Breakdown voltage of air gap is an important basis for the external insulation design in high-voltage transmission and transformation projects. Current external insulation design only relies on experimental results of discharge characteristics of air gap. However

the breakdown tests of air gap are rather time-consuming and expensive

and fail to include all kinds of air gaps in actual transmission and transformation projects. So it is necessary to further research discharge mechanism and model of air gap for predicting breakdown voltage of air gap

and using the prediction results to guide the external insulation design for the purpose of reducing the number of required tests. In this paper

current applications of empirical and semi-empirical formulas

artificial intelligence method

physical model

and fractal method to prediction of breakdown voltage of air gap are summarized.Furthermore

two key research directions toward prediction of breakdown voltage of air gap

i.e. the fine physical model and the artificial intelligence method comprehensively considering gap structure

voltage waveform and environmental parameter

are suggested.