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1 Aalborg Univeritet Grid impedance etimation baed hybrid ilanding detection method for AC microgrid Ghzaiel, Walid ; Jebali-Ben Ghorbal, Manel; Slama-Belkhodja, Ilhem ; Guerrero, Joep M. Publihed in: Mathematic and Computer in Simulation DOI (link to publication from Publiher):.6/j.matcom.5..7 Publication date: 7 Document Verion Early verion, alo known a pre-print Link to publication from Aalborg Univerity Citation for publihed verion (APA): Ghzaiel, W., Jebali-Ben Ghorbal, M., Slama-Belkhodja, I., & Guerrero, J. M. (7). Grid impedance etimation baed hybrid ilanding detection method for AC microgrid. Mathematic and Computer in Simulation, 3, DOI:.6/j.matcom.5..7 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-making activity or commercial gain? You may freely ditribute the URL identifying the publication in the public portal? Take down policy If you believe that thi document breache copyright pleae contact u at providing detail, and we will remove acce to the work immediately and invetigate your claim. Downloaded from vbn.aau.dk on: januar 9, 8

2 Accepted Manucript Grid impedance etimation baed hybrid ilanding detection method for AC microgrid Walid Ghzaiel, Manel Jebali Ben Ghorbal, Ilhem Slama Belkhodja, Joep M. Guerrero PII: S (5)-9 DOI: Reference: MATCOM 453 To appear in: Mathematic and Computer in Simulation Received date: October 4 Revied date: 8 Augut 5 Accepted date: 6 October 5 Pleae cite thi article a: W. Ghzaiel, M. Jebali Ben Ghorbal, I. Slama Belkhodja, J.M. Guerrero, Grid impedance etimation baed hybrid ilanding detection method for AC microgrid, Math. Comput. Simulation (5), Thi i a PDF file of an unedited manucript that ha been accepted for publication. A a ervice to our cutomer we are providing thi early verion of the manucript. The manucript will undergo copyediting, typeetting, and review of the reulting proof before it i publihed in it final form. Pleae note that during the production proce error may be dicovered which could affect the content, and all legal diclaimer that apply to the journal pertain.

3 Manucript Click here to view linked Reference Grid impedance etimation baed hybrid ilanding detection method for AC microgrid Walid Ghzaiel a, Manel Jebali Ben Ghorbal a, Ilhem Slama Belkhodja a and Joep M. Guerrero b a Univerité de Tuni El Manar, ENIT, L.S.E.-, LR ES 5, BP 37-, Tuni le Belvédère, Tuniie b Aalborg Univerity, Dept. Energy Technology, 9 Aalborg, Denmark Abtract Thi paper focue on a hybrid ilanding detection algorithm for parallel-inverter-baed microgrid. The propoed algorithm i implemented on the unit enuring the control of the intelligent bypa witch connecting or diconnecting the microgrid from the utility. Thi method employ a grid impedance etimation technique that ue reonance excitation once grid fault occur. It include two tage: the firt one i to detect grid impedance variation reulting from a grid fault; and the econd one i to excite the reonance by uing a virtual reitance in order to extract the grid impedance parameter. Grid impedance variation detection algorithm i baed on grid current meaurement temporal redundancie with fat current acquiition period. Once the grid impedance variation i detected, the excitation reonance i performed by injecting a reonance frequency in only one inverter control to avoid interaction with other unit. The elected inverter will be the one cloet to the controllable ditributed generation ytem or to a healthy grid ide in cae of mehed microgrid with multiple-grid connection. The detection algorithm i applied to quickly detect the reonance phenomena, o that the reonance excitation i canceled and the reitive and inductive grid impedance part are etimated. Simulation reult are carried out to illutrate the effectivene of the propoed method. Keyword: microgrid, ilanding, grid impedance variation, reonance excitation, droop control method, virtual reitance.. Introduction Microgrid (MG) are emerging a a way to improve both power quality of the electrical grid by making them marter and more flexible in the future. MG hould be able to operate connected to the utility grid or in iland under grid fault condition [6. The tranition between grid-connected and ilanded mode relie on the ilanding detection algorithm. The ilanding operation depend on the electrical power quality at the point of common coupling (PCC) and the grid exitence. Hence, two ilanding cenario can be conidered: - Intentional or pre-planned ilanding: Symmetrical or unymmetrical fault taking place omewhere in the grid will lead to voltage dip, frequency variation or unbalance problem that can be non-detected by the protection device. In thi cae, local ilanded detection algorithm are needed to diconnect the MG. - Non-intentional or unplanned ilanding: In thi cae, MG continue to upply local load, while the main grid i diconnected from the PCC [8. A non-detected ilanding condition may lead to dangerou ituation for utility peronnel and caue damage to the ditributed generation ytem compoing the MG. Hence, a reliable ilanding detection method hould be et in order to detect any ilanding cenario at leat a fat a required by tandard. German tandard (VDE- 6), for example, require diconnection from the utility grid under grid impedance variation of Ω within 5 [. The detection hould be available even under the wort condition defined a a Non-Detection Zone (NDZ), in which MG active and reactive produced power are completely conumed by the load. In fact, pre-

4 planned ilanding could be eaily detected by repecting ome tringent pre-defined tandard. However, unplanned ilanding, that i to ay the failure of the grid, i more complicated to be detected ince the voltage and frequency at PCC can be utained in NDZ limit. In the literature, everal method have been propoed for ilanding detection due to the importance of thi iue and it impact on the interconnected ytem behavior [9,, 4, 3, 8, 6. Thee method are mainly divided into two main categorie: communication-baed method between the protection device and the power generation ytem, and local method that ue only local meaurement. Local method can be claified into three ubcategorie: paive method challenged by a large NDZ and an accurate uitable threhold; active method that uffer from power quality degradation; and hybrid method that tend to ummarize the benefit of both paive and active method. In thi ene, a novel hybrid ilanding detection method baed on grid current meaurement and reonance excitation i propoed in thi paper. Indeed, at firt, a paive method baed on continuou grid current meaurement allow the detection of any abrupt grid current variation at the PCC. Afterward, thi paive method i combined with an active method baed on the injection of the proper reonance frequency. Reonance excitation i carried out by implementing and varying a virtual reitance. Thi reitance help, in grid connected mode, to improve the output power quality in healthy grid mode and to drive the ytem near the reonance once an abrupt grid current variation i detected. It alo allow the dicrimination of the variation caue: grid impedance variation [3 or load variation. Thi method i able to detect ilanding condition under any faulty condition even under the NDZ, the wort cae, where other method failed [8. However, in a MG that comprie parallel inverter connected to the grid, the method application i more difficult due to the different proper reonance frequencie reulting from the impedance of each generation ytem. A conequence, reonance excitation of each inverter can damage the ytem and reduce the reliability of the MG. To overcome thi problem, a MG control tructure i propoed which derived from the tructure of the parallel-inverter connected to double grid: the grid fault occurrence detection algorithm allow tiff grid determination and faulty grid one. Afterward, in reonance excitation phae, only the cloet ditributed generation ytem (DGS) to the healthy grid remain connected to thi utility grid while the other are diconnected. However, they are kept interconnected with controllable DGS, uch a ga engine, dieel generator or ditributed torage ytem. Thee controllable DGS can upport voltage and frequency in ilanded mode. The developed grid impedance parameter determination algorithm will be executed by the healthy grid cloet DGS control. Thi paper i organized a follow. The double grid-connected MG tructure and the propoed MG control tructure are reported in ection II. The hybrid ilanding method i introduced and dicued in ection III with the different algorithm tep. Section IV i devoted to imulation reult.. Microgrid architecture and control trategy MG generally include parallel ditributed generation ytem modeled by three-phae inverter connected to the utility grid through an LCL-filter for the non-controllable DGS and LC-filter for the controllable DGS. For implicity, LCL-filter are ued for both type of DGS in thi paper. The grid ide filter inductance i conidered a a line inductance, which increae the inductive character of output ytem impedance and help for a proper parallel operation of controllable DGS ince they operate a voltage ource... Microgrid with two grid connection point For a mehed MG, the MG can be connected to a double grid through two intelligent bypa witche (IBS) in order to enure better ervice continuity under grid fault. Fig. preent a double grid-connected MG tructure: parallel-inverter and load remain grid-connected when grid-condition guarantee a proper operation. In cae of grid fault occurrence detected on one ide of the double grid, the witche Ki, i =,...4 will cloe and open according to the grid failure in order to enure healthy working of load and DGS and afety tranfer of the electrical energy.

5 The propoed ilanding method i baed on a reonance injection for a pecific period. Thi will have a negative impact due to the interaction between the parallel-inverter under reonance tate. Thi interaction i a conequence of the line impedance between each inverter and the AC common bu. A a olution, an inverter with load can remain connected to the healthy grid, wherea the other DGS excite the reonance to determine the grid impedance of the faulty grid. Hence, the etimated impedance value help keeping the witch tate deciion... Propoed microgrid tructure A controllable DGS (DG3), uch a ga engine, dieel generator and ditributed torage ytem, i included. It i connected to parallel inverter and behave like a grid tracker in order to upport the voltage and frequency in ilanded mode. The non-controllable DGS are inverter baed on renewable power ource uch a wind turbine (WTS) and photovoltaic ytem (PVS). They act a current ource in grid connection mode. The ytem compoed of controllable DGS, non-controllable DGS (DG) and load, i connected to the cloet DGS from the grid (DG) through a witch device noted K and all the ytem i connected to the grid through an IBS, a depicted in Fig.. Indeed, grid impedance variation challenge the AC bu interfacing inverter control. In thi way, the witch K hould be opened once a grid impedance variation wa detected to avoid reonance problem in the MG caued by the interaction with the grid. Hence, controllable DG will, together with the parallel inverter DG, upply the load while the cloet DG will execute the algorithm of grid impedance parameter determination to detect the ilanding condition. A a conequence, different poible cenario can occur a illutrated in Table. Table. Propoed MG cenario IBS K Scenario ON ON Connected MG ON OFF Connected DG ON Off Re-ON OFF Reconnected MG Ilanded MG 3 Fig.. Configuration of a MG connected to a double-grid Fig.. Propoed MG tructure control trategy

6 3. Propoed microgrid control trategy 4 In grid-connected mode, parallel non-controllable DGS, uch a PVS and WTS, behave like current ource (CS). Their control require maximum power point tracking (MPPT) algorithm in order to ufficiently upply the load and to inject the maximum power to grid. In thi grid-connected mode, both frequency and voltage are upported by the grid. The controllable DGS act a a voltage tracker, behaving like a voltage ource, where it reference are the grid voltage and frequency [8. In ilanded mode and during the execution of the algorithm explained hereinafter, the controllable DGS will change it reference to upport by itelf the voltage and frequency while the parallel non-controllable DGS are till working a current ource. Indeed, MG i uually deigned o a to repect the following relation in order to avoid tranmiion power loe a follow P P and Q Q () MG Load However, in ilanded mode, and with non-controllable DGS baed on MPPT, the produced MG power hould not exceed the total demanded load power. In thi way, the intalled non-controllable DGS hould have a maximum power le than the minimum total load power a follow P P () Load _ min DG _ max Q Load _ min QDG _ max (3) The upported controllable DG act hence a a grid ource and a power balancer. 3.. Non-controllable DGS trategy A the non-controllable DGS behave like a current ource, it control trategy i baed on controlling the grid ide current. The voltage and frequency are impoed by the grid in grid-connected mode and upported by the controllable DGS in ilanded mode. The continuou DC voltage i conidered a contant by a prime ource ide converter and the grid model i an inductive reitive branch in erie with an ideal voltage ource noted V g [8,. Noteworthy that the high order LCL-filter can provoke the ytem intability which need the integration of a damping element. For thi reaon, active damping i adopted a a olution and it i carried out by implementing a virtual reitance in the control trategy. In fact, it i done by multiplying the ened filtercapacitor current by a predetermined gain noted R v [3, and the reult of thi multiplication i ubtracted from the converter voltage reference a depicted in Fig. 3. In the propoed microgrid tructure, DG; in which the propoed ilanding detection algorithm i implemented and DG have the ame current control trategy. MG Load 3 Bode Diagram [ T [ T [ T G iex () Fig. 3. Non-controllable DGS control trategy M a g n itu d e (d B ) P h a e (d e g ) Frequency (Hz) Wc=5 Wc= Wc=5 Fig. 4. Bode plot of multi-frequencie non-ideal PR For reliability and power quality reaon, a multi-frequencie non-ideal ProportionalReonant (PR) controller i ued to control the output current in both DG and DG. It good regulation capability and it

7 harmonic uppreion effect are proven in [3. In the non-controllable DGS reponible for reonance excitation DG, the tranfer function of the adopted (PR) controller i given by: G ( ) = k iex pex krex ωc ω ω c k ω rexh c h= 5,7, ωc ω h where k pex i the proportional gain reponible for the ytem dynamic while k rex i the reonant gain reponible for reducing the teady-tate error. ω, ω h, and ω c are repectively the fundamental frequency, the harmonic frequencie and the cut-off frequency ued for frequency fluctuation reducing a depicted in Fig. 4. For more implification, grid-ide current and reference voltage are expreed in the tationary reference a depicted in Fig. 3 uing the Concordia tranformation: = [ T V abc and i = [ T i abc [ = T [ T V 3.. Controllable DGS control trategy = In grid-connected controllable DGS; conidered a a voltage ource, the active and reactive power noted (P and Q) flowing from the filter output to the PCC through an inductor can be expreed a follow [7, 5: E V V E V P = ( coφ ) coθ in φ in θ (5) Z Z Z EV V EV Q = ( coφ )inθ in φ coθ (6) Z Z Z φ i the angle phae between the controllable DG filter output and the PCC, Z i the inductor impedance, E i the DGS capacitor voltage amplitude, V i the PCC voltage amplitude and θ i the inductor impedance angle. In thi paper, droop method i ued to control the controllable DGS and it principle i baed on the two following aumption: ) The inductor impedance i purely inductive (Z=X, θ=9 ). Thi aumption i true due to the large filtre inductance and power line impedance. But the reitive character of the power line impedance in low voltage cae can be a challenge. A a olution, virtual impedance can be added to the ytem control loop or uing LCL-filter intead of LC one. ) The angle φ i very mall and aumed to be null (φ =, coφ =). Baed on thee two aumption, Eq. (5) and Eq. (6) can be expreed a hown below, where the DGS frequency i ued intead of it angle phae ω = ω * m( P P*) (7) E = E * n( Q Q*) (8) From Eq. (5) and Eq. (6), the relation P/f and Q/E can be deduced. In fact, an increae in the active and reactive power lead to a decreae in the frequency and the voltage level repectively a can be hown in Fig. 5. Hence, the frequency and voltage can be conidered a information about demanded power load variation hared between the parallel inverter in real time. However, the conventional droop method ha the challenge of a low tranient repone. In thi ene, everal (4) 5

8 modified droop method have been propoed in literature [4, 7, 5. In thi paper, derivative and integrator term are added in droop equation to improve the ytem tranient repone. Hence, the equation will be expreed a: d( P P*) ω = ω * m( P P*) md (9) φ = φ * G p ( )( P P*) () dt E = E * n( Q Q*) nd ( Q Q*) () E = E * Gq ( )( Q Q*) () ɸ i the phae of the V c *, φ * = ω * dt = ω * t, md m G p ( ) = (3) nd n Gq ( ) = (4) E ω * ω ω 6 Fig. 5. Voltage and frequency droop principle [ T [ T Fig.6. Controllable DGS control trategy [ T G ( ) G i () G v () G q ( ) [ T [ T [ T LPF p ω* φ* φ V * in( ) c = E φ

9 7 Fig. 7. Control tructure baed on Droop control Droop control input, active and reactive power, are calculated from the unfiltered p and q power rich in ripple uing firt order low-pa-filter (LPF) a expreed in Eq. (5) and Eq. (6). ωc p = Vc α iα Vcβ iβ P = p (5) ω c ωc q = Vc β iα Vcα iβ Q = q (6) ω c The three voltage reference i deduced from the etimated voltage amplitude E* and frequency ɸ a hown in Fig. 6 and it can be given by: Vc* = Ein( ωt) (7) The filter capacitor voltage and inverter current are both regulated by multi-frequencie non-ideal PR controller where it tranfer function i preented by: G ( ) = k v pv k ω rv c ω ω c k ω rvh c h= 5,7, ω c ω h (8) G( ) = k i pi kri ωc ω ω c k ω rih c h= 5,7, ω c h ω (9) 3.3. Synchronization Many grid ynchronization algorithm exit in the literature and their importance i growing due to it advanced role in performing an accurate ynchronization and avoiding overcurrent. One of thee ynchronization algorithm i the econd order generalized integrator SOGI that i adopted in thi paper ince it i characterized by it high efficiency under unbalance or perturbation a proven in literature [ and [3. The SOGI behave like a band pa filter with reonance frequency equal to the grid frequency (f=5hz), hence it i capable to extract the poitive and negative component. The poitive angle-phae component i ued to ynchronize properly the MG. However, the grid frequency can be varied; hence an adaptive frequency loop named FLL block (frequency locked loop) i added to adapt the reonance frequency of the SOGI in real time. The SOGI algorithm conit of two filter; a band pa filter allowing the recontitution of the filtered input ignal V and a low pa filter permitting the contitution of the in-quadrature component qv. The tranfer function of the econd order generalized integrator relating V a well a qv to the V i given by V ' k ω' = () V k ω' ω' qv V ' = k ω' k ω' ω' Where ɷ i the SOGI reonance frequency and k i the damping factor. V ocillate with the ame frequency ɷ of the input voltage while qv i the in-quadrature component of V. Thee two parameter (V and qv ) are ued to calculate the poitive and negative equence for three phae ytem, while they are themelve the poitive and negative equence in ingle phae ytem, and to calculate the proper ()

10 angle-phae. 8 ω' γ V abc [ T V α V β K K ω ff qvα' qvβ' FLL V β ' qvβ' Poitive equence αv ' V / α / / / / V β V α V β Negative equence Arctg(y/x) θ Arctg(y/x) θ Fig. 8. Three-phae SOGI-FLL algorithm 3.4. Hybrid ilanding method The propoed ilanding detection method i baed on two part, the firt one aim to detect the variation of grid impedance parameter while the econd one allow the determination of the grid impedance variation: A. Paive part: The method baed on temporal redundancie of grid current meaurement i widely ued and it efficiency in fault detection wa proven [. It principle conit on the detection of any abrupt variation between conecutive grid current meaurement. Any abrupt current variation will be depicted a a reidual with different amplitude level. Indeed, in healthy tate, with no grid impedance variation, the meaured quantity current evolution ha no dicontinuity, hence the final reidual noted Re igk, calculated from three conecutive non-filtered reidual noted r k i lower than a mall threhold ε. Once the grid impedance varie, in faulty tate, the meaured current quantity evolution will preent a dicontinuity hown a a reidual pike at the intant of the grid fault. The reidual expreion are given by Eq. () and Eq. (3). Re () igk = rk rk rk r (3) k = igk igk igk i gk, i gk- and i gk- are the conecutive meaured current at the acquiition ampling time T a ; k T a, (k-) T a and (k-) T a. The threhold ε i defined a the maximum value that can reach Re igk when current evolution ha no dicontinuity (Re igk,< ε ) and ince it i proportional to the quare of the ampling, it will be alway very mall a expreed in Eq. (4) and Eq. (5). ε = 3ω Ta I m (4) I m VPCCm Vgm = (5) Z g Where VPCCm i the maximum voltage at PCC, Vgm i the maximum grid voltage, and τ ( ω = π / τ ) i the ytem time contant, with τ <<< Ta.

11 B. Active part: A proven in [3, the ytem reonance frequency depend on the grid impedance, epecially on the inductive part of the grid impedance. The active part of the propoed hybrid ilanding detection method allow the injection of the proper ytem reonance frequency needed to etimate the grid impedance parameter value once a grid fault occur. In fact, the reonance i excited properly by a virtual damping reitance noted R v that drive the ytem near the reonance once a reidual pike i detected. Indeed, the principle of the method conit on taking a high proportional gain in the beginning, and then reducing the virtual reitance gradually until reaching the reonance. Then, in the next tep the reonance frequency can be extracted by uing the Fat Fourier Tranformation (FFT). The reonance frequency of cloet DGS reponible for ilanding detection (indicated a part I in Fig.9), the etimated grid inductive part and the deduced grid reitive part are expreed a follow: 9 f re L L Lline Lg = (6) π L ( L L L ) C line g f Λ ( L L L L = g line (4π L Cf fre ) (4π L ( L L ) line ) C f fre ) (7) Λ R g = 4 9ω Ta ε 4 ( V PCCm V gm ) ( ωl ) g (8) 4. Simulation reult Simulation were carried out by uing PSIM oftware. The propoed ilanding detection method decribed for detection and etimation of grid impedance variation i teted on the ytem depicted in Fig.9. The parallelinverter and different kind of local load are connected to a PCC. The linear reitive load i continuouly connected but the linear reitive-inductive load (L_L) i connected at t ON =.8 and the non-linear one at t ON =.. The load are ized to tet their effect on ilanding algorithm. The ytem parameter ued in imulation are hown in Table, 3 and 4. In the econd and third cenario, the voltage and frequency reference are propoed by the ytem itelf with E ref =35V and ɷ ref =34.59 rad/. Table. DGS parameter PDG PDG PDG3 VDC(V) fpwm L L Cf Zg Zd kw kw 4kW 65 khz mh mh 5µ.4Ω,.9mH mh Table 3. Load parameter Rp Rl Ll Rnl Lnl Cnl 55Ω 8Ω.4H 8Ω.4H µf Table 4. Control parameter k pex k iex k p k i k pv k iv ω c ω f R v m md n nd T a µ

12 DG Zline3 Zline [ T [ T [ T [ T [ T PWM PW M [ T - () G v - () G i () G i - [ T ON at t=. G q ( ) G p ( ) Grid-connected Microgrid φ* φ * E in( φ ) V c = Threhold DG ON at t=.8 Zline E = E * ω = ω* Q DG *= P DG *= Pule witch Reaching Reonance calculation Zg variation detection FFT etimation L g etimation R g L g PW M [ T - [ T Ilanding Algorithm [ T E= E g ω = ω g Q DG = Q DG * P DG= P DG * calculation Routh criteria Z gk in limit Ilanded Microgrid Z g : calculation algorithm G iex () - Fig.9. Propoed MG control tructure An abrupt grid fault i carried out by varying the grid impedance (from Z g to Z g Z d ) at t=.4. The implemented paive part of the propoed ilanding method baed on reidual calculation detect thi variation uccefully by preenting a reidual pike. Then, the witch K i opened to iolate the part of ytem compoed of the cloet DGS from the grid (DG), the controllable DG and the non-controllable DG from the whole microgrid ytem. Finally, the active part of the propoed hybrid method i applied by exciting the reonance a hown in Fig..

13 After the excitation during a pecific period choen here time of T ampling, the algorithm will take the appropriate deciion; recloing the witch K to reconnect the whole microgrid to the grid (cenario3) or going to cenario 4 and diconnect it from the utility grid to operate in ilanding mode. A Z d =mh, which preent an impedance variation of.638ω ( Z g < Ω), the witch K i recloed and cenario 3 i achieved. Noteworthy that before recloing the witch K, the adaptive virtual reitance R v can change it value to reconfigure the ytem with the new grid impedance parameter to maintain the ytem power quality. Fig. how the pectral analyi of the non-controllable DG current output. A pike at the frequency f=86hz i preented. Thi frequency correpond to the excited reonance frequency. Hence, the new grid impedance i etimated (f re =86Hz Etimated L g =.7mH ) Fig. preent the active and reactive power of the non-controllable DGand DG acting a a current ource and the controllable DG. The active power of non-controllable DGS are et to kw, PD=PD=kW, while the controllable active power reference i choen 4kW. A hown in Fig., in gridconnected controllable DGS, there i no-grid impedance variation and K i cloed. In thi cae, the controllable DGS propoe it power reference. In cenario, under reonance excitation, the load require the needed power, hence the active P DG and reactive Q DG power do not track their reference and they jut inject the ret of the required load power none upplied by DG igdg igdg ig3dg Igref_DG Igref_DG Igref3_DG reidual pule-witch k Time () Fig.. Zg variation detection and reonance excitation IgDG IgDG Ig3DG 5 5 Frequency (Hz) Fig.. Reonance frequency (Zd=mH) Pule-witch k Pule-witch k.8.4 PDG PDG 4 3 PDG Pref 4.5K 4K 3.5K 3K.5K QDG Qref K K K -K -K.8..4 Time () Fig.. DGS output during the tranition between cenario Reigk 8 4 Vca_ref Vcb_ref Vcc_ref VCDG_a VCDG_b VCDG_c E_reference Time () Fig. 3. Controllable DGS output voltage during reonance

14 The LCL-filter capacitor voltage in the controllable DGS noted by V c i illutrated in Fig. 3, where the reonance effect doe not appear. It amplitude follow the voltage amplitude obtained at the output of the droop control algorithm, noted a E-reference. E-reference decreae during the reonance tate due to the change of voltage amplitude reference a preented in Fig. 9, part Control algorithm. It hould be noted that the reonance excitation duration i choen here, and in all next imulation reult, equal to 8 time the T ampling. Thi duration i lower than the time required by tandard (.) to detect ilanding mode. It depend on the period of FFT window that neceitie N ( N =6, 56, 5 ) ampling time and can be le than the choen duration. Indeed, the choice of thi long period aim to how up the capability of the virtual reitance for reonance controlling by puhing the ytem near the reonance without diverge. Fig. 4 how the voltage at PCC upported by the controllable DGS under reonance excitation and by the utility grid over the reonance. The PCC voltage track it reference, noted E pcc, reduced during the reonance tate due to the diconnection of the controllable DG3 from the utility grid and to it relation to the reactive power demanded by the load. Notice that the frequency preent a little variation in thi tate due to the relation P/f with the active power conumed by the load. Pule-witch k Vpcc_a Vpcc_b Vpcc_c Epcc Wpcc Time () Fig.4. PCC voltage and frequency during reonance excitation Reigk 8 4 igdg igdg ig3dg Igref_DG Igref_DG Igref3_DG - - Ia_lr Ib_lr Ic_lr Ia_ll Ib_ll Ic_ll Time () Fig. 5. Load power quality during algorithm execution A preented in Fig. 5, linear reitive and reitive-inductive load are connected to the AC bu at different time to emphaize the effect of an abrupt load variation on the ilanding detection method. The imulation reult how that the propoed ilanding detection method doe not have any negative effect on the load current. 6K 5K PRPLL PDGPDG IgSG IgDG Ig3DG 4K 3K K QDGQDG 4 3 PDG 4 QRQLL Time () Fig. 6. Load power under reonance excitation 5 5 Frequency (Hz) Fig. 7 Reonance frequency (Zd=mH)

15 3 The relation preented in Eq. () and Eq. () are preented in Fig. 6, where the total load power i higher than the power upplied by both of the non-controllable DG and DG. The active power P DG of the controllable DGS i contant and equal to it reference (P DGref =4kW). Fig. 7 preent the pectral repone of the grid ide current in DG that depict the preence of a pike at f=9hz due to the reonance phenomenon appearance. By identification, a grid inductance of L g =.75mH i etimated. Indeed, the propoed hybrid method can be ued to improve the MG output quality after the ilanding detection. In fact, the method allow the etimation of grid impedance that varie under grid fault or grid blackout; hence if the etimated grid impedance value exceed the limit the IBS will open to diconnect the MG. In other way, if it i below the limit ( Z g <Ω), the DGS control trategy hould adopt the grid parameter to maintain the output power quality and to avoid the interaction effect; hence ytem control reconfiguration i needed. In the propoed MG tructure, the virtual reitance i ued not only to damp the ytem (DG and DG) and to drive the ytem near the reonance to extract the grid parameter but alo for ytem reconfiguration reaon. The virtual reitance ha the ability to maintain the ytem pole location and hence maintaining the ytem tability and reliability by returning the ytem pole, changed due to grid impedance variation, to their initial place. 5. Concluion In the next decade, power quality and ervice continuity will be the mot important feature on which the reearch interet relie. For thi reaon, ilanding condition detection i well invetigated in the microgrid power quality improvement, to enure the eamle tranition between the operating mode grid-connected and the ilanded mode, and alo for protection reaon. In thi way, a hybrid ilanding detection method i preented in pecific MG tructure. Thi ilanding method i baed on reonance injection after grid parameter variation detection and it i able to detect the ilanding under wort condition. Indeed, the algorithm i implemented in the cloet DGS of MG to the grid to avoid the interaction between the parallelinverter under reonance tate and it i capable to reconfigure the ytem control trategy by introducing the new grid parameter. During reonance excitation and throughout ilanded mode, controllable DGS i ued to upport the frequency and voltage of MG while the parallel inverter act a current ource. The propoed detection algorithm i a proper and efficient olution for both intentional and unintentional ilanding detection while the propoed MG tructure i well flexible in term of high MG power quality maintaining. Reference [ A. H. K. Alaboudy, H. H. Zeineldin, Ilanding detection for inverter-baed dg coupled with frequency-dependent tatic load, IEEE Tran. Power. Deliv. () [ H. Berriri, M.W. Naouar, I. Slama-Belkhodja, Senor fault tolerant control for wind turbine ytem with doubly fed induction generator, ELECTRIMACS, 6-8th June, Cergy-Pontoie, France,. [3 W. Ghzaiel, M. Jebali-Ben Ghorbal, I. Slama-Belkhodja, J. M. Guerrero, A novel grid impedance etimation technique baed on adaptive virtual reitance control loop applied to ditributed generation inverter, IEEE conference in Power Electronic and Application (3). [4 J. M. Guerrero, J. Mata, L. G. de Vicuna, M. Catilla, J. Miret, De-centralized control of parallel operation of ditributed generation inverter uing reitive output impedance, IEEE Tran. Ind. Appl. (7) [5 J. M. Guerrero, J. C. Vaquez, J. Mata, M. Catilla, L. G. de Vicuna, Control trategy for flexible microgrid baed on parallel lineinteractive UPS ytem, IEEE Tran. Ind. Electron. (9) [6 J. B. Jeong, H. J. Kim, Active anti-ilanding method for PV ytem uing reactive power control, IET Electronic Letter (6) 4 5. [7 Y. A. -R. I. Mohamed, E. F. El-Saadany, Adaptive decentralized droop controller to preerve power haring tability of paralleled inverter in ditributed generation microgrid, IEEE Tran. Power. Electron. (8) [8 J. Rocabert, Gutavo M. S. Azevedo, A. Luna, J. M. Guerrero, J. I. Candela, P. Rodrıguez, Intelligent connection agent for threephae grid-connected microgrid, IEEE Tran. Power. Electron. () [9 P. Rodríguez, A. Luna, I. Candela, R. Mujal, R.Teodorecu, F. Blaabjerg, Multireonant frequency-locked loop for grid ynchronization of power converter under ditorted grid condition, IEEE Tran. Ind. Electron. ( ) 7-38.

16 [ P. Rodriguez, A. Luna, R. S. Munoz-Aguilar, I. Etxeberria-Otadui, R. Teodorecu, F. Blaabjerg, A tationary reference frame grid ynchronization ytem for three-phae grid-connected power converter under advere grid condition, IEEE Tran. Power. Electron. () [ S. Seman, J. Niiranen, A. Arkkio, Ride-through analyi of doubly fed induction wind-power generator under unymmetrical network diturbance, IEEE Tran. Power. Syt. ( 6) [ A.V. Timbu, P. Rodriguez, R. Teodorecu, M. Ciobotaru, Line impedance etimation uing active and reactive power variation, IEEE Power Electronic Specialit Conference (7) [3 R. Teodorecu, F. Blaabjerg, M. Lierre, P.C. Loh, Proportional-reonant controller and filter for grid-connected voltage-ource converter, IET Electric Power Application, (6) [4 H. Vahedi, M. Karrari, Adaptive fuzzy andia frequency-hift method for ilanding protection of inverter-baed ditributed generation, IEEE Tran. Power. Deliv, (3) [5 J.C. Vaquez, J.M. Guerrero, A. Luna, P. Rodriguez, R. Teodorecu, Adaptive droop control applied to voltage-ource inverter operating in grid-connected and ilanded mode, IEEE Tran. Ind. Electron. (9) [6 J. C. Vaquez, J. M. Guerrero, J. Miret, M. Catilla, L. G. Vicuna, Hierarchical control of intelligent Microgrid, IEEE Ind. Electron. Mag. () 3-9. [7 J. C. Vaquez, J. M. Guerrero, M. Savaghebi, R. Teodorecu, Modeling, analyi, and deign of tationary reference frame droop controlled parallel three-phae voltage ource inverter, IEEE Tran. Ind. Electron. (3) 7 8. [8 X. Wang, J. M. Guerrero, Z Chen, Control of grid interactive AC Mmicrogrid, IEEE conference in Indutrial Electronic (ISIE) () - 6 [9 Y. Zhu, D. Xu, N. He, J. Ma, J. Zhang, Y. Zhang,G. Shen, C. Hu, A Novel RPV (Reactive-Power-Variation) Antiilanding method baed on adapted reactive power perturbation, IEEE Tran. Power. Electron. (3)

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