Percentage differential protection of double-circuit line using wavelet transform

This article presents development of a new percentage differential protection scheme for double- circuit transmission line using wavelet transform (WT). The work presented addresses the problems encountered by conventional distance relays when protecting double- circuit transmission lines sharing the same right of way. By the use of powerful analyzing and decomposing features of WT, a new technique is presented that takes care of the problems faced by pilot independent distance relay. The suggested technique depends on the six line currents of the two parallel lines at each end. The idea of the protection algorithm is based on the percentage of the difference of the two line current of each corresponding phase at each terminal. The measured six line currents at the relay location are decomposed into three levels of decomposition using the db4 mother wavelet. The summation of third level approximation coefficients are used as bias current whereas third level detailed coefficients are used as differential current. The proposed method eliminates many of the problems associated with double- circuit lines, such as high fault resistance, cross- country fault, mutual coupling effects, remote infeed, and avoids the problem of single- end supply. The stability of the relay under varied system configurations and operating conditions is examined. The relay algorithm is tested for various probable fault inception angles and loading conditions and with varying fault resistance between 0-150 ohms. The proposed wavelet based scheme provides encouraging results for different fault and system conditions. Relaying signals under different fault conditions were obtained by simulating the system using PSCAD/EMTDC.