Study Committee B5 Colloquium 2005 September Calgary, CANADA

Transcription

1 36 Study oittee B olloquiu Septeber 4-6 algary, ND ero Sequence urrent opensation for Distance Protection applied to Series opensated Parallel Lines TKHRO KSE* PHL G BEUMONT Toshiba nternational (Europe Ltd. United Kingdo SUMMRY The effects of zero sequence utual coupling on the perforance of distance protection installed on parallel transission lines can be significant. t is well understood and docuented that the effects of utual coupling can cause distance relays to over-reach or under-reach depending on the operational status of the adjacent line. n principle, this proble can be solved by introducing the zero sequence current fro the parallel line to the relay on the faulted line. Nevertheless, anufacturers and users have adopted a variety of coping strategies in order to copensate for these effects in cases where the adjacent current is not available. However, recent experience has shown that there is another proble that ust be considered when distance relays are applied to series copensated parallel lines. When a fault occurs on a series copensated transission line, a transient oscillating current will flow between the L and coponents. This oscillation has a sub-synchronous frequency. Generally speaking, it is difficult to reove the sub-synchronous frequency copletely fro the easured current and voltage, hence the sub-synchronous coponents will affect distance easureent. n this paper, the undesirable effect of utual coupling on distance easureent is discussed theoretically together with practical siulation cases and the benefit of introducing zero sequence copensation on parallel transission lines for distance calculation is also deonstrated using siulation studies. Keywords: Distance relay Series capacitor ero sequence copensation Parallel line Mutual ipedance sub-synchronous frequency EMTP TP. NTRODUTON Series copensated lines provide significant advantages in ters of iproveents in power syste stability, an increase in the capacity of transission lines etc. However, fro a line protection perspective, series copensated lines can cause any difficulties, particularly for distance protection relays. n order to overcoe probles nuerous studies have been undertaken []-[4]. The ain probles for distance protections applied to copensated lines are overreach, directional eleent errors and oscillation of ipedance easureent. n previous studies, it has been recognized that probles occur in the case of faults beyond the series capacitor. Therefore studies have focused on phenoena which were apparent for *kaset@til.toshiba-global.co 36 -

2 faults occurring beyond the series capacitor and solutions have been proposed. n these studies, it would appear that distance protections were able to calculate the ipedance correctly for faults up to the series capacitor. Meanwhile, any studies in the application of distance relays to parallel lines have also been undertaken [], [6]. The ain proble in the application of distance relays to parallel lines is the effect of the utual ipedance of the parallel line on the phase-to-ground ipedance easureent for earth faults. Theoretically, it is apparent that the introduction of the residual current of the adjacent line is the easiest and the ost effective way of overcoing this proble. Many alternative ethods have been proposed because soeties it is neither possible nor preferred to introduce the residual current of the adjacent line. Our studies have revealed that the cobination of parallel lines and series capacitors has caused new probles for ipedance easureent even when the fault occurs before the series capacitor. n this paper, the new probles are introduced both theoretically and by siulation. t is shown that the introduction of the residual current of the adjacent line is the solution to the proble.. MUTUL ERO SEQUENE OMPENSTON Firstly, the principle of distance easureent when distance protection is applied to parallel lines with utual ipedance is shown in Fig.. for a typical parallel line odel with double end infeed. P ter M, Q ter E P E Q P V P VTs Ts, M Q F, F F Fig.. Typical parallel line odel with double end infeed n Fig.. it is assued that an -phase-to-ground fault occurs. The phase voltage at the relay can then be calculated as follows. V P = = F F ( + ( + k s F + k F M + MF M = ( + F F F + where, V P : -phase voltage at P ter, : -phase current of faulted line : zero sequence current of faulted line, M : zero sequence current of adjacent line F : positive sequence ipedance fro relay to fault point F : zero sequence ipedance fro relay to fault point MF : zero sequence utual ipedance fro relay to fault point (, k / k s = / = M Therefore the ipedance to the fault point can be calculated as follows. V MF F M (. = P F + k s + k (. M f the zero-sequence current of the adjacent line is not available, distance relays ust calculate the ipedance as follows. ' F VP = (.3 + k s Exaining (. and (.3, it is apparent that F is larger than F when is positive (i.e. in the sae 36 -

3 direction and that F is saller than F when is negative (i.e. opposite direction. When faults occur towards the far end of the line is generally positive and distance relays ay underreach. Overreach for near faults is not so serious a proble for distance protection. Therefore one possible solution to this proble is to use a larger value of ks than the calculated value. However, the direction of zero sequence current can vary with the operational condition of the adjacent line, for exaple, the adjacent line ay be in operation or open or earthed. n alternative idea for adjusting the setting of k s is to introduce the status of the adjacent line to the relay. t is well known that the distance relay on the healthy line can overreach because of utual zero sequence copensation, and one solution to this proble is to block the utual zero sequence copensation for faults on the adjacent line. This can be detected by coparing the zero-sequence current of the protected line to the zero-sequence current of the adjacent line. This solution has been applied for any years in Japan. 3. SERES OMPENSTED PRLLEL LNE t is well recognized that faults beyond the series capacitor, as shown in Fig.3. cause two ain probles for distance easureent. The first proble is the overreach caused by the reduction in ipedance by the insertion of the capacitor in the line. The other proble is that sub-synchronous frequencies are superiposed. This can be understood by solving the differential equation (3., and the fault current can be given by (3. generally:- di E P sinω t = Ri + L + idt (3. dt F = where, sin( ωt + φ + e αt ( sinφ cosω t + n sinω t ω : Syste angular velocity = E / R + ( ωl / ω, α = R/ L, ω = / L ( R/ L, φ = tan ( P ωl / ω, α + ω α n = cosφ + sinφ R ω ω ω Therefore the sub-synchronous frequency coponent, which is expressed by ω, is superiposed on the fundaental frequency coponent. Generally speaking, it is difficult to reove this frequency using a digital filter, because the long data window required will cause a delay in operation. Hence the algorith for distance easureent ust be iune to this lower frequency [7]. Fig.3. shows the parallel series copensated line odel. n this odel series capacitors are installed at one end. t is clear that the zero-sequence current of the adjacent line will flow to the fault point through the series capacitors for faults located up to the series capacitor. The influence of utual ipedance includes the sub-synchronous frequency caused by the series capacitor. Furtherore the zero sequence current of the adjacent line can be capacitive if the total capacitive reactance is larger than the line reactance. This phenoenon is ore likely to happen when the fault point is close to the series capacitor. (3. VTs Ts R, L P ter M, Q ter EP Fig.3. Fault at series copensated line F EQ EP P VP VTs Ts, x, x F M (-x, (-x Fig.3. Parallel series copensated line S S Q EQ S : series capacitance x 36-3

4 x (-x P x (-x Q EP P x (-x Q EQ EP x positive-sequence st circuit (-x EQ positive-sequence circuit positive-sequence nd circuit x (-x P x (-x Q P Q x (-x negative-sequence st circuit x (-x negative-sequence circuit negative-sequence nd circuit x (-x P x (-x Q P Q x M (-x zero-sequence st circuit x (-x zero-sequence circuit zero-sequence nd circuit Fig.3.3 Equivalent circuit using sequence coponents Fig.3.4 Equivalent circuit using sequence coponent by -phase coponent ethod Fig.3.3 shows the equivalent circuit using sequence coponents in the case of an -phase-to-ground fault. n Fig.3.3, the utual ipedance in the zero-sequence circuit coplicates the calculation. t is well known that by applying the -phase coponent ethod [8], the utual ipedance in the zero-sequence circuit can be decoupled. The calculation ethod of conversion to -phase coponents is shown in (3.3 and the ethod of re-conversion to noral sequence coponents is shown in (3.4. By eans of the -phase coponent ethod, Fig.3.3 is odified to Fig.3.4. V V V V k k = k,l k,l = V V V V k,l k,l k k, k k = k,l k,l (3.3 k,l, k = (3.4 k,l k where, k=,, (positive, negative, zero sequence coponent respectively, : converted value of first circuit and second circuit respectively L, L : easured value of Line and Line respectively For exaple, V is the zero sequence voltage of the second circuit shown in Fig.3.4 and,l is the positive sequence current of Line. t should be noted that the utual ipedance is eliinated in Fig.3.4, and consequently it becoes easier to see how the fault current flows. n Fig.3.4, and can be calculated as follows using the conversion above., (3. = + M = M The fault current flows to P-ter side and Q-ter side dependent on the ratio of the ipedances of both sides. The of each line can be calculated as follows if f is assued to be the total fault current, for exaple:-, (3.6 L + L =, =, where, 36-4

5 ( x + j(/ ω Q =, f + P + Q j(/ ω ( x j(/ ω = (3.7 f j(/ ω s described before, when the direction of,l is different fro the direction of,l relays can overreach when using (.3 instead of (. for ipedance calculation. t is apparent that the relay would cause overreach when the value of x is close to, which eans that the fault is near to the series capacitor. t can also be seen that the phase angles of,l and,l can be different, and this difference causes phase errors in the ipedance easureent. n addition to the above, it is clear that the sub-frequency coponent can vary between,l and,l. Furtherore they can be different fro a, which is calculated fro + +. Therefore the sub-frequency coponent can vary between a +k s and a +k s +k, which are the denoinators of (. and (.3 respectively, although their respective voltage is the sae. 4. PRTL SMULTONS ND NLYSS Probles predicted in the application of distance protection to series copensated parallel lines have been described theoretically in previous sections. n this section, siulations using EMTP/TP have been undertaken in order to understand the influence of utual ipedance. The odel syste used in the siulation is shown in Fig.4.. Paraeters used in siulations are shown in Table 4.. P-ter Ts S Q-ter P, P VTs Ts F, M F8 S MOV Q, Q D x, x (-x, (-x MOV S : Series apacitors MOV : Metal Oxide Varistors Fig. 4. Siulation odel Line ipedance [ohs] Ter-P source Table.4. Siulation paraeters Nae Value Line length [k] Voltage [kv] 7 Frequency [Hz] Positive sequence ipedance ( + j76 ero sequence ipedance ( 4 + j84 Mutual zero sequence ipedance ( M 3 + J84 Positive sequence ipedance ( P j36 ipedance ero sequence ipedance ( P +j7 Ter-Q source Positive sequence ipedance ( Q j47 ipedance [ohs] ero sequence ipedance ( Q j37 Series capacitor ( [ohs] -j46 Table.4. Fault points Distance fro Fault reactance point x [%] [ohs] F F. 9. F 9 F F4 38 F F6 7 7 F F8 76 Fault points checked in the siulations are shown in Table 4.. Fro the results four cases have been shown in this paper, (Figs. 4. to 4. inc. with wave fors of and (the respective zero-sequence currents of the protected and adjacent lines together with the results of distance easureent. The figures show the coparison between two ethods of zero sequence copensation, one in which only the self zero-sequence copensation is calculated using (.3, and the other one in which the self and utual zero-sequence copensation is calculated using (

6 urrent [ Measured 6 4 ctual ctual Measured ctual Measured ctual Measured Fig.4. Fault point F8(% (Upper: Wavefor, Left: With utual copensation, Right: Only self copensation The following can be identified fro Fig The phase difference between and is alost degrees. - The ipedance easureent using and is very stable and precise - significant oscillation and overreach by about % of ipedance easureent can be seen for the case where only is used. urrent [ Fig.4.3 Fault point F6(7% (Upper: Wavefor, Left: With utual copensation, Right: Only self copensation The following observations can be ade upon exaining Fig The phase difference between and is alost 9 degrees. - The ipedance easureent using and is very stable and precise - significant oscillation whose aplitude is % of the theoretical ipedance of ipedance easureent can be seen for the case where only is used. - Overreach of resistance easureent can be seen for the case using only, although the reactance easureent is close to the theoretical value. This is the phase error in the ipedance easureent. 36-6

7 urrent [ Fig.4.4 Fault point F4(% (Upper: Wavefor, Left: With utual copensation, Right: Only self copensation The following can be identified fro Fig The phase difference between and is alost 6 degrees. - The ipedance easureent using and is very stable and precise - significant oscillation of ipedance easureent can be seen for the case where only is used. - Underreach of reactance eleent and overreach of resistance easureent can be seen for the case where only is used. urrent [ Fig.4. Fault point F(% (Upper: Wavefor, Left: With utual copensation, Right: Only self copensation The following can be identified fro Fig The agnitude of is alost zero, s after the fault. - The ipedance easureent using and is very stable and precise - The ipedance easureent using only is stable and precise ccording to these results, it is expected that the following probles will be experienced in distance easureent if is not used in the calculation. 36-7

8 - The sub-synchronous frequency coponent can cause significant oscillations in distance easureent. - Overreach for faults towards the far end of the line can occur due to the effect of utual ipedance, which is not seen by the relay unless copensation for the parallel line is applied. - The angular difference between and can be seen and can be considered as the reason for the error in distance easureent. - The effect of utual ipedance on distance easureent varies significantly with the fault point. n order to copare the frequency coponents of the zero-sequence current in the protected line and the adjacent line, the -phase current, the denoinator of (. and (.3 and the -phase voltage, the results of the FFTs(Fast Fourier Transfor of these wavefors are shown below. The data window is s fro fault inception. The fault point is F6, which is the sae as that shown in Fig.4.3. The horizontal axis of the following graph is f/f, (f is the syste frequency, and the vertical axis is the contribution of each frequency coponent, in which the value of the basic frequency coponent is noralized to Fig.4.6 Frequency diagra ( Fig.4.7 Frequency diagra ( Fig.4.8 Frequency diagra ( a Fig.4.9 Frequency diagra ( a +k s Fig.4. Frequency diagra ( a +k s +k Fig.4. Frequency diagra (V a On exaining Figs. 4.6 to 4. inc., it can be seen that a large sub-synchronous coponent exists in and, copared to the sub-synchronous coponent which is sallest in ( a +k s +k, the copensated current. Hence, the introduction of the zero-sequence current of the adjacent line effectively reduced the sub-synchronous coponent. f it is considered that the D coponent in the current can be 36-8

9 eliinated alost copletely by using a digital filter, it can be said that the frequency coponent in a +k s +k is siilar to that of the voltage. This fact can be considered as the ain reason why the introduction of the zero-sequence current of the adjacent line enables stable and precise distance easureent.. ONLUSON This paper describes the probles associated with distance easureent when distance protection is applied to series copensated parallel lines. The following points are confired both theoretically and by practical siulation. - Faults up to the series capacitor can cause probles such as oscillation, overreach for reote end faults and phase error in distance easureent. - These probles are caused by the utual zero-sequence ipedance. Sub-synchronous frequency coponents in the zero-sequence current of the adjacent line cause oscillation in the easureent. The angular difference between the zero-sequence current of the protected line and the zero-sequence current of the adjacent line can cause overreach, underreach and phase error in the ipedance easureent. - The relationship between and varies with the fault point. This eans that the effect of utual zero sequence ipedance on distance easureent also varies with fault point. - These probles can be solved copletely by using utual zero-sequence copensation by the introduction of the zero-sequence current of the adjacent line. 6. REFERENES [] T.Maekawa, Y.Obata, M.Yaaura, Y.Kurosawa and H.Takani Fault location for series copensated parallel lines, EEE/PES Transission and Distribution onference and Exhibition, onference Proceedings,,pp [] M.M.Saha, B.Kasztenny, E.Rosolowski and J.zykowski First zone algorith for protection of series copensated lines, EEE Trans. on Power Delivery Vol.6, No., pp.-7 pr.. [3] pplication guide on protection of coplex transission network configuration, GRE aterials, GRE S-34 WG-4, ug. 99 [4] D.Novosel,.Phadke, M.M.Saha and S.Lindahl Probles and solutions for icroprocessor protection of series copensated lines, Sixth nternational onference on Developents in Power Syste Protection, onference Publication no.434, pp.8-3 [] Yi Hu, D.Novosel, M.M.Saha and V.Leitloff n adaptive schee for parallel-line distance protection, EEE Trans. on Power Delivery Vol.7, No., pp.- Jan.. [6].G.Jongepier and L.van der Sluis The behavior of distance relays applied to double-circuit lines in practice and the need for adaptive protection, GRE S34 Stockhol June,99 [7] Y.Ohura, T.Matsuda, M.Suzuki, M.Yaaura, Y,Kurosawa and T.Yokoyaa, Digital distance relay with iproved characteristics against distorted transient wavefors, EEE Trans. on Power Delivery Vol.4, No.4, pp.-3 Oct.989. [8] Y.Hase Practical theory hand book for power syste techniques, MRUEN, 4 (in Japanese 36-9