Very fast transient over-voltages and transient enclosure voltages in gas insulated substations

Gas Insulated Substations have found a broad range of applications in power systems over the last three decades because of their high reliability, easy maintenance, small ground space requirements etc. Although GIS have been in operation for several years, some of the problems are of more attention. These problems include generation of Very Fast Transient Overvoltages (VFTO) during switching operations or earthing faults, the secondary breakdown inside GIS caused by VFTO and Transient Enclosure Voltages (TEA) outside the GIS. In this paper the VFTO levels are estimated by using PSPICE models for all the equivalent circuits of the GIS components including the spark channel development. The results show that the spark lengths below 4 cm, will not affect the VFTO level in a GIS. Further, the maximum possible value of the VFTO level is 2.72 per unit in the case of a 245 KV GIS has been observed. Moreover, the maximum over voltages occur at the open ends of the gas insulated bus bars. However, the VFTO levels after the VFTO breakdown under highly non-uniform fields are low i.e.-1.7 per unit after introduction of spark resistance and hence the Over voltages are further reduced to 1.2 per unit.