ANALYSIS OF VERY FAST TRANSIENT OVER VOLTAGES IN GAS INSULATED SUBSTATION

In a Gas Insulated Substaions (GIS), Very Fast Transient Over voltages (VFTOs) are generated mainly due to switching operations. The s witching operation may be of a disconnector switch or a circuit breaker or an earthing switch. For Ultra High Voltage GIS, switching overvoltage levels are possible up to 3.0 p.u. depending on switching configuration and are comparable to basic insulation level. In other words, the margin betwee n protection level and insulation level becomes narrow and reliability of equipment becomes a major concern. In view of this, overvoltage levels shall be controlled by means of suitable measures like control switching, switching through resistance etc.