K-Factor Value and Front-Time-Related Characteristics in Negative Polarity Lightning Impulse Test for UHV-Class Air Insulation

In IEC-60060-1 as revised in 2010, a conversion method for lightning impulse voltage test waveforms using the test voltage function (K-factor function) was adopted. Presently, studies are being conducted on the test standards for ultra-high voltage (UHV)-class equipment. Since the existing K-factor function was established based on the experimental results for more small models compared with the insulation structure of UHV-class equipment, the issue has arisen as to whether this K-factor function can also be applied for UHV-class equipment. The authors experimentally obtained K-factor values using the largest possible model assuming UHV-class equipment in the previous studies. This paper reports the K-factor values and the insulation characteristics with respect to the front time obtained for air insulation under a negative-polarity lightning impulse voltage. According to the results of this experiment, even though the K-factor values for an air gap were likely to be slightly lower than the existing K-factor function in the negative polarity, the tendency of the change with respect to the superimposed oscillation frequency was similar. Furthermore, it was found that even though the breakdown voltage for waveforms with an extended front time was likely to decrease further than that for the standard lightning impulse waveforms, the decline was about 3.1% even for a front time of 4.8  μ s. Consequently, it was indicated that a lightning impulse voltage test waveform with an extended front time may help properly verify the insulation performance of a UHV-class air bushing.