Investigation of very fast transient over voltages in gas insulated substations

Gas Insulated Substations (GISs) have found a broad range of applications in power system over the last three decades because of their high reliability, easy maintenance, small ground space requirements, etc. Although GIS have been involved in power systems since long time ago, some of the problems are of more attention. These problems include generation and propagation of very fast transient over-voltages (VFTO) during switching of circuit breakers or disconnector switches or earth faults. The generated VFTO causes stress in the air-insulated switchgear and secondary equipment. This paper presents a modeling guideline for VFTO in GIS. The origin of VFTO, its propagation and impacts on GIS have been discussed in this paper. Estimation of VFTO has been carried out using EMTP-RV for various switching conditions. The variations in VFTO peak along the GIS bus nodes regarding the 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 DS operation as compared to that of CB operation. Moreover, the increase of trapped charge on the line increases VFTO levels. Finally, the beneficiary of several methods has been discussed to reduce VFTO amplitudes such as damping resistor, Ferrite rings and Nanocristalline rings.