Characterization of internal disturbances and external faults in transformers using an S-transform--based algorithm

In this paper, the S-transform is used to discriminate between the internal disturbances and external faults of transformers. The proposed algorithm consists of 2 stages. The internal disturbances and the external faults are distinguished in the first stage. Next, the S-transform is applied to differential currents of faulty phases and the absolute deviation of the S-matrix is calculated. The relay scheme issues a trip signal in the case of an internal fault, according to the absolute deviation of the S-matrix. The scheme is implemented in a MATLAB environment and the inputs are differential currents, derived from EMTP software. In order to simulate the internal turn-to-turn and turn-to-earth faults, the power transformer is modeled using 8 x 8 RL matrices, obtained from the subroutine BCTRAN of the EMTP software. The differential current signals are contaminated by noise and the robustness of the algorithm in a noisy environment is investigated. The performance of the S-transform and the wavelet transform are compared. It is shown that the proposed algorithm is superior to wavelet transform-based methods.