Enhanced Surface Insulation Strength of Modified Polyimide Under High-Frequency Electric Stress by Fabricating the Al2 O3/PI Composite Coating

Polyimide (PI) is widely used in inter-turn insulation of the power electronic transformer (PET) due to its excellent high temperature and radiation resistance. With the development of PET to high voltage and large capacity, the traditional Kapton-type PI films under high-frequency electric stress do not make it a good fit to fulfill the insulation requirements of PET. Surface discharge is considered to be one of the main causes of gas-solid insulation failure in PET. The surface modification of PI films by the ion-exchange method is an effective solution. The purpose of this paper is to propose the fabrication of an Al2O3/PI composite coating with micron-level thickness to improve the corona resistance and surface insulation strength of modified PI (A/PI) films. The results show that the minimum corona resistance life of Kapton-type PI was only 32.68 minutes, while the maximum effective lifetime of modified A/PI films was 174.26 minutes, which was 5.3 times higher than that of the former. And the high-frequency flashover voltage is, at best, raised to 40.06% of the Kapton-type PI under 5 kHz. The enhanced mechanism can be explained from the perspective of the trap energy level. Finally, the surface charge density and surface electric field distribution of PI and A/PI at the gas-solid interface were calculated by the finite element method. The experimental results show that A/PI has better resistance to high-frequency electrical stress than pure PI. This research is expected to provide a reference for the protection design of high-frequency power electronic equipment inter-turn insulation surface discharge.