Study on the K-factor function in the lightning impulse test 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. For the lightning impulse voltage test, a study is being conducted on the application of a method of evaluating the test waveform through conversion using the test voltage function (K-factor function) that was adopted in IEC 60060-1. The existing K-factor function was established based on the experimental results for more compact models, as compared with the insulating structure of UHV-class equipment, mainly with a breakdown voltage of about 100 kV. To determine whether this K-factor function can also be used for the test of UHV-class equipment, the experimental results for large-sized models were needed. To address this issue, the authors experimentally obtained K-factor values using the largest possible model (UHV model) assuming UHV-class equipment in the previous studies. In the present paper, a study was conducted on a new K-factor function based on these experimental results using UHV models. Initially in the study, several ideas for the K-factor function were shown and applied to various waveforms to clarify their advantages and disadvantages. Subsequently, in addition to these results, a study was conducted on a K-factor function suitable for UHV-class equipment with considering the actual UHV facilities. Consequently, it was concluded that the form of the function should be the same as that of the existing one but that it would be reasonable to adopt a relatively lower K-factor function for UHV-class equipment by revising the constant.