Time-frequency analysis of very fast transient over voltages for effective shielding of control circuits in a 245kv gas Insulated Substation

Very fast transient over voltages (VFTO) and very fast transient over currents (VFTC) are generated during switching operation of disconnectors in Gas insulated substation (GIS). In this paper the accurate GIS model has been developed and VFTO´S and VFTC´S were estimated at bushing termination using EMTP program. The wavelet analysis of VFTO and VFTC signals have been carried out and frequency spectrum of VFTO wave form is obtained. In order to estimate the frequency spectrum more accurate Gabor wavelet function is employed. By using above method the time-frequency spectrum of the VFTO waveform has been calculated at bushing termination in a 245kV GIS system during switching condition. The frequency characteristics of the VFTC have been analyzed using the Fast Fourier Transform (FFT) technique the validity of the wavelet function is confirmed by comparing both the results. This analysis helpful for an effective shielding of the control circuits present within the GIS against transient voltages and currents.