Resonant behavior of EHV transformer windings under system originated oscillatory transient over voltages

Grid connected EHV transformers experience various terminal disturbances when in service. The present work attempts to investigate the voltage stresses that may develop on the transformer insulations under a variety of terminal disturbances. A number of standard and non-standard wave shapes like lightning impulse, chopped lightning impulse, steep-front long tail switching surge and oscillatory transient over voltages have been simulated and impressed on the terminals of a 400 kV EHV power transformer operating in the Indian power grid to ascertain how the winding insulations are stressed under these disturbances. Relevant section of the Indian power grid and the transformer has been modeled using Alternative Transient Program (ATP). It has been established that oscillatory system transients can trigger natural resonate frequencies of the transformers causing high voltage stresses on the insulations. Short Time Fourier Transform (STFT) analysis of the oscillatory voltage response of the windings confirmed the presence of resonant frequencies indicating forced resonance. Some remedial measures involving winding design modifications have been suggested in the paper to overcome the problem.