Coherence function method of detection of fault in a power transformer during impulse test

The paper discusses the coherence function method of fault detection in a power transformer during impulse test. It is based on calculation of neutral current in a transformer on the basis of derived equivalent electrical network of the winding. Turn faults are theoretically simulated in the high voltage coil of 220 KV, 100 MVA winding. The neutral current and its Fourier Transform are calculated for simulated HV coil without and with turn fault. The output of frequency response is subjected to coherence function calculation. The results indicate a higher sensitivity of detection of fault with coherence function compared to that of frequency response. A method to calibrate various types of noise superimposed on impulse voltage is also discussed with a view to explain coherence results obtained above.