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

At present, a study is being conducted on the lightning impulse (LI) voltage test standard for UHV-class electric power equipment at WG 19 "Adaptation of TC 42 standards to UHV test requirements" in IEC TC42 "High-voltage and high-current test techniques". In the present paper, the past LI voltage test results for UHV-class equipment were initially investigated to determine test waveforms that could be generated. As a result, for a UHV-class transformer and gas insulated switchgear assembly, the front time T₁ exceeded 2.2 μs or more, or even considerably 3.0 μs in some cases in order to generate a waveform with an overshoot rate β\´ of 10% or less using existing test facilities. Subsequently, experiments were conducted with regard to the extension of the T₁, which is a prospective way to revise the LI voltage test waveform. Breakdown characteristics with respect to the T₁ were measured using the largest possible models assuming UHV-class equipment for three types of insulating media, namely an SF₆ gas gap, an oil gap, and a long air gap. Consequently, it was confirmed that extending the T₁ up to about 3.6 μs had a minor influence on the breakdown characteristics. In addition, a study was also conducted on the influence of the deregulation of the standard value for the T₁ from a viewpoint of insulation properties of equipment for the wave front steepness. As a result, it was concluded that the equipment insulation performance could be appropriately verified even if waveforms with an extended T₁ were used. Based on these data, the idea of the standard for lightning impulse test waveforms for UHV-class equipment was proposed as follows: The standard voltage waveform shall be the same as the conventional one for less-than-800 kV equipment with a T₁ of 1.2 μs and a time to half value of 50 μs but the upper allowable range of the T_{1- /sub> may be up to 3.6 μs; provided, however, that a note shall be added besides the exceptional provision that, whenever possible, the test should be conducted using a waveform of which the T1 is adjusted to 1.56 μs or less, the same as that for less-than-800 kV equipment.}