Influence of switching conditions on the VFTO magnitudes in a GIS

Estimation of very fast transient overvoltage (VFTO) has been carried out using EMTP for various switching conditions in a 420 kV gas insulated substation. The variation of VFTO peak along the GIS bus nodes for disconnector and circuit breaker switching operations as well as the variation of VFTO peak with different magnitudes of trapped charges have been studied. The results indicate a distinct pattern of variation of VFTO peak along the nodes of the GIS bus in the case of disconnector switch operation as compared to that of circuit breaker operation. It has also been noticed that the variation of VFTO peak levels are not in direct proportion to the trapped charge present on the HV bus.