A Current Differential Line Protection Using a Synchronous Reference Frame Approach

Transcription

1 A Current Dfferental Lne rotecton Usng a Synchronous Reference Frame Approach L. Sousa Martns *, Carlos Fortunato *, and V.Fernão res * * Escola Sup. Tecnologa Setúbal / Inst. oltécnco Setúbal, Setúbal, ortugal ED Energas de ortugal, Lsboa, ortugal Keywords: Current dfferental protecton, Synchronous reference frame, ark transformaton, Lne protecton. Abstract Ths paper presents a new approach for a current dfferental protecton of the transmsson lnes. Ths approach s based on the ark transformaton or ABC-dqo transformaton. Usng the ABC-dqo transformaton the three phase quanttes are transformed nto a synchronous rotatng reference frame. In ths way, the lne currents of the three phase system are measured and transformed nto three dc components. So, mmunty to problems such as samplng msalgnment and tme delay of the communcaton channel wll be mproved. Several test results are presented n order to show the effectveness of the proposed approach. 1 Introducton Wth the ntroducton of dgtal relays, t was possble to mplement more effectve protecton schemes. Some technques that can be used for dgtal protecton of transmsson lnes are symmetrcal components [1], dfferental equaton algorthm and travellng wave algorthm [2,3]. Furthermore, ths technology also allows mplementng current dfferental protecton schemes to transmsson lnes, specally usng dgtal technques coupled to modern communcaton lnks for data transmsson. Current dfferental relays are wdely used for the protecton of electrcal equpment such as transformers and generators aganst nternal faults. The man reason for usng ths relay type s due to ther smplcty and hgh senstvty. So, wth the development of dgtal relays and communcaton technology, current dfferental relays have also been used for the protecton of transmsson lnes [4,5,6]. However, when ths relay type s appled to transmsson lnes there are some problems such as samplng msalgnment, tme delay of the communcaton channel, lne capactve chargng current and errors n current transformers make current comparson dffcult to acheve. So, several alternatve approaches to conventonally appled current dfferental relays have been proposed. So, approaches based on composton of the modal voltage and current measurements at both ends [7] and based on the comparson of the ntegraton of current sgnals under half cycles at both ends [8] have been proposed. The frst one presents the advantage of mprovement to the relay senstvty but the msalgnment between samples can orgnate some problems. The other one presents the soluton for the msalgnment between samples but depends totally on the zero crossng. Other approaches have been used as a soluton for the msalgnment between samples problem [9,1,11]. However, n some specfc condtons these approaches can cause a fault detecton falure [12]. Ths paper presents a new approach for a current dfferental protecton of the transmsson lnes. Ths approach s based on the ark transformaton or ABC-dqo transformaton n order to transform the three phase quanttes nto a synchronous rotatng reference frame. So, the current measurements at both ends are transformed nto a synchronous rotatng frame usng. Ths paper s organzed as follows. Secton 2 descrbes the new proposed approach for the current dfferental protecton. In order to show the effectveness of the proposed methodology several smulaton results are presented n secton 3. Secton 4 presents the conclusons of the work. 2 roposed current dfferental lne protecton The prncple of the current dfferental lne protecton rests n the comparson of the sum of the ncomng and outgong currents at the termnatons, wth a restrant current functon dependng on the same currents. Local determnaton of the dfferental and restrant currents calls for the transmsson of currents from the other termnatons. Therefore t s requred a constant use of transmsson channels (Fgure 1). Fgure 1: Current dfferental protecton scheme. However, normally the current dfferental relays present problems such as samplng msalgnment and tme delay of the communcaton channel. So, to mprove the mmunty to such problems t s proposed an approach based on the ark transformaton. The ncomng and outgong currents at the termnatons are all transformed nto new quanttes by the

2 ABC-dqo transformaton. Fg. 2 shows the proposed approach for the current dfferental relay. components of d and q represent the postve sequence fundamental, mmunty to problems such as samplng msalgnment and tme delay of the communcaton channel wll be mproved. Each of the components of the resultng current space vector n the fundamental frequency rotatng coordnate system wll be used to dscrmnate a lne fault. So, three dfferental currents wll be obtaned, accordng the dqo components, as can be seen by the followng expressons: Fgure 2: roposed approach for the dfferental relay. Usng the ABC-dqo transformaton the three phase quanttes are transformed nto a synchronous rotatng reference frame. To acheve ths, the lne currents of the three phase system are measured and transformed nto a synchronous rotatng frame usng the Clarke transformaton followed by the rotatng matrx. Equatons (1) and (2) show the result of those transformatons. d s q s o s = s1 s2 s3 and d r q r o r = r1 r2 r3 (1) 2π cosθ cos θ cos θ π = snθ sn θ sn θ (2) The tme varyng angle θ represents the angular poston of the reference frame whch s rotatng at constant speed n synchronsm wth the three-phase ac voltages. The components d and q represents the resultng current space vector n the fundamental frequency rotatng coordnate system. The dc components of and represent the postve sequence fundamental. The used angle θ n the ark transformaton to transform currents s1,2, 3 nto s dqo s related wth the bus S three-phase ac voltages. The angle θ that s used n the ark transformaton to transform currents r1,2, 3 nto r dqo s related wth the bus R three-phase ac voltages. Transformed currents s dqo and r dqo are then used n the transmsson channel between the two relays. Snce dc d q d df q df o df = d s q s o s d r q r Analysng each of the dqo dfferental currents, t s possble to dscrmnate the fault type. Here, t wll be used the d component to dentfy any lne fault. So, the dfferental relay operaton s mplemented comparng d df wth a restrant current. The restranng current o r I r s defned as: 1 I r = d s + d r 2 The operaton condton of the dfferental relay operaton s: (3) (4) Iop k I r (5) where K s a constant coeffcent representng the slope of the relay characterstc. To provde a mnmum pck-up current to the relay, the followng condton was also consdered: Iop k o (6) Therefore, the fnal defnton of the dfferental relay operaton s: I op k Ir + ko (7) After the relay operaton t wll be analyzed the o component n order to dscrmnate between phase to phase fault and phase to earth fault. So, the followng condton allows dentfyng the fault type: o df o df > ko ko,, earth fault no earth fault (8)

3 To dentfy the faulted and un-faulted lnes a new transformaton wll be used. So, currents s dqo and r dqo wll be transformed nto new quanttes by the dqo-abc transformaton or the nverse ark transformaton. Equatons (9) and (1) show those transformatons. s1 s2 = s3 1 d s q s o s and r1 r2 = r3 1 d r q r o r (9) cosθ snθ 1 2π 1 cos θ sn θ = (1) 2π cos θ + sn θ The used angle θ n the nverse ark transformaton located n relay S s related wth the bus S three-phase ac voltages, as can be seen by Fg. 3. The angle θ that s used n the ark transformaton located n relay R s related wth the bus R three-phase ac voltages. expected from ths result, there s a sgnfcant change n the ampltude of the dfferental currents after the three-phase short crcut. dia (A) dib (A) dic (A) Fgure 4: Dfferental lne currents before and after a three phase fault. Fg. 5 shows the test results of the dqo dfferental currents for a three-phase short crcut fault. From these results t s possble to verfy that those currents are dc components. The ampltude of d component s related wth the ampltude of the abc dfferental currents. Snce the d component s related wth the actve power, ths component s fundamental n order to conclude about a lne fault. Ths result also shows that there s a sgnfcant change n the d component ampltude before and after the fault. So, ths type of fault s easly dentfed by the analyss of the d component. Snce there s not an earth fault, as expected the current dfferental o component s nearly zero. 3 Id (A) Fgure 3: roposed approach for the dfferental relay. Iq (A) Smulaton results The purpose of ths secton s to llustrate the performance of the proposed current dfferental relay under dfferent condtons. The presented results are for a 52 kv, 5 km transmsson lne. Ths system has been mplemented by the Matlab/Smulnk software program and the ower System Blockset. Dfferent types of nternal faults are evaluated to verfy the effectveness of the proposed approach. Fg. 4 shows the lne dfferental currents for a three-phase short crcut. As Io (A) Fgure 5: Dfferental lne currents after ABC-dqo transformaton before and after a three phase fault. Fg. 6 shows the dfferental lne current for a short crcut fault between phase a and b. As expected from ths result, there s a change n the ampltude of the current n the fault phases.

4 3 2 dia (A) dia (A) -2 2 N dib (A) dib (A) dic (A) dic (A) Fgure 6: Dfferental lne currents before and after a phase to phase fault. The dfferental lne currents after ABC- dqo transformaton before and after a phase to phase fault are presented n Fg. 7. From ths result t s possble to verfy that those currents present a smlar result then the three-phase short crcut. In fact, after the short crcut fault between phase a and b there s a suddenly change n d and q components. The ampltude of the d component ncreases and the q component decreases after the fault. Ths s expected snce the d component s related wth the actve power the q component s related wth the reactve power. However, n ths case, after the short crcut the dq dfferental currents present a dc and an ac component. Ths ndcates that the system t s not equlbrated. So, snce the current dfferental o component s nearly zero and there s an ac component a phase to phase fault t s clearly dentfed. Id (A) Iq (A) Io (A) t (s) Fgure 7: Dfferental lne currents after ABC-dqo transformaton before and after a phase to phase fault. Internal phase to earth faults were also smulated. Fg. 8 shows the dfferental lne currents before and after for a short crcut fault between phase a and earth. From ths result, t s possble to verfy that there s a change n the ampltude of the phase a dfferental. Fgure 8: Dfferental lne currents before and after a phase to earth fault. Fg. 9 shows the test results of the dqo dfferental currents before and after for a short crcut fault between phase a and earth. Ths result shows that dq components present a sgnfcant change. Agan, the ampltude of the d component ncreases and the q component decreases after the fault. So, all fault types can easly be dentfed by the analyss of the d component. However, n ths fault type (phase to ground fault) the current dfferental o component s not anymore nearly zero, presentng now a sgnfcant ac component. So, by the analyss of the o component t possble to easly dentfy earth faults. Id (A) Iq (A) Io (A) N Fgure 9: Dfferental lne currents after ABC-dqo transformaton before and after a phase to earth fault. The effect of fault resstance s nvestgated for nternal faults. Fg. 1 shows the effect of fault resstor varaton on the dfferental characterstc. In ths fgure t s presented the d component of the dfferental currents for a no fault stuaton and for a phase to earth fault wth a fault resstor of 2Ω and 1Ω. It s also presented d component of the dfferental currents for a phase to phase fault wth a fault resstor of 1Ω. It s clear that n all test cases the operatng characterstc s always greater than the d component of the

5 dfferental current for a no fault stuaton. On other hand, the operatng characterstc s always smaller than the d component of the dfferental current for a fault stuaton Operatng regon Iop=Ko+k.IRT N 2 1 N 1 Restranng regon IRT (A) Fgure 1: Effect of fault resstor varaton on the percentage dfferental characterstc. In several dgtal transmsson lne protecton based on current dfferental, the samplng at the ends of the transmsson lne s unsynchronzed. Therefore the samples from the two transmsson lne ends may not be algned. As a consequence, nstablty of the protecton scheme could be happen [8]. However, n ths methodology ths s not crtcal. Ths can be llustrated by an example of a phase to phase fault and a tme delay of the transmtted sgnals. So, n Fg. 11 t s llustrated the d component of the dfferental currents for a phase to phase fault wth and wthout tme delay of the transmtted sgnals. In ths case t was used a tme delay of 2 ms and 1 ms. As can be seen by ths fgure, the tme delay of the transmtted sgnals does not present any problem for the stablty of the proposed protecton scheme. Idf (A) Fgure 11: roposed approach for the dfferental relay. 4 Conclusons An nvestgaton of a new approach for a current dfferental protecton of the transmsson lnes was presented and analyzed. In ths new approach ark transformaton or ABCdqo transformaton has been used. Usng ths concept the three phase quanttes are transformed nto a synchronous rotatng reference frame. Ths wll transform the three phase lne currents nto three dc components. Therefore, mmunty to problems such as samplng msalgnment and tme delay of the communcaton channel wll be mproved. For the dfferental characterstc t s only requred to analyse the d component of the dfferental lne currents. The effectveness of the proposed approach was analyzed by several smulaton results. References [1] A. G. hadke, M. Ibrahm, T. Hlbka. Fundamental Bass for Dstance Relayng wth Symmetrcal Components, IEEE Transacton on ower Apparatus and Systems, Vol. AS-96, No. 2, March/Aprl, pp , (1977). [2] M. Akke, J. T. Thorp. Some Improvements n the Three-hase Dfferental Equaton Algorthm for Fast Transmsson Lne rotecton, IEE roc.-gener. Transm. Dstrbuton, vol. 14, no. 1, pp , (1993). [3] M. H. J. Bollen. Travellng-Wave-based rotecton of Double-Crcut Lnes, IEEE Transactons on ower Delvery, vol. 13, no. 1, pp , (1998). [4] J. Wheatley. A mcroprocessor-based current dfferental protecton, n roc. 4th Int. Conf. Developments n ower System rotecton, IEE Conf. ub. 32, pp , (1989). [5] N.. Albrecht, W. C. Fleck, K. J. Fodero, R. J. Ince. Charge comparson protecton of transmsson lnescommuncatons concepts, IEEE Transactons on ower Delvery, vol. 7, no. 4, pp , (1992). [6] Z. Y. Xu, Z. Q. Du, L. Ran, Y. K. Wu, Q. X. Yang, J. L. He. A Current Dfferental Relay for a 1-kV UHV Transmsson Lne, IEEE Transactons on ower Delvery, vol. 22, no. 3, pp , (27). [7] R. K. Aggarwal, A. T. Johns. A dfferental lne protecton scheme for power systems based on composte voltage and current measurements, IEEE Transactons on ower Delvery, vol. 4, no. 3, pp , (1989). [8] L. J. Ernst, W. L. Hnman, D. H. Quam, J. S. Thorp. Charge comparson protecton of transmsson lnes relayng concepts, IEEE Transactons on ower Delvery, vol. 7, no. 4, pp , (1992). [9] IEEE Commttee Report. Synchronzed samplng and phasors measurements for relayng and control, IEEE Transactons on ower Delvery, vol. 9, no. 1, pp , (1994). [1] J. Lambert, A. G. hadke, D. M. Nabb. Accurate voltage phasor measurement n a seres compensated network, IEEE Transactons on ower Delvery, vol. 9, no. 1, pp , (1994). [11] H. Y. L, E.. Southern,. A. Crossley, S. otts, S. D. A. ckerng, B. R. J. Caunce, G. C. Weller. A new type of dfferental feeder protecton relay usng the global postonng system for data synchronzaton,

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