Disconnector switching induced transient voltage and radiated fields in a 1100Â kV gas insulated substation

Very fast transient overvoltages (VFTO) generated in a gas insulated substation (GIS) during disconnector switching operations are one of the major concerns in insulation design of the GIS. Such over voltages can cause malfunctioning of the protection and control circuits in addition to initiating faults inside the gas insulated bus ducts especially in the presence of metallic particles. In this paper, VFTOs have been estimated at various locations using EMTP in a 1100 kV rated GIS during disconnector switching operations. For the substation studied, the maximum computed overvoltage is 1.76 p.u without considering the trapped charge on the bus bar and it reaches 2.54 p.u with a trapped charge of −1 p.u. Major frequency components in the computed VFTO lie in the range of 800 kHz to 15 MHz. Studies on mitigation of VFTO by the use of damping resistors in the disconnector showed that a resistance close to 500 Ω gives the best results. For the bus sections with trapped charge, increasing the resistance value above 500 Ω results in a negative peaking of VFTO. Transient radiated fields through the bushings of the GIS are also studied for electro magnetic compatibility (EMC) design of the substation.