Influence of VFT on Shell-Type Transformer

When a disconnecting switch in a gas insulated switchgear (GIS) operates, a high frequency oscillating surge called very fast transient (VFT) is generated. The VFT may set off overvoltage in a transformer winding. In the paper the authors evaluate the influence of VFT on a shell-type transformer. To simulate waveforms of VFT invading a transformer, a simple equivalent circuit for a shell-type transformer in a high frequency range is derived experimentally. Using this model the overvoltage waveforms are analyzed for various configurations of GIS. Then, the influence of VFT on a 500 kV shell-type transformer winding is investigated. The results lead to a conclusion that VFT does not threaten the reliability of insulation of the transformer winding