DEVELOPMENT OF PROTECTION ALGORITHMS FOR COMPENSATED TRANSMISSION LINE

Abstract

Flexible Alternating Current Transmission System (FACTS) controllers can be connected throughout the complete length of transmission line. Moreover, the preferred location is at the middle of the line because voltage fluctuation is maximum at that point. Particularly, shunt FACTS controllers are preferably connected in the middle as they provide voltage support to the line. When FACTS controllers are connected at the sending or receiving end substations the protection of transmission line becomes somewhat easier because measurements can be taken locally without any considerable time delay. If the FACTS controller is placed at the middle of transmission line, the protection of transmission line becomes a challenging task for protection engineers. In such condition transmission line has two sections: one before the FACTS device which is uncompensated portion whose impedance is linear and other after the device which is being compensated and it's having variable impedance (non-linear). Therefore, ultimate challenge for protection engineers is to protect such a line whose impedance is not linear throughout the whole length. To protect transmission line having midpoint compensation, first requirement is to know the section of line in which fault occurred. After identifying fault section, an adaptive protection algorithm is needed to protect the complete line. In this work, fault section is identified by a combined wavelet-SVM based technique which takes very less time to analyze the pattern and select the faulty section. Wavelet transform decomposes all the frequency bands present in fault signal and SVM analyze the frequency patterns for different fault sections. Any transmission line fault signal has various frequency signals up-to 80 kHz. Relay, which is placed at the bus, receive all the frequency signals including fundamental frequency signal through current transformer. Any fault signal occurred before or after the FACTS device has different frequency spectrum because of the filtering by FACTS device itself. FACTS device filters the particular frequency band from the fault signal according to its inductance or capacitance value at that time (or according to its compensation level). In case of series compensation, inductance or capacitance value inserted by FACTS device has been used as filtering element, therefore, it was claimed that particular frequency signal will be missing if fault occurred in section-Il. Although, in case of shunt compensation like SVC and STATCOM, impedance inserted by the device is not constant, so it cannot be said surely that which frequency band will be missing in frequency spectrum. Therefore, to make transmission line protection system reliable, it is needed to find fault section clearly. For this purpose, a high pass filter is installed at the FACTS device location which surely filters the high frequency signals generated in section two in case of fault. In case of fault in section one, wavelet transform provides detail about high frequency components, while, fault in section two has no or less detail about high frequency signals. One drawback of the wavelet transform is its sensitivity for noise signals. Therefore, to make relay performance less prone to noise signal, a SVM classifier is added to make protection algorithm more adaptive. SVM can draw an optimal hyper plane to categorize two classes, therefore, it can classify the wavelet signals optimally for section one or two faults. Further, SVM parameters are optimized by Genetic Algorithm to make classifier an optimal classifier. Results are compared with the RBFNN based classifier. Presence of TCSC in the middle of transmission line creates major protection issues particularly in distance protection. First section of line has linear impedance trajectory and can be easily protected by the convention mho relay algorithm. After TCSC, point impedance of line is a shifted trajectory. In capacitive mode of operation, it shifts into downward direction and in inductive mode of operation it shifts toward upward direction and in both the cases amount of shift depends upon the level of compensation. Therefore, requirement of the transmission line protection system is such that it should be adaptive with the shifted impedance trajectory of the second portion of transmission line. Another challenge in protection of transmission line having TCSC is that, TCSC itself protected by MOV varistor which is a non-linear resistor. When fault current exceeds a certain limit, overvoltage across TCSC capacitor become too high so, to protect TCSC from overvoltage, MOV discharge excessive energy through itself. In this case the equivalent impedance of TCSC with MOV is not linear and measured fault impedance has uncertain error. It is the requirement of the power engineer to place TCSC safely in the transmission line even in fault condition such that as soon as fault cleared it should function on the line accordingly to reduce economic losses. Therefore, in any short duration fault or transient condition, TCSC remain in the line and inserts its own impedance. To make transmission line protection system adaptive, it is required to calculate TCSC impedance and with respect to this impedance it is required to make transmission line protection system adjustable. TCSC equivalent impedance depends on firing angle of thyristor, therefore, it can be calculated by measuring a single variable. MOV setting is kept such that it operates in highly severe condition and bypasses high amount of current. In this case, TCSC and MOV equivalent impedance comes into picture. ii Similarly, the presence of shunt FACTS device such as SVC and STATCOM also creates some major protection issues when placed at the middle of transmission line. Certainly, the preferred location for shunt FACTS device is at the middle because at this point voltage sag is maximum. Shunt devices provide voltage support at the point of connection by injecting a quadrature current into the line. Shunt devices does not affect the performance of relay for any fault before the point of connection. For any fault after the point of connection it introduces certain error in fault impedance calculation according to current injected by the device. Shunt devices inject both the leading or lagging current at the point of common coupling depending on the system requirement hence creates under and over reaching problems. Therefore, to make transmission line protection system adaptable with the action of shunt FACTS device it is required to calculate the error introduced by the device. A compensation unit has been inserted in the conventional Mho relay which calculates error in terms of current injected by the shunt device into the transmission line. Fault zone identification in compensated transmission line is constrained by several factors like fault resistance, loading conditions and mainly by the level of compensation at the time of fault. For a secure transmission line protection system, it is needed to deal with all above mentioned factors when deciding the faulty zone. In uncompensated transmission line, some fault zone identification techniques are proposed which make transmission line protection system adaptable with loading conditions and different values of fault resistance. In these zone identification techniques, fault impedance coordinates have been calculated for different types of faults and power system operating conditions. Decision is taken by training any intelligence tool like SVM to decide the particular fault zone. These techniques are adaptive to changing loading conditions and various values of fault resistance. In case of compensated transmission line, inductive portion of transmission line is also variable and can emerge in any direction depending on the mode of operation of the compensator. Fault impedance coordinates of different zones for these lines are overlapping over the imaginary axis at the intersection of zones. Even the learning of intelligence tool by these coordinates cannot solve the problem completely because for two different fault locations same coordinates may exist. So, to solve zone identification problem in compensated transmission line, work is needed to be done on both the sides; first on fault impedance coordinates and second on intelligence tool. In this work, impedance inserted by compensator into the transmission line is estimated and modified impedance coordinates of fault impedance is calculated for the learning of intelligence tool. 111 Due to several advantageous features, SVM is used for fault zone identification in compensated transmission line. Results are compared with the RBFNN based classifier.