Insulation characteristics of GIS under oscillatory waveforms of nonstandard lightning impulse

To rationalize insulation and reduce the cost of gas insulated substations (GIS), it is important to review test voltages, which makes it necessary to investigate insulation characteristics under surge waveforms in more detail and recognize equivalence with standard lightning impulse tests. This paper describes a study concentrating on disconnector switching surges to lower lightning impulse withstand voltages (LIWV) for GIS in cable-system substations. The first step was to review the results of a GIS disconnector switching surge waveform analysis and extract single-frequency oscillatory waveforms (2-5 MHz) and double-frequency oscillatory waveforms (lower frequency: about 1 MHz, higher frequency: about 5 MHz) as typical waveforms. Then, insulation characteristics under the single- and double-frequency oscillatory waveforms at three frequencies each were obtained and compared with those under standard lightning impulses. As a result, the minimum breakdown voltages were 4-13% higher. Finally, using the insulation characteristics mentioned, a study was carried out to reduce the specification. Two typical waveforms of disconnector switching surges were converted into equivalent standard lightning impulses. This yielded peak decreases of 11% and 19% respectively, which makes reduction of insulation