ADAPTIVE LINE DIFFERENTIAL PROTECTION ENHANCED BY PHASE ANGLE INFORMATION

Transcription

1 ADAPTIVE INE DIEENTIA POTECTION ENHANCED BY PHASE ANGE INOMATION Youyi I Jianping WANG Kai IU Ivo BNCIC hanpeng SHI ABB Sweden ABB Sweden ABB China ABB Sweden ABB - Sweden youyi.li@se.abb.com jianping.wang@se.abb.com aken-kai.liu@cn.abb.com ivo.brncic@se.abb.com zhanpeng.shi@se.abb.com ABSTACT Good balance among speed, sensitivity security at the same time is a big challenge or line dierential protection. Its security may be inluenced seriously by CT saturation. Meanwhile its speed sensitivity may be inluenced seriously by high resistance aults heavy load conditions. To meet these challenges, a new line dierential is described in this paper. By using adaptive characteristic rom incremental currents, proposed protection can achieve high security, speed sensitivity simultaneously. INTODUCTION Classical line dierential protection with percentage restrained characteristics is widely used in power systems as the main protection, because it can achieve ast operation or the whole line with high sensitivity phase-segmented operation clear selectivity inherently []. Despite the great advantages mentioned above, there are some problems o sensitivity security with line dierential protection. The criterion o a typical line dierential protection is shown as below. I I k I I As shown in ig.a, I I are the measured currents at local remote sides respectively. I is the threshold o dierential current k is the slope ratio in operation characteristic. Generally, this classical line dierential protection works well or normal operation conditions. It may have low sensitivity speed or the aults with heavy load condition high resistance. In such cases, the restrain current may become too large operate (dierential current may become too small. It may even lead to ailure o operation or internal aults. Current transormer (CT saturation is an additional challenge or dierential protections. The measurement error by CT saturation may bring big alse dierential current even or external ault cases, which may lead to mal-trip. Some additional methods such as harmonics blocking unction could be used to block such potential mal-trips, but it makes the solution complex slows down the speed obviously. ( Another problem o classical line dierential protection is balance o security, sensitivity speed. or classical line dierential protection, its operating characteristic is ixed ater the settings are made. High threshold means high security, low sensitivity slow speed. ow threshold means low security, high sensitivity ast speed. It s diicult to get good security, sensitivity speed at the same time with the same group o settings. ault component based dierential protection has been proposed to improve the perormance [][3], which may be described by the criterion as shown below. I I I I k I I I Compared with classical dierential protection, the main change in the criterion in equation ( is that the ault component currents (incremental currents or sudden change currents I I are used instead o ull component currents I I. It has better sensitivity speed especially or the cases with heavy load condition high resistance, because the load currents are removed in this method. On the other h, the problem o CT saturation remains in this ault component based line dierential protection. In addition, the diiculty o balance or sensitivity, speed reliability also remains. Thereby, it is clear that perormance improvement o line dierential protection is still very attractive. ots o studies have been done to improve the perormance or both sensitivity o internal aults security o external aults by employing directional inormation o ault components [4][5], which shows the possibility to get good balance o sensitivity security at the same time. Our research shows that the line dierential protection can get even better perormance by using the adaptive operating characteristic based on the change o phase angle between ault component currents. The proposed new protection can achieve some very attractive eatures: - Good sensitivity speed or high resistance aults heavy load conditions. - Be immune to CT saturation inherently. - Good balance o security, sensitivity speed. BASIC PINCIPE Proposed solution is based on the additional inormation o ault component currents. It will calculate the phase angle dierence between the incremental currents rom ( CIED 5 /5

2 both ends o the protected line. I it is close to, a preliminary internal ault will be indicated. Then the sensitivity o the protection will be increased. Otherwise, i the angle is close to 8, a preliminary external ault will be indicated. Then the operate threshold will be increased to enhance the security. Based on the adjusted characteristic, the protection can achieve better sensitivity or internal aults better security or external aults. Basic theory ault component theory has already been widely used in dierent kinds o protections. In these protections, the sensitivity speed is improved by removing the load components. The general idea o ault component theory can be described in ig. as shown below. I I (a System with aults (b System in normal operation I S U (c ault component network (internal ault S I I (d ault component network (external ault I ig. ault component network In ig. above other parts in this paper, are the internal ault external ault respectively. I I are the measured currents on terminal respectively. I I are the corresponding ault component currents. U is the voltage o the additional source at the ault point in ault component networks. U S S S, S are the equivalent source impedances line impedance. are the phase angles o the ault component currents o terminal respectively. As shown in ig., the network with a ault (ig.a can be divided into two networks, one is the network in normal operation (ig.b the other one is ault component network (ig.c or ig.d. When an internal ault occurs, the phase angle dierence between the ault component currents is: arg I U arg U S arg / / S S S arg I I arg I In EHV/UHV power systems, the phase angle o equivalent source impedances S, S line impedance have relatively similar phase angles. The phase dierence calculated by ormula (3 is a small value is close to normally. When an external ault occurs, the phase angle dierence between the ault component currents on both terminals is: arg I arg I I arg I 8 I arg I Obviously, the phase dierences are dierent or internal ault cases external ault cases. The corresponding vector diagram is shown below. I I I (a Internal ault cases (b External ault cases I ig. Vector diagram o incremental currents Such obvious dierence o phase angles between internal ault external ault (the gap is almost 8 can be employed in line dierential protection to enhance the protection perormance or both sensitivity security. In practical applications, there may be lots o errors rom dierent actors, such as CT saturation, measurement error, digital ilter error, charging current etc., especially during the ault transient period. ortunately, the big gap o phase dierence between internal ault external ault can ensure enough security even with all these errors in practical applications. (3 (4 CIED 5 /5

3 Main algorithm Proposed protection can be described by dierent mathematical orms, two o them are shown as below by general equation (5 or (6. I, I arg( I arg( I I (, I I (, arg( I arg( I ( T T (, Here, I is the operate current (dierential current I T is restrain current. They may be based on ull component values or ault component values with dierent mathematical orms. or example, I may be I or I, etc. I may be T I or max(, I, etc. The adaptive eature o proposed method is mainly controlled by the two general unctions (, (,. (, is used to control the slope ratio o the percentage restraint characteristic (, is used to control the value o minimum operating value or the oset o total operating threshold. The diagram o the characteristic in equation (5 (6 are shown in ig.3(a ig.3(b respectively as below. I (, I (, Operating estraining (a Characteristic Operating estraining (b Characteristic (, I T (, I T ig. 3 Adaptive operating characteristic (5 (6 When is close to, it means a possible internal ault generally. The unction values o (, (, will be decreased automatically to reduce the operate threshold. Otherwise, when is close to 8, it means a possible external ault. The unction values o (, (, will be increased to ampliy the operate threshold. As shown in ig.3, the characteristic o this protection is, to a great extent, determined by the unction values o (, (,. And the two adaptive eature unctions are based on ault component values. Thereby, the inluence o load current is reduced greatly. It is well-known that CT saturation will cause large measurement error o currents or both amplitude phase angle, which might lead to mal-trip in some cases. ortunately, its inluence on current phase is limited in some sense. During the external aults with CT saturation, the phase angle dierence o is no more 8, but the angle oset is not too much normally. Even or the external aults with serious CT saturation, is more close to 8 instead o. The adaptive eature based on still works well with proper design o unctions (, (,. Proposed method is immune to CT saturation. Here, the detailed algorithm designs o (, (, may be linear unction, nonlinear unction or piecewise linear (nonlinear unction depending on the detailed requirements. urthermore, (, (or (, may be predeined ixed value(s instead adaptive unction(s i it is needed. I both adaptive unctions are ixed values, the algorithms in equation (5 (6 become the classical dierential protections. Our study also shows that ater the ault occurs, it will get a stable calculation result o within very short time period. It means that the adaptive operating characteristic based on is ready in a very short time ater the ault occurs it will not delay the ault detection. As mentioned beore, there are lots o choices or adaptive eature unctions o (, (,. One o the possible detailed designs or equation (5 is shown as below. I I k k arg( I arg( I cos max( I, I I Here, (, is an adaptive unction based on cos as shown in equation (8 (, is a ixed value as shown in equation (9. k k are used (7 CIED 5 3/5

4 to control the adaptive eature. (, k k cos (8 (, Id min (9 Here, (, is a ixed value I in equation (7 only or simplicity in design. I an adaptive unction like equation ( is used instead o ixed value, it may result in an even better adaptive eature. (, Id k k cos ( min 3 4 SIMUATION VEIICATION Proposed solution is veriied by both simulation prototype. ots o simulation analysis with PSCAD /Matlab have been done to veriy the perormance o the proposed method. The structure o the simulation system is shown in ig.4. described in both polar coordinate classical Cartesian coordinates as shown in ig.5 (a ig.5 (b. (a Characteristic in polar coordinate 4 Characteristic o operate I delta phase, phase A 5 5 A 6 A 5 A Operate characteristic, phase A S I I S operate I (A 8 6 ig. 4 Simulation system model 4 This is a 5km length 5kV transmission line system. is an internal ault located at 5 km rom terminal. And is in an external ault on the busbar o terminal. The main parameters o the simulation system are shown as below: e, e j85 S 5 j84 S e, e j85 S 5 j84 S 4.95 j.8 m 4.68 j8.6 m / / 8 X C.359 / m 8 X C 3.46 / m In the simulation cases o this paper, the algorithm in equation (7 is used. The setting are given below: I 4A, k k. 8. The sampling requency is khz calculating cycle o the protection algorithm is ms. Proposed method will update the adaptive threshold calculate the algorithm in equation (7 in every calculating cycle. Internal ault An internal A-G ault occurs at in ig.4 with 3Ω resistance. The load current is 3 ka. Because o high resistance heavy load, it may be diicult or classical line dierential protection to operate quickly in this case. But proposed method can operate within 6 ms. The characteristic o proposed protection or this case is restrain I (A (b Characteristic in classical coordinate ig. 5 Characteristic or internal ault In ig.5 (a, the radial coordinate is operate current I the angular coordinate is dierential phase angle. Ater ault occurs, becomes rapidly steadily together with big incremental o dierential current I. Based on the two obvious eatures, an internal ault can be detected. ig.5 (b shows the characteristic o the protection in equation (7 in classical I / I coordinate. Ater the T ault occurs, becomes. This method can detect the change o phase angle rapidly, modiy the characteristic dynamically automatically. Thereby, the slope in characteristic described in equation (8 could be calculated as:.8.8 cos. As a result, the required operate threshold or trip becomes much smaller than that o classical percentage dierential characteristic. By this means, the speed sensitivity is improved. More simulation cases on this 5 kv system show that proposed method can operate within one cycle or high resistance ault even up to kω with proper settings. CIED 5 4/5

5 External ault An external A-G solid ault occurs at in ig.4 with serious CT saturation o I. The characteristic o proposed protection is shown in ig.6. Characteristic o operate I delta phase, phase A CONCUSION The dierential phase angle o incremental currents can indicate internal/external ault rapidly reliably. This eature can be employed to change the characteristic dynamically. By this means, adaptive dierential characteristic is achieved, which improves the speed sensitivity or internal aults improves the security or external ault even with heavy CT saturation. Some general mathematical equations o the new idea are described in this paper. inally, simulation tests demonstrate that the principle has not only very good dependability or internal aults but also high security or external aults. 33 EEENCES operate I (A 4 (a Characteristic in polar coordinate Operate characteristic, phase A (b Characteristic in classical coordinate ig. 6 Characteristic or external ault It is clear in ig.6 (a that there is a big dierential current up to more than ka or this external ault because o CT saturation. Meanwhile, the dierential phase o incremental currents is no more 8 because o CT saturation. It reaches 5 during the external ault period. But it is still in the second quadrant as expected ar away rom. or the worst condition in this case, when the dierential phase reaches 5, the slope in characteristic is, restrain I (A [] S. C. Sun,. E. ay, 983, "A current dierential relay system using iber optics communications ", IEEE Transactions on Power Apparatus Systems. vol. PAS-, [] Wu Yekai, Yuan Baoji, 996, "Split-phase current dierential protection using ault components", Electric Power. vol. 4, 4-9, 9. [3] Yi Xianggen, Chen Deshu, hang he, 996, "ault component based digital dierential protection", Automation o Electric Power Systems. vol., 3-7. [4] hang hen-yu, He Jiali, Wang Guoxin, Guo zheng, 5, "Directional current dierential protection based on ault components", Electric Power. vol. 38, -6. [5] B. Kasztenny, G. Brunello,. Servov,, Digital low-impedance bus dierential protection with reduced requirements or CTs", Proc. IEEE/PES Transmission Distribution Con., vol., cos.8.8 cos(5. 49 This high slope can ensure good security o proposed protection rom mal-trip. ig.6 (b also shows the characteristic in a classical way. Although there is a big dierential current ater ault occurs, the required trip threshold also become very large thanks to the adaptive eature, which is much more than the operate value. CIED 5 5/5