Partial Discharge Behavior of Oil-Paper Insulation in Converter Transformers Under Complex Electrical and Thermal Stresses

Converter transformers in ultra-high voltage direct current transmission systems face complex and variable operating conditions, posing significant challenges to their performance stability. As the core component of converter transformers, oil-paper insulation is susceptible to accelerated degradation under the combined effects of complex harmonics and thermal stress, leading to partial discharge. This study investigated the effects of power-frequency AC superimposed harmonics and thermal aging on the partial discharge characteristics of oil-paper insulation through a combined experimental and molecular simulation approach. The results demonstrated that increasing harmonic frequency and content significantly reduced the partial discharge inception voltage and enhanced discharge magnitude and frequency. Under 450 Hz and 30% harmonic conditions, the maximum discharge magnitude increased by 300%, and the discharge repetition rate rose by 220% compared to baseline values. Thermal aging was found to induce notable changes in discharge characteristics, including reduced PDIV and increased average discharge magnitude, maximum discharge magnitude, and discharge repetition rate. Molecular simulations revealed microstructural degradation during thermal aging, such as fiber fracture, interfacial debonding, and formation of oxidative byproducts. The thermal decomposition products were shown to collectively accelerate insulation failure through conductive ion generation and electric field distortion. These findings provide a theoretical foundation for structural optimization and condition assessment of oil-paper insulation materials, offering significant insights for enhancing the operational reliability of converter transformers.