Very high frequency overvoltages at open air EHV substations during disconnect switch operations

EHV open air substations experience flashovers produced by very high frequency overvoltages (VHF OV) during disconnect switch operations. It was found during field tests that the highest measured value of sparkover voltage across the EHV switch gap between contacts during its operation was 1.9 p.u. The most unfavourable scheme, theoretically, occurs when a bus is fed from a single transformer, without outgoing lines. Calculations showed that in 787 and 525 kV substations with branched buses VHF OV could result in flashovers on a bus open end with unfavourable ratio of bus lengths. For transformer insulation protected with an arrester VHF OV are not dangerous. Some protective measures are recommended.