Response of EHV Grid Transformers to System-Originated Oscillatory Switching Transients

This paper investigates the conditions for transformer winding resonances with an emphasis on oscillatory excitation voltages originating from various switching events in the power grids. A section of the Eastern Regional Power Grid in India has been simulated in this paper by using the Alternative Transient Program 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 matches the terminal excitation frequencies. Short time Fourier transform (STFT) has been used to analyze the frequency contents of the oscillatory terminal excitation as well as the voltage response of the windings. The results of the STFT confirmed the presence of resonant frequency in the oscillatory terminal voltage and in the 400-kV main and tap changer windings of the transformer. It has been demonstrated that the voltage stresses in the windings under certain critical switching operations can exceed the stresses developed under the standard 1.2/50- μs lightning impulse at the transformer´s basic insulation level voltage. The reduction in winding voltage stresses under resonance is possible by deploying some remedial measures involving winding design modifications which have also been suggested in this paper.