Influence of Very Fast Transients on Transformer

Abstract

Very fast transients (VFTs) occur in a gas insulated switchgear (GIS) at a disconnecting switch operation. VFTs are high frequency surges and are characterized by a fast steep wavefront and an oscillating waveform. VFTs cause voltage oscillations in a transformer winding connected directly to a GIS.
  VFTs in the GIS are simulated using the Laplace transform. The fast steep wavefront and the oscillating waveform attaking on the transformer are shown as a function of GIS constitutional parameters. For a case study, influence of the fast steep wavefront and the oscillating waveform is investigated on 550kV class transformer. In the case of VFTs at the transformer, a voltage level of the fast steep wavefront is larger than an amplitude of the oscillation waveform. However the interturn voltage oscillation in the winding caused by the oscillation waveform can be larger than that by the fast steep wavefront. The authors deduce that the voltage oscillations do not threaten the reliability of the interturn insulation.