A wavelet-based restricted earth-fault power transformer differential protection

Abstract

In order to enhance power transformer differential protection, especially with respect to its reliability and speed, several schemes have been proposed in the last years. Faults inside the transformer protection zone may cause critical damages to this device if its protection unit does not operate. It is known the traditional phase current differential element presents low sensitivity to detect turn-to-ground faults close to the transformer neutral point, requiring a specific restricted earth-fault protection unit. This work aims to present a restricted earth-fault protection based on the wavelet transform (REFW) for detecting turn-to-ground faults to give support to the conventional phase differential protection. The proposed REFW protection uses only high-frequency components instead of traditional low-frequency components obtained by means of phasor estimation methods, speeding up the detection of turn-to-ground faults. Several events close to the power transformer were simulated, such as turn-to-ground faults, faulted transformer energization, and external faults, in order to assess the behavior of the REFW unit and compare its performance with the conventional REF unit. The REFW unit presented efficiency in the detection of faults close to the transformer neutral point. The proposed method obtained a success rate of 100% for all simulated databases analyzed in this work, even during faults in presence of inrush currents, whereas the conventional REF unit presented good sensitivity for turnto-ground faults, except in cases of transformer energization with turn-to-ground faults. In addition, the proposed method presented a faster detection than the conventional one. The proposed method was validated for events generated in an experimental test bench. The experimental results proved the efficiency and speed of the REFW unit.