances [] Ths advanced, ntegrated dstrbuton system addresses the need for applcaton n locatons wth electrc supply and/or delvery constrants, n remote stes, and for protecton of crtcal loads and economcally senstve development Mcrogrds should be composed by dfferent mcro sources namely, Mcroturbne, wnd turbne, battery storage, supercapactors fuel cell etc The mcro source such as wnd power and photovoltac cells, whch output power s nfluenced by the weather, have obvous ntermttent [2] Control of mcrogrds becomes a crtcal ssue for ensurng relable operaton of the mcrogrd and each AC and DC mcrogrds have dfferent herarchcal controls, but those could be generalze nto three levels by referrng to the herarchcal control whch are classfed as follows: () the prmary control s based on the droop method, ncludng an output-mpedance vrtual loop; (2) the secondary control allows the restoraton of the devatons produced by the prmary control and to mprove power qualty va unbalance and mtgaton of harmoncs n network; and (3) the tertary control manages the power flow between the mcrogrd (MG) and the external electrcal dstrbuton system [3] The controller and droop control should be able to control and manage power properly n both operatng mode, grd-connected mode and stand-alone mode Mcrogrd n stand-alone mode would lead to more challenges, partcularly when the mbalance of generaton and consumpton happen because of flexble load and DERs An ntegraton of energy storage system nto the controller and droop control wll mprove the controller performance; meanwhle, DC lnk capactors support the voltage regulaton and energy management has a strong mpact on power system stablty whch wll lead to the coordnaton of chargng and dschargng constrants aganst overchargng and deep chargng and ncrease lfetme of some storage devces such as battery and so on ;sometmes the storage devces aforementoned can be used as back-up system n case of the man mcro sources become ncapable to reach load demand Mcrogrd loads are commonly categorzed nto two types: fxed and flexble Fxed loads cannot be altered and must be satsfed under normal operatng conons whle flexble loads are responsve to controllng sgnals Flexble loads could be curtaled n response to economc ncentves or slandng requrements consst of dstrbuted generaton unts (DG) and dstrbuted energy storage systems (ESS) whch could be nstalled at electrc utlty facltes and/or electrcty consumers premses [4], [5] Ths paper shows a revew on Coordnaton and energy management of mcrogrds broad based on controller and droop control The paper s organzed as followng Secton II gves a bref revew of droop control wth dfferent aspect wth respect to the form of power transmsson and dstrbuton wthn a mcrogrd Secton III, whch s followed by controller wth dfferent structures, Secton I, Concluson and future trends of mcrogrds 2/8

3 2 Lterature Revew 2 Characterstcs of Droop Control An ntegraton of multple renewable resources can be consdered as mcrogrd and can also behave lke smart grd accordng to the dfferent electrcal apparatus ncorporated n that ower system ndustry use a control technology whch has been avalable many decades called droop control Droop control can be dvded nto two broad categores namely conventonal droop control and modfed droop control [5] In electrcal power system analyss, most of generators operaton based on power transfer, frequency and mpedance of lne mean whle some countres use 6 Hz whereas other countres use 5 Hz Each resources need power electroncs nterface to transfer the energy to the common bus through mpedance of lne The general equatons based on the above statements are wrtten as follows E = sn φ () X 2 E cosφ = (2) X By consderng the value of power angle and trgonometrc rules, we can derve the followng clarfcaton of phase shft when power angle s very small 2 φ snφ φ And the cos( cos) =, the Equatons () and (2) become X φ (3) E and X ( E ) (4) after dervng the aforementoned equaton we can conclude that actve power depends upon angle meanwhle reactve power mostly depends upon to the output voltages [2] So the droop control can be defned from ampltude and frequency of nverter output voltages and Fgure s representng equvalent dagram of two parallel nverters by consderng dfferent parameters namely transmsson lnes, voltage of each nverters from sendng up to the recevng end and angles Table lsts typcal lne parameters It proves that the lne mpedance of the hgh voltage system s nductve due to ts value of nductance whch s bgger E δ I I 2 E δ 2 2 R jωl jωl R load Fgure Equvalent dagram of two parallel nverters [24] 3/8

4 Table Typcal lne parameters [6], [22] Lne type R ( Ω km ) X ( km ) Ω RX Low voltage lne Medum voltage lne Hgh voltage lne than the value of resstance, and the low voltage system s near pure resstve because of the value of resstance s bgger than the value of nductance meanwhle medum voltage s also nductance due to the value of nductance the aforementoned characterstcs of each lne wll be shown and summarzed n Table due to the voltage level, therefore, the traonal droop characterstcs cannot be appled n the low voltage mcrogrds, t needs mprovement 22 Characterstcs of Conventonal Droop Control Due to the standard, the frequency and the voltage ampltude are controlled by respectng the real and reactve power generaton of the system For ths reason, the power sharng strateges n mcrogrd s succeeded by the output power generaton accordng to ts dstrbuted generator s power ratng 22 Conventonal Droop Control wth a ure Inductance Lne Means ωl R ths forms the frequency and voltage are ncreasng meanwhle actve and reactve power also ncrease The reactve power s not varyng whereas the changng of φ s makng actve power also change Ths Fgure 2 s presentng how voltage E s varyng at the same tme actve and reactve power change based on the followng control equatons [7]: f = f K (5) = K (6) q Fgure 2 s showng the voltage and frequency that has some lmtatons based on standard where the actve power ncrease meanwhle frequency drop whereas reactve power ncrease meanwhle voltage drop Therefore the above Equatons (5) and (6) are sketched as follows 222 Conventonal Droop Control wth a ure Resstve Lne Ths characterstc of s smlar to the above but the dfference s based on value of phase whch s small and vary between 9 < φ < 9 and cosφ, snφ φ E ( E) Eφ thus the droop equatons are modfed as follow as, R R then R ωl so Table 2 s the summary of the benefts and drawbacks of conventonal droop wth a pure resstve lne by comparng benefts and drawbacks of conventonal droop control 223 Conventonal Droop Control wth Complex Impedance The actve power and reactve are ncreasng n clockwse and antclockwse ac- 4/8

5 Table 2 Traonal and modfed droop control Type of droop control Equatons Benefts Drawbacks Reference Conventonal droop control f = f K = K q No communcaton wres between dstrbuted energy generatons (DER) Easly to adjust the coeffcents Has more relablty and stablty margn Applcable for low voltage Boundary based on sgn of voltage and frequency [] Droop modfed = K n q ( ) f = f K Actve power s decreasng due to the varaton of R Economcally ths system can provoke much losses n transmsson s ncreasng due to the varaton of φ Actve power s ndependent on φ [7] f f ±4%* f n ω ± 5%* n Capactve load Inductve load max max max max Fgure 2 Droop characterstcs based on pure nductve lne [23] cordng to the varaton of angle φ ths type of droop control has some enhancement on mcrogrd where t can facltate the decouplng of actve and reactve power n terms of controls In adon, t has some mprovement based on voltage regulaton Whereas t has some negatve mpact on mcrogrd stablty where lne mpedances should be calculated n advance by observng the varaton of actve power and reactve power where actve and reactve power can be calculated by usng Equatons (7) and (8) [8] U ( E U)cosφ Eφsn φ Z (7) U ( E U)snφ Eφcos φ Z (8) 23 Type of Droop Control Modfed Assumptons sutable n hgh voltages transmsson are not applcable n mcrogrd whereby the value of resstance should be ncluded n calculatons and the droop modfed s based on changng some parameters of conventonal droop control n the followng paragraph wll descrbe all type of droop control modfed 23 A Modfed Droop Control Method for arallel Operaton of SI s n Mcrogrd In order to enhance dynamcs response of parallel connected nverters, the trad- 5/8

6 Table 3 Traonal and modfed droop control tonal droop control s mproved by addng dervatve terms on traonal droop Ths method s capable to acheve best performance By any means, t s not easy to track a sutable coeffcent for the dervatve term n the supplemental droop equatons and can be gven as follows: * d ω = ω mf nf (9) * d E= E m e ne () Table 3 s descrbng the benefts and drawbacks of modfed droop control method for parallel operaton of voltage source nverter s (SI) n mcrogrd by comparng wth conventonal traonal droop control 232 Droop Control Modfed wth Adaptve Transent Droop Gan Ths type of droop control modfed s combnng statc droop characterstcs and transent droop control at the same tme The transent droop functon use some dervatves of actve power and reactve power and added to droop functons n order to reduce an oscllatons of power sharng between mcro sources To fgure out the problems related to the power oscllatons and voltages we need to add an adaptve transent droop gan m d on conventonal droop control as follows: d δ ω = ωn ω md () d t dδ By replacng by ts value we get new equaton whch s: d d ω = ωn m mdp md (2) where mdp and md are adaptve dervatve gans n the voltage frequency droop control characterstcs Equaton of voltage and reactve power are modfed also by addng adaptve transent droop gan n d to the equaton of traonal droop control Type of droop control Equatons Benefts Drawbacks reference Traonal f = f K = K q No communcaton sgnals between unts Best modularty Best relablty and stablty Ubqutous and applcable n dfferent mcro sources Load rely on frequency, voltage and ampltude devatons Impedance of lne wll affect sharng between mcro sources It has oor transent sutable for pure nductve lne [] Modfed * d m n f f ω = ω * d E= E m n e e Fast transent response recson and accurate regulaton Reducton of overloadng and underloadng Hardly to track the sutable coeffcents for dervatve term that ensures stablty of the system Frequency are leadng toward to the nomnal value [9] 6/8

7 d δ = n q nd (3) By nsertng the value of n d we get new equaton whch can be shown below: d d = n n q nd nd (4) Benefts of droop control modfed wth adaptve transent droop gan Droop coeffcents are broad based on small sgnal stablty araton of voltage output magntude and angle are varyng n vce-versa Remarkable changng of dampng modes power sharng strateges Crculatng current and power oscllaton are reduced completely Drawbacks of droop control modfed wth adaptve transent droop gan Implementaton of ths method need a prerequste of small sgnal stablty It s hard to track the value of dervatve terms of actve power and reactve power aratons of voltage, frequency and reactve power are not presented [] 233 Robust Droop Multple Loop Control Method Ths type of droop control s the same as conventonal droop control but the aforementoned droop has been added wth outer robust power and the nner voltage and current loop Ths type of control consder the nfluence of output mpedance and lne mpedance n order to mprove a relablty and robustness of power sharng between two parallel nverters The smplfed dagram of two parallels nverters are shown n ths Fgure 3 where E s the ampltude of the nverter output voltage e, s the phase devaton between e and u L Consderng the output mpedance and lne mpedance [], actve and reactve power can be respectvely expressed as: 2 = ( EU Lcosφ UL) cos β EU Lsnφsn β (5) Z Z lne 2 = ( EU cosφ U ) sn β EU snφ cos β L L L Z Zlne where Zlne = Rlne jx lne s the lne mpedance Z = R jx s the equvalent output mpedance of nverter, and E s the mpedance angle between Z and Z lne In the low-voltage mcrogrds Rlne Xlne, so snφ φ and cosφ = Fgure 3 s schematc dagram of robust power droop controller where all equatons of robust power droop controller are wrtten and nserted n (6) & DFT U Dscrete FourerTransform m n E K e ω U K I s K I s LL Feedbackcoeffcent ωt E ϕ 2E sn( ωt ϕ) U ref Fgure 3 Schematc dagram of the robust power droop controller [] 7/8

8 dagram meanwhle Table 4 s short statement of robust power droop controller about sortng out the benefts and drawbacks of robust power droop controller over benefts and drawbacks of conventonal droop control 234 Droop Control wth Low ass Flter Modfed In power system stablty and control, frequency of system s vtal sgnal because of many apparatuses should operate at the range of nomnal frequency accordng to the standard of IEE and other research nsttutons As aforethought, common controllers have not presented any soluton to resolve frequency drop n the SIs The purpose of proposed control method s returnng frequency to the nomnal value The new control method should reduce the overshoot and oscllaton of response The modfcaton of low pass flter by addng zero and pole on low pass flter wll automatcally change the output value but the output sgnal s stll remanng n standard Regardng frequency wll go towards nstead to decrease due to the varaton of load [2] 235 Sne rtual Actve ower-frequency Droop Control The droop control modfed can decouple actve and reactve power through rotatng orthogonal transformaton, and extend conventonal droop control to low voltage mcrogrd Ths type of droop modfed called sne vrtual actve power frequency droop control may restrct the maxmum power of nverters wthout addng droop controllng unt of ampltude lmts, and can enhance the accuracy of reactve power sharng By consderng rotatng orthogonal transformaton matrx T, the actve and reactve power can be transformed nto the vrtual actve power and vrtual reactve power va the coordnate transformaton T for rotatng coordnates orthogonal transformaton matrx can be expressed as: X R Z Z snθ cosθ T = = R X cosθ snθ Z Z (7) the actve and reactve power nject nto the bus by every nverter can be wrten as: Table 4 Conventonal droop control modfed and robust droop multple loop control Type of Droop control Equatons Benefts Drawbacks References Traonal f = f K = K q No communcaton sgnals between unts Hgh stablty Best relablty Best proftablty Sutable for nductve lne Modfcaton durng grd connected mode are needed oltage and frequency depend on coeffcents settngs [] Robust Droop multple oop control * U nu L L ( s) ( E U ) = L R R EU meu L L = R R s s * ( s) ( ω ω ) Hgh transent response Hgh stablty and relablty An accuracy of power sharng Trackng of voltage ower wll depend on voltage nstead of to depend on frequency vce versa Reactve power wll depend on frequency [] 8/8

9 U = R 2 2 ( U cosδ) X sn δ R X U = X 2 2 ( U cosδ) R sn δ R X By combnng Equatons (8) and (9) thus yeld = T Sne vrtual actve frequency droop control can be represented as follow as: ( ) (8) (9) (2) ω = ω ksn l (2) Ths modfcaton has done by changng only power and frequency equaton thus the Fgure 4 proves the sne droop control and general dagram Sne vrtual actve power-frequency droop control s robust and feasble to be mplemented Ths control method has a strongest enhancement on large mcrogrds whereas traonal droop control s lttle bt ncapable to control the power sharng strateges between mcro sources [3] 236 Angle Droop Control Angle droop control s a conventonal droop control whereby the frequency has been replaced by angle and ts tremendous are shown clearly where the frequency drop are less than the droop control wth frequency and stablty margn are very good In adon, the varaton of output power s very low than the power from traonal droop Eventually, ths control method has best frequency regulaton and good stablty margn by comparng to the conventonal droop technques Fgure 5 s mplementaton of the equatons of angle droop control and schematzed n the followng fgure meanwhle Table 5 s bref of drawbacks and T Ksn[ l( )] ω ω T K K q q U U Fgure 4 Block dagram of sne droop control Droop control equatons ower calculaton = I qq I d d = Iq q I d d LF LF - - K p K q - - δ δ * = Esn( ωt ϕ) ~ E Fgure 5 Angle droop control 9/8

10 Table 5 Conventonal droop control and angle droop control Type of droop control General Equatons Advantages drawbacks References Conventonal droop control f = f K = K q Easy to be mplemented No need of a communcaton cables between mcrosources Low nerta oor dynamc response [] Angle droop control δ = δ K q = K Stablzes the operaton mode of the droop control that have hgher Constant frequency regulaton Best stablty margn More robust and effectveness Requrement of GS oor performance of actve and reactve power sharng between unts [4] benefts of angle droop control by comparng the benefts and drawbacks of conventonal droop control [4] 237 Droop Control Based Synchronzed Operaton The feasblty of ths control method has two man clue namely: error reducton operaton and voltage recovery operaton The power sharng performance s mproved by the sharng error reducton operaton, whch s hghly actvated by the lower band wh synchronzaton sgnals However, the error reducton operaton wll be avalable n reducton of output voltage ampltude Moreover, the voltage recovery operaton s proposed to compensate the decrease In ths control, communcaton s compulsory even t s very smple to be mplemented however, the plug-and-play s reserved Eventually The mproved droop control equatons can be wrtten as [5] [6] * ω = ω K (22) K * n K n= n= (23) E t = E n t K G n E where K denotes the tme of synchronzaton event untl tme t where the voltage recovery operaton sgnal at the n-th synchronzaton nterval, n G s has two possble values: or If G n =, t means the voltage recovery operaton s performed Benefts droop control based synchronzed operaton Improved power sharng system Not put-on by the physcal parameters Sturdy to communcaton delay Dsadvantages droop control based synchronzed operaton Requrement of low bandwh communcaton Hardly to be mplemented 238 Dot Droop Control Method Ths control method s based on relatonshps between a reactve power and the varatons of output voltage n order to mprove the reactve power sharng strateges The proposed control method can avod ths couplng dependence The change rate of voltage wll lead contnuously untl the desred flows, and ts performance cannot be much affected by the lne mpedance n G /8

11 [5] The droop controller can be expressed as: = K (24) q t * xd τ t = (25) where K q s the droop coeffcent, s nomnal voltage, s set to zero, s set to zero and related to the reactve power of dstrbuted generaton, and s the reactve power set pont at the nomnal value In steady state the should be reset back to zero n order to prevent the varaton of output voltage magntude Eventually the restoraton mechansm has been desgned as: d = Kres R ( ) (26) The control dagram of proposed and droop control and the dstrbuted generaton (DG) control block dagram are shown n Fgure 6 The control method of proposed droop control and the dstrbuted generaton block dagram are shown n ths Fgure 6 Benefts of dot droop control method ower calculaton = * I * I d = * I * I d I d q q q q d p q LF LF Droop controller U f Droop controller ω E E sn( ω t) * - dot droop oltage and current loop controller I I ref ref WM ower Converter k res R s n s Droop controller restoraton Fgure 6 droop controller and the control block dagram of sngle dstrbuted generaton The output results are related to the ntal conons of the voltage change rate Inspte of the system s stable and the steady-state solutons may be t does not exstin adon,the power sharng performasnces are not crucal superor to those of conventonal methods /8

12 Restorng process of voltage wth a local control loop reventon of couplng dependablty Dsadvantages of dot droop control method Depend upon the ntal conons The controller s not possble n real mcrogrd applcatons Steady-state soluton may be not avalable Stablty of the controller through a mcrogrds s not good 239 State of Charge Based on Droop Control The man purpose of ths control method s to enhance dynamc performance of the system and sort out the problem related to the sharng power n the dstrbuted energy storage system and to mprove the lfe tme of some storage devces [7] The state of charge s one way to recognze tme for chargng and dschargng accordng to the crtera determned by a desgner and for some storage devces can be used n backup system when the man mcro sources become ncapable to satsfy the load demand [] The energy storage unts wth hghest SoC generate more actve power than the lowest Soc But the SoC-based droop control, the frequency gradually devate from ts nomnal value So, an auxlary control has been ntegrated n order to restore the frequency and ampltude of the AC bus voltage so ths effect, made the frequency and ampltude of the pont of common couplng to be restored [7] 23 Arctan Droop Control In ths type of droop control modfed the power frequency droop slope has been replaced by arctan based algorthm In order to carry out the arctan based algorthm, the mcrogrd operators should make sure f the frequency of the system s n accepted range accordng to the standard In fact the determnaton of droop coeffcents s based on frequency constrants meanwhle usng a fxed gradent resently fxed gradent droop s not a curve nether an angle t s just a straght lne Therefore, the dfferental of gradent (concavty) s all the tme zero The arctan droop permts varaton n both gradent and concavty of the power profle The gradent vares va the power boundares and t s feasble to acheve natural frequency boundng wth no dependence of a secondary or tertary level controller Scentfcally arctan droop control supples an adequate best control than the gradent of droop about power set pont In adon, t has best desrable horzontal asymptotes whch s a smooth functon over the doman beng used [8] Eventually t has exstng functon lbrares n hghest level codng languages and the general equatons of droop can be wrtten as: a p ( ( ρ ( ))) f = f arctan (27) π And s equal to ( ( )) ω = ω 2a arctan ρ (28) p Benefts of Arctan droop controller 2/8

13 a p a p Naturally the frequency s the range of f and f 2 2 Reducton of an angle untl t reaches as the same as droop control Best performance of controllng power sharng scheme The system s more robust and more stable No need of communcaton wres Drawback of Arctan droop controller The system s more complex In terms of economy the system s costly Hz 23 New erspectves on Droop Control n ac Mcrogrd The man dea behnd on ths type of droop control s modfcaton of ω by addng proportonal dervatve Whereas are added a coeffcent of vrtual mpedance n order to mprove reactve sharng mechansm Therefore, ths type of droop modfed has some advantages and drawbacks whch are lsted below [9]: Advantages new perspectves on droop control n ac mcrogrd Reactve power sharng accurately Enhancement of transent process Robustness regardng parameters system Drawback new perspectves on droop control n ac mcrogrd Requrement of hgh bandwh for controller s the man challenges Frequency devaton s small 24 Control Modes for Dstrbuted Generatons The selecton of control strategy depends upon the types of mcro sources nstalled n the mcrogrd If the energy from mcro sources are nfluenced by weather for nstance array and wnd turbne generator etc Those types of mcro sources should be operated by controller even though controller cannot work alone where t needs voltage, frequency, reactve power and nonreactve power reference from droop controller whch s called f f Control s composed by three loops whch are power, voltage and current loop Therefore by usng dqo transformaton the current loop takes drect-axs reference current dref and quadrature-axs reference current qref from power loop and voltage loop as well In power control loop, the reference actve power follows the change of frequency /f control could adjust the reference power of dstrbuted generatons accordng to the voltage and frequency of mcro-grd by respectng the standard If the mcrogrd s composed by large storage capacty, the best control strategy s pure droop or fcttous mpedance, snce the load followng s shared among mcro sources [] Type of Controller controller must acheve a specfc qualty n order to control mcro sources In the structure of controller, the actve and reactve power should be decoupled to get the reference value of the nductor current dref, and qref whch 3/8

14 wll be compared wth the actual values The steady state error sgnal passes through I or ID controller of the nstantaneous current loop as the nverter brdge modulaton voltage sgnal I or ID controller has a capacty of keepng steady state error zero by adjustng ts values, and the dspatched generaton of the control receve the support of the frequency by the soft phase locked loop Actve and reactve power reference are the reference values of the actve power and reactve power; then ω s the frequency; current reference from drect axs and reference current from quadrature axs by power decouplng and dref, qref are the d-axs and the q-axs current reference value by the power decouplng; d and q are the modulated voltage of the d-axs and the q-axs by the current loop control [] The schematc dagram of controller s presented by Fgure 7 ) controller based on conventonal roportonal Integral (I) Ths type of controller s composed by two loops whch are power and current loop Moreover, controller can be realzed by decouplng process where I controller has been used for gettng a steady state error zero The adjustment of I controller wll produce dref and qref as nput for current loop The structure of ths controller s drawn n Fgure 8 and schematzed as follows: 2) control strategy based on multvarable I-controller Contrary to the controller conventonal I-controller that composed by feed forward sgnals to mnmze the couplng between two axes, moreover the Current controller Loop ref ref 2 3 abc dqo 2 3 d q / / I dref I Ld I Lq I qref I ω Lf ω L f I d dqo abc q abc Fgure 7 Schematc dagram of controller [] ref ref ower loop I abc d dqo q I I dref I Ld I Lq I qref Current controller Loop I ω Lf ω L f I d dqo abc q abc Fgure 8 Schematc dagram of controller [7] [2] 4/8

15 multvarable current controller the plant nverson technque has been mplemented and schematzed as follows accordng to the Fgure 9 [2] 3) Another type of controller The purpose of control s to supply constant actve and reactve power at a desred power factor The reference values of power are determned by a local or central controller from the mcrogrd control center Ths schematc dagram can be mplemented as a voltage controlled current source Current or voltage components n drect axs ( & ) Id d And n quadrature axs ( q & q) I wth nverter termnal voltage are calculated based on specfc method [2] Therefore the Table 6 s comparson of benefts and dsadvantages of each type of controller 3 Future Trend and Concluson Ths academc revew presents eleven () droop control modfed and four type (4) of controllers for controllng mcrogrd The Essental operatng prncples of each of the strategy has been emphaszed The advantage and drawbacks of each controller has been found out as relable scentfc evdence whch determne the choce of optonal control strateges of mcrogrd Therefore, the results of ths work have provded a state-of-the-art revew of research efforts n mcrogrd control systems for future researchers wllng to do a research about mcrogrd Eventually, type of controller archtectures for mcrogrds power energy managements, and voltage and frequency regulaton has been descrbed, and hghlghted that modern mcrogrd has to become ntellgent and more ref dref d _ st n st U c, d d _ v d abc abc dqo d q Gan K d Kq ref ωnn T st ref ref _ q qref ωnn T st ST ST n _ v q Uc,q q Fgure 9 Schematc dagram of the controller based on multvarable current controller 5/8

16 Table 6 Benefts and dsadvantages of each type of controller [], [2] Type of controller controller controller based on I controller controller based on multvarable I-controller Another Type of controller Benefts Suppresson of transent fluctuaton of voltage and frequency are not perfect Best correctness and effectveness way to manage mcrogrd n dfferent way such as slanded mode and connected to the utlty grd Suppress the transent fluctuaton of voltage and frequency Improvement of stablty and relablty Not need of communcaton between unts Good dynamc responses Mantenance of economc operaton Fast dynamcs response Less transent Best capablty of decouplng actve and reactve power Rejecton performance of dsturbance Better trackng of the reference values Don t have zero crossng detecton, The computaton of actve and reactve power go smoothly The smplcty of the algorthm erformance of shftng between states are seamlessly wthout any nterrupton, seamlessly Dsadvantages ropagaton of steady state error roblems related to the best stablty and relablty ropagaton of steady state error n the system Inexpensve economcally Easly to desgn t Adjustment of I controller need some are very complcated Expensve economcally Hardly to be mplemented More dffcult to adjust of I controllers ncorporated n voltage and current loop and t needs more sklls about desgn of flters oor dynamc response Low nerta It s very hard to desgn and to mplement ths controller flexble by the help of telecommuncaton system based on broadband technology The sgnfcance of the load characterstcs for mcrogrds operaton and control are demonstrated and deeply explaned Under the mpact of load and lne, characterstcs has been analyzed; several dfferent load sharng strateges among dstrbuted energy resource (DER) unts has been compared for nstance reactve and actve power managements Three robust controllers namely conventonal droop control, droop modfed and controller has been shown ther benefts and drawbacks aganst each other Acknowledgements Frst of all, I would lke to prase my Almghty God, for supportng me health, wsdom and strength n my work and for hs perfect gudance of my lfe and n all my actvtes I would lke to express my sncere thanks and best regards to my beloved and respected famly and my laboratory mates for ther nvaluable help and encouragement durng my research for ther moral and fnancal support throughout my entre academc career and I would lke to thank to Saood aseem hd student due to hs academc support, Fnally I would lke to thank chna government scholarshp councl (CSC) helped me for my successfulness References [] Saleem, HA (24) Mcrogrd Modelng and Grd Interconnecton Studes [2] Mou, X, B, D and Ren, X (2) Study on Control Strateges of a Low oltage Mcrogrd Danl Xtong Zdonghua (Automaton of Electrc ower Systems), 34, 6/8

17 9-96 [3] andoorn, TL, asquez, JC, De Koonng, J, Guerrero, JM and andevelde, L (23) Mcrogrds: Herarchcal Control and an Overvew of the Control and Reserve Management Strateges IEEE Industral Electroncs Magazne, 7, [4] Wang, Y, Tan, KT and So, L (23) Coordnated Control of Battery Energy Storage System n a Mcrogrd ower and Energy Engneerng Conference (AEEC), 23 IEEE ES Asa-acfc, IEEE, -6 [5] CmnLs8feNsTfU [6] Engler, A and Soultans, N (25) Droop Control n L-Grds 25 Internatonal Conference on Future ower Systems, IEEE, 6 p [7] an, Y, L,, L, X, Le, B and Xu, Z (2) Strategy of Research and Applcaton for the Mcrogrd Coordnated Control 2 Internatonal Conference on Advanced ower System Automaton and rotecton (AA), Bejng, 6-2 October 2, [8] Yao, W, Chen, M, Matas, J, Guerrero, JM and an, ZM (2) Desgn and Analyss of the Droop Control Method for arallel Inverters Consderng the Impact of the Complex Impedance on the ower Sharng IEEE Transactons on Industral Electroncs, 58, [9] Trved, A, Jan, DK and Sngh, M (23) A Modfed Droop Control Method for arallel Operaton of SI s n Mcrogrd Innovatve Smart Grd Technologes-Asa (ISGT Asa), 23 IEEE, -5 [] Hassanzahraee, M and Bakhsha, A (2) Transent Droop Control Strategy for arallel Operaton of oltage Source Converters n an Islanded Mode Mcrogrd Telecommuncatons Energy Conference (INTELEC), 2 IEEE 33rd Internatonal IEEE, -9 [] Chen, Y, Luo, A and eng, Z (23) Robust Droop Multple Loop Control Method for Resstance Output arallel Inverters 2nd IET Renewable ower Generaton Conference (RG 23), Bejng, 9- September 23, -4 [2] Tavakol, A, Gharehpetan, GB, Moghan, JS, Karshenas, HM and Sanjar, MJ (23) A New Control Approach to Improve Frequency Response of oltage Source Inverters Smart Grd Conference (SGC), Tehran, 7-8 December 23, -6 [3] Ln, R, Yang, M and L, H (25) Analyss of arallel hotovoltac Inverters wth Improved Droop Control Method Internatonal Conference on Modellng, Smulaton and Appled Mathematcs (MSAM 25), [4] Roslan, MA, Hasan, S, Hard, S and Nsja, I (25) Survey of Droop Control Technque for arallel Inverter Operaton In: Appled Mechancs and Materals ol 793, Trans Tech ublcatons, [5] Han, H, Hou, X, Yang, J, Wu, J, Su, M and Guerrero, JM (26) Revew of ower Sharng Control Strateges for Islandng Operaton of AC Mcrogrds IEEE Transactons on Smart Grd, 7, [6] L,, Chen, F, Chen, M, Guerrero, JM and Abbott, D (26) Agent-Based Decentralzed Control Method for Islanded Mcrogrds IEEE Transactons on Smart 7/8

18 Grd, 7, [7] Natesan, C, Ajthan, S, Man, S and Kandhasamy, (24) Applcablty of Droop Regulaton Technque n Mcrogrd A Survey Engneerng Journal, 8, [8] Rowe, CN, Summers, TJ, Betz, RE, Cornforth, DJ and Moore, TG (23) Arctan ower-frequency Droop for Improved Mcrogrd Stablty IEEE Transactons on ower Electroncs, 28, [9] Sun, Y, Hou, X, Yang, J, Han, H, Su, M and Guerrero, JM (27) New erspectves on Droop Control n AC Mcrogrd IEEE Transactons on Industral Electroncs, 64, [2] Dong, Y, Bng, C, un, L and Janhua, L (24) Control Strategy of Low oltage Mcrogrd Based on Equvalent Reference Current Control 24 Chna Internatonal Conference on Electrcty Dstrbuton (CICED), Shenzhen, September 24, [2] Rahbarmagham, H, AMr, EM, ahd, B, Gharehpetan, GB and Abed, M (25) Superor Decoupled Control of Actve and Reactve ower for Three-hase oltage Source Converters Turksh Journal of Electrcal Engneerng & Computer Scences, 23, [22] Araújo, LS, Narváez, DI, Squera, TG and llalva, MG (26) Study and Development of a Modfed Droop Control Strategy for Autonomous Mcrogrds roceedngs of the World Congress on Engneerng and Computer Scence, ol, San Francsco, 9-2 October 26 [23] Ramezan, M, Jooraban, M and Golestan, S (2) Droop Based Control of arallel Connected Three hase Inverters Usng Modfed Droop Method Internatonal Revew of Automatc Control, 4, [24] Chen, Z, e, X, Zhu, W, Lu, X and eng, L (25) A Two-Stage Combned Control Strategy for arallel Operaton of Inverter 25 IEEE Energy Converson Congress and Exposton (ECCE), Montreal, C, 2-24 September 25, Submt or recommend next manuscrpt to and we wll provde best servce for you: ublcaton frequency: Monthly 9 subject areas of scence, technology and medcne Far and rgorous peer-revew system Fast publcaton process Artcle promoton n varous socal networkng stes (LnkedIn, Facebook, Twtter, etc) Maxmum dssemnaton of your research work Submt Your aper Onlne: Clck Here to Submt Or Contact servce@oalbcom 8/8