Study on VFTO in UHV GIS substation

In GIS substations, operations of disconnectors may cause very fast transient overvoltage (VFTO) with high amplitude and steep wave front. Considering the narrower insulation margin of UHV GIS equipments and larger absolute magnitude of VFTO in UHV GIS systems, VFTO in UHV GIS systems could cause larger damage than those in EHV and lower ones. Therefore study on VFTO in UHV GIS systems is of great significance to choose more economical and more effective measures to prevent VFTO, in order to finally improve the safety and reliability of UHV GIS systems. In this paper, characteristics and hazards of VFTO in UHV GIS systems are analyzed by comparing it to that of EHV GIS systems. Comparison shows that the absolute value of VFTO in UHV GIS system is almost as twice as that of EHV GIS systems, but the insulation level of UHV GIS equipments is only 1.55 times as that of EHV ones, resulting in much more danger in the UHV GIS equipments. Then, several preventive measures are analyzed and summarized in this paper, by simulating Jindongnan UHV GIS substation based on EMTP. The maximum amplitude of VFTO with no suppressing measures is up to 2.72 p.u., dangerously exceeding the limit level of insulation, which is about 2.32 p.u. in UHV GIS systems. Analysis shows that installing proper shunt resistors in disconnectors is the most feasible and effective preventive measure at present. In spite of the lack of practical applications, ferromagnetic rings have great prospects of effectively suppressing VFTO. All possible switching sequences in Jindongnan UHV GIS substation are analyzed and simulated, among which the one that generates the severest VFTO is found out. Because the transformer of Jindongnan UHV GIS substation is connected to bus through overhead lines for long distance, the steepness of wave front is effectively reduced, thus VFTO will not threaten the transformer.