Study on switching impulse test waveform for UHV-class electric power equipment

Ultra high voltage (UHV) systems are increasingly being planned and constructed, hence studies are promoted on the standard for high-voltage test techniques for UHV-class equipment, the scope of which includes the switching impulse voltage test waveform. Influences that must be considered to study this test waveform are those of actual switching overvoltage waveforms generated in the UHV system and applied to equipment and the change in the breakdown characteristics associated with a longer insulation length such as an air gap clearance. In the present paper, to determine the switching impulse test waveform, switching overvoltage waveforms applied to the UHV equipment were initially investigated. As a result, it emerged that the time to peak T
_p
was long, ranging from 700 μs to 1000 μs, for closing overvoltage which is relatively high switching overvoltage. When other types of overvoltage in the switching surge range were considered, the T
_p
varied widely from 100 μs to 1000 μs. Subsequently, the insulation characteristics of UHV-class long air gaps for switching impulse waveforms were investigated. It emerged that, for the fundamental model, namely rod-plane electrodes, the T
_p
of switching impulse waveforms with the lowest breakdown voltage (critical time to peak, T
_cr
) ranged from 350 μs to 550 μs whereas it shifted to a shorter time of 200 μs to 350 μs when environmental conditions, such as humidity and precipitation, were taken into consideration. For basic structure models assuming actual facilities and real scale structure models, the T
_cr
ranged from 100 μs to 300 μs. Furthermore, the internal insulation characteristics for switching impulse waveforms, such as SF
₆
gas and oil-paper insulation, were also investigated. Consequently, in the case of the internal insulation of equipment such as gas insulated switchgears (GIS) and transformers, the T
_p
of switching impulse waveforms had a minor influence on the insulation characteristics, even if it varied from 100 μs to 1000 μs. Following comprehensive evaluation of these characteristics, it was concluded that, in the switching impulse test for UHV-class equipment, it is reasonable to apply a waveform of 250/2500 μs, which is the same as that for equipment of 800 kV or less.