Influence mechanism of electro-thermal coupling effect on inter-turn insulation damage of high-frequency transformer

Existing studies have demonstrated that high-frequency discharge, thermal effect, and charge transport mechanism are the primary causes of high-frequency insulation failure, yet it remains unclear which mechanism plays the dominant role. Therefore, this paper establishes an experimental and simulation platform for partial discharge (PD) in inter-turn insulation under high-frequency (HF) square-wave voltage to investigate the damage mechanism of inter-turn insulation induced by high-frequency electro-thermal coupling. Experimental results show that a transition region exists around 10 kHz. Below 10 kHz, the PD amplitude changes significantly while the temperature rise is relatively gentle, and the failure of HF transformer inter-turn insulation is mainly attributed to the coupling effect of HF PD and thermal effects. Above 10 kHz, the PD amplitude exhibits negligible variation whereas the temperature rises sharply. Notably, the inter-turn insulation breaks down rapidly with an extremely short time to breakdown at 30 kHz, indicating that high-frequency thermal effect becomes the dominant factor. The electric field acts in synergy with the thermal effect to accelerate the degradation of Nomex paper, thereby further promoting the thermal degradation and premature breakdown of the insulation. The findings of this study provide a solid theoretical basis for the life assessment and multi-physics modeling of HF transformer inter-turn insulation.