AC creeping flashover characteristics of oil-paper interface caused by floating discharge under parallel component

Abstract

Floating discharge is a typical fault in oil-immersed power transformers. Test models used in previous research were primarily simple vertical components, mainly simulating the barrier area. However, in the actual operation of transformers, areas with parallel components (such as the angle ring region) are equally critical. While the creeping flashover characteristics of parallel components had been relatively well understood, studies on creeping flashover at the oil-paper interface caused by floating discharge under parallel components remain limited. To investigate the flashover characteristics and mechanisms, this study designed a test model featuring multi-ring electrodes and copper sheet floating electrodes. We conducted AC flashover tests on insulating pressboard subjected to floating discharge. The results show that, for the creepage distances of 60–80 mm, the flashover voltage increased linearly with the creepage distance, while the average breakdown field strength fluctuated slightly around 3.54 kV/mm. Statistical t tests (at the 0.05 significance level) were performed to rigorously confirm this linear trend. Furthermore, compared to slightly inhomogeneous electric fields without floating electrodes (which typically exhibit breakdown strengths around 11.5 kV/mm), the average breakdown field strength here dropped significantly to approximately 3.54 kV/mm. This comparison highlights the severe tangential and normal electric field distortion introduced by the floating electrode. Analysis of the distinct asymmetry in damage on both sides of the suspended electrode was characterised. Additionally, high-speed imaging recorded the flashover process, including streamer discharge, relative to the pressboard surface. This study clarified the entire flashover process, which includes the streamer discharge stage, the transition discharge stage, and the complete flashover stage. Through finite element modelling of the corresponding stages and analysis of the charge carrier generation mechanisms during streamer discharge, the intrinsic relationship between pressboard trace characteristics and the flashover process is elucidated. Overall, these findings contribute to a better understanding the development of floating discharge and creeping discharge processes under parallel components.