Dynamic analysis of inverter dominated unbalanced LV micro-grids

Transcription

1 Dynmi nlysis of inverter dominted unlned LV miro-grids N. L. Soultnis, A. I. Tsouhniks, N. D. Htzirgyriou, J. Mhseredjin Astrt This pper presents simultion of the dynmi ehvior of low voltge miro-grids oth under grid onneted nd utonomous opertion. In LV miro-grids it is most likely tht the soures prtiipting in the system re onneted to the network with inverters. This results in the formtion of n inerti - less system. Modeling hs to llow for the representtion of ll hrteristi unlnes enountered in the LV network. The simultion is onduted using the eletromgneti trnsient nlysis progrm EMTP-RV. Index Terms distriuted genertion (DG), miro-grids, inverter ontrol, low voltge networks D I. INTRODUCTION RIVING fores suh s environmentl onerns nd the need for energy effiieny, tied with tehnil progress in new forms of energy prodution, fvor the shift towrds deentrlized power system. Distriuted genertors (DG) with prime soures sed either on renewle energy or on nturl gs re instlled in distriution or higher voltge networks. A lrge sle integrtion of DGs is ntiipted for lower voltge networks, promising distint dvntges for the onsumers regrding reliility nd power qulity [][]. Low voltge feeders with severl miro-soures [3][4], storge devies nd ontrollle lods tht pper to the upstrem distriution network s ontrollle entity, form the so lled mirogrids [][]. The miro-grid n operte either onneted with the MV distriution grid exhnging power with it ording to the existing onditions or utonomously in the event of upstrem network unvilility. The miro-grid is n intensive pplition of distriuted genertion, therefore dynmi nlysis is required to ensure tht it opertes in stle mnner nd tht voltge nd frequeny flututions re mintined within eptle limits []-[3]. The isoltion from the upstrem grid nd the susequent utonomous opertion re hllenging. Proper lod shring mong the miro-soures hs to e ensured long with good dynmi response hrteristis following ny disturne. This work ws performed with the EU reserh projet Miro-grids (Contrt No ENK 5-CT ). The uthors grtefully knowledge the support reeived from the Europen Union for this reserh. N. L. Soultnis, A. I. Tsouhnis nd N. D. Htzirgyriou re with the Ntionl Tehnil University of Athens, Athens, 5780 Greee (phone: ; fx: ; e-mil: nsoultn@power.ee.ntu.gr / ggelos@power.ee.ntu.gr / nh@power.ee.ntu.gr).. Jen Mhseredjin is with Éole Polytehnique de Montrél. A LV miro-grid my e essentilly n inerti-less system, sine ll soures my e onneted to the network vi inverters to gin in ontrol flexiility or to ridge inomptiilities etween the output of the soures nd the network. In ddition the speil fetures of LV networks, suh s uneven phse loding, the presene of single nd threephse lines hve to e ounted for in the network representtion. In this pper eletromgneti trnsient nlysis progrm EMTP-RV [5] is used to exmine the dynmis of the miro-grid. The EMTP-RV n e used to simulte the dynmis of ny poly-phse system urtely. There is no limittion in the system topology nd the lines n hve ny numer of wires. The instntneous vlues of voltges nd urrents n hve ny ritrry form eyond the sinusoidl fundmentl wveform nd every system omponent is desried y set of differentil equtions. Frequeny is nturl property of the system quntities nd the inerti of rotting mhines is not neessry for its omputtion. Moreover, the opertion nd ontrol of the eletroni onverters n e modeled in detil [6]. EMTP-RV provides stedy stte poly-phse lod flow for the initiliztion of the stte vriles, so tht the time domin simultion strts from speified stedy stte ondition voiding unneessry trnsients. In [7] stility lgorithm for the dynmi nlysis of inverter dominted miro-grids in islnd opertion hs een proposed nd the simultion results hve een verified y EMTP-RV. This pper fouses on the EMTP-RV pplition to the dynmi nlysis of miro-grids nd presents new dt, simultion results nd nlysis. II. CHARACTERISTICS OF LV MICRO-GRIDS Low voltge networks of urn nd rurl res er some distint fetures tht need to e ounted for in their nlysis (stedy stte lod flows, fult lultions or trnsient simultions). Unlned loding nd exittion re very ommon nd result from the uneven lod distriution mong the three phses nd from the existene of single-phse soures. Moreover, the network struture itself my e unlned, minly due to single-phse lterls or the inherent symmetry of the three phse lines. Unlike high voltge networks, line impednes re predominntly resistive, while their shunt pitne is typilly ignored. Bsilly, the struture of the LV network differs from HV networks in tht ny LV line, in ddition to the phse ondutors, will lso inlude grounded or ungrounded neutrl ondutor nd

2 possily seprte protetive erth (grounding) ondutor. In puli LV distriution networks the neutrl is typilly multigrounded, t the soure, onsumer servie points nd possily other seleted lotions long the lines. In suh se, prllel return pth exists through the erth for neutrl (zero sequene) urrent, even in the sene of ny fult. In unfulted onditions, however, this return pth is ustomrily ignored with negligile error, sine the neutrl ondutor presents muh lower return impedne [8]. As fr s the soures tht prtiipte in the system re onerned, they n e lssified on the sis of their pility to produe ontrolled tive power on demnd. Soures whih lk this pility (suh s photovoltis or wind turines) re normlly operted under PQ ontrol [], ensuring tht mximum tive power P is extrted from the prime soure, under onstnt retive power Q or power ftor (possily regulted). This ontrol type remins the sme, whether the system opertes onneted to the upstrem network or not. Soures sed on storge devies or regulted prime movers, on the other hnd, permit the ontrolled vrition of their output power, within the limits set y the primry energy uffer nd their rted pity. Suh soures re ssigned the tsk of frequeny regultion during utonomous opertion of miro-grid nd re often desried s grid-forming units, sine they re responsile for setting up nd regulting the network where the PQ-ontrolled soures re onneted [9]. In the isolted mode of opertion, the system lod is fed only y the lol miro-soures. Thus the grid forming units re lled upon to regulte frequeny nd voltge for the lods nd the PQ-ontrolled soures, funtion in priniple equivlent to mintining the tive nd retive power lne within the islnd system. Grid-forming units re interfed to the network through voltge soure inverters, whose frequeny nd voltge is regulted using frequeny tive power nd voltge retive power hrteristis, emulting the droop urves of trditionl genertors [9]-[4]. This llows proper lod shring mong multiple regulting units within the sme network. This type of ontrol is rried out only with lol mesurement of terminl quntities, without ny ommunition requirements etween individul units nd n operte when the miro-grid is onneted to the min grid s well. The two ontrol types disussed ove re implemented in the inverter tht interfes the soures to the network. In prtiulr, the ontrol lgorithms regulte the mgnitude nd phse ngle of the inverter internl voltge (tully of the fundmentl omponent of the respetive PWM wveform). Hene in the se of storge units the frequeny/ngle is ontrolled y tive power frequeny droop nd the mgnitude y retive power voltge droop s shown for the EMTP-RV implementtion of ontrol in Fig.. Two first order trnsfer funtions with time onstnts T m nd T e re used for the tive nd retive power mesurement dely respetively. The time onstnts n e lso djusted to deouple the tive nd retive power ontrol hnnels. The phse ngle derived y integrting the frequeny nd the mgnitude resulting from the retive power droop ontrol onstitute the desired internl emf voltge vetor. The stilizing ftor is gin with units rd/w whih dds phse led to the system nd improves the response nd stility espeilly in the se of network lines with high resistne to retne rtio. Soures operted under PQ ontrol re onsidered to e onneted to the network vi voltge soure inverters, s well. In this se, the ngle nd mgnitude of the internl AC voltge re ontrolled in order to mintin the d side voltge t speified referene vlue nd the retive power output t desired set point [5], s shown in Fig.. 3ph Inverter Rel Power () () () fo Mesurement Dely f(s) Droop Gin 3ph Inverter Retive Power Stilizing Ftor π Vo Mesurement Dely f(s) Droop Gin COS PROD Internl Fig.. Control of storge units nd regulted soures vi droop urves (one for frequeny tive power nd one for internl voltge retive power). V 0, f 0, orrespond to zero Q, P output. () () () DC Vd Referene 3ph Inverter Retive Power Q Referene - - k k3 COS k Δθ DC PROD Fig.. PQ-type ontrol (non regulted Mirosoures). θt Internl PROD The power lne etween the inverter output power nd the power produed y the primry soure determines the d side voltge. The ngle differene Δθ etween the inverter internl nd terminl voltges is derived from the d voltge error vi ontroller. Δθ nd the phse ngle θ t of the terminl voltge yield the phse ngle θ = θ t Δθ of the internl emf. The devition of the retive power from its setpoint yields the mgnitude of the internl emf vi n I- ontroller. III. APPLICATIONS A. Test System The study se LV miro-grid uilt in EMTP-RV is shown

3 3 in Fig. 3. Detils n e found in [6]. Network dt re provided in the Appendix. Two storge units re onneted t nodes A nd B, providing regultion in the isolted mode of opertion. Both of them re ttery nks oupled to the network through voltge soure inverters, ontrolled s desried in Fig.. Two PV units re onneted t nodes C nd D through inverters ontrolled s shown in Fig.. A wind turine equipped with n indution genertor diretly oupled to the grid is onneted t node E. The rted pities of the storge units t nodes A nd B re respetively 35 kva nd 5 kva, t lg. Their droop urves re speified so tht the units produe zero tive nd retive power when the miro-grid is onneted to the min grid. The slopes of the droop urves, given in the Appendix, determine the output power (generted or onsumed) in reltion to the miro-grid frequeny nd unit internl voltge. E.g. for the seleted slopes of the P-f urves, the mximum output power of eh unit orresponds to frequeny devition of ± 0.5 Hz, while tive nd retive power prodution is in proportion to their rting. B. Simultion results Two ses were exmined nd seleted grphs of the system quntities re presented. In the first of the simulted ses, only the two ttery units re in opertion, ll other soures eing disonneted. The miro-grid is initilly onneted to the min grid, disonneting t se. Then, the lod t nodes C, D, nd E inreses y 5% t.5 se. Node voltges, urrents nd power flows re plotted in Figs. 4 to. Initilly, the lod is fed y the upstrem network. The sum of the tive power output of the three phses in eh of the two ttery inverters is zero (Figs. 7, 9), s ditted y the P-f droops for system frequeny equl to 50 Hz (Fig. 4). At the sme time, the internl voltges of the two inverters ssume vlue orresponding to zero totl retive power output (sum of three phses) (Figs. 5, 8, 0). On seprtion of the mirogrid from the min grid, t t= se, the power output of the storge units ruptly inreses to meet the totl lod demnd (Figs 6,7,9). This inrese in tive power results in redution of the miro-grid frequeny (Fig. 4), s ditted y the P-f droop urves. Eventully, new equilirium point is estlished, where the two storge units operte t lower system frequeny, shring the totl miro-grid lod in proportion to their rted pities. Retive power demnd from the two inverters lso inreses (Figs 6,8,0), to meet the lod demnd, resulting in inresed internl voltges (Fig. 5), ording to their Q-V droop urves. The voltge level throughout the network drops somewht, ut is kept t n eptle level, s seen in Fig. for the lod node C. The tive nd retive power demnds of this lod re shown in Fig. nd they remin prtilly unffeted. At t=.5 s 5% lod inrese t nodes C, D nd E ours, lned y further inrese in the tive nd retive power prodution of the two storge units (Figs. 6-0). The mirogrid frequeny drops further (Fig. 4), while the internl voltges of the inverters re inresed (Fig. 5). RLC RLC RLC / /.7 kva PV PV RLC RLC.6 /.4 / 4 kva RLC RLC RLC RLC.6 / 3. / 4 kva Node D Node C 0/0.4 kv 400 kva, 6% kv Distriution Grid Fig. 3. The study se Miro-grid network. 9 4 Node E / 6.4 / 8 kva 7 Node A Bttery Inverter RLC 0 / 0 /.7 kva Node B Wind Turine IG RLC RLC RLC Bttery Inverter Fig. 4. Miro-grid frequeny (mesured t the terminls of the two ttery units).

4 4 Fig. 5. Internl phse voltges of the ttery inverter t node A. Fig. 8. Retive power flow per phse of the inverter t node A (lod onvention). Fig. 6. Currents of the ttery inverter t node A. Fig. 9. Ative power flow per phse of the inverter t node B (lod onvention). Fig. 7. Ative power flow per phse, of the inverter t node A (lod onvention). Fig. 0. Retive power flow per phse of the inverter t node B (lod onvention).

5 orresponding redution in the tive power prodution of the two storge units (Fig. 7) nd smll inrese in the frequeny (Fig. 3). The hnges of the ggregte (lod nd PV) powers nd urrents to node C re shown in Figs. 6 nd 5. The tive nd retive power nd the rotor speed of the WT re presented in Figs. 9 nd 0. Notly, the wind power flututions re lso refleted on the system frequeny nd on the prodution of the ttery inverters during the isolted mode of opertion. 5 Fig.. Phse voltges t node C. Fig. 3. Miro-grid frequeny (mesured t the terminls of the two ttery units). Fig.. P, Q flows per phse through the supply le to node C (setion in Fig. 3). For the seond se studied, the sme disturne (isoltion from the min grid) is simulted, ut the two PV rrys nd the WT of the miro-grid re now in opertion from the eginning of the simultion. The PV units t nodes C nd D produe 3 kw nd 4 kw respetively, t unity power ftor. The WT produes on verge 5 kw nd onsumes. kvr. Its slip is nd its input mehnil power 5.5 kw. To represent vritions of the wind, ±0% periodi osilltion of frequeny 0.5 Hz is superimposed on the verge mehnil torque. The miro-grid is disonneted from the upstrem network t.5 s nd the output of the PV units is inresed y 50% t 5 s, following n ssumed inrese in insoltion. Seleted wveforms re presented in Figs. 3 to 0. Before the disonnetion from the min grid, t t=.5 s, the miro-grid tive lod demnd is overed only in prt y the lol prodution of the miro-soures, requiring lrge power import from the upstrem grid, whih supplies the retive power requirements, s well. Upon isoltion, the two storge units strt inresing their outputs to ompenste the loss of the grid import (Fig. 7,8 for the storge unit t node B) nd the miro-grid frequeny drops to lower vlue (Fig. 3). The tive power prodution of the two PV units is inresed y 50% t t=5 s (Fig. 4 for the PV unit t node C), using Fig. 4. P, Q output per phse of the PV inverter t node C (lod onvention). Fig. 5. Currents through the supply le to node C (setion in Fig. 3).

6 lod hnges nd vritions in the prodution of the mirosoures. 6 Fig. 8. Retive power flow per phse of the inverter t node B (lod onvention). Fig. 6. P, Q per phse through the supply le to node C (setion in Fig. 3). Fig. 9. P, Q output of the WT indution genertor (lod onvention). Fig. 7. Ative power flow per phse of the inverter t node B (lod onvention). IV. CONCLUSIONS Simultion of n inverter dominted LV miro-grid using EMTP-RV ws presented. Test ses demonstrted, inlude oth grid-interonneted nd isolted mode of opertion. Prtiulr hrteristis of LV networks, suh s network, lod nd soure unlnes nd neutrl ondutors, re ounted for in the system representtion. The disturnes onsidered inlude sudden isoltion from the upstrem grid, Fig. 0. Rotor speed of the WT. V. APPENDIX The output filter retne of eh inverter is 0.04 p.u. on its rting (lines 7, in Fig. 3). Control prmeters (Fig. ) for the storge unit inverters (on se power 00 kva/phse):

7 7 Slope of f P droop urve (Hz/p.u. P) Slope of V Q droop urve (p.u. V/p.u. Q) Unit t node A Unit t node B T m 0. s T e 0.08 s A vlue of ten times less the vlue of the f-p droop ws used for the stilizing gin of eh unit. Control prmeters for the PV inverters (Fig. ): k = 0, k =0, k 3 = 400, C = 0 mf, V d-ref =.4 pu WT indution genertor prmeters: R s =.5 Ω, R r =.67 Ω, X ls = X lr =3.39 Ω, X m = 97 Ω J = Kg m, 4 poles Tle I provides dt for the lines of the study se network of Fig. 3 (sequene quntities for omptness), derived y omputing the 4x4 primitive impedne mtrix of eh le type [7], sed on the les hrteristis. For the singlephse line setion 9 of Fig. 3, the tul loop impedne vlue is given. TABLE I LINE DATA OF THE MICRO-GRID NETWORK Line No, 5, 8, , 3 Line Type Overhed Bundled le 4X0MM AL XLPE Overhed Bundled le 4X0MM AL XLPE Overhed Twisted le 3X70MM AL XLPE 54.6MM AAAC Connetion le XLPE 3X50MM AL Z (Ω/km) 0.84 j j j Z 00 (Ω/km).36 j j j CU 35MM j j Connetion le X5MM CU j j Connetion le 6.8 X6MM CU j 0.9 Connetion le X6MM CU j j 0.48 Connetion le 4X6MM CU 3.4 j j 0.47 Length (m) VI. REFERENCES [] R. Lsseter, A. Akhil, C. Mrny, J. Stephens, J. Dgle, R. Guttromson, A. Melliopoulos, R. Yinger, J. Eto, "White pper on integrtion of distriuted energy resoures - The CERTS miro-grid onept" Offie of Power Tehnologies of the US Deprtment of Energy, Contrt, DE- AC03-76SF00098, April 00. [] MICRO-GRIDS Lrge Sle Integrtion of Miro-Genertion to Low Grids, EU Contrt ENK5-CT , Tehnil Annex, My 00, lso t [3] Tehnil Brohure CIGRE Tsk Fore , Modeling new forms of genertion nd storge Nov [4] N. Htzirgyriou, G. Kriniotkis, N. Jenkins, J. Pes Lopes, J. Oyrzl Modeling of Mirosoures for seurity studies, CIGRE Pris 30 Aug. 3 Sep [5] J. Mhseredjin, S. Dennetiere, L. Due, B. Khodkhhin, L. Gerin- Ljoie, On new pproh for the simultion of trnsients in power systems presented t the IPST 05 Interntionl Conferene, Montrel Cnd, June 005. [6] J. Mhseredjin, L. Due, M. Zou, S. Dennetiere, G. Joos, Elimintion of numeril delys in the solution of ontrol systems in EMTP presented t the IPST 05 Interntionl Conferene, Montrel Cnd, June 005. [7] N. L. Soultnis, S. A. Ppthnsiou, N. D. Htzirgyriou, A Stility Algorithm for the Dynmi Anlysis of Inverter Dominted Unlned LV Mirogrids., IEEE Trnstions on Power Systems, Volume, Issue, Fe. 007, pp [8] E. Clrke, Ciruit Anlysis of A-C Power Systems, vol. I. New York: Wiley, 964 [9] A. Engler, "Control of ttery inverters in modulr nd expndle islnd grids." (In Germn), PhD disserttion, Univ. Kssel, Germny 00. [0] A. Engler, "Appliility of droops in low voltge grids." DER Journl No, Jn [] S. Brsli, M. Cerolo, P. Pelhi, D. Poli, "Control tehniques of dispersed genertors to improve the ontinuity of eletriity supply" in Pro. 00 IEEE Power Engineering Soiety Winter Meeting, Vol., pp. 7-3, Jn. 00. [] D. Georgkis, S. Ppthnsiou, N. Htzirgyriou, A. Engler, C. Hrdt. Opertion of prototype Miro-grid system sed on miro soures equipped with fst-ting power eletronis interfes. Proeedings of PESC 04, June 004, Ahen, Germny [3] M. C. Chndorkr, D. M. Divn, R. Adp, Control of prllel onneted Inverters in stnd lone AC supply systems, IEEE Trns. Ind. Applitions, pp 36-43, Jn [4] M. C. Chndorkr, D. M. Divn, B. Bnerjee, Control of distriuted UPS systems, Power Eletronis Speilists Conferene PESC 94, 5 th Annul IEEE, Vol., pp 97-04, June 994. [5] O. Wsynzuk, N. A. Anwh, "Modeling nd dynmi performne of self ommutted photovolti inverter system" IEEE Trns. Energy Conversion Vol. 4, No 3, pp 3-38, Sep [6] S. Ppthnssiou, N. Htzirgyriou, K. Strunz, A Benhmrk Low Miro-grid Network Pro. CIGRE Symposium Power Systems with Dispersed Genertion, Athens, April 005. [7] J. D. Glover, M. Srm, Power System Anlysis & Design PWS- Pulishing 994 VII. BIOGRAPHIES Nikos L. Soultnis reeived the Diplom in Eletril Engineering from NTUA in 989 nd the M.S. degree from UMIST Mnhester, UK in 000. Presently he is PhD student in NTUA. His reserh interests re dispersed genertion nd power system nlysis. He is memer of the IEE nd of the Tehnil Chmer of Greee. A. I. Tsouhniks reeived his Eletril nd Computer Engineering degree t Ntionl Tehnil University of Athens (NTUA), in 004. Currently he is PhD student in the Eletril Engineering Deprtment of NTUA. His reserh interests inlude renewle energy soures nd dispersed genertion. Nikos D. Htzirgyriou ws orn in Athens, Greee. He reeived the Diplom in Eletril nd Mehnil Engineering from NTUA nd the MS nd PhD degrees from UMIST, Mnhester, UK. He is professor t the Power Division of the Eletril nd Computer Engineering Deprtment of NTUA. His reserh interests inlude Dispersed nd Renewle Genertion, Dynmi Seurity Assessment, nd pplition of Artifiil Intelligene Tehniques to power systems. He is senior memer of IEEE, memer of CIGRE SCC6 nd the Tehnil Chmer of Greee. Jen Mhseredjin (M'87) grduted from Éole Polytehnique de Montrél with Ph.D. in 99. From 987 to 004 he worked t IREQ (Hydro-Quée) on reserh nd development tivities relted to the simultion nd nlysis of eletromgneti trnsients. In Deemer 004 he joined the fulty of eletril engineering t Éole Polytehnique de Montrél.