Breakdown Characteristics and Damage Mechanism of Typical Defects in Oil-Paper Insulation Under HEMP

High-altitude electromagnetic pulses (HEMPs) can couple with power transmission and distribution lines, creating a nanosecond-level HEMP conductive environment that can degrade the insulation performance and even cause breakdown of distribution transformers, posing a threat to the safe operation of power systems. In this article, based on the main vulnerable parts identified in HEMP effect tests on distribution transformers, a typical defect model of oil-paper insulation was constructed, and an experimental platform was set up to study the breakdown characteristics of oil-paper insulation under HEMP. The process of charge accumulation was analyzed using finite element simulation, revealing the damage mechanism of oil-paper insulation under HEMP and other nanosecond pulses. The results show that the breakdown characteristics of oil-paper insulation under HEMP have a significant polarity effect, which is related to the accumulation of charges at the interface of oil-paper insulation. The accumulation of charges at the interface of oil-paper insulation is comparable to the nanosecond rise time of the HEMP waveform, resulting in a certain balance relationship in the location of breakdown points. Under negative-polarity HEMP, needle-plate defect structures have a higher proportion of breakdown at the trailing edge of the waveform, indicating that the energy provided within the rise time of the HEMP waveform is slightly insufficient.