Time-Frequency Spectrum Analysis of Very Fast Transient Currents (VFTC) Generated in a GIS

Abstract

Very Fast Transient Overvoltages (VFTO) and the associated Very Fast Transient Currents (VFTC) generated during switching operations in a Gas Insulated Substation (GIS) couple conductively and inductively to the secondary equipment connected to the GIS. The frequency spectrum of the VFTC gives the dominant frequencies present but cannot provide the time varying current amplitude w.r.t. any particular frequency. Since the transient response of the control circuits is a function of the frequency content of the VFTC, it becomes necessary to segregate the VFTC waveform both in time and frequency scale simultaneously. This has been achieved by employing the GABOR wavelet function and the time-frequency spectrum of the VFTC waveform has been calculated at various locations for a 245 kV GIS during a switching event. The validity of the wavelet model is confirmed by reconstructing the VFTC signal at the GIS-cable termination as well as at the switching point using the time varying current waveforms obtained for different frequencies. Finally, the transient current vs frequency as well as the current vs distance charts are developed for the 245 kV GIS under study during various switching operations.