Wavelet Transform Based Differential Protection Scheme for Tapped Transmission Line

Using the powerful analyzing and decomposing features of wavelet transform, a new differential protection scheme for tapped transmission line is presented. The idea of proposed scheme is based on the third decomposition of the measured signals (three line currents) at each relay location using the db4 mother wavelet. Thereafter, third level approximation coefficients (CA3) are reconstructed and used for deriving operating and restraining quantities. Based on extensive simulations it has been found that the operating quantity is the absolute value of the difference of third level reconstructed signal (A3) for each phase. The restraining quantity is one third of the absolute value of the summation of third level reconstructed signal for each phase. The wavelet transform is used to capture important features of the decomposed signal that are sensitive to high resistance fault and provides proper discrimination between external and internal faults. The proposed scheme has been tested under different fault and system conditions. A series of test using PSCAD/EMTDC output data in a 400 kV tapped transmission lines have proved the accuracy and effectiveness of the proposed scheme.