Aalborg Universitet. Published in: I E E E Transactions on Smart Grid. DOI (link to publication from Publisher): /TSG.2015.

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1 Aalborg Univeritet Control Strategie for Ilanded Microgrid uing Enhanced Hierarchical Control Structure with Multiple Current-Loop Damping Scheme Han, Yang; Shen, Pan; Zhao, Xin; Guerrero, Joep M. Publihed in: I E E E Tranaction on Smart Grid DOI (lin to publication from Publiher):.9/TSG Publication date: 27 Document Verion Early verion, alo nown a pre-print Lin to publication from Aalborg Univerity Citation for publihed verion (APA): Han, Y., Shen, P., Zhao, X., & Guerrero, J. M. (27). Control Strategie for Ilanded Microgrid uing Enhanced Hierarchical Control Structure with Multiple Current-Loop Damping Scheme. I E E E Tranaction on Smart Grid, 8(3), DOI:.9/TSG General right Copyright and moral right for the publication made acceible in the public portal are retained by the author and/or other copyright owner and it i a condition of acceing publication that uer recognie and abide by the legal requirement aociated with thee right.? Uer may download and print one copy of any publication from the public portal for the purpoe of private tudy or reearch.? You may not further ditribute the material or ue it for any profit-maing activity or commercial gain? You may freely ditribute the URL identifying the publication in the public portal? Tae down policy If you believe that thi document breache copyright pleae contact u at vbn@aub.aau.d providing detail, and we will remove acce to the wor immediately and invetigate your claim. Downloaded from vbn.aau.d on: eptember 5, 28

2 IEEE TRANSACTIONS ON SMART GRID 25 Control Strategie for Ilanded Microgrid uing Enhanced Hierarchical Control Structure with Multiple Current-Loop Damping Scheme Yang Han, Member, IEEE, Pan Shen, Xin Zhao, and Joep M. Guerrero, Fellow, IEEE Abtract In thi paper, the modeling, controller deign, and tability analyi of the ilanded microgrid (MG) uing enhanced hierarchical control tructure with multiple current loop damping cheme i propoed. The ilanded MG i conited of the parallel-connected voltage ource inverter uing LCL output filter, and the propoed control tructure include: the primary control with additional phae-hift loop, the econdary control for voltage amplitude and frequency retoration, the virtual impedance loop which contain virtual poitive- and negative-equence impedance loop at fundamental frequency, and virtual variable harmonic impedance loop at harmonic frequencie, and the inner voltage and current loop controller. A mall-ignal model for the primary and econdary control with additional phae-hift loop i preented, which how an over-damped feature from eigenvalue analyi of the tate matrix. The moving average filter-baed equence decompoition method i propoed to extract the fundamental poitive and negative equence, and harmonic component. The multiple inner current loop damping cheme i preented, including the virtual poitive, virtual negative and variable harmonic equence impedance loop for reactive and harmonic power haring purpoe and the propoed active damping cheme uing capacitor current feedbac loop of the LCL-filter, which how enhanced damping characteritic and improved inner-loop tability. Finally, the experimental reult are provided to validate the feaibility of the propoed approach. Manucript received April, 25; revied July 3, 25; accepted September 8, 25. Date of current verion ; date of current verion. Thi wor wa upported in part by the National Natural Science Foundation of China under Grant 5375, and in part by the State Key Laboratory of Power Tranmiion Equipment & Sytem Security and New Technology under Grant 27DA527345, and in part by the Open Reearch Subject of Sichuan Province Key Laboratory of Power Electronic Energy-Saving Technologie & Equipment under Grant zjj25-67, and in part by the Open Reearch Subject of Artificial Intelligence Key Laboratory of Sichuan Province under Grant 25RZJ2. Paper no. TSG Y. Han and P. Shen are with the Department of Power Electronic, School of Mechatronic Engineering, Univerity of Electronic Science and Technology of China, No.26, Xiyuan Avenue, Wet Hi-Tech Zone, Chengdu 673, China ( hanyang@uetc.edu.cn; panhen@26.com), alo with the State Key Laboratory of Power Tranmiion Equipment & Sytem Security and New Technology, Chongqing Univerity, Chongqing 444, China, and alo with Artificial Intelligence Key Laboratory of Sichuan Province, Sichuan Univerity of Science and Engineering, Zigong 643, China. X. Zhao and J. M. Guerrero are with the Department of Energy Technology, Aalborg Univerity, Aalborg 922, Denmar ( xzh@et.aau.d, joz@et.aau.d). Color verion of one or more of the figure in thi paper are available online at Digital Object Identifier /TSG. Index Term Microgrid, droop control, econdary control, phae-hift control, mall-ignal model, power haring, virtual impedance, active damping, voltage control. NOMENCLATURE DG Ditributed generator. MG Microgrid. VSI Voltage ource inverter. AD Active damping. THD Total harmonic ditortion. PV ESS SoC APF Photovoltaic cell. Energy torage ytem. State of charge. Active power filter. LPF PLL Low-pa filter. Phae loced loop. P, Q Active and reactive output power. ω, E Angular frequency and amplitude of the output voltage reference. p, q Intantaneou active and reactive power. v Cα, v Cβ αβ-axi output voltage. i oα, i oβ αβ-axi output current. ω c Cut-off frequency of the LPF. pf, if Parameter of the frequency retoration control. pe, ie Parameter of the voltage retoration control. ω MG, E MG Angular frequency and voltage amplitude of MG. ω ec, E ec Angular frequency and voltage amplitude of the retoration controller. τ PLL time contant. d Additional phae-hift coefficient. δ The phae angle of the MG ytem. δ d The phae angle diplacement of the additional phae-hift loop. δ p The phae angle of the power-frequency droop controller. p, q Frequency and voltage droop coefficient. Δ Small deviation of the variable. T ω Window length of the moving average filter. Z vαβ Reitive-inductive virtual impedance of MG. Z oαβ Output impedance of MG. v dc voltage of MG. ω, f Fundamental and witching frequencie. L, L o Filter and output inductance.

3 IEEE TRANSACTIONS ON SMART GRID 25 2 C R L L NL,R NL,C NL AD pv, rv, hv pi, ri, hi R Filter capacitor. Balanced reitive load. Nonlinear load parameter. Active damping coefficient. Parameter of the voltage controller. Parameter of the current controller. I. INTRODUCTION ECENTLY, ditributed generator (DG) baed on renewable energy, uch a olar power plant, and wind turbine, etc., are attracting more and more attention for their environmental friendly characteritic []. High penetration level of DG may caue invere power flow, voltage fluctuation, and other problem in ditribution ytem. Microgrid (MG), which contain a number of ytematically organized DG unit, have been emerging a a framewor to overcome the problem caued by the high level of penetration of DG and mae large-cale application of DG poible [2]. A an interface between the DG and the power grid or local load, a voltage ource inverter (VSI) i the mot common topology which can operate either in grid-connected or ilanded mode to provide a controlled and high-quality power exchange with the grid or local load. In ilanded mode, the local load hould be upplied by the DG unit, which now act a controlled voltage ource (CVS) [3]. And the MG need ome form of control in order to avoid circulating current between the DG unit and enure table and efficient operation. The important role that can be achieved uing thee control tructure are active and reactive power control capabilitie among the DG, energy management, frequency and voltage regulation, and economic optimization [4]. Many control trategie of parallel VSI forming an MG have been invetigated [5-9], where the decentralized and cooperative controller uch a the droop method have been propoed and are conidered a preferred option due to everal attractive advantage uch a flexibility, and no need of high bandwidth communication []. The droop control i a ind of cooperative control that allow parallel connection of VSI haring active and reactive power []. In order to analyze the performance of thee method, recent literature addreing the tability and dynamic performance of the droop-controlled MG by uing tate-pace and mall-ignal model have been preented [8], [2]. The virtual impedance can be added in the control loop to enhance the reliability and performance of the droop-controlled VSI, enuring the inductive behavior of the output impedance of the DG. The tranient during fat load witching and voltage quality under nonlinear load in MG may alo be influenced by virtual impedance effect [7], [9], [3]. It i reported in [4] that the droop control method involve an inherent trade-off between power haring and voltage and frequency regulation. The econdary controller were propoed to compenate the deviation of voltage and frequency in [4], [5], [5]. The ilanded MG can retore the frequency and voltage amplitude in pite of deviation created by the total amount of active and reactive power demanded by the load [5] while the influence of thi econdary control on the tability and dynamic performance of MG wa not conidered. The increaing proliferation of nonlinear load could reult in ignificant harmonic ditortion in the ditribution ytem. And a MG hould be able to operate under nonlinear load condition without performance degradation. Baed on the IEEE tandard [6], the voltage total harmonic ditortion (THD) for enitive load hould be maintained below 5%. In indutrial application, LC filter are uually ued a an interface between the inverter and the local load to effectively mitigate the harmonic content of the inverter output waveform [7]. The pure LC or LCL circuit are highly uceptible to reonance with harmonic component generated by the inverter or the ditorted nonlinear load. In order to mitigate ytem reonance, a damping reitor can be placed in the LC or LCL circuit, but reult in power lo [8]. To avoid drawbac of paive damping, variou active damping (AD) method baed on the inner loop feedbac variable have been developed [9], [2]. Among AD method, the method involving feedbac of the capacitor current of the LCL filter ha attracted attention due to it effectivene, imple implementation, and wide application [2]. The variou hierarchical control trategie for MG have been preented in our previou wor [2-26], where the power ocillation, accuracy of the droop control, and the power haring problem are eldom conidered. In [2], the parallel-connected bidirectional converter for AC and hybrid MG application are analyzed in tandalone operation mode and the conventional hierarchical control in tationary frame under reitive condition i adopted. A general approach of hierarchical control for MG i preented in [22], the tertiary control could provide high-level inertia to interconnect more MG, acting a the primary control of the cluter and the tertiary cluter control can fix the active and reactive power to be provided by thi cluter or act lie a primary control to interconnect more MG cluter. In another hierarchical control tructure, an autonomou active power control trategy i modeled for MG with photovoltaic cell (PV) generation, energy torage ytem (ESS) and load to achieve power management in a decentralized manner [23], and the tate of charge (SoC) of the ESS can be ept within the afe limit by automatically adjuting the power generation from the PV ytem and load conumption. In [24], the coordinated control of DG inverter and active power filter (APF) to compenate voltage harmonic in MG i addreed, the APF participate in harmonic compenation and conequently the compenation effort of DG decreae to avoid exceive harmonic or loading of the DG inverter. In [25], the advanced decentralized and hierarchical control method are reviewed, and future trend in hierarchical control for MG and the cluter of MG are given. In thi paper, the enhanced hierarchical control for ilanded MG i preented, uing multiple inner loop active damping cheme, which how improved characteritic in term of hierarchical control tructure including the droop control with additional phae-hift loop and the centralized econdary control cheme for voltage amplitude and frequency retoration purpoe of MG. A mall ignal model i developed for the power control loop, which tae the droop control with additional phae-hift loop and econdary control into conideration. The power control loop i deigned to be overdamped to uppre power ocillation. The propoed multiple active damping and virtual impedance cheme are

4 IEEE TRANSACTIONS ON SMART GRID 25 3 adopted for the inner loop, i.e., the capacitor current feedbac loop plu the output virtual impedance loop, which achieve the purpoe of inverter ide LC reonance active damping, reactive power and harmonic power haring. The propoed approach employ the moving average filter-baed equence decompoition which i compoed of the virtual poitive- and negative-equence impedance loop at fundamental frequency, and the virtual variable harmonic impedance loop at harmonic frequencie [27], [28]. The virtual poitive- and negative-equence impedance loop improve the performance of the active power-frequency (P-ω) and reactive power-voltage magnitude (Q-E) droop controller and reduce the fundamental negative equence circulating current. A proper haring of harmonic power among all the DG inverter i achieved by uing the virtual variable harmonic impedance loop at characteritic harmonic frequencie. The feaibility of the propoed approach i validated by the experimental reult obtained from two parallel-connected 2.2W Danfo inverter under linear and nonlinear load condition. The main noveltie in thi paper are lited below. ) Development of the accurate mall-ignal model of the power controller and integration with the econdary controller. A tate equation model of the ilanded MG i preented, uing the primary and econdary control, including an additional phae-hift loop for power ocillation damping. 2) Implementation of the virtual poitive-equence and negative-equence impedance loop at fundamental frequency, and the virtual variable harmonic impedance loop at harmonic frequencie for reactive and harmonic power haring among the DG inverter. The moving average filter-baed equence decompoition method i propoed to extract the fundamental poitive- and negative-equence, and harmonic component. wind turbine E MG PV array E MG DG AC- - - ie pe ω MG ω MG if pf Secondary Control E ec ω ec v dc i o,αβ v C,αβ v a PWM x abc VSI v b AD v c Power Calculation i C,αβ L PR Current Controller P Q i o,αβ i La i Lb i Lc abc i L,αβ Droop Control with Phae Shift i αβ PR Voltage Controller δ E MG v Cb v Cc abc vc,αβ Voltage Reference Generator V Main Grid V/38 V Static Switch Energy Source Ilanded Microgrid DG DG 2 DG n Load Energy Source Load Load Fig.. Typical tructure of MG with DG and multiple load. Energy Source AC Bu 3) Development of the AD trategy for reonance damping of the ilanded MG. The multiple inner loop with AD trategy i propoed to avoid reonance and improve tability of the inner loop controller of the enhanced hierarchical tructure of the ilanded MG ytem. The remainder of thi paper i tructured a follow. The enhanced hierarchical control tructure and trategy of the ilanded MG ytem are analyzed in detail in Section II, including the droop control, econdary control, mall-ignal analyi of the power controller, virtual impedance loop, inner voltage and current control loop. Section III provide comprehenive experimental reult. Finally, Section IV conclude thi paper. II. ENHANCED HIERARCHICAL CONTROL STRATEGY A typical tructure of MG with n DG and load i given in Fig.. Although the propoed control trategie can operate in either the grid-connected mode or ilanded mode, only the ilanded operation mode will be conidered in thi paper. The three-phae VSI with LCL filter are uually ued a the DG interface to connect with the local AC bu, and the power tage of two DG and the propoed control trategy for their interface inverter connected in an ilanded mode are hown in Fig. 2. Each DG unit with it LCL filter can be conidered a a ubytem of the MG. C v Ca v vαβ Virtual v Impedance Loop αβ L o i oa i ob i oc abc i o,αβ Switch The Moving Average Filter-Baed Sequence Detection i i i o, f o, f o, h VSI Primary Control Strategy AC Bu Switch 2 Switch 3 R L Reitive Load L NL DG 2 wind turbine PV array AC- - - v dc v 2a VSI 2 v 2b v 2c Low BW Communication L 2 i L2,a i L2,b i L2,c abc i L2,αβ v C2,c C 2 v C2,a v C2,b abc vc2,αβ VSI 2 Primary Control Strategy L o2 i o2,a i o2,b i o2,c abc i o2,αβ C NL Nonlinear Load R NL Fig. 2. MG power tage and control ytem.

5 IEEE TRANSACTIONS ON SMART GRID 25 4 Virtual Variable Harmonic Impedance Loop at Harmonic Frequencie h=-5,7,-,3 i oα,h v R v,h α,h i oβ,h i oα,f i oβ,f i - oα,f i - oβ,f hω L v,h hω L v,h R v,h R v,f ω L v,f ω L v,f R v,f R - v,f ω L - v,f ω L - v,f R - v,f v β,h v vα,f v vβ,f v - vα,f v - vβ,f v vα v vβ Virtual Poitive- and Negative-Sequence Impedance Loop at Fundamental Frequency Virtual Impedance Loop h= -5,7,-,3 HC Switch v Power Controller v E MG δ Voltage Reference Generator HC Switch G v () HC Switch hv 2 2 ( h) G v () hv 2 2 ( h) PLL ω MG Droop Control δ Secondary Control τ δ d E MG δ p E ec HC Switch G i () / E ω ec hi 2 2 ( h) hi 2 2 ( h) ω HC Switch G i () ω d q G d () v dc AD v v α GPWM() v GPWM() v β Voltage Controller Current Controller PWM Proce Power Stage p Q P P Q G d () K pe ie / K pf if / ω c /(ω c ) LPF ω c /(ω c ) Secondary Control E MG AD Inner Voltage and Current Loop Fig. 3. Bloc diagram of the propoed enhanced hierarchical control trategy with multiple inner-loop damping cheme. p q ω MG Power Calculation p=v Cα i oα v Cβ i oβ q=v Cβ i oα -v Cα i oβ L L i Lα i Cα i oα i oβ C v Cα i Lβ i Cβ v Cβ C A hown in Fig. 3, the control trategy of an individual DG unit i implemented in the tationary reference frame. The dynamic of the DG unit are influenced by the output LCL filter, the droop controller, the econdary controller with frequency and voltage amplitude retoration, the power calculation, the virtual impedance loop which contain the virtual poitive- and negative-equence impedance loop at fundamental frequency, and the virtual variable harmonic impedance loop at harmonic frequencie, and the PR-baed inner voltage and current loop. The propoed control trategie are preented a follow. A. Droop Control for MG The droop control i utilized to avoid communication wire while obtaining good power haring, which i reponible for adjuting the frequency and amplitude of the voltage reference according to the active and reactive power (P and Q), enuring P and Q flow control [], [3]. The active power frequency (P-ω) and reactive power voltage magnitude (Q-E) droop control cheme are defined a ( P P ), E E ( Q Q ) () p where ω and E repreent the frequency and amplitude of the output voltage reference, ω and E are the nominal frequency and amplitude, P and Q are the active and reactive power reference normally et to zero in ilanded MG [26], [29], and p and q are the droop coefficient. Referring to [], the intantaneou active power (p) and reactive power (q) are calculated from the αβ-axi output voltage (v Cαβ ) and current (i oαβ ) a p vc io vc io, q vc io vc io (2) The intantaneou power are then paed through low-pa filter with the cut-off frequency ω c to obtain the filtered output real and reactive power (P and Q) a follow q P c p, Q c q (3) c c The bandwidth of the low pa filter i much maller than that of the inner controller of DG unit and the performance of the ytem i trongly influenced by thi fact [8]. B. The Secondary Control for MG The inherent trade-off between power haring and voltage and frequency regulation i one drawbac of the droop method. The conventional droop control i local and doe not have communication with other DG unit. In order to mitigate thee diadvantage, a retoration control can be added to remove any teady-tate error introduced by the conventional droop and achieve global controllability of the MG that enure nominal value of voltage and frequency in the MG [4], [5], []. A hown in Fig. 2, the primary and econdary control are implemented in each DG unit. The econdary control i realized by low bandwidth communication among the DG unit. By uing thi approach, the frequency and voltage amplitude retoration compenator can be derived a [4] ec pf ( MG MG ) if ( MG MG ) dt (4) Eec pe ( EMG EMG ) ie ( EMG EMG ) dt where pf, if, pe, and ie are the control parameter of the proportional integral (PI) compenator of the frequency and voltage retoration control, repectively. The angular frequency level in the MG (ω MG ) are meaured and compared to the reference (ω MG) and the error proceed by the PI compenator are ent to all the DG in order to retore the frequency of MG. The control ignal (E ec ) i ent to primary control level of each DG in order to remove teady-tate error of the droop control. ) Frequency control: Taing the idea from large electrical power ytem, in order to compenate the frequency deviation

6 IEEE TRANSACTIONS ON SMART GRID 25 5 produced by the local P-ω droop controller, econdary frequency controller have been propoed in []. A model of the frequency econdary control i hown in Fig. 4, which i alo depicted in Fig. 3 in detail. ω MG PLL(G PLL()) τ if pf G fec () ω ec Secondary Control ω ω Droop Control with additional phae-hift Loop p δ p δ d Fig. 4. Bloc diagram of frequency control for a DG unit. MG ω A P P 2 Primary Repone B E MG E C C Primary Repone D D Pmax d δ P2max G LPF() Secondary Repone Q Q,max P Secondary Repone P ω MG -Q, max Q Fig. 5. Secondary control veru primary control repone. Frequency retoration. Voltage amplitude retoration. The control bloc diagram in Fig. 4 include the droop control and econdary control. For droop control model, a low-pa filter (G LPF ()) with cutting frequency of 5 Hz ha been conidered for power calculation. The econdary control ha been modeled by mean of a implified PLL firt-order tranfer function (G PLL ()) with the gain (τ) ued to extract the frequency of the MG, the econdary PI controller (G fec ()) i ued to retore the frequency deviation [], and a proportional gain ( d ) of the additional phae-hift loop i uper-impoed to the active power control loop to uppre power ocillation [8]. The additional phae-hift loop perform a phae diplacement δ d which i added to the phae determined by the P-ω droop δ p, reulting in the angle δ of the inverter voltage E MG. Thi trategy increae ytem damping, and the phae of inverter δ which i ued by the reference generator bloc, i calculated by (5) From the bloc diagram of Fig. 4, ω MG i derived a Gf ec ( ) ( ) ( ) p d GLPF MG MG P G ( ) G ( ) G ( ) G ( ) p f ec PLL f ec PLL d (6) where G LPF (), G fec () and G PLL () are expreed a c GLPF (), () if G f ec pf, GPLL () (7) c Fig. 5 how the operation principle of econdary control, which remove frequency and voltage amplitude deviation caued by the primary control loop [5]. The characteritic of econdary control for the frequency retoration i hown in Fig. 5. It can be een that econdary control hift up the primary repone o that frequency reache to the nominal value. A hown in Fig. 5, the point of A and B are the nominal frequencie of the DG and DG2, repectively. The operation point of DG and DG2 deviate from the nominal frequencie and operate at the point of C and D when a tranient increae of load i applied in the ytem. The idling frequency change and the operation point of DG and DG2 hift to new operating point of C and D after the econdary controller i applied in the control ytem. Without thi action, the frequency of the MG i load dependent. E MG pe ie G eec () G d () E ec Secondary Control E E MG q Droop Control G LPF () Fig. 6. Bloc diagram of voltage amplitude control for a DG unit. 2) Voltage control: A imilar approach can be ued a in the frequency econdary control, in which each DG unit meaure the voltage error, and trie to compenate the voltage deviation caued by the Q-E droop [4], []. A hown in Fig. 5, the econdary control i able to remove voltage deviation caued by primary control in DG unit and the voltage amplitude retoration can be achieved. Fig. 6 how the implified control diagram in thi cae. The cloed-loop voltage dynamic model can be obtained a G () eec ( ) Gd( ) qglpf EMG EMG Q, ec G ( ) G ( ) () ie Ge pe (8) eec eec C. Small-Signal Analyi of the Power Controller Thi ection preent the mall-ignal model of the primary and econdary controller with additional phae-hift loop, emphaizing tability of the MG power controller. The mall-ignal dynamic of the retoration control can be obtained by linearizing (4) if ie ec pf MG, MG Eec peemg E (9) MG where the ymbol Δ in (9) denote the mall deviation of the variable from the equilibrium point [8]. Taing ()-(5) into account, the droop control with centralized econdary retoration control can be obtained a ec p ( P P ), EMG Eec EMG q ( Q Q ) () By linearizing (), and ubtituting (9) for (), the mall-ignal model can be written a if pf MG MG p P () ie EMG pe EMG EMG q Q Q

7 IEEE TRANSACTIONS ON SMART GRID 25 6 The linearized mall-ignal model of P and Q can be written a P c P c ( Io vc Io vc VC io VC io ) (2) Q cq c ( Io vc Io vc VC io VC io ) ΔP Δω Δδ p - p - d Δδ d Δδ Δω MG Fig. 7. Linearized model of the additional phae-hift loop for P-ω droop control. Fig. 7 how the linearized model for frequency and power angle loop, and the linearized inverter phae angle i given by p d d P (3) Subtituting Δω of () for (3), we get if pf MG MG pp d P (4) At thi point, a hown in Fig. 7, note that the firt derivative of the inverter phae angle (dδ/dt) i not the droop frequency (ω) but the MG frequency (ω MG ) and the firt tate variable for the tate equation model can be obtained a ( t) MG ( t) or ( ) MG ( ) (5) Conidering that the frequency i the firt-order derivative of the phae angle, we get (6) p d MG d Baed on (), (3), and (6), the equation that relate the frequency hift and econdary control of the inverter can be calculated due to the active power deviation from the equilibrium point, thu we get ( ) MG ( ) P (7) pf if MG p d Finally, according to ΔE MG of (), the derivative of E MG can be obtained a ( ) EMG E Q (8) pe ie MG q By rearranging (9)-(8), the mall-ignal power controller model can be written in a tate-pace form a in (9), which decribe the behavior of the tate Δδ, Δω MG, ΔE MG and ΔP, and ΔQ on the -th (=, 2) inverter in function of the deviation of the active power and reactive power from the equilibrium point. MG MG vc E M E MG N MG i o P P Q Q (9) Or ymbolically, repreented a X MX N S (2) where M and N are derived a if pf M ie qc, pe pe c c Io Io VC V C N Io Io VC V C cio cio cvc cvc cio cio cvc cv C (2) with ( p d ) c qc,. (22) pf Taing the Laplace tranformation on both ide of (2), uing initial condition of x init =, we get X ( ) M X ( ) N S ( ) (23) ( ) X ( ) S ( ) I M N (24) 55 where I 5 5 i a fifth order identity matrix. Then, auming (I 5 5 -M ) i noningular, ΔX () can be calculated a X ( ) ( I M ) N S ( ) (25) 55 By uing adjoint matrix adj(i 5 5 -M ), ΔX () can be rewritten a adj( I5 5 - M) NS ( ) X () ( I - M ) 55 pe (26) To enure ytem table, the pole of the denominator of (26) mut lie in the left-hand ide of the -plane, thu we get D( ) ( I - M ) (27) 5 5 TABLE I THE PARAMETERS OF THE PRIMARY AND SECONDARY CONTROLLERS Symbol Parameter Value ωc Meauring filter cut-off frequency π rad/ p, q Frequency and voltage droop coefficient. rad//w,. V/Var pf, if Frequency proportional and integral term of the econdary compenator.8, - pe, ie Amplitude proportional and integral term of the econdary compenator.8, - d Additional phae-hift coefficient.5 rad/w τ PLL time contant 5 m R - v,f, Rv,5, Rv,7, Rv,, and Rv,3 L v,f, Lv,5, Lv,7, Lv,, and Lv,3 Virtual reitance Virtual inductance 6,,,, and Ω 6, 2, 2,.5 and.5 mh Subtituting parameter of Table I to (27), the eigenvalue of the matrix M defined by (2) are calculated a λ =,λ 2 =λ 3 = ,λ 4 =λ 5 = (28) Note that all the non-zero pole of the matrix M are real and the ytem i over-damped. According to [8], the calculation of δ derivative preent a high variation level due to the active power ripple, epecially under nonlinear load condition. ω

8 IEEE TRANSACTIONS ON SMART GRID 25 7 intead of ω MG i ued a the frequency feedbac for the econdary controller in the experiment. D. Virtual Impedance Loop Virtual reitance enhance ytem damping without additional power lo, ince it i provided by a control loop and it i poible to implement it without decreaing ytem efficiency [4]. When virtual inductance i utilized, the DG output impedance become more inductive, decreaing P and Q coupling, enhancing the ytem tability, and reducing power ocillation and circulating current [4]. A hown in Fig. 3, the voltage drop acro the virtual poitive- and negative-equence impedance, and the virtual variable harmonic impedance loop in αβ reference frame are derived a vv, f () Rv, f io, f Lv, f io, f (29) vv, f () Rv, f io, f Lv, f io, f v () R i L i v () R i L i v, f v, f o, f v, f o, f v, f v, f o, f v, f o, f v () R i h L i v () R i h L i v, h v, h o, h v, h o, h v, h v, h o, h v, h o, h (3) (3) where R v,f and L v,f are the virtual fundamental frequency poitive equence reitance and inductance, R - v,f and L - v,f repreent the virtual fundamental frequency negative equence reitance and inductance, and h denote the dominant harmonic component, which are -5, 7, -, 3, etc., and ω repreent the ytem fundamental frequency. At the fundamental frequency, the virtual poitive equence impedance loop i deigned to be mainly inductive to improve the reactive power haring baed on the Q-E droop [3]. And the problem of the preence of the high R/X ratio which caue a coupling in the control of active and reactive power when uing the conventional droop controller ha been reolved. The virtual negative equence impedance i deigned to be reitive to minimize the negative equence circulating current among the DG [27]. And the ize of negative inductance need to be ept maller than the effective inductance to guarantee the tability of the virtual variable harmonic impedance loop at harmonic frequencie, and the larger the poitive reitance in the virtual variable harmonic impedance loop, the better the haring of harmonic power can be achieved [3]. In order to extract the fundamental poitive equence and negative equence current a well a the dominant harmonic current, a et of Par tranformation and the moving average filter are preented for realizing the load current decompoition, which i hown in Fig. 8. The moving average filter are linear-phae finite impule repone filter that are eay to realize in practice, are cot effective in term of the computational burden, and can act a ideal low-pa filter if certain condition hold [28]. The tranfer function of the moving average filter can be imply preented a T e GMAF () (32) T where T ω i referred to a the window length. The moving average filter pae the dc component, and completely bloc the frequency component of integer multiple of /T ω in hertz. i oαβ ω t -ω t i oαβ,f -5ω t 7ω t -ω t 3ω t Magnitude (db) Phae (deg) The Moving Average Filter The Moving Average Filter The Moving Average Filter The Moving Average Filter The Moving Average Filter The Moving Average Filter The Moving Average Filter ω t -ω t -5ω t 7ω t -ω t 3ω t Firt-order LPF i oαβ,f i oαβ,f i - oαβ,f i oαβ,5 i oαβ,7 i oαβ, i oαβ, Fig. 8. The propoed moving average filter-baed equence decompoition of fundamental poitive- and negative-equence, and harmonic component. Bloc diagram. Bode plot for the moving average filter. To provide a mean of comparion, the tranfer function of the firt-order counterpart of the moving average filter i obtained a (33) by approximating the delay term in (32) by the firt-order Padéapproximation. GMAF () T T /2 (33) e T /2 T/ 2 And the moving average filter with. window length (T ω ) i ued in the equence decompoition and the bode plot of the moving average filter and the firt-order LPF are hown in Fig. 8. It can be oberved that, the moving average filter reult in notche at the concerned harmonic frequencie. Conequently, the accuracy of the equence decompoition i ignificantly improved by uing the moving average filter. E. Inner Voltage and Current Control Loop v αβ v vαβ v vαβ,h v vαβ,f v - vαβ,f G v ()G i ()G PWM () Virtual Impedance Loop at Fundamental Frequency R v,f jω L v,f R - v,f - jω L - v,f i oαβ,h(r v,h - jω L v,h) h= -5,7,-,3, Virtual Variable Harmonic Impedance Loop i oαβ,f i - oαβ,f i oαβ,h L G i () C AD i Lαβ i Cαβ v Cαβ C The Moving Average Filter-Baed Sequence Detection (Fig. 8) i oαβ

9 IEEE TRANSACTIONS ON SMART GRID 25 8 Z oαβ () G()v αβ G()Z vαβ () Z Cαβ ()=Z oαβ ()G()(Z vαβ,f ()Z vαβ,h ()) i oαβ () v Cαβ Fig. 9. Voltage and current loop integrated with virtual impedance loop. Simplified model of the inner loop. Equivalent impedance model of a VSI. From ytem control diagram of Fig. 3, the implified model of the inner loop i derived, a hown in Fig. 9. In order to overcome the drawbac of the paive damping method, the active damping of a virtual reitor in parallel with the capacitor i ued to avoid reonance and enhance tability of the inner loop controller. The voltage loop reference ignal are modified by the virtual impedance loop which contain the virtual poitive- and negative-equence impedance, and the virtual variable harmonic impedance loop are hown in Fig. 9. Then, the output voltage of the a DG unit can be derived a v ( ) G( ) v ( G( ) Z ( ) Z ( )) i (34) C v o o where G(), Z vαβ, and Z oαβ are the cloed-loop voltage tranfer function, reitive-inductive virtual impedance, and the output impedance without virtual impedance loop, repectively [7], [3]. The virtual poitive equence impedance loop at fundamental frequency i only for attenuating circulating current, which can be omitted [27]. The tranfer function in (34) are derived a Gv ( ) Gi ( ) GPWM ( ) G () (35) 2 LC ( C G ( )) G ( ) G ( ) C Z ( ) Z ( ) Z ( ) Z v v, f v, h o () R v i PWM AD L R h L v, f v, f v, h v, h L h v, f R v, f Lv, h R v, h L G () i 2 LC C Gv A Gi GPWM ( ( ) ) ( ) ( ) (36) (37) Under nonlinear load condition, the dominant harmonic component hould be taen into conideration for the voltage and current controller in order to uppre output voltage harmonic. The tranfer function of the voltage and current controller are Gv () pv (38) h rv hv h5,7,,3 ( ) Gi () pi (39) h ri hi h5,7,,3 ( ) where pv and pi are the proportional coefficient, rv and ri are the reonant coefficient at the fundamental frequency, hv and hi repreent the voltage and current reonant controller coefficient for the h th order harmonic component. The total output impedance with the virtual impedance loop can be derived a Z ( ) G( )( Z ( ) Z ( )) Z ( ) (4) C v, f v, h o From (35)-(4), the equivalent impedance model of a DG unit can be derived, a hown in Fig. 9. Phae (deg) Magnitude (db) Phae (deg) Magnitude (db) th 7th th 3th Without Z v and AD With Z v and without AD Without Damping With Active Damping Fig.. Bode plot of the cloed-loop voltage gain and the output impedance of a DG unit. Cloed-loop voltage gain of G() with PR plu multi-reonant controller. The output impedance with PR and multi-reonant controller. Magnitude (db) With Z v, AD and muti-reonant controller 5th 7th th 3th By uing the cloed-loop model decribed by (35)-(4), the bode plot of the cloed-loop voltage gain and the output impedance with and without AD method are illutrated in Fig.. From Fig., it can be oberved that the AD method enure effective damping at LCL reonance frequency and the ytem how ufficient tability margin. The gain of the cloed-loop voltage controller are unity at the fundamental and 5 th, 7 th, th, and 3 th harmonic frequencie, repectively, which mean the ytem obtain the zero-error tracing capability at both the fundamental frequency and target characteritic harmonic frequencie. The total output impedance Z Cαβ () of a DG unit under PR controller plu virtual impedance loop which contain the virtual poitive- and negative-equence impedance, and the virtual variable harmonic impedance loop without AD i given in Fig.. It how that, the total output impedance i about 4 db without AD under reonance frequency of the LC filter and about 2 db under the conidered frequencie uch a -5f, 7f, -f, 3f, etc. Large output impedance lead to a large harmonic voltage drop under current harmonic, which ditort the output voltage [7]. The total output impedance of the DG under PR plu multi-reonant controller with virtual impedance loop uing AD method, Z Cαβ () i greatly reduced and lower voltage ditortion can be expected under nonlinear load.

10 IEEE TRANSACTIONS ON SMART GRID 25 9 III. EXPERIMENTAL RESULTS In order to validate the feaibility of the propoed enhanced hierarchical control trategy, the experimental reult obtained from two paralleled-connected DG unit are preented and compared. The experimental etup wa built and teted in the Microgrid Reearch Lab of Aalborg Univerity [32], which conit of two 2.2W Danfo inverter connected in parallel with linear and nonlinear load, and dspace6 platform wa ued to implement the control algorithm. The controller parameter of the MG are hown in Table I and II. The chematic and photo of experimental etup are hown in Fig. and Fig.2, repectively. TABLE II SYSTEM PARAMETERS OF EXPERIMENTAL SETUP Symbol Parameter Value vdc, vmg and MG voltage 65 V, 3 V f, f MG and witching frequencie 5 Hz, Hz L, Lo Filter and output inductance.8 mh,.8 mh C Filter capacitor 25 μf The experimental reult of the VSI in the ilanded MG ytem with and without uing the propoed virtual impedance loop and AD method under reitive load condition are compared in Fig. 3 and Fig. 4. A hown in Fig. 3, mall output voltage of inverter and inverter 2 would reult in evere output current ocillation, and higher output voltage reference would trip the converter due to the overcurrent protection. Fig. 4 how the experimental reult of the MG under reitive load condition, when the inner loop AD cheme with AD =28.5 and the propoed virtual impedance loop with the virtual poitive- and negative-equence impedance, and the virtual variable harmonic impedance loop are ued. It i oberved that the ocillation of the output current are alleviated with the propoed AD method and virtual impedance loop, which confirm theoretical analyi. vc,abc (V) RL Balanced reitive load 5/23 Ω LNL, RNL,CNL Nonlinear load 84 μh, 46 Ω, 235 μf AD Active damping coefficient 28.5 pv, rv, 5, 7,,3v Voltage loop PR parameter.75, 2, 5, 4, 2, 2 pi, ri, 5, 7,,3i Current loop PR parameter 3, 5,,, 5, 5 power upply 65 V v dc Inverter LCL filter i o,abc Switch L NL R NL C NL io,abc (V) i L,abc i C,abc v C,abc power v dc upply 65 V PC-Simulin RTW & dspace Control De i L2,abc i C2,abc v C2,abc LCL filter i o2,abc Switch 3 Switch 2 Nonlinear Load R L Reitive Load Fig.. Schematic of the experimental etup uing parallel DG unit. vc2,abc (V) dspace6 (c) Danfo Inverter Reitive Load io2,abc (V) Fig. 2 Photo of the experimental etup. Diode Rectifier (d) Fig. 3. Experimental reult of the ilanded MG ytem without uing the propoed control method under reitive load condition. The output voltage of inverter. The output current of inverter. (c) The output voltage of inverter 2. (d) The output current of inverter 2.

11 IEEE TRANSACTIONS ON SMART GRID 25 Normal Operation Load Step Inverter Diconnection vc,abc (V) io2,abc (A) io,abc (A) Fig. 5. Tranient repone of the output current during reitive load tep change (t=3.53) and udden diconnection of inverter (t=7.35). The output current of inverter. The output current of inverter 2. vc2,abc (V) Fig. 5 how the output currrent waveform of the MG haring the reitive load by uing the droop method with the additional phae-hift loop ( d ) and AD ( AD ) method. Both inverter are haring the reitive load in the normal operation mode, and a load tep increae of 23 Ω i uddenly applied at t=3.35 and inverter i diconnected at t=7.35, while only inverter 2 i upplying the total load current. A hown in Fig. 5, the inverter 2 can upply the load and the ytem remain table when inverter trip. THD=5.45% (c) vc,abc (V) io2,abc (A) THD=.2% (d) Fig. 4. Experimental reult of the ilanded MG ytem with uing the propoed control method under reitive load condition. The output voltage of inverter. The output current of inverter. (c) The output voltage of inverter 2. (d) The output current of inverter 2. Normal Operation Load Step Inverter Diconnection vc,abc (V) Fig. 6. Experimental reult of the output voltage of a VSI. The output voltage of inverter without uing the propoed method. The output voltage of inverter with uing the propoed method. io,abc (A) Fig.6 how a comparion of the experimental reult of the ilanded MG ytem under nonlinear load condition with and without uing the virtual impedance loop which contain the virtual poitive- and negative-equence impedance, and the virtual variable harmonic impedance loop and harmonic compenation in the voltage and current loop. A hown in Fig. 6, when the harmonic compenation i not activated and only the virtual poitive- and negative-equence impedance loop are activated, the output voltage are everely ditorted by nonlinear load and the total harmonic ditortion (THD) of output voltage i about 5.45%. Fig. 6 how the

12 IEEE TRANSACTIONS ON SMART GRID 25 experimental reult when harmonic compenation i enabled and the propoed control method i ued, where the multiple PR controller are tuned at the 5 th, 7 th, th, and 3 th harmonic frequencie in the voltage and current loop. In thi cae, THD of output voltage i reduced to.2%. It can be concluded that the THD of the output voltage are effectively reduced with the propoed control trategie which contain AD method, harmonic compenation and the virtual poitive- and negative-equence impedance, and the virtual variable harmonic impedance loop. t=7.35. A hown in Fig. 9, the active power (P, P 2 ) and reactive power (Q, Q 2 ) haring of the two DG unit are achieved. And mall amount of reactive power can be oberved due to the effect of output inductance (L o ). Normal Operation Load Step Inverter Diconnection io,abc (A) io,abc (A) Normal Operation Load Step Inverter Diconnection io2,abc (A) io2,abc (A) Fig. 7. Experimental reult of the ilanded MG ytem with the conventional droop control. The output current of inverter. The output current of inverter 2. Fig. 7 how the performance of the MG when the control of the virtual poitive- and negative-equence impedance, and the virtual variable harmonic impedance loop are not activated. It i hown that the current of inverter and 2 are not identical, due to the circulating current between thee DG unit. When the power haring error are compenated by uing the propoed method, the tranient repone of the current under nonlinear load are given in Fig. 8. From Fig. 7 and Fig. 8, it i hown that the current haring error are effectively reduced and inverter and 2 have imilar output current and the reactive and harmonic power haring performance are improved. Note that the virtual harmonic impedance at the dominant harmonic frequencie, i.e., 5 th, 7 th, th, and 3 th are controlled. The higher harmonic frequencie can alo be controlled in the virtual variable harmonic impedance loop when needed. The tability of the MG i alo guaranteed under nonlinear load condition when the load tep and udden diconnection of one DG unit are applied. The experimental reult of the ilanded MG ytem with and without uing the econdary controller for the propoed control cheme under reitive load condition are hown in Fig. 9. Fig. 9~(c) how the experimental reult without uing the econdary controller and when a load change i uddenly applied at t=3.35 and the firt inverter i diconnected at Fig. 8. Tranient repone of the output current during load tep change (t=3.53) and udden diconnection of inverter (t=7.35) under nonlinear load with the propoed virtual impedance loop and harmonic compenation control. The output current of inverter. The output current of inverter 2. A depicted in Fig. 9, the pea voltage of inverter and 2 are not exactly the ame under normal operation condition, and mall deviation i oberved. With an increae of the load i applied at t=3.35, the pea voltage of inverter 2 drop and the voltage of inverter increae. When inverter i withed off at t=7.35, the pea voltage of inverter 2 drop again due an increae of load. A hown in Fig. 9(c), the frequency i deviated from 5Hz, i.e., a teady-tate error about.5hz can be oberved under normal operation condition. With an increae of load, the frequencie of both inverter and inverter 2 drop for about.6 Hz. After the tripping of inverter, the frequency of the MG drop for.hz to reach a teady tate of Hz. Fig. 9(d)~(f) how the performance of the ilanded MG with uing the econdary controller. Fig. 9(d) how the dynamic repone of the active power and reactive power in the DG for each cenario. Note that there i a mall increae in active power to retore the frequency deviation when the econdary control i activated. The effect of the econdary control trategy to retore voltage and frequency deviation of the DG are depicted in Fig. 9(e) and (f). Notice that the deviation in voltage amplitude and frequency due to droop control and virtual impedance loop are recovered to the nominal value. Fig. 9(e) how the pea

13 IEEE TRANSACTIONS ON SMART GRID 25 2 Normal Operation Load Step Inverter Diconnection Normal Operation Load Step Inverter Diconnection P(W), Q(Var) P 2 P Q 2 Q P(W), Q(Var) P 2 P Q2 Q Amplitude (V) Inverter Amplitude (V) Inverter Inverter Inverter (c) (c) P(W), Q(Var) P 2 P Q 2 Q P(W), Q(Var) P 2 P Q 2 Q (d) (d) Amplitude (V) Inverter Amplitude (V) Inverter (e) Inverter (e) Inverter (f) Fig. 9. Performance of the ilanded MG conit of two DG without (a~c) and with (d~e) uing econdary controller under the reitive load condition. and (d) Active and reactive power. and (e) Voltage amplitude. (c) and (f) Frequency. (f) Fig. 2. Performance of the ilanded MG conit of two DG without (a~c) and with (d~e) uing econdary controller under the nonlinear load condition. and (d) Active and reactive power. and (e) Voltage amplitude. (c) and (f) Frequency.

14 IEEE TRANSACTIONS ON SMART GRID 25 3 voltage of inverter and 2 are identical under the normal operation cenario. The voltage amplitude retoration can be oberved when a udden increae of load i applied, and voltage amplitude recover to the nominal value uccefully even after diconnection of the inverter. Fig. 9(f) how that the frequencie of inverter and 2 are controlled to 5 Hz imultaneouly under normal operation condition. When the load are uddenly increaed, both frequency curve drop and gradually recover to 5 Hz. In the lat cenario, the frequency retoration of inverter 2 i alo achieved when the inverter i witched off from the MG at t=6.8, which recover to the pre-defined frequency after a few econd. Fig. 2 how the experimental reult of the ilanded MG ytem for the propoed control cheme under nonlinear load condition. Fig. 2~(c) how the performance of the MG without uing econdary controller and when a balanced reitive load i uddenly applied at t=3.35 and inverter i diconnected at t=7.35. And Fig. 2(d)~(f) how the performance of the ilanded MG with uing the econdary controller, repectively. Fig. 2 and (d) how that the active power and reactive power can be hared between DG by mean of droop control and enhanced virtual impedance loop, no matter with or without uing the econdary control. Thee reult illutrate that the P-ω droop control i ufficient to hare the active power once the virtual impedance loop and inner AD cheme are adopted, ince the frequency i a global variable in the MG ytem [5]. The propoed econdary control i able to eep the reactive power hared between DG unit under load variation. After diconnection of the inverter from the MG ytem in the lat cenario, inverter 2 feed the load current by injecting the doubled active power. By comparing Fig. 2, (c) and (e), (f), it can be oberved that frequency and voltage amplitude retoration of the DG unit can be achieved by mean of the econdary control trategy. IV. CONCLUSION Thi paper preent an enhanced hierarchical control for 3-phae parallel-connected VSI-baed ilanded MG. The propoed method utilize the primary control which i baed on the virtual impedance loop with the virtual poitive- and negative-equence impedance loop at fundamental frequency, and the virtual variable harmonic impedance loop at harmonic frequencie, and droop control cheme with an additional phae-hift loop to enhance the haring of reactive power and harmonic power between the DG unit. The moving average filter-baed equence decompoition ha been propoed to accurately extract the fundamental poitive and negative equence, and harmonic component for the virtual impedance loop. With the centralized controller of the econdary control, the voltage amplitude and frequency retoration are achieved. The developed mall-ignal model for the primary and econdary control how that, an overdamped feature of the power loop i achieved to improve the whole MG ytem damping. A multi-loop control trategy with the inner-loop AD method for reitive and nonlinear load condition i alo preented. The capacitor current of the LCL filter are ued a feedbac ignal to actively damp the high frequency reonance while an outer voltage loop with output virtual impedance regulate the output voltage and enure ytem tability over a wide range of operating condition. Experimental reult from two parallel-connected DG unit verified the effectivene of the propoed control trategie. REFERENCES [] R. J. Wai, C. Y. Lin, Y. C. Huang, and Y. R. Chang, Deign of high performance tand-alone and grid-connected inverter for ditributed generation application, IEEE Tran. Ind. Electron., vol. 6, no. 4, pp , Apr. 23. [2] D. E. Olivare, A. Mehrizi-Sani, A. H. Etemadi, C. A. Canizare, R. Iravani, and M. 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He joined the Department of Power Electronic, School of Mechatronic Engineering, Univerity of Electronic Science and Technology of China (UESTC) in 2, and where he ha been an Aociate Profeor ince 23. From March 24 to March 25, he wa a Viiting Scholar at the Department of Energy Technology, Aalborg Univerity, Aalborg, Denmar. Hi reearch interet include AC/ microgrid, grid-connected converter for renewable and DG, phae-loced loop (PLL), power quality, active power filter and tatic ynchronou compenator (STATCOM). He ha authored more than 2 ISI-indexed journal paper in the area of power electronic, power quality conditioner, and mart grid. He received Bet Paper Award from 23 Annual Conference of HV and Power Electronic Committee of Chinee Society of Electrical Engineer (CSEE) in Chongqing, China, and the 4th International Conference on Power Quality in 28, in Yangzhou, China. Pan Shen wa born in Hefei, China in 99. He received hi B.S. in Electrical Engineering and Automation from Anhui Agricultural Univerity, Hefei, China, in 23. He i currently woring toward the M.S. degree in Power Electronic and Electric Drive at the Univerity of Electronic Science and Technology of China (UESTC), Chengdu, China. Hi current reearch interet include ac/dc microgrid, power quality, power converter, power ytem automation, and active power filter. and microgrid. Xin Zhao received the B.S. and M.S. degree in Power Electronic & Electrical Drive from Northwetern Polytechnical Univerity, Xi an, China, in 2 and 23, repectively. He i currently woring toward the Ph.D. degree at Department of Energy Technology, Aalborg Univerity, Denmar. Hi reearch interet include control of power converter, power quality Joep M. Guerrero (S -M 4-SM 8-FM 5) received the B.S. degree in telecommunication engineering, the M.S. degree in electronic engineering, and the Ph.D. degree in power electronic from the Technical Univerity of Catalonia, Barcelona, in 997, 2 and 23, repectively. Since 2, he ha been a Full Profeor with the Department of Energy Technology, Aalborg Univerity, Denmar, where he i reponible for the Microgrid Reearch Program. From 22 he i a guet Profeor at the Chinee Academy of Science and the Nanjing Univerity of Aeronautic and Atronautic; from 24 he i chair Profeor in Shandong Univerity; and from 25 he i a ditinguihed guet Profeor in Hunan Univerity. Hi reearch interet i oriented to different microgrid apect, including power electronic, ditributed energy-torage ytem, hierarchical and cooperative control, energy management ytem, and optimization of microgrid and ilanded minigrid. He i an Aociate Editor for the IEEE TRANSACTIONS ON POWER ELECTRONICS, the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, and the IEEE Indutrial Electronic Magazine, and an Editor for the IEEE TRANSACTIONS on SMART GRID and IEEE TRANSACTIONS on ENERGY CONVERSION. He ha been Guet Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS Special Iue: Power Electronic for Wind Energy Converion and Power Electronic for Microgrid; the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS Special Section: Uninterruptible Power Supplie ytem, Renewable Energy Sytem, Ditributed Generation and Microgrid, and Indutrial Application and Implementation Iue of the Kalman Filter; and the IEEE TRANSACTIONS on SMART GRID Special Iue on Smart Ditribution Sytem. He wa the chair of the Renewable Energy Sytem Technical Committee of the IEEE Indutrial Electronic Society. In 24 he wa awarded by Thomon Reuter a Highly Cited Reearcher, and in 25 he wa elevated a IEEE Fellow for hi contribution on ditributed power ytem and microgrid.