Interaction Analysis in Islanded Power Systems with HVDC Interconnections

Transcription

1 3rd Interntionl Hyrid Power Systems Workshop Tenerife, Spin 8 9 My 218 Intertion Anlysis in Islnded Power Systems with HVDC Interonnetions Crlos Colldos-Rodríguez, Edurdo Prieto-Arujo, Mr Cheh-Mne, Rird Ferrer-Sn-José, Oriol Gomis-Bellmunt CITCEA-UPC, Spin Emil: rlos.olldos@ite.up.edu Silvi Snz, Crmen Longás, Antonio Cordón, Luis Corondo Red Elétri de Espñ, Spin Astrt Islnded power systems re often onneted to lrger minlnd power systems using HVDC les. The prolifertion of new HVDC interonnetors in those islnded systems might led to redution of the onventionl synhronous genertion-sed power plnts onneted to the grid, with the ssoited derese of rotting inerti nd short-iruit urrent pity in the resulting system, posing new hllenges on the system stility nd the ehvior during system fults. In these situtions, the power system dynmis hevily depend on the onverter ontrol lgorithms, ft tht requires new methodologies to study the system stility nd the potentil intertions etween the different system elements, onsidering the power eletronis. This pper nlyses intertions in multi-infeed HVDC islnded system with nd VSC- HVDC links. In prtiulr, frequeny stility is evluted in se study tht represents n islnd with two HVDC links, two synhronous genertors nd n ggregted lod. Also, frequeny stility limits re determined when the synhronous genertion of the islnd is redued. I. INTRODUCTION The numer of power eletroni devies onneted to the grid is inresing due to the integrtion of renewle genertion nd High Voltge Diret Current (HVDC) trnsmission systems. An exmple of grid with high penetrtion of onverters is found in Northern Europe, where severl ountries nd offshore wind power plnts re onneted through HVDC trnsmission systems. Ares with high penetrtion of power eletronis present new hllenges in reltion to system stility nd response during fults. In prtiulr, instilities might e used y intertions etween power onverters nd other omponents of the grid, e.g. trnsformers nd trnsmission lines [1] [5]. The onverter ontrol plys key role to ensure proper opertion of the system nd void undesirle intertions etween omponents. Systems with multiples -HVDC links hve een studied in [6] [8], where five min intertions re identified: trnsient overvoltge, ommuttion filure, hrmoni intertion, power voltge stility nd ontrol intertions. Also, frequeny stility my e onsidered in se of smll systems with low inerti [8]. In order to limit the impt of these intertions, mitigtion strtegies were proposed sed on onverter design nd ontrol oordintion [7]. A numer of HVDC links hs een used to onnet islnded systems to the min AC grid. Interonneting islnds to minlnd is solution to redue the high osts of lol genertion nd improve the seurity of supply. In these ses, onventionl power plnts sed on synhronous genertors n e removed from the grid, reduing the inerti nd shortiruit urrent pity in the islnded system. Currently, most of the islnded systems interonneted through HVDC re sed on Line-Commutted Converters (): Gotlnd [9], Jeju [1], Mjor [11], Srdini nd Corsi [12]. Voltge Soure Converters (VSC) n provide dditionl enefits for n islnded power system thnks to their higher ontrolility. Compred to s, VSCs do not present ommuttion filure due to disturnes on the AC grid nd n provide oth voltge nd frequeny support, ontriuting to improve the system stility. Therefore, VSCs n operte onneted to wek grids, whih my represent smll islnds with low numer of synhronous genertors. However, when the synhronous genertion of the system is redued, the inerti or short-iruit urrent my not e suffiient to ensure stle opertion with onventionl VSC ontrols. As solution, the VSC ontrol n e improved, e.g. the urrent referene loop is modified in [13] to operte onneted to grids with low short-iruit urrent. Also, VSCs n operte in grid-forming mode, i.e. generting the voltge mgnitude nd ngle of the grid [14]. It is importnt to identify the opertionl limits of onventionl VSC ontrol tehniques for frequeny nd voltge support nd when grid-forming tehniques should e pplied to ensure stle opertion of the islnded system. The multiterminl HVDC grid used in Zhousnd islnds represents n exmple of islnded system onneted through VSC-HVDC [15]. Multi-infeed HVDC systems with nd VSC-HVDC links my eome more ommon in islnded system (see Fig. 1), sine VSCs n provide dditionl enefits for s, e.g redution of ommuttion filure or hrmoni emission without lrge pssive filters [16], nd n improve the stility. Currenly, few multi-infeed systems re plnned or in opertion: Gotlnd is interonneted through nd VSC-HVDC link, wheres n dditionl VSC-HVDC link might e instlled in Jeju [1]. This pper nlyses intertions in multi-infeed HVDC islnded system with nd VSC-HVDC links. In prtiulr, frequeny stility is evluted in se study tht represents n islnd with two HVDC links, two synhronous genertors nd n ggregted lod. The frequeny response ontriution of the HVDC links is nlysed when onventionl frequeny-power droop ontrol is implemented. Also, frequeny stility limits re determined when the synhronous genertion of the islnd is redued. Time-domin simultions in PSCAD/EMTDC hve een rried out to demonstrte the frequeny response of the islnded system

2 3rd Interntionl Hyrid Power Systems Workshop Tenerife, Spin 8 9 My 218 AC grid 1 AC-DC onverter 1 VSC AC-DC onverter 1 - HVDC link VSC-HVDC link AC-DC onverter 2 VSC AC-DC onverter 2 AC grid 2 ommuttion filure. The ontrol struture is sed on PI ontrollers tht define the firing ngles for the thyristors, s shown in Fig. 3. retifier 6 R = π TωL AC V r = 6 2 π TV AC2 osα r Smooth retor V DC-r V DC inverter 3 AC filters Fig. 1: Exmple of multi-infeed HVDC islnded system fter sudden disonnetion of synhronous genertion. II. MULTI-INFEED HVDC ISLANDED SSTEM Fig. 2 shows the onfigurtion of the system under study, whih represents n islnded AC grid fed y n nd VSC-HVDC link importing power from AC min grid. Swithing models hve een used for the inverter HVDC terminls onneted to the islnd, while the retifier HVDC terminls re represented s voltge soures. The islnded grid is omposed y two synhronous genertors tht re onneted to the sme us, vrile lod nd three overhed lines, L1, L2 nd L3, whih re represented with frequeny dependent models. 3 4 L2 L3 2 1 L1 Lod SG1 SG2 Fig. 2: Cse study of multi-infeed islnded system with n nd VSC-HVDC links A. Configurtion nd Control of -HVDC Link The -HVDC link is sed on 12-pulse ridges with symmetril monopole onfigurtion. The retifier side is represented with n verge model, s shown in Fig. 3, whih is modelled s [17]: VDC r = 6 2 π T V AC2 os α IDC 6 π T ωl AC (1) where, V AC2 nd ω re the line-to-line voltge nd ngulr frequeny of the AC grid, IDC is the DC urrent through the -HVDC link, T is the trnsformer rtio nd L AC is the equivlent indutne of the trnsformer. The inverter side, is represented with detiled model inluding thyristors, trnsformers nd retive nd hrmoni ompenstion filters. A onventionl opertion is onsidered, where the -retifier ontrols DC urrent nd the -inverter ontrols DC voltge [18]. An extintion ngle γ ontrol is lso implemented in the inverter side to redue the risk of 9 * PI + α r 9 * V DC PI + β min V mx DC 9 γ * + PI β min γ Fig. 3: Representtion of -HVDC link B. Configurtion nd ontrol of -HVDC link The VSC-HVDC link is sed on Modulr Multilevel Converters () with hlf-ridge sumodules nd symmetril monopole onfigurtion. The retifier side is modelled s DC voltge soure, s shown in Fig. 4, wheres the inverter side is represented with the elerted model presented in [19]. This model represents ll the sumodules of the onverter individully s pitors tht re onneted or disonneted depending on their swithing sttes. The VSC-HVDC link opertion onsiders the VSCretifier ontrolling DC voltge nd the VSC-inverter ontrolling tive power nd AC voltge of the islnded system. The VSC-retifier ontrol is not represented, wheres the VSC-inverter inludes detiled ontrol. The ontrol strtegy for the is shown in Fig. 4 nd is sed on [2]. The min ojetive of the ontrol is to exhnge power etween the AC nd DC grids, while ensuring lning of the energy stored in ll the rms without lrge devitions. Also, Nerest Level Modultion (NLM) is used s modultion tehnique, whih n redue the verge ommuttion frequeny [21]. C. Modelling nd ontrol of synhronous genertors The synhronous genertion of the islnded system is represented y two onventionl stem power plnts, inluding turines, synhronous mhines, exiters nd governors, s shown in Fig. 5. The mehnil prt of the genertion unit is represented y single-mss models with speifi inerti. D. Frequeny ontrol Frequeny ontrol is sed on onventionl powerfrequeny droop. Fig. 6 shows the ontrol struture of the power-frequeny droop implemented in the onverters nd synhronous genertors. The power-frequeny droop gin, K droop, n e defined s in [17]: K droop = f/f P/P 1 = 1 k fp P f 1 (2) where f nd P re the frequeny nd power vritions, f is the nominl synhronous frequeny, P is the rted power of the onverter or synhronous genertor nd k fp is the ontrol gin shown in Fig. 6.

3 3rd Interntionl Hyrid Power Systems Workshop Tenerife, Spin 8 9 My 218 V DC 4 TABLE I: Systems prmeters Prmeter Vlue Unit AC grid voltge (RMS, ph-ph) 22 kv rted power MW rted power MW Genertors droop 5 % droop 2 % droop 2 % Lines length (L1, L2 nd L3) 15 km Mesurements Grid side ontrol PV opertion mode Internl energy ontrol referenes PQ referene lultion Internl energy ontrol Energy ontrol Grid urrent referene lultion Additive urrent referene lultion Grid urrent ontrol Additive urrent ontrol Fig. 4: onfigurtion nd ontrol Exiter Sumodules gte signl genertion Single-mss 1 Gte signls A. Cse Study 1: Contriution of HVDC links to Frequeny Support The ontriution of HVDC links to frequeny support of the islnded system is nlysed onsidering the lod nd genertion speified in Tle II, where 41.7 % of the totl lod is supplied y synhronous genertion. A sudden loss of MW of synhronous genertion (SG2) is onsidered t 1 s, whih represents 8.3% of the totl genertion. Different senrios hve een tested depending on the elements tht ontriute to frequeny support: Only Synhronous genertors (SG) Synhronous genertors nd -HVDC link (SG+) Synhronous genertors nd VSC-HVDC link (SG+) Synhronous genertors nd oth HVDC links (SG++) SG TABLE II: Opertionl prmeters in first se study. f P Governor Stem turine Fig. 5: Frequeny response depending on the remin synhronous genertion power III. CASE STUD Two se studies re nlysed onsidering the islnded systems presented in Setion II nd with the prmeters shown Tle I. First, the ontriution of the HVDC links to the frequeny response is nlysed. Then, the frequeny stility of the islnded system is evluted y reduing the synhronous genertion, i.e. the inerti of the system. In order to nlyse the frequeny response of the system, sudden loss of synhronous genertion is simulted in PSCAD/EMTDC, disonneting genertor SG2 from the system. f + f f Fig. 6: Frequeny response depending on the remin synhronous genertion power k fp P Prmeter Vlue Unit SG1 power MW SG2 power MW -HVDC link power 1 MW VSC-HVDC link power 2 MW Lod 6 MW Fig. 7 shows the frequeny response for ll the senrios. When the onverters do not provide frequeny support, the genertor SG1 hs to ompenste the power imlne used y the loss of SG2. The frequeny is redued elow 49.2 Hz, whih is not within eptle opertionl rnges [22]. The stedy stte frequeny is Hz nd is rehed round 2 seonds fter the genertion loss. It is ler tht the onverters n ontriute signifintly to the frequeny support, reduing the mximum frequeny devition (the frequeny is lwys ove Hz) nd rehing the stedy stte in shorter time. This is euse the onverters hve fster dynmi response, ompred to the synhronous genertors, to ompenste the power imlne fter the genertion loss. B. Cse Study 2: Frequeny stility limits The frequeny stility limits re nlysed onsidering redution in the totl synhronous genertion of the islnded system, i.e. redution in the totl inerti. In this se study, the lod is 5 MW nd sudden loss of 1 MW of synhronous genertion (SG2) is pplied t 1s, whih represents 18.2 % of the totl genertion. Initilly the power trnsferred from the onverters is the sme s in the previous se study nd eh synhronous genertor (SG1

4 3rd Interntionl Hyrid Power Systems Workshop Tenerife, Spin 8 9 My SG SG + SG + SG Fig. 7: Frequeny response omprison with or without support of the onverters nd SG2) provides 1 MW, i.e % of the totl lod is supplied y synhronous genertion. In order to test the frequeny stility limits the genertion from SG1 is redued progressively, while the totl lod is mintined nd repled y dditionl power from the onverters shred eqully etween the nd. Tle III shows the 4 senrios onsidered in this se study, where the totl synhronous genertion efore nd fter the power imlne is indited. TABLE III: Senrios to nlyse frequeny stility limits. Sen. Initil SGs (MW) Initil SGs (%) Finl SGs (%) Fig. 8 nd Fig. 9 show the results of the power ontriution from eh synhronous genertor nd onverter nd the frequeny response. When the SG2 is disonneted, SG1 strts ompensting the power, reduing the frequeny of the system s the mehnil torque provided y the turine is slower thn the eletril one. When this redution on the frequeny is deteted y the onverters, they strt injeting power ording to the droop ontrol. When the synhronous genertion tht remins onneted to the system is high enough, the onverters n respond orretly to the hnge (see Fig. 8). When the inerti is redued, some osilltions ppers in the power of the onverters, whih n e trnslted to frequeny osilltions. When SG1 genertes MW, the power of the egins osillting ut the system hs enough dmping nd the effet on the frequeny is not signifint, s shown in Fig. 8. When the power from SG1 is redued to 25 MW, this osilltion is inresed nd the frequeny is ffeted, s shown in Fig. 8. The frequeny of this osilltion is round 14 Hz, whih is in the rnge of eletromehnil intertions. When the power from SG is redued to 12.5 MW, the system is is stle, ut the opertion is not eptle s the lrge osilltions would fore the disonnetion of the onverters, s shown in Fig. 8d. Fig. 9 shows the frequeny for ll the ses, where lower inerti vlues of the system re trnslted to derese on the minimum frequeny. SG1 SG2 () Senrio 1 () Senrio 2 () Senrio 3 (d) Senrio 4 Fig. 8: Simultion results when the inerti of the system is redued

5 3rd Interntionl Hyrid Power Systems Workshop Tenerife, Spin 8 9 My 218 Fig. 9: Frequeny response depending on the remin synhronous genertion power IV. CONCLUSION This pper hs presented intertions in multi-infeed HVDC islnded systems tht result into frequeny instility. HVDC links provide fst frequeny response nd redue signifintly the frequeny devitions fter power imlne used y the loss of synhronous genertion. However, when totl synhronous genertion is repled y power trnsferred from the HVDC links the totl inerti will redued until the system eomes unstle. In senrio with low inerti eletromehnil intertions might use frequeny instility nd must e studied in detil. A potentil solution will e to operte the s gridforming onverter, whih my void intertions with the synhronous genertors. ACKNOWLEDGMENT This work hs een funded in prt y the Spnish Ministry of Eonomy nd Competitiveness under Projet ENE C4-1-R [1] S. Hwng, M. oon, nd G. Jng, Evlution of STATCOM Cpility on Trnsient Stility in Jeju-islnd with Lrge-sle Wind Frm, in CIGRE: AORC Tehnil meeting 214, 214, pp. 5. [11] J. Prieto, R. Grndino, E. Betten, G. Curtotti, C. Velzquez, C. Geler, H. Weinkuf, S. Ahenh, nd A. Glrz, The RÓMULO projet, Spnish peninsul Mllor ( 243 km, 2 kv, 2x MW ): first Spnish HVDC link, in CIGRE: Pris 21, 21. [12] V.C. Billon ; J.P. Tisne ; V. Aridiono ; F. Mzzoldi, The Corsin tpping: from design to ommissioning tests of the third terminl of the Srdini-Corsi-Itly HVDC, IEEE Trnstions on Power Delivery, vol. 4, no. 1, pp , [13] A. Ege-Alvrez, S. Fekrisl, F. Hssn, nd O. Gomis-Bellmunt, Advned Vetor Control for Voltge Soure Converters Conneted to Wek Grids, IEEE Trnstions on Power Systems, vol. 3, no. 6, pp , 215. [14] J. Roert, A. Lun, F. Bljerg, nd I. Pper, Control of Power Converters in AC Mirogrids, IEEE Trnstions on Power Eletronis, vol. 27, no. 11, pp , 212. [15] C. Li, X. Hu, J. Guo, nd J. Ling, The DC grid reliility nd ost evlution with Zhoushn five-terminl HVDC se study, Proeedings of the Universities Power Engineering Conferene, vol. 215-Novem, 215. [16] N. Flourentzou, V. Agelidis, nd G. Demetrides, VSC-Bsed HVDC Power Trnsmission Systems: An Overview, IEEE Trnstions on Power Eletronis, vol. 24, no. 3, pp , 9. [Online]. Aville: [17] P. Kundur, Power System Stility nd Control, [18] D. Jovi nd K. Ahmed, High Voltge Diret Current Trnsmission: Converters, Systems nd DC Grids. Wiley, 215. [19] J. Xu, C. Zho, W. Liu, nd C. Guo, Aelerted Model of Modulr Multilevel Converters in PSCAD/EMTDC, IEEE Trnstions on Power Delivery, vol. 28 (1), pp , jn 213. [2] E. Prieto-Arujo, A. Junyent-Ferré, C. Colldos-Rodríguez, G. Clrin-Colet, nd O. Gomis-Bellmunt, Control design of Modulr Multilevel Converters in norml nd AC fult onditions for HVDC grids, Eletri Power Systems Reserh, vol. 152, pp , 217. [Online]. Aville: [21] Q. Tu nd Z. Xu, Impt of smpling frequeny on hrmoni distortion for modulr multilevel onverter, IEEE Trnstions on Power Delivery, vol. 26, no. 1, pp , 211. [22] ENTSO-E, Network Code on Lod-Frequeny Control nd Reserves, 213. [Online]. Aville: LFCR-Issue1.pdf REFERENCES [1] G. Li nd J. Sun, Control Hrdwre-in-the-Loop Simultion for Turine Impedne Modelling nd Verifition, in 16th Wind Integrtion Workshop, Berlin, 217. [2] H. Liu, X. Xie, J. He, T. Xu, Z. u, C. Wng, nd C. Zhng, Susynhronous Intertion Between Diret-Drive PMSG Bsed Wind Frms nd Wek AC Networks, IEEE Trnstions on Power Systems, vol. 32, no. 6, pp , nov 217. [Online]. Aville: [3] L. P. Kunjumuhmmed, B. C. Pl, C. Otes, nd K. J. Dyke, Eletril osilltions in wind frm systems: Anlysis nd insight sed on detiled modeling, IEEE Trnstions on Sustinle Energy, vol. 7, no. 1, pp , 216. [4] J.-S. oon, S.-. Kim,.-H. Kim, K.-C. Lee, nd C.-k. Lee, The nlysis of STATCOM nd SVC opertion effet, in Trnsmission & Distriution Conferene & Exposition: Asi nd Pifi, 9, 9. [5] H. Sd, S. Dennetiere, nd B. Cler, Intertions investigtions etween power eletronis devies emedded in HVAC network, in 13th IET Interntionl Conferene on AC nd DC Power Trnsmission, 217. [6] G. Andersson, P. Fisher de Toledo, nd G. Liss, HVDC Multi-Infeed Performne, pp [7] CIGRE, Systems with multiple DC infeed, Cigré, no. WG B4.41, pp , 7. [8] C. K. Kim nd G. Jng, Opertion strtegy of Cheju AC network inluded multi-infeed HVDC system, Journl of Eletril Engineering nd Tehnology, vol. 8, no. 3, pp , 213. [9] G. Asplund, L. Crlsson, nd O. Tollerz, ers HVDC - Prt II, Teh. Rep., 3.