Aalborg Universitet. Published in: I E E E Transactions on Industrial Electronics. DOI (link to publication from Publisher): /TIE.2016.

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1 Aalborg Univeritet Droop Control with Improved Diturbance Adaption for PV Sytem with Two Power Converion Stage Liu, Hongpeng; Loh, Poh Chiang; Wang, Xiongfei; Yang, Yongheng; Wang, Wei; Xu, Dianguo Publihed in: I E E E Tranaction on Indutrial Electronic DOI (link to publication from Publiher): 1.119/TIE Publication date: 216 Document Verion Accepted author manucript, peer reviewed verion Link to publication from Aalborg Univerity Citation for publihed verion (APA): Liu, H., Loh, P. C., Wang, X., Yang, Y., Wang, W., & Xu, D. (216). Droop Control with Improved Diturbance Adaption for PV Sytem with Two Power Converion Stage. I E E E Tranaction on Indutrial Electronic, 63(1), DOI: 1.119/TIE 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 vbn@aub.aau.dk providing detail, and we will remove acce to the work immediately and invetigate your claim. Downloaded from vbn.aau.dk on: april 7, 218

2 Droop Control with Improved Diturbance Adaption for PV Sytem with Two Power Converion Stage Hongpeng Liu, Member, IEEE, Poh Chiang Loh, Xiongfei Wang, Member, IEEE, Yongheng Yang, Member, IEEE, Wei Wang, Member, IEEE, and Dianguo Xu, Senior Member, IEEE Abtract Droop control ha commonly been ued with ditributed generator for relating their terminal parameter with power generation. The generator have alo been aumed to have enough capacitie for upplying the required power. Thi i however not alway true, epecially with renewable ource with no or inufficient torage for cuhioning climatic change. In addition, mot droop-controlled literature have aumed a ingle dc-ac inverter with it input dc ource fixed. Front-end dc-dc converter added to a two-tage photovoltaic (PV) ytem ha therefore uually been ignored. To addre thee unreolved iue, an improved droop cheme for a two-tage PV ytem ha been developed in the paper. The developed cheme ue the ame control tructure in both grid-connected and ilanded mode, which together with properly tuned ynchronizer, allow mode tranfer to be eamlely triggered. Moreover, the propoed cheme adapt well with internal PV and external grid fluctuation, and i hence more precie with it tracking, a compared with the traditional droop cheme. Simulation and experimental reult have verified thee expectation, and hence the effectivene of the propoed cheme. Index Term Microgrid, PV ytem, droop control, ilanded mode, grid-connected mode, eamle tranfer. I. INTRODUCTION ORMATION of microgrid (MG) i an attractive approach for Folving energy challenge experienced around the globe [1], [2]. It ha thu been widely tudied in either it grid-connected or ilanded mode [3]-[5]. In the former, interfacing inverter are normally operated a controlled current ource, injecting Manucript received September 16, 215; revied February 13, 216 and April 6, 216; accepted May 4, 216. Thi work wa upported by the National Natural Science Foundation of China under Grant , and in part by the Lite-On Power Electronic Technology Reearch Fund under Grant PRC H. Liu, W. Wang, and D. Xu are with the Department of Electrical Engineering, Harbin Intitute of Technology, Harbin 151, China ( lhp62@hit.edu.cn; wangwei62@hit.edu.cn; xudiang@hit.edu. cn). P. C. Loh, X. Wang, and Y. Yang are with the Department of Energy Technology, Aalborg Univerity, Aalborg D-922, Denmark ( pcl@et.aau.dk; xwa@et.aau.dk; yoy@et.aau.dk). active and reactive power to the grid [6]. Alternatively, they can be voltage-controlled like propoed in [7]-[2]. For example, in [7], a ingle-phae PV ytem ha been voltagecontrolled to upport the fundamental voltage, while eliminate harmonic at it point of common coupling (PCC). The approach however add an extra inductor in erie with the grid to make it more inductive at the expene of ize and efficiency. Another well-received approach i indirect current control, where the grid current ha been controlled by regulating magnitude and phae of the ac filter capacitor voltage [8], [9]. It deign i however complex becaue of nonlinear factor, and ine and coine table included in the control. A implification ha ubequently been developed in [1] without the ine and coine table. The controller can then be deigned uing claical control technique. Depite that, the load voltage waveform may till be ditorted ince it i not regulated directly by a voltage control loop. A further improvement can be found in [11], where the control ytem ha been enhanced by connecting a local load to the filter capacitor, before feeding back the capacitor current for control. The method work fine, but it dc-link dynamic ha not been addreed. The next voltage-controlled technique propoed i the droop method, which in effect, ha mapped the generator terminal parameter with it active and reactive power generation [12]-[14]. The droop method i originally borrowed from large ynchronou generator connected to the power grid [15], but when applied to maller ditributed generator (DG), i prone to fluctuation becaue of the abence of large inertia and fuel reerve. Depite that, the droop concept ha been commonly ued with [16] and [17] preenting a droop controller for DG that can guarantee zero teady-tate error for it output reactive power. It output active power may however deviate from the deired reference value. To olve the problem, integral term have been added to the conventional droop cheme for forcing the DG active and reactive power to track their reference cloely [18]. The droop principle ha alo been recommended by the Conortium for Electric Reliability Technology Solution (CERTS) for regulating photovoltaic (PV) inverter with a table bu voltage, even during load tranient [19]. More recently, a univeral controller ha been propoed in [2], where maximum-power-point-tracking (MPPT), droop control

3 and dc-link voltage regulation have been managed imultaneouly without major control reconfiguration. The limitation introduced i other non-renewable ource or torage unit mut be preent for balancing upply and demand, which if not catered, will everely narrow variation range of the load. Depite thee difference, the tudie in [16] to [2] have not conidered poible fluctuation of the dc ource connected to the inverter. Such conideration i relevant to renewable ource like PV, where variation may be unintentionally caued by change of irradiation or intentionally activated by the MPPT cheme. In addition, the tudie have only focued on ingle-tage converter, which make them not directly applicable to two-tage converter ued with ome PV ytem. The other concern invetigated i the uninterrupted upply of power, which will then require eamle tranfer between grid-connected and ilanded mode [21]-[28]. For example, [23] ha propoed a high-performance inverter controlled by an adaptive liding-mode cheme. Sliding-mode control i generally fat, and can hence ride through the tranfer moothly. However, it i nonlinear and mathematically complex, which may make analyi difficult. In [24], a voltage-currentweighted control ha been propoed with different weight ued for it grid-connected and ilanded mode during the tranfer. It ue four controller in total, and i hence equally complex. In [25] and [26], the tranfer i accompanied by a reconfiguration of controller, which uppoedly, i a non-optimized olution when compared with cheme that ue a ingle common control tructure throughout the tranfer. Such common control cheme are however not widely dicued at preent. Moreover, eamle tranfer require ynchronization with the grid, prior to connecting an ilanded DG with the main [29], [3]. For ingle-phae inverter, zero-croing detection of the grid voltage i a poible technique, but enitive to grid voltage ditortion [31]. Another ynchronization technique ha therefore been preented in [32] for P f and Q V droopcontrolled DG. The technique ue one ynchronizer for eliminating voltage magnitude difference and another for eliminating phae difference between the DG and grid. It i however formulated for an inductive grid only, and not the predominantly reitive low-voltage (LV) ditribution network. Similar ynchronization concept can however till be ued, but only after ome minor modification are added to account for it reitive characteritic. Summarizing the above, preently unreolved iue are lited, a follow. Studie, including [12] to [19], have conidered ingletage inverter with ufficient generation capacitie. Thi may not be true for PV ytem, where two-tage converter with no or limited torage may ometime be ued. Effect of internal and external fluctuation on the droop performance have uually been neglected. Thi may not be appropriate for PV ytem, where climatic change are common. A common droop tructure with the firt two iue reolved ha not been developed for enuring eamle Z V E Fig. 1. Simplified circuit for illutrating power tranfer. tranfer between operating mode. Thee challenge have been addreed in the paper by propoing an improved droop cheme for two-tage PV ytem. Simulation and experimental reult have verified the expected performance of the propoed cheme. II. GENERAL POWER TRANSFER AND POSSIBLE VARIATIONS Fig. 1 how a imple circuit for formulating power tranfer. The notation ued are V, Z θ and E -δ (V and E are peak value) for repreenting output voltage from the PV ytem, line impedance and voltage at the PCC, repectively. In the grid-connected mode, E -δ i decided by the grid, while in the ilanded mode, it i determined by the local generator and load. Regardle of that, power angle δ i uually mall, which mean inδ δ and coδ 1 can be ued for implification. In addition, for a LV ditribution network, the line impedance i mainly reitive, which mean θ. The active and reactive power (P and Q) tranferred can thu be expreed a: V ( V E) P (1) Z VE Q. Z (2) Thee equation how that active power can be controlled by adjuting V, while reactive power can be controlled by varying δ, even though it i alo affected by V. Additionally, both power are affected by diturbance from E and Z, which trictly, are external of the PV ytem. Internally within the PV ytem, active power limitation can alo happen becaue of a drop in irradiation, for example. Specific detail related to thee iue are dicued eparately in the next two ection for the ilanded and grid-connected mode. III. ISLANDED MODE A. Control Principle Beginning with active power tranfer repreented by (1), a P-V curve can be plotted, a hown in Fig. 2. Thi curve will hift when E and / or Z vary, a undertood from (1). Regardle of that, the P-V curve alone i not ufficient for defining a pecific operating point for the network. To mark out thi table operating point, the PV ytem mut be controlled to add a econd P-V curve to Fig. 2, which conventionally, ha aumed the linear droop characteritic repreented by (3) [33]. V V k ( P P ) (3) p f f k ( Q Q ) (4) q where f and f are the meaured and rated frequencie of the PV ytem, V and V are the meaured and rated output voltage

4 L D Lac DC-DC Converter DC-AC Inverter Fig. 2. P-V interaction during change of PV power and grid voltage. amplitude, P and Q are the rated active and reactive power, and k p and k q are the active and reactive droop coefficient, repectively. An interection at point a i thu created, whoe correponding operating point on the PV curve i alo marked with the ame notation of a. In the teady tate, both operating point mut have the ame power value P a, in order to keep the dc-link voltage tabilized. The ame reaoning can be applied to reactive power tranfer, which then neceitate the Q-f droop expreion in (4) for controlling the PV ytem. It i however not poible to draw (2) and (4) on a ingle diagram becaue of their difference in one of the parameter (δ and f, where δ = 2ft + δ, and δ i the initial power angle). Implementation of (3) and (4) then reult in a control cheme imilar to that hown in Fig. 3 for a two-tage PV ytem when in it ilanded mode. The only difference i Fig. 3 include a modified active power droop expreion, intead of (3). The modified droop expreion will be explained later. Regardle of that, Fig. 3 how the output voltage v ac and current i ac of the rear-end inverter being meaured for computing P and Q. The active and reactive droop expreion can then be ued for mapping out the deired V and f, from which the demanded voltage reference v ref i computed for tracking by the uual double-loop controller. On the other hand, the front-end boot converter i controlled by a ingle-loop voltage converter notated a G B (). Input to G B () i the difference between the meaured dc-link voltage V DC and it reference V DCref. To nullify thi difference, G B () i uually a proportional-integral (PI) controller, which may not be neceary for the propoed cheme becaue of reaon explained next. A decribed earlier, the P-V curve and line in Fig. 2 help to define a common operating point with P = P a. In the literature, thi active power ha alway been aumed a upplied by the DG without difficulty [14], [34]. Such aumption i however true only when a large reerve of fuel or torage i available for cuhioning variation of parameter, which for a PV ytem, are it irradiation and temperature. Auming now that the reerve i not catered and the PV maximum power P PVmax ha been reduced uch that P PVmax < P a in Fig. 2 (ee point b 1 ), power delivered to the dc-link by the V PV T boot converter will then be leer than power drawn out from it by the inverter. The dc-link voltage will therefore drop until the inverter power i reduced to P = P b = P PVmax, which in Fig. 2, i achieved by lowering the droop line by V I (ee point b 2 ). The challenge i finding the value for VI, which the implet method i to compute it uing a PI controller. Input to the PI controller can be the difference between power delivered by the dc-dc converter to the dc-link capacitor and power extracted from the ame capacitor by the inverter. However, in the teady tate, thi power imbalance will reduce to zero (ignoring loe) before the dc-link capacitor voltage can tabilize. Depite that, the integral term of the PI controller will till output a finite VI for lowering the droop line in Fig. 2. Value of the PI controller output i however not pecific, becaue of it ingle-to-multiple mapping characteritic, which i uually not encouraged when multiple converter are connected in parallel. Becaue of that, the proportional droop cheme without any integral term i ued intead for finding VI. Intead, the dc-link voltage V DC i allowed to vary within a very mall range, notated a V DCmin V DC V DCref. The lower limit of the range mut at leat be higher than the maximum inverter output voltage to avoid over-modulation. The neceary expreion governing it can thu be derived from (3) a V DCmin V + k p P, after ubtituting P =. The required V I can then be computed from (5), baed on the ame proportional droop principle. kv VDC VDCref, VDC VDCref VI (5), VDC VDCref k P V p V DC I V DCref C Fig. 3. Control cheme for two-tage PV ytem in ilanded mode (tranformer ued in practice and experiment not hown for conciene). where k V i the droop coefficient for computing V I. Some inight drawn from (5) are ummarized, a follow. Offet V I varie in the range of k p P V I. Lower limit k p P i obtained by ubtracting the maximum of (3) from it minimum (V (V + k p P )). The actual V DC, when fallen below V DCref, repreent a lack of PV capacity (e.g. inufficient irradiation) for retoring the dc-link voltage. The inverter power mut hence be lowered by introducing a negative V I. The actual V DC, when rien above V DCref, repreent an exce PV capacity (e.g. trong irradiation), which can be lowered to meet the lower load demand. No offet (V I = ) i thu added for changing the inverter power. A V DC i now required to vary, controller G B () for the v ref i L V f C ac vac P iac Q

5 boot converter cannot be a PI controller. It hould be a proportional droop controller with gain given by (V DCref V DCmin ) / P. Coefficient k V will uually be high if only a narrow dc-link voltage range i permitted (partly due to the large dc-link capacitor C included for decoupling purpoe). The modified P-V droop expreion i then written a (6), where the firt term i the uual droop expreion for power haring, and the econd term i for regulating internal power limitation within the generator. V V k p ( P P) kv ( VDC VDCref ) (6) The Q-f expreion, on the other hand, remain unchanged, ince internal active power variation will not limit the amount of reactive power that the rear-end inverter can deliver. Thi explain why the reactive power droop expreion in Fig. 3 ha been retained a (4), while the active power droop expreion ha been changed to (6). Moreover, it hould be emphaized that Fig. 3 work even with a limited amount of torage (mainly for catering much lower night uage) added to the dc-link of each PV ytem through a econd dc-dc converter. Thi torage can charge when it charger detect V DC above V DCref, which a aforementioned repreent an exce PV capacity. On the other hand, it may dicharge when V DC fall below V DCref to reinforce the lack of PV capacity, if neceary. The dc-link voltage will then tabilize, but if the interval of inufficient PV capacity i longer than that the torage capacity can upport, the dc-link voltage will eventually continue it drop, allowing (6) to work a intended. The control cheme in Fig. 3 with (6) included can thu be ued without change, even with ome torage introduced through a econd dc-dc converter at the dc-link. B. Small-Signal Analyi Small-ignal analyi can next be performed on (4) and (6) for tudying the behavior of paralleled converter, whoe number ha been kept at two to avoid exceive mathematical complexity. Output voltage of thee two converter can further be notated a and, repectively. Their angle and magnitude can then be expreed a: Vqi i arctan (7) Vdi 2 2 V V jv V V (8) i di qi di qi where i = 1 or 2 i the aigned converter index number. The variation of energy E i in each dc-link capacitor of the converter can alo be written a: Ei Pi t dt CVDCref CV (9) DCi 2 2 where V DCi and C are the dc-link voltage and capacitance, repectively. The linearly perturbed expreion for relating the dc-link voltage and it power can thu be derived a: 1 k DC V DCi Pi Pi mc (1) where m i the equilibrium value of V DCi, and i for repreenting perturbation. Additionally, Δf i = Δδ i /2π. Combining thee perturbed expreion with the mall-ignal expreion of (4) and (6) then reult in the tate equation (11) for repreenting each converter. V DCi VDCi fi f i Pi A (11) i Bi V V di di Q i V V qi qi where A i and B i repreent the coefficient matrixe. To again avoid exceive complexity, line impedance of the two converter are aumed equal, which together with the load impedance, are expreed a Z = r + jx and Z L =R L + jx L, repectively. The output current expreion of the converter can thu be expreed a: I Y V (12) with I Id1 Iq1 Id2 I q2 T V Vd1 Vq 1 Vd2 V q2 where [Y] i the nodal admittance matrix, and I di and I qi are real and imaginary component of each converter output current. Active and reactive power of each converter can then be computed uing (13), which upon perturbed, lead to (14). Pi VdiIdi VqiIqi (13) Qi VqiIdi VdiIqi S IV V I (14) where S P Q P Q T The tate equation of the whole ytem with two converter can hence be repreented a: A1 B1 X IVYkX(15) A 2 B 2 where T X VDC1 f1 Vd1 Vq1 VDC 2 f2 Vd 2 V, q2 1 1 k. 1 1 From (15), the tate pace matrix M for the whole ytem can eventually be extracted a: A1 B1 M IVYk. (16) A 2 B 2 Uing (16), root loci can be plotted for analyzing the ytem repone with 218 3V, for the converter, Z L = 5+j.2Ω for the load, Z = 2+j.1Ω for the ditribution line, and ω f = rad/ for the common cut-off frequency of the low-pa filter ued for computing the average power. With droop coefficient next et a k p =.4, k q =.1 and k V changing from.5 to.5, Fig. 4 how the firt et of root loci plotted. The dominant pole λ 2 and λ 3 noted in the figure are T

6 2.5e-6 5.5e e e-5 1.8e e-5 4e-5 8e-5 8e-5 4e-5 2.6e-5 1.8e e e e e-6 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS Fig. 4. Root locu diagram for.5 k V Fig. 5. Root locu diagram for.1 k p.1 with improved droop cheme. found to hift away from the imaginary axi, a k V increae. The expectation i thu an improvement of ytem dynamic, while retaining tability ince all pole are till in the left-half -plane. Fig. 5 follow with another et of root loci obtained with k q =.1, k V =.4 and k p varying from.1 to.1. Only two pole λ 2 and λ 3 are again found to affect the ytem dynamic, ince the other pole are far away from the imaginary axi. Depite that, the ytem i till table. Finally, Fig. 6 how the lat et of root loci, where the coefficient changed i k q from.1 to.1, while keeping the other contant at k p =.4 and k V =.4. Since thi combination of coefficient can lead to intability if not deigned properly, it i plotted for both improved and traditional droop cheme for comparion. The traditional cheme referred to here i the cheme, which ue (3) and (4) for droop purpoe. It i thu the ame baic cheme ued in [19] and many other found in the literature. Fig. 6 how the root loci of the improved droop cheme, where it can be een that dominant pole λ 2 and λ 3 hift cloer to the imaginary axi, a k q increae. The ytem repone will therefore become more ocillatory, and eventually detabilize, a k q rie above.377 (λ 2 and λ 3 enter the right-half -plane). In contrat, repone of the traditional droop cheme i dominated by the ingle pole λ 3, which will move cloer to the imaginary axi, a k q increae. The ytem eventually become untable, a k q rie above The table range of k q of the improved droop cheme i thu wider than that of the traditional droop cheme. Beide thi, tabilitie of both droop cheme are not affected by normal variation of the converter and inverter output filter, ince both cheme ue only low-pa filtered or average power value. IV. GRID-CONNECTED MODE For moother operation, the ame droop expreion hould be ued for the grid-connected mode [16], which alo require the PV inverter to output the maximum power P PVmax. Thi maximum power can however be lower than the rated power P in (3), depending on the operating condition. The traditional droop expreion in (3) and (4) are therefore not directly applicable, and mut hence be changed accordingly. One traightforward approach i given in (17): kip V V k p ( P P) ( k pp )( PPVmax P) (17) where k pp and k ip are proportional and integral gain, repectively. The modification noted here i the econd term of (17), which in effect, ha added an offet V G (k p P V G ) to the original droop line. The purpoe i to make the ytem output the PV maximum power P PVmax, rather than it rated x 1-5 Fig. 6. Root locu diagram for.1 k q.1 improved droop cheme and traditional droop cheme. 35

7 power P (P PVmax P ). Moreover, unlike the ilanded mode, power haring i not the main concern in the grid-connected mode, which hence permit an integral term to be ued in (17). The ame offet in (17) can alo help to filter off influence from grid voltage and line impedance variation. For example, a hown in Fig. 2, the PV ytem i initially operating at point c with maximum PV power harneed. The grid voltage E i ubequently aumed to change lightly, cauing the power tranfer curve from (1) to lower lightly. The new operating point of the inverter will then be at c 1, while the dc-dc converter remain at c. Thi condition i however unutainable, ince an imbalance in power exit between the dc-dc converter and inverter. The repone from (17) will then be the lowering of droop line by V G, until the final operating point c 2 i reached. Finding V G from (17) i however not direct, ince P PVmax i unknown, if irradiation and temperature are not meaured. Moreover, (17) doe not permit the rear-end inverter, controlled by it, to regulate the dc-link voltage V DC, which trictly, i neceary, ince the front-end dc-dc converter i already taked to track the PV maximum power point. Expreion (17) mut hence be changed to (18), where it econd term can be ued for regulating the dc-link voltage by indirectly forcing the inverter output power to follow the PV maximum power. kip V V k p ( P P) ( k pp )( VDC VDCref ) (18) Additionally, the gain k pp in (18) hould be choen uch that the maximum tranient energy flowing through the dc-link capacitor doe not caue it voltage to rie above it trip level. The gain k ip hould, on the other hand, be choen to minimize the teady-tate error. For the implemented ytem, they are repectively choen a k pp =.3 and k iq =.5, which are comparably low gain uitable for the lower outer droop control loop. The ame reaoning can be applied to (4), leading to the following modified Q-f expreion: kiq f f kq( Q Q) ( k pq )( Q Qref ) (19) where k pq and k iq are proportional and integral gain, repectively. Again, the firt term of (19) i the uual droop expreion, hifted vertically by the added econd term to follow the demanded reference Q ref (et to zero if unity power factor i preferred). Additionally, k pq hould be choen to give a atifactory dynamic repone, while k iq hould be choen to minimize the teady-tate error. For the implemented PV ytem, thee gain are et a k pq =.1 and k iq =.4 to give a ufficiently long repone time needed for decoupling the lower outer droop power loop from the fater inner voltage and current tracking loop. Fig. 7 how the eventual grid-connected cheme implemented for the PV ytem. A uual, the boot converter i controlled by meauring V PV and I PV from the PV panel for tracking in the MPPT block. The inverter i however droop-controlled uing almot the ame control block a in Fig. V PV I PV L T V PVref D C Fig. 7. Control cheme for two-tage PV ytem in grid-connected mode (tranformer ued in practice and experiment not hown for conciene). 3 for the ilanded mode, except with the improved droop expreion in (18) and (19) ued intead. In addition, a harmonic detection block baed on intantaneou power theory [35] ha been ued for extracting harmonic voltage v h from the inverter terminal voltage v ac. The voltage reference for tracking i then obtained by umming v f and v h, where v f i the fundamental voltage reference produced by the droop controller. Voltage v h i therefore not compenated, which according to (2), will reult in a maller grid current ditortion i h, caued by the grid voltage harmonic v Gh. vh v v Gh Gh ih (2) Z Z vh forced to zero The grid-connected cheme in Fig. 7 will hence function well, even though at time, it may encounter problem caued by large delivered PV power. More pecifically, the large delivered PV power may caue the inverter output voltage v ac to rie higher than threhold permitted by the local load. When that happen, the front dc-dc boot converter hould top it upward tepping to the maximum power point. Intead, the curtailed power value hould be ued or the gradual tepping down of power hould be activated until v ac i brought below the permitted threhold. V. SEAMLESS TRANSFER Comparing (6) and (4) for the ilanded mode with (18) and (19) for the grid-connected mode, the generalized droop expreion for both mode can be comprehended, a follow: IP V V k p ( P P) ( PP )( VDC VDCref ) k pp, grid connected mode PP kv, ilanded mode kip, grid connected mode IP, ilanded mode f IQ f kq( Q Q) ( PQ )( Q Qref ) k pq, grid connected mode PQ, ilanded mode kiq, grid connected mode IQ, ilanded mode i L V f Lac C ac vac P iac Q V DC Z (21) (22)

8 2 C P PV V DCref V DC V PV V DCref PP IP I PV V PVref P Q k q k p f f V k f grid V PQ E k IQ 2 E Z VE Z Q ref P Q v ac i ac V DC Q V DCref P Q Q ref P Q kip k pp kiq k pq V f V y 2 f y kiv k pv p Fig. 1. Overview of improved droop cheme. f V k k i V E grid v ref v h v ac Fig. 8. Block diagram illutrating improved P-V and Q-f droop control. Fig. 9. Droop line for illutrating grid voltage and grid frequency ynchronization. Together with (1) and (2), block diagram repreenting (21) and (22) can ubequently be drawn, a hown in Fig. 8 and. The ame droop expreion in (21) and (22) can therefore be ued for both mode with eamle tranfer between them expected, ince there are no major change or wopping of control cheme. The boot converter will however experience change from MPPT to dc-link voltage control, and vice vera. Thee change are however decoupled by the dc-link capacitor, and will hence not diturb the grid ignificantly. In addition, eamle tranfer from the ilanded to grid-connected mode require ynchronization with the grid, before tatic witch between the PV ytem and grid can be cloed. The neceary ynchronization can be explained with Fig. 9. Before ynchronization, Fig. 9 how the inverter operating at point d 1 with output voltage V d and output power P d generated for the local load. After initiating ynchronization, P d mut be held contant to continuouly meet the local load demand, while lifting the inverter voltage from V d to the grid voltage E. Such lifting can again be realized by adding an autonomouly generated vertical offet, expreed a follow. kiv Vy ( k pv )( E V) (23) where k pv and k iv are proportional and integral gain of the voltage ynchronizer. The ame applie to Fig. 9, where the initial reactive power Q e mut be maintained, while lifting the inverter frequency from f e to the grid frequency f grid. Lifting the frequency alone i however not ufficient, ince the phae of the inverter θ may not be equal to that of the grid θ grid. It i therefore important for the inverter to track the phae of the grid uing (24), which upon achieved, will indirectly force the inverter to have the ame frequency a the grid. ki fy ( k p )( grid ) (24) where k pθ and k iθ are proportional and integral gain of the phae ynchronizer. Thee parameter and thoe in (23) are choen a k pθ = 1, k iθ = 2, k pv = 3 and k iv = 5 to give a repone time of around.2 for both ynchronizer. Expreion (23) and (24) are however not explicitly added to (21) and (22), ince they are needed only during the hort ilanded to grid-connected tranition. Intead, they are ummarized with the other droop and offet expreion in Fig. 1. The neceary tranfer procedure are alo comprehended, a follow. Grid-connected to ilanded mode Detection of diconnection command. MPPT to dc-link voltage control for the boot converter. Same improved droop control for the inverter, but with parameter changed appropriately (ee (21) and (22)). Ilanded to grid-connected mode Detection of connection command. Activation of ynchronizer in (23) and (24). Cloing of tatic witch after ynchronization. Deactivation of ynchronizer. DC-link voltage to MPPT control for the boot converter. Same droop control for the inverter, but with parameter changed appropriately (ee (21) and (22)).

9 TABLE I PARAMETERS OF TESTED SYSTEM Parameter Value -4V 1 P / W 4V -4V 4V VDC / V Panel voltage variation (V PV ) Grid voltage (E) Grid frequency (f grid ) DC-link voltage (V DC) DC-link voltage capacitance (C) Boot inductance (L) Output filter inductance (L ac ) Output filter capacitance (C ac) Inverter witching frequency (f ) Impedance of tranmiion line (Z) v inv1 v inv2 i inv1 i inv2 P 2 P 1 V DC2 V DC1 19V-3V 22V(rm) 5Hz 4V 94μF 4mH 5mH 1μF 1kHz 2Ω,.8μH 4 3 V PV2 2 1 V PV1 t /.5 Fig. 11. Simulated reult of two paralleled PV inverter uing improved droop cheme. VPV / V VI. SIMULATION RESULTS 2A -2A 2A Two imilarly rated PV ytem, tied to a LV ditribution network, have been imulated in Matlab/Simulink uing parameter lited in Table I for each ytem. Reult obtained from them are decribed below. A. Ilanded Mode for Evaluation of Internal Power Change Fig. 11 how the reult of the two ilanded PV ytem when controlled by the improved droop cheme. Before.1, the two ytem are aumed to have the ame maximum rating of 1kW. They hould hence hare the active load demand evenly with their repective dc-link voltage kept at 4V (nominal value read from Table I). After.1, the maximum -2A P / W VDC / V vgrid / V t / -2.6 Fig. 12. Simulated reult with grid voltage fluctuation when controlled with traditional and improved droop cheme. power of one ytem ha been reduced from 1kW to 6W, while that of the other remain unchanged. The compromied ytem, being unable to provide ufficient active power, will then have it dc-link voltage pulled down. The drop caue the droop line of the compromied ytem to be hifted down, like in Fig. 2. The droop-demanded output active power of the compromied ytem i then lowered until it reache 6W. In the meantime, the econd uncompromied ytem upplie more active power to the load, which trictly, i neceary before upply and demand can be balanced. The overall network i therefore table with only light droop oberved with the dc-link and inverter output voltage, even though the PV terminal voltage may change more prominently. B. Grid-Connected Mode for Evaluation of External Change A explained with (1) and (2), external diturbance are introduced through E and Z. Both parameter are prominently influenced by the grid. Reult from the grid-connected mode are therefore choen for demontration (even though the ame teting ha alo be performed in the ilanded mode). A hown in Fig. 12, the grid voltage read are v grid = 22V from to.2, 225V from.2 to.4, and 22V from.4 to.6. Throughout the three interval, the maximum PV power ha been et to P PVmax = 1W. However, with the traditional droop cheme, P PVmax i not harneed when the grid voltage increae in the econd interval. The actual power generated i only 8W, delivered by a much maller grid current i grid. On the other hand, Fig. 12 how the reult obtained with the improved droop cheme when ubject to the ame change. The harneed power i now alway 1W, even though ome light variation can be oberved at the intant of voltage change. The propoed cheme i therefore more effective in term of active power tracking. 2 igrid / A

10 Fig. 13. Simulated reult with grid frequency fluctuation when controlled with traditional and improved droop cheme. Fig. 14. Simulated reult for howing ilanded to grid-connected tranfer and grid-connected to ilanded tranfer when controlled with traditional control cheme. Fig. 13 and next how the reult obtained when the grid frequency increae from 5Hz to 5.2Hz during the interval from.2 to.4. The former ue the traditional droop Fig. 15. Simulated reult for howing ilanded to grid-connected tranfer and grid-connected to ilanded tranfer when controlled with improved droop cheme. cheme, while the latter ue the improved droop cheme. The traditional droop cheme i obviouly not able to regulate the reactive power at zero during the econd interval (Q = 5VAr), which the improved cheme can achieve after ome light tranient variation. The propoed cheme i thu le affected by grid frequency fluctuation, which urely, i encouraged. Some reult are then included in Fig. 14 and for howing how the traditional controlled ytem repond during mode tranfer. Quite obviouly, Fig. 14 how the inverter voltage v inv urging during the ilanded to grid-connected tranfer at.115, while Fig. 14 how the inverter current i inv urging during the revere grid-connected to ilanded tranfer. Such urging i however not een in Fig. 15 and at the tranfer time of.2. The tranfer are hence eamle with the improved droop cheme.

11 Fig. 16. Laboratory etup howing A -ocillocope, B - PV inverter, C - load, D - line tranformer, and E -point of common coupling. Fig. 18. Experimental reult with maximum power fluctuation from PV panel when uing improved droop cheme. Fig. 19. Experimental reult with grid voltage ditortion. without harmonic detection unit and with harmonic detection unit. (c) Fig. 17. Experimental reult with load change. Full view, increaing load, and (c) decreaing load. VII. EXPERIMENTAL RESULTS Experimental teting with a 1-kW PV ytem ha been performed for proving the practicality of the propoed cheme. The hardware etup i hown in Fig. 16, while it parameter are given in Table I. The dc ource ued i a Chroma 6215H-6S programmable upply, programmed to output typical PV characteritic, while the controller ued i a 32-bit STM32F13VBT6 microcontroller from STMicroelectronic. Reult from the etup when firt ilanded are hown in Fig. 17 when the improved droop cheme i ued, together with a load increae and then decreae. The load power i more pecifically read a P Load = 28W from to 7.42, 56W from 7.42 to 21.35, and 28W from to 25. Thee power are upplied by the PV ource, whoe terminal voltage will hence change from V PV = 25V to 24V, and then back to 25V. Depite the change, the reult how that the PV inverter operate a intended with it dc-link voltage V DC kept at the nominal of 4V, ince no drop in PV capacity ha been introduced yet. Fig. 18 how power fluctuation from the PV imulator, whoe value have been read a P PVmax = 7W from to 9, 5W from 9 to 19, and 7W from 19 to 25. Thee power change, in turn, caue the panel voltage to change from V PV = 246V to 24V and then back to 246V. Regardle of that, the

12 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS 5V vgrid vinv iinv iload 1A igrid -5V 15W -1A Pinv -15W t/ 3 (c) Fig. 2. Experimental reult with grid voltage fluctuation when uing traditional droop cheme. Full view, zoomed-in view of firt tranition, and (c) zoomed-in view of econd tranition. (c) Fig. 21. Experimental reult with grid voltage fluctuation when uing improved droop cheme. Full view, zoomed-in view of firt tranition, and (c) zoomed-in view of econd tranition. dc-link voltage i well-regulated at 4V by the improved droop cheme, while harneing PPVmax during the three interval. Fig. 19 next how the reult without the harmonic detection block in Fig. 7 for filtering out the grid voltage ditortion. The oberved inverter current and grid current are obviouly ditorted, a anticipated. Thee current ditortion are however not een in Fig. 19 when the harmonic detection block in Fig. 7 i enabled. Fig. 2 then how the reult obtained with grid voltage vgrid fluctuation when controlled by the traditional droop cheme from (3) and (4) (vgrid = 22V from to 14.48, 225V from to 24.38, and 22V from to 3). A oberved, maximum PV power of PPVmax = 7W i only generated in the firt and third interval. Power generated in the econd interval i only 56W becaue of the higher vgrid. Grid voltage fluctuation will therefore affect the maximum power harneing ability of the ytem when controlled by the traditional droop cheme. For comparion, Fig. 21 repeat the experiment with grid voltage fluctuation when controlled by the improved droop cheme from (21) and (22) (vgrid = 22V from to 14.74, 23V from to 24.34, and 22V from to 3). At the intant of grid voltage increae, the inverter power in Fig. 21 i noted to drop to 52W, before gradually returning to the maximum PV power of PPVmax = 7W. In contrat, the decreae of grid voltage in Fig. 21(c) ha caued the inverter power to increae to 88W, before returning to 7W. The extra power of 18W at the tranition i upplied by the dc-link capacitor ince the maximum PV power i only 7W. The propoed cheme i thu able to harne maximum active power even when ubjected to grid voltage variation. Intead of grid voltage, Fig. 22 how the reult obtained with the traditional droop cheme when ubjected to grid

13 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS Fig. 22. Experimental reult with grid frequency fluctuation when uing traditional droop cheme. Grid frequency increaing and decreaing. 1A 5V -5V 5V VDC VPV vinv iinv Fig. 23. Experimental reult with grid frequency fluctuation when uing improved droop cheme. Grid frequency increaing and decreaing. frequency fluctuation. A een from Fig. 22, the increae of grid frequency from 5Hz to 5.1Hz ha caued the grid current to gradually lag behind the grid voltage. The output reactive power from the inverter i thu non-zero. It only return to zero in Fig. 22 after the grid frequency return from 5.1Hz to 5Hz. The ame grid frequency teting ha been repeated in Fig. 23, where Fig. 23 how the grid current lagging initially, before returning to it in-phae poition when the grid frequency increae from 5Hz to 5.1Hz. Fig. 23 alo how the gradual returning of reactive power to zero when the grid frequency decreae from 5.1Hz to 5 Hz. The improved vgrid igrid -5V A 1A -5V 5V IPV -1A 1A t/ -1A (c) Fig. 24. Experimental reult howing mode tranfer with the improved droop cheme. Full view, ilanded to grid-connected tranfer, and (c) grid-connected to ilanded tranfer. droop cheme i thu able to track reactive power more accurately even when the grid frequency fluctuate. Lat but not leat, Fig. 24 how the experimental tranfer from ilanded to grid-connected mode at 5.47 and the revere tranfer from grid-connected to ilanded mode at Their zoomed-in view are provided in Fig. 24 and (c), repectively. A een from Fig. 24, the inverter with it

14 table dc-link voltage i found to upply only 28W to the local load, before the tranfer. The grid current read i alo at zero initially. The output voltage of the inverter i then activated to track the grid voltage, which upon ynchronized, permit the tatic witch to cloe to tart the tranfer. After the tranfer, the inverter and grid current are noted to increae ince the full maximum PV power i harneed with 28W flowing to the load and 42W flowing to the grid (total = P PVmax =7W). During thi time, the PV terminal voltage i alo oberved to drop from 25V to 225V, caued by it larger amount of power generated. Baed on the ame reaon, the revere tranfer in Fig. 24(c) alo correctly how the inverter current decreaing and grid current falling to zero after entering the ilanded mode. The inverter now generate only 28W for the local load even though the maximum power available i 7W. Overall, the two tranfer have been eamle with no diturbing tranient oberved. Thi i partly due to the ynchronizer ued, and partly due to the common improved droop tructure ued for both operating mode. VIII. CONCLUSION An improved droop cheme ha been propoed in the paper for a PV ytem connected to a reitive LV ditribution network. The approach i developed for a two-tage PV ytem, permitting it to tranfer eamlely between the ilanded and grid-connected mode, after enuring proper ynchronization. It alo allow the PV ytem to generate maximum active power, while remain not affected by power fluctuation, and other grid voltage and frequency diturbance when in the grid-connected mode. Such immunity i realized by dynamically hifting the droop line in both grid-connected and ilanded mode. Simulation and experimental reult have verified the performance of the propoed method. REFERENCES [1] S. A. Arefifar, and Y. A. R. I. Mohamed, Probabilitic optimal reactive power planning in ditribution ytem with renewable reource in grid-connected and ilanded mode, IEEE Tran. Ind. 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Power Delt., vol. 27, no. 3, pp , Jul [34] H. Han, Y. Liu, M. Su, and J. M. Gueerrero, An improved droop control trategy for reactive power haring in ilanded microgrid, IEEE Tran. Power Electron., vol. 3, no. 6, pp , Jun [35] Y. Q. Wang, L. Y. Yao, J. F. Peng, Y. L. Wang, and X. H. Mao Analyi of Harmonic current uppreion and reactive power compenation on 125 MVA motor generator, IEEE Tran. Plama Sci., vol. 4, no. 3, pp , Mar.212. Hongpeng Liu (M 13) received hi B.S. degree in Electrical Engineering from Harbin Univerity of Science and Technology, Harbin, China, in 2, and hi M.S. and Ph.D. degree in Electrical Engineering from Harbin Intitute of Technology, Harbin, China, in 26 and 211, repectively. In 211, he joined Harbin Intitute of Technology a an Aitant Profeor in the Department of Electrical Engineering. Hi current reearch interet include photovoltaic generation, Micro-grid, and PWM converter/inverter ytem. Poh Chiang Loh received hi B. Eng (Hon) and M.Eng from the National Univerity of Singapore in 1998 and 2 repectively, and hi Ph.D from Monah Univerity, Autralia, in 22, all in electrical engineering. Hi interet are in power converter and their grid application. Xiongfei Wang (S 1-M 13) received the B.S. degree from Yanhan Univerity, Qinhuangdao, China, in 26, the M.S. degree from Harbin Intitute of Technology, Harbin, China, in 28, both in electrical engineering, and the Ph.D. degree from Aalborg Univerity, Aalborg, Denmark, in 213. Since 29, he ha been with the Aalborg Univerity, Aalborg, Denmark, where he i currently an Aitant Profeor in the Department of Energy Technology. Hi reearch interet include modeling and control of grid-connected converter, harmonic analyi and control, paive and active filter, tability of power electronic baed power ytem. He received an IEEE Power Electronic Tranaction Prize Paper award in 214. He erve a the Aociate Editor of IEEE Tranaction on Indutry Application and the Guet Aociate Editor of IEEE Journal of Emerging and Selected Topic in Power Electronic Special Iue on Ditributed Generation. Yongheng Yang (S 12 - M 15) received the B.Eng. degree from Northwetern Polytechnical Univerity, Xian, China, in 29, and the Ph.D. degree from Aalborg Univerity, Aalborg, Denmark, in 214. He wa a Pot-Graduate with Southeat Univerity, Nanjing, China, from 29 to 211. In 213, he wa a Viiting Scholar with the Department of Electrical and Computer Engineering, Texa A&M Univerity, College Station, USA. Since 214, he ha been with the Department of Energy Technology, Aalborg Univerity, where he i currently an Aitant Profeor. Hi reearch interet are focued on grid integration of renewable energy ytem, power converter deign, analyi and control, harmonic identification and mitigation, and reliability in power electronic. Dr. Yang i a Member of the IEEE Power Electronic Society Student and Young Profeional Committee, where he i reponible for the webinar erie. He erve a a Guet Aociate Editor of the IEEE Journal of Emerging and Selected Topic in Power Electronic pecial iue on Power Electronic for Energy Efficient Building. Dr. Yang ha alo been invited a a Guet Editor of Applied Science pecial iue on Advancing Grid-Connected Renewable Generation Sytem. He i an active reviewer for relevant top-tier journal. Wei Wang (M 13) received her B.S. degree in Automatic Tet and Control from Harbin Intitute of Technology, Harbin, China, in 1984, her M.S. degree in Electrical Engineering from Harbin Intitute of Technology in 199, and her Ph.D. degree in Mechanical Electronic Engineering from Harbin Intitute of Technology in 22. In 1984, he joined Harbin Intitute of Technology, a an Aitant Profeor in the Department of Electrical Engineering, where he wa an Aociate Profeor from 1995 to 23, and where he ha been a Profeor ince 23. She current reearch interet include regenerative energy converter technique, micro-grid, oft-witching converter, and lighting electronic technology. Dianguo Xu (M 97, SM 12) received the B.S. degree in Control Engineering from Harbin Engineering Univerity, Harbin, China, in 1982, and the M.S. and Ph.D. degree in Electrical Engineering from Harbin Intitute of Technology (HIT), Harbin, China, in 1984 and 1989 repectively. In 1984, he joined the Department of Electrical Engineering, HIT a an aitant profeor. Since 1994, he ha been a profeor in the Department of Electrical Engineering, HIT. He wa the Dean of School of Electrical Engineering and Automation, HIT from 2 to 21. He i now the aitant preident of HIT. Hi reearch interet include renewable energy generation technology, power quality mitigation, enorle vector controlled motor drive, high performance PMSM ervo ytem. He publihed over 6 technical paper. Dr. Xu i a enior member of IEEE, an Aociate Editor for the IEEE Tranaction on Indutrial Electronic. He erve a Chairman of IEEE Harbin Section.