Elimination of DC component and identification of inrush current using harmonic analysis for power transformer protection

This paper provides an estimation of transients and method of elimination of decaying DC component for the purpose of power transformer protection. Further, the presented strategy discriminates different harmonics generated during power transformer energization and distinguishes inrush from short circuit. The algorithm proposed in the relay logic disconnects the equipment reliably and accurately on sensing fault based on switching instance, switching angle of the circuit breaker and amplitude estimation of the current waveform generated during the fault. In order to verify the performance of the proposed algorithm, off line simulation of the power system model is carried out using MATLAB and observed the performance of the relay during inrush and short circuit phenomenon. In this paper a method has been proposed to estimate harmonics up to 5^th order using MATLAB and a laboratory test results has been exhibited for analysis and interpretation with respect to a set threshold value. The laboratory simulation discriminates internal and external fault from inrush current, thereby preventing mal-operation of the relay during inrush while tripping on a short circuit.