Variance Index of DWT Based Symmetrical Fault during Power Swing

Distance relays are designed to operate reliably and secured manner, and are strictly obeyed to avoid mal-operation during stressed condition. To avoid these conditions, power swing blocking functions (PSB) are incorporated in the distance relay design. However, any kind of fault is initiated during swing, the PSB function should unblock it and provides trip signal to the circuit breakers (CB). Unsymmetrical fault cases were easily unblocked, but in the case of symmetrical fault during power swing condition is difficult task for relay unblocking due to balanced phenomenon. Variance indexes of discrete wavelet transform (VIDWT) based symmetrical fault detection during power swing is presented in this paper. The method utilizes measured three-phase current signals from relay end. Initially, high frequency components are extracted from three-phase current signals using DWT. Next, variance index are estimated for the details coefficients of current signals helps to discriminate the symmetrical fault from swing event. To validate the response of proposed technique, a 400 kV, 50 Hz single machine infinite bus system is considered and simulated in MATLAB/SIMULINK platform. Different critical cases such as fault inception time, slip frequency, fault resistance and fault location were considered for simulation and comparative assessment. From the results, it examines that the proposed method is more efficient as compared to other available methods.