Effects of low amplitude oscillatory switching transients on EHV grid transformers

A section of the Eastern Regional Power Grid in India has been simulated in this paper using Alternative Transient Program (ATP) for the purpose of defining high frequency transient voltages which may occur during various switching events. The investigation reveals that such oscillatory voltages could excite natural frequencies of a 400/220/132 kV power transformer which closely match with the terminal excitation frequencies. Short Time Fourier Transform (STFT) has been used to analyze the frequency contents of the voltage response of the windings. Results of the STFT confirmed the presence of resonant frequency in the 400 kV main winding of the transformer. Reduction in winding voltage stresses under resonance is possible by deploying some remedial measures which have also been suggested in this paper.