A Novel Fault-Analysis Method for the Power Grid with Inverter-Interfaced Distribution Generators

Transcription

1 ELEKTROTEHNŠK ESTNK 81(5): 56-6, 014 ORGNAL SCENTFC PAPER A Novel Fault-Analyss Method or the Power Grd wth nverter-nteraced Dstrbuton Generators Zengl Yang 1, Ljun Wang 1, Hubng Zhou 1, Youhua Wang 1,Xangpng Kong,Zhe Zhang 3, Xanggen Yn 3, and Fan Xao 3 1 State Grd Hube Electrc Power Company, Wuhan , Chna State Grd Jangsu Electrc Power Research nsttute, Nanjng 11103, Chna 3 State Key Laboratory o Advanced Electromagnetc Engneerng and Technology (Huazhong Unversty o Scence and Technology), Wuhan , Chna Abstract. Because o the specal ault-current characterstcs o the nverter-nteraced dstrbuton generator (DG), the conventonal ault-analyss method based on the synchronous generator cannot be used or the power grd wth DGs. The paper ponts out the problems o usng the conventonal ault-analyss method or the power grd wth DGs and proposes a novel ault-analyss method or the power grd wth DGs. Numerous smulaton cases show that the proposed ault-analyss method s accuracte to meet the requrements o short-crcut calculaton o the power grd and relay protecton. Keywords: nverter-nteraced dstrbuton generator (DG), ault-current characterstcs, ault-analyss method Nova metoda za analzo kratkostčnh tokov v omrežju z nvertersko prključenm razpršenm vr prspevku je prkazana problematka konvenconalne analze kratkostčnh tokov v elektroenergetskem omrežju z nvertersko prključenm razpršenm vr. Na osnov analze delovanja elektroenergetskega omrežja z nvertersko prključenm razpršenm vr, v prspevku predlagamo novo metodo za analzo kratkostčnh tokov. Ekspermentaln rezultat potrjujejo pravlnost metode n njeno uporabnost pr zaščt omrežja. 1 NTRODUCTON Nowadays, more and more dstrbuted generators (DGs) are beng ntegrated nto the power grd or ollowng three reasons. Frstly, DGs provde on eectve technologcal means or utlzaton o envronmentally rendly energy sources, thus good or the envronmental protecton. Secondly, DGs can be located dspersedly and lexbly to meet the ncreasng power demand, and can reduce n ths way the nvestment nto expandng the transmsson and dstrbuton system. Thrdly, DGs can be reserve o the man power supply to mprove the power supply-relablty. t s known that most DG types (lke photovoltac generaton system, uel cell, wnd turbne, etc) are nterace wth the power grd wth nteraced nverters. The are the so-called nverter-nteraced DGs [1-3]. Due to the ncreasng ntegrated capacty o DGs, the ault characterstcs o the power grd have changed greatly and the perormance o the conventonal relay protecton has aced severe challenges [4-6]. n order to assure saety o the power grd and DGs, the study and mprovement o the relay protecton are o great sgncance. As we know, the relay protecton dentes the ault element accordng to the changed characterstcs o the electrcal magntude (or nonelectrcal magntude) o the grd ault occurance. Hence, the ault-analyss method and the subsequent ault-characterstcs study o the power grd are the bass or settng the relay protecton system. For the conventonal ault-analyss method, the power s suppled by the conventonal synchronous generator. However, the ault-current characterstcs (ncludng the transent and steady-state characterstcs) o DG are much derent rom those o the conventonal synchronous generator [7], whch means that the conventonal ault-analyss method s o the use or the power grd wth DGs. To solve the ssues o conventonal relay protecton usng DGs. Searchng or an ecent ault-analyss method or the power grd wth DGs scholars have carred out many research works. By equvalently representng DG wth ts steady-state model or loadlow calculaton, the ault-analyss methods or the dstrbuton wth DGs and other DGs are proposed n [8, 9]. However, due to the dynamc response o the nverter durng a grd ault, the normal Receved 1 September 014 Accepted 0 October 014

2 A NOEL FAULT-ANALYSS METHOD FOR THE POWER GRD WTH NERTER-NTERFACED DSTRBUTON 57 DG operatng mode cannot be provded [10]. Hence, DG cannot be replaced by ts steady-state model or load-low calculaton. n [10], the ault characterstcs o DG wth the P-Q or - control are studed wth smulaton and the conventonal ault-analyss method s extended to take the DG contrbuton nto consderaton. n [11], a model o DG durng a grd ault and an mproved ault-analyss method are proposed. However, the low-voltage rde-through (LRT) [1] s not consdered n [10, 11] and the DG behavor cannot meet the requrements o the grd code. Hence, the short-crcut calculaton results o the proposed ault-analyss methods are not n accordance wth the realstc stuaton. n [13], the DG aultcurrent characterstcs are studed theoretcally to meet the LRT requrements and a ault-analyss method or the dstrbuton wth a sngle DG s proposed. However, the proposed ault-analyss method s not applcable or a mult DGs. n order to ll the gap, the rst to study the conventonal ault-analyss method to dene the problem o usng t or the power grd wth DGs, and to proposed a novel ault-analyss method. Fnally, the smulaton cases are studed wth PSCAD/EMTDC to valdate the eectveness o the novel ault-analyss method. CONENTONAL FAULT-ANALYSS METHOD The conventonal ault-analyss method based on the node-voltage equaton s studed to provde gudance or proposng a novel the ault-analyss method to be used or the power grd wth DGs..1 Symmetrcal-ault analyss As shown n Fg.1, node o the actve s aulted through ault mpedance z. t s noted that ault mpedance z s not taken nto consderaton when the bus mpedance or admttance matrx s bult or aultanalyss. Actve Fg.1. Symmetrcal-ault analyss z Actve (a) ault model o actve (b) the orgnal (c) ault branch Accordng to the superposton prncple, the ault mode o the actve shown n Fg.1(a) s dvded nto two parts: the orgnal as shown n Fg.1(b) and the ault branch shown n Fg.1(c). As seen rom Fg.1(b), the short crcut o the orgnal can be regarded as an addtonal njecton current low z nto the through node. Hence, the node voltage can be expressed as Zj j Z (1) jg where G s a set o the actve nodes, Zj s the mutual mpedance between node and node j. As shown n (1), the node voltage conssts o two tems. The rst tem s Z j j. t represents the node voltage jg beore the ault occurance s caused by all the current sources. Ths s the so-called normal component 0 o the node voltage denoted by. The second tem s Z. t s the ault component o the node voltage caused by short crcut current when all the current sources are dsconnected. The superposton o the two voltage components equals the real node voltage ater the ault occurance,.e. 0 Z () For aulted node, Where 0 Z j j jg 0 Z (3) s the normal voltage o node beore the ault occurance and Z s the sel-mpedance o aulted node. Accordng to the ault branch shown n Fg.1(c), the ollowng equaton s obtaned. z 0 (4) Hence, ault current s Wth ault current 0 Z z (5) and (3), the voltage o any node can be calculated and the current o any branch then obtaned.. Asymmetrcal-ault analyss When the symmetrcal ault takes place, the sequence s can be equvalently represented by two-port s rom the ault port, as shown n Fg.. For the conventonal ault-analyss method, the negatve-sequence current s supposed to low through the same elements as the postve-sequence current. Hence, the negatve-sequence has the same structure wth the postve-sequence, but the derence s that the negatve-sequence potentals o all the power sources are zero.

3 58 ZENGL, LJUN, HUBNG, YOUHUA, XANGPNG, ZHE, XANGGEN, FAN Postvesequence Fg. Asymmetrcal ault-analyss Smlarly as the symmetrcal-ault analyss, the sequence voltages o node o the occurance o a asymmetrcal ault can be expressed as 0 Z Z (6) 0 Z 0 0 The sequence voltages o any node and the sequence currents o any branch n the can be obtaned by solvng (6) and the ault boundary condtons. 3 CONENTONAL FAULT-ANALYSS METHOD Accordng to the DG ault-current characterstcs study [7], only the postve-sequence current s provded by DG when the symmetrcal or asymmetrcal voltage dps. Moreover, o the sudden occur o the generator voltage, the transent component n the ault current provded by DG s so that small and damped so quckly that t can be neglected. When a ault occurs n a real power grd, the generator voltage don't drop suddenly wth transent perod to the DG ault current relatvely long. However, t don't aect the steady-state component o the ault current whch means that there s only the postve-sequence component n the steady state ault current provded by DG o the grd-ault occurance. Hence, DG can be equvalently replaced by a constant postve-sequence current source o the grdault occurance as shown n Fg. 3. Fg. 3. model 1 1 (a) postve-sequence 1 Negatvesequence (b) negatve-sequence 1 y 0 Equvalent DG postve-sequence current-source n Fg. 3, c represents the equvalent postve -sequence current source, and s the 1 v postve-sequence component o the generator voltage. The equvalent DG mathematcal model can be expressed as: 1) 09., then d0, and 0. Acvtve Zerosequence (c) zero-sequence cv c v ) , and 1 1., d 0 1 then,and arctan d ) then 1. cv. d 0, and 1 1., 1,and cv arctan ) 0.4, then 1., and cv 90. d 0 As noted, d0s the DG actve-current reerence value beore the ault occurance. As seen rom the equvalent DG mathematcal model, the conventonal ault-analyss method cannot be used or the power grd wth DGs or the ollowng three reasons. 1) For the conventonal ault-analyss method, the negatve-sequence has the same structure as the postve-sequence. However, t s unenable or DG. The DG branch should exst n the postvesequence and not n the negatve-sequence, snce there s only the postve-sequence component n the DG ault current. ) As show n (), or the conventonal ault-analyss method, the ault component o the node voltage s Z. t s caused by short crcut current when all the current sources are dsconnected. For the above calculaton method o the node voltage ault component to be eectve, the subtransent reactance and subtransent potental o the synchronous generator shuold be kept constant beore and ater the ault occurrence. However, the condton s not unenable or DG. Accordng to the equvalent DG mathematc model, the magntude o the equvalent DG postvesequence current source changes beore and ater the ault occurrence. Hence, n order to calculate the ault components o the node voltages n the power grd wth DGs, the current sources cannot be dsconnected. Lkewse, the calculaton method o the branch-current ault component n the conventonal ault-analyss method cannot be used or the power grd wth DGs. 3) As seen rom (), (5) and (6), all the used elements n the conventonal ault-analyss method are those o the -th column n the mpedance matrces. Hence, or the conventonal ault-analyss method, only the elements wth the column number equal to the number o the aulted node are needed or the short-crcut calculaton. The reason s ault current that s the only one njected nto the supermposed. However, as stated above, the magntude o the equvalent DG postve-sequence current source changes be ore and ater the ault occurrence, hence, the current sources cannot be dsconnected n the supermposed. t means that not only the elements wth the.

4 A NOEL FAULT-ANALYSS METHOD FOR THE POWER GRD WTH NERTER-NTERFACED DSTRBUTON 59 column number s equal to the number o the aulted node, but also the elements wth the column numbers are equal to the numbers o the DGs-nteraced nodes are needed or the short-crcut calculaton o the power grd wth DGs. Besdes, accordng to the equvalent DG mathematcal model, the magntude o the equvalent DG postve-sequence current source s related to the postve-sequence component o the generator voltage. Hence, both the bus mpedance matrces and equvalent DG mathematc model are needed to mplement the ault-analyss, whch makes the ault-analyss much more complcated and the conventonal ault-analyss method napplcable. Generally speakng, the DG penetraton mpacts the conventonal ault-analyss method greatly. n order to meet the ault-analyss requrements o the power grd wth DGs and establsh a sold bass or the relay protecton study, t s necessary to propose a novel aultanalyss method to be used or the power grd wth DGs. 4 NOEL FAULT-ANALYSS METHOD n ths chapter, a novel ault-analyss method to be used or the power grd wth DGs s proposed. Frstly, as t s only the postve-sequence current that s provded by DG whatever the ault type s, replace DG wth a postve-sequence current source equvalently, and then establsh sequence s. Secondly, or the negatve-sequence and zero-sequence s, the ault-analyss method s the same as the conventonal analyss method. The negatve-sequence and zero-sequence voltages o node are Z (7) Z Fnally, or the postve-sequence, the orgnal shown n Fg.1(b) s decomposed nto two parts: the normal beore the ault occurrence and supermposed, as shown n Fg.4. Actve 1 (a) orgnal 1 Actve 1 (b) normal Fg. 4. Decomposton o the postve-sequence For the normal shown n Fg. 4(b), the normal component o the postve-sequence voltage o node s 0 1 Z j j jg (8) where G 1 s a set o actve nodes (ncludng all the conventonal synchronous generators and DGsnteraced nodes ). 1 Actve 1 1 (c) supermposed For the supermposed shown n Fg. 4(c), all the current sources representng the conventonal synchronous generators are dsconnected, but the controlled current sources representng DGs reman connected. The controlled current sources representng DGs n the supermposed by DG,k. DG,k s the ault component o the output currents o DGs whch can be expressed as = (9) Where DG,k DG,k DG,k DG 0,k s the output current o DG nteraced wth node k ater the ault occurrence and DG 0,k s the output current o DG nteraced wth node k beore the ault occurrence. Hence, the ault component o the postve-sequence voltage o node s 1 Zk DG,k Z 1 (10) kg Where G s a set o the DGs-nteraced nodes n the. The real postve-sequence voltage o node ater the ault occurrence s Zk DG,k Z (11) 1 kg Combnng (7) and (11), the sequence voltages o node are Zk DG,k Z 1 kg (1) Z 0 Z 0 0 The ault components o the DG output currents DG,k are unknown and are related to the postvesequence voltages o the generator termnals. Hence, (1) cannot be solved only wth the ault boundary condtons. n order to mplement the ault-analyss or the power grd wth DGs, the ollowng approaches should be adopted. Frstly, accordng to (11) and (1), the postvesequence voltages o the DGs nteraced nodes and sequence voltages o the aulted node are expressed n (13) and (14), respectvely. 0 Z Z m G (13), m 1 m 1 mk DG,k m 1 kg k DG,k 1 kg Z Z Z (14) 0 Z 0 0 Then, the DG output currents are avalable beore the ault occurrence, and sequence currents at the DG,k ault pont can be obtaned by solvng (13), (14), the

5 60 ZENGL, LJUN, HUBNG, YOUHUA, XANGPNG, ZHE, XANGGEN, FAN ault boundary condtons and the equvalent DG mathematcal model. Fnally, the sequence voltages o any node and sequence currents o any branch n the can be obtaned wth (1). As seen rom (1), only the elements wth the column numbers are equal to the numbers o the aulted node and DGs-nteraced nodes are needed or the aultanalyss o the power grd wth DGs. Fg. 5 shows a schematc dagram o the novel aultanalyss method. Begn mport data Establsh sequence s and correspondng bus admttance matrx accordng to the ault type, then apply trangular decomposton to the admttance matrx For the bus mpedance matrx o postve-sequence, obtan the elements whose numbers are equal to the numbers o DGs nteraced nodes Select aulted node For the bus mpedance matrx o sequence s, obtan the elements whose numbers are equal to the numbers o aulted node Solve (13), (14), the ault boundary condtons and the equvalent mathematc model o DG smultaneously Obtan the ault components o output currents o DGs and sequence currents at the ault pont Obtan the sequence voltages o speced nodes and sequence currents o speced branches wth (1) End Fg. 5. Dagram o the ault-analyss method o the power grd wth DGs 5 CASE STUDY n order to valdate the novel ault-analyss method, a smulaton model o the power grd wth DGs was bult wth PSCAD/EMTDC, as shown n Fg. 6. LD1 LD n Fg. 6, the transmsson lnes are o the same type and the lne parameters are r (1) =r () =0.17Ω/km and x (1) =x () =0.394Ω/km. The total lengths o L1, L, L3, L4, L5 and L6 are 5 km, 6 km, 10 km, km, 0.5 km and 0.5 km respectvely. The capacty o the two grd-connected DGs s 1 MW each. The parameters o T1 and T are the same. The rated capacty s 1.5/1.5MA, the turn rato s 0.38 k/10.5 k, the wndng type s Y/D and the leakage reactance s pu. The equvalent mpedances o LD1 and LD are 10j39.11 Ω, and the equvalent mpedance o LD3 s 80j6.08 Ω. Table 1 and Table compare the calculated and measured o the ault currents o the three-phase ault occurance at 1 and respectvely. Table 3 and Table 4 compare the calculated and measured o ault currents o the Phase-B-to-Phase-C ault occurance at 1 and respectvely. t s noted that the unt o the magntude s the ampere (A), and the unt o the angel s the degree ( ). From Table 1, Table, Table 3 and Table 4, t can be ound the theoretcal agree wth measured very well, whch valdates the eectveness o the novel ault-analyss method. Hence, the novel ault-analyss method can satsy the requrements o ault-analyss and relay protecton study o the power grd wth DGs. Table 1. Branch currents on condton that three-phase ault occurance at 1 current o DG1 current o DG Current at the ault pont Mag. Ang. Mag. Ang. Mag. Ang Table. Branch currents on condton that three-phase ault occurance at current o DG1 current o DG Current at the ault pont Mag. Ang. Mag. Ang. Mag. Ang / L1 L L3 L4 G 1 L5 L6 LD3 T1 T DG1 DG Fg. 6 Dagram o the smpled power grd wth DGs

6 A NOEL FAULT-ANALYSS METHOD FOR THE POWER GRD WTH NERTER-NTERFACED DSTRBUTON 61 Table 3. Branch currents o the Phase-B-to-Phase-C ault occurance at 1 current o DG1 current o DG Current at the ault pont Mag. Ang. Mag. Ang. Mag. Ang. Mag. Ang. Mag. Ang. Mag. Ang / / Table 4. Branch currents o the Phase-B-to-Phase-C ault occurance at current o DG1 current o DG Current at the ault pont Mag. Ang. Mag. Ang. Mag. Ang. Mag. Ang. Mag. Ang. Mag. Ang / / ERFCATON AND ALDATON OF THE SMULATON MODEL The smulaton model was bult accordng to Fg. 6, and the DG model accordng to Re. [7] was vered and valdated. n ths context, the mathematcal equatons and logc o the smulaton model were vered. Moreover, n the smulaton model, the grd s represented by an deal voltage source, and the loads LD1, LD and LD3 are represented by a branch consstng o a resstor and nductor. The smulaton model s composed o the basc modules (such as transmsson lne, transormer, etc) provded by PSCAD/EMTDC were valdated by numerous users all over the world over many years. Ths means that the smulaton model s correct. Furthermore, accordng to Tables 1,, 3 and 4, the smulaton results agree wth the theoretcal-analyss results qute well, snce the largest absolute error o the magntude between the smulaton results and the theoretcal-analyss results s only 0.63% and the largest absolute error o the angle between the smulaton results and the theoretcal-analyss results s only 1.77%. Hence, the smulaton results and the theoretcalanalyss results very each other. 6 CONCLUSONS As there s a great derence between the ault-current characterstcs o DG and o conventonal synchronous generator, the conventonal ault-analyss method cannot be used or the power grd wth DGs. A novel ault-analyss method s thereor proposed n ths paper. For the novel ault-analyss method, DG s represented by a postve-sequence current source o the magntude correspondng to the postve-sequence generator-termnal voltage. Then the ault-analyss s mplemented by solvng the node-voltage equatons o the sequence s and the ault boundary condtons and settng up an equvalent DG mathematcal model. Besdes, only the elements wth the column numbers equal to the numbers o the aulted node and DGs-nteraced nodes n the bus mpedance matrces are needed or the ault-analyss o the power grd wth DGs. The smulaton results show that the calculaton accuracy o the novel ault-analyss method well meet the requrements o ault-analyss and relay protecton study o the power grd wth DGs. ACKNOWLEDGEMENT Ths work was supported by the Natonal Natural Scence Foundaton o Chna (Grant No ). REFERENCES [1] S. Alepuz, A. Calle, S. Busquets-Monge, S. Kouro, B. Wu, Use o stored energy n PMSG rotor nerta or low voltage rdethrough n back-to-back NPC converter based wnd power systems, EEE Transactons on ndustral Electroncs, ol. 60, ssue 5, 013, pp [] W. Zhang, D. Xu, X. L, R. Xe, H. L, D. Dong, C. Sun, M. Chen, Seamless transer control strategy or uel cell unnterruptble power supply system, EEE Transactons on Power Electroncs, ol. 8, ssue, 013, pp [3] Y. Gu, W. L, Y. Zhao, B. Yang, C. L, X. He, Transormerless nverter wth vrtual DC bus concept or cost-eectve grdconnected P power systems, EEE Transactons on Power Electroncs, ol. 8, ssue, 013, pp [4]. Calderaro,. Gald, A. Pccolo, P. Sano, A Petr net based protecton montorng system or dstrbuton s wth dstrbuted generaton, Electrc Power Systems Research, ol. 79, ssue 9, 009, pp

7 6 ZENGL, LJUN, HUBNG, YOUHUA, XANGPNG, ZHE, XANGGEN, FAN [5] A. F. Naem, Y. Hegazy, A. Y. Abdelazz, M. A. Elsharkawy, A classcaton technque or recloser-use coordnaton n dstrbuton systems wth dstrbuted generaton, EEE Transacton on Power Delvery, ol. 7, ssue 1, 01, pp [6] H. Wan, K. K. L, K. P. Wong, An adaptve multagent approach to protecton relay coordnaton wth dstrbuted generators n ndustral power dstrbuton system, EEE Transacton on ndustral Applcaton, ol. 46, ssue 5, 010, pp [7] X. Kong, Z. Zhang, X. Yn, Fault current study o nverter nteraced dstrbuted generators, The Dstrbuted Generaton and Alternatve Energy Journal, to be publshed n 015. [8] X. Fu, Decouplng phase doman method or ault-analyss o dstrbuton system wth dstrbuted generaton (n Chnese), Electrc Power Automaton Equpment, ol. 9, ssue 6, 009, pp [9] S. Wang, X. Jang, C. Wang, A superposton method o aultanalyss or dstrbuton systems contanng dstrbuted generatons (n Chnese), Automaton o Electrc Power Systems, ol. 3, ssue 5, 008, pp [10] M. E. Baran,. E-Markaby, Fault-analyss on dstrbuton eeders wth dstrbuted generators,, EEE Transacton on Power Systems, ol. 0, ssue 4, 005, pp [11] C. Wang, X. Sun, An mproved short crcut calculaton method or dstrbuton wth dstrbuted generatons (n Chnese), Automaton o Electrc Power Systems, ol. 36, ssue 3, 01, pp [1] Techncal rule or photovoltac power staton connected to power systems, Q/GDW , May 011. [13] Z. Wu, G. Wang, H. L, G. Pan, X. Gao, Fault Characterstcs analyss o dstrbuton s consderng control scheme o nverter nteraced dstrbuted (n Chnese), Automaton o Electrc Power Systems, ol. 36, ssue 18, 01, pp Zhe Zhang receved hs Ph.D. degree n electrcal engneerng rom HUST, Wuhan, Chna, n 199. Currently, he s a proessor at the School o Electrcal and Electronc Engneerng, HUST. Hs nterest s n protectve relayng. Emal: zz_mal00@163.com. Xanggen Yn receved hs Ph.D. degree n electrcal engneerng rom HUST, Wuhan, Chna, n Currently, he s a proessor at the School o Electrcal and Electronc Engneerng, HUST. Hs major areas nclude protectve relayng and power system stablty control. Emal: xgyn@hust.edu.cn. Fan Xao was born n Hunan, Chna, n He s currently pursung the Ph.D. degree at HUST, Wuhan, Chna. Hs research nterest s n protecton o power grd wth accesson o dstrbuted generators. Emal: Zengl Yang s a senor engneer o State Grd Hube Electrc Power Company, Wuhan, Chna. Hs nterest s n protectve relayng. Emal: yangzl8@hb.sgcc.com.cn. Youhua Wang s a senor engneer o State Grd Hube Electrc Power Company, Wuhan, Chna. Hs nterest s n protectve relayng. Emal: wangyh@hb.sgcc.com.cn. Hubng Zhou s a senor engneer o State Grd Hube Electrc Power Company, Wuhan, Chna. Hs nterest s n protectve relayng. Emal: zhouhb@hb.sgcc.com.cn. Ljun Wang s a senor engneer o State Grd Hube Electrc Power Company, Wuhan, Chna. Hs nterest s n protectve relayng. Emal: wanglj@hb.sgcc.com.cn. Xangpng Kong receved hs Ph.D. degree n electrcal engneerng rom Huazhong Unversty o Scence and Technology (HUST), Wuhan, Chna, n 014. Currently, he s workng at State Grd Jangsu Electrc Power Research nsttute. Hs nterest s n protectve relayng.