High Frequency Switching Transients in 420kV Gas Insulated Substation Due To Disconnector Operation

Abstract

Very Fast Transient Over voltages (VFTOs) are generated in GIS due to any switching operation but their effect is significant during opening and closing operation of Disconnector. VFTOs are significant especially in disconnector is due to two reasons- Due to slow operating speed of disconnector and Length of GIS bus bar is relatively small compared to AIS Busbar. Due to slow operating speed of disconnector, the sparks are generated and extinguished several times. The transients which are generated at the disconnector travel through the length of GIS bus bar. At the end of the bus bar, they get reflected and refracted many times in a very less time and will have frequency range in Megahertz. Hence these are called very fast transient over voltages.

VFTOs are of major concern for High operating voltage GIS in which ratio of lightning impulse withstand voltage and to system voltage decreases. As this ratio decreases, VFTOs cause many problems like insulation damage, secondary breakdown of GIS.

Thus, the detailed study of VFTOs is necessary for insulation coordination studies in GIS and for adopting mitigating methods. This detailed study of VFTOs is possible by modelling of each component of GIS using electrical equivalent circuits which provides accurate results.

In this paper VFT over voltages at various locations in a 420 kV gas insulated substation is analyzed  using Electromagnetic Transient Program (EMTP-ATP). The peak value of VFTO is measured for different trapped charge values on the bus section. The maximum peak with a trapped charge of -1 P.U is 3.09 P.U. From the Fast Fourier Transform (FFT) analysis of the VFT overvoltage waveforms it has been found that the major frequency components lie in the range of 1 MHz to 70 MHz the effect of trapped charge, terminal components on VFTO magnitude and steepness is studied, the electrical equivalent of ferrite rings is also connected to the 420kV system to reduce the effect of VFTOs. In this paper a 420kV system is considered for VFTO study, circuit-based model is developed in EMTP-ATP.VFTOs are observed at different locations with Disconnector operation. The influence of switching conditions on VFTOs and various terminal equipment is also observed. Electromagnetic field associated with VFTCs is estimated using a field-based model in COMSOL multi physics software.