Theoretical Analysis of External Insulation of Typical Electric Equipment under Impulse Voltage and Research on Design and Verification Method of Margin

Abstract Lightning impulse voltage, one of the typical characteristic working conditions for assessing high-voltage power equipment, can be divided into two forms, namely full wave, and chopped wave. The external structure size of the power equipment directly depends on the lightning impulse voltage, but how lightning impulse voltage relates to the external structure size of the power equipment needs to be carefully studied. Firstly, the theoretical analytical expressions of full wave and chopped wave lightning waveforms are given from the perspective of theoretical formulas. And the lightning waveform is considered to be a polynomial superposition of exponential functions. Based on this, the discussion is divided into two typical insulation structure scenarios: 1) the verification and analysis of the space net distance of a 10 m class super long composite insulator; 2) the analysis of the relationship between the breakdown electric field strength of 5 mm tiny oil gap and oil gap spacing. Further, the lightning impulse voltage test was conducted for large hardware structures, and the corona layer mathematical model was obtained through the precise observation of the structure with the ultraviolet imager. The research shows that the lightning impulse voltage mainly checks the external insulation performance of large insulation structures such as bushings and post-insulators. For the internal oil gap duct structure of the large converter transformer, the lightning impulse voltage mainly checks the micro gap insulation strength. Lightning impulse voltage also has a breakdown effect on SF6 insulation gas, but its internal gas decomposition mechanism needs theoretical and experimental analysis urgently. The research content of this paper has certain theoretical and practical guiding value for the optimization design of large insulation structures under lightning impulse voltage.