Study on Partial Discharge Characteristics under the Thermal-Electrical Coupling Effect of Epoxy-impregnated Paper Insulation in High-Voltage Dry-type Valve Side Bushing

As a critical component connecting the converter transformer and the converter valve, the valve-side dry-type bushing serves both electrical insulation and mechanical support functions. However, the harsh operating environment of the valve-side bushing often leads to insulation failures due to partial discharge under long-term electro-thermal coupling effects, resulting in significant economic losses. This study investigates the degradation process of the dry-type valve-side bushing under actual operating conditions. Experimental samples of epoxy resin impregnated paper composite insulation were prepared using embedded needle electrodes. A partial discharge testing platform was established, utilizing the pulse current method to measure partial discharge characteristics under electro-thermal coupling conditions. The study examines partial discharge characteristics of epoxy resin impregnated paper composite insulation samples under positive DC bias voltage with superimposed harmonics at different frequencies, within a temperature range of 80°C to 120°C. The impact of electro-thermal coupling on the insulation was analyzed in relation to electrical tree growth. Results indicate that increasing environmental temperature significantly reduces the partial discharge inception threshold, facilitating easier initiation and development of partial discharge, thereby accelerating sample degradation. Although high-frequency harmonic content is relatively low under actual operating conditions, partial discharge activity is more intense under the superimposed effect of high-frequency harmonics. This research provides theoretical insights for the optimized design and diagnostic evaluation of valve-side dry-type bushing insulation in converter transformers.

I. Introduction

In recent years, the valve-side dry-type bushing (hereafter referred to as the valve-side bushing) has been extensively utilized in power systems due to its oil-free operation, explosion-proof design, and high reliability [1]. Nevertheless, the operational environment for valve-side bushings is particularly severe. The primary insulation of these bushings is composed of epoxy resin impregnated paper (ERIP), which is exposed to operating voltages with high-order harmonic components. Due to the inadequate heat dissipation capabilities of solid insulation, increasing power transmission exacerbates heating issues, thereby becoming a critical bottleneck for the safe and stable operation of valve-side dry-type bushings [2]. Extended electro-thermal coupling effects frequently result in insulation failures due to partial discharges, leading to substantial direct and indirect economic losses.