Electric Field Distortion and Insulation Response Characteristics of Typical Defects in 35 kV Oil-Impregnated Paper Bushings

Oil-impregnated paper (OIP) capacitive bushings are critical insulation components of power transformers, and internal defects may induce partial discharge (PD) and further lead to insulation breakdown, threatening equipment safety and power supply reliability. This study investigates the electric field distribution and insulation response characteristics of four typical defects in a 35 kV OIP bushing, including serrated cuts at the edge of the capacitor foil, metallic tips between capacitor screens, internal creepage along the lower porcelain insulator, and poor grounding of the end screen. First, full-scale three-dimensional models of normal and defective bushings are established to analyse the effects of different defects on electric field distribution and dielectric response parameters. Corresponding defective specimens are then fabricated, and dielectric parameter measurements and partial discharge tests are performed to verify the simulation results. The simulation results indicate that the maximum electric field strengths at the defect locations of the serrated foil, metallic tip, and internal creepage defects reach 8.92 kV/mm, 2.82 kV/mm, and 12.19 kV/mm, respectively, while their influences on dielectric response parameters are limited. In contrast, the loss of end-screen grounding increases the end-screen potential to approximately 26 kV and causes significant variations in dielectric parameters. The experimental results are highly consistent with the simulation analysis. This study reveals the electric field distortion mechanisms and response differences associated with typical bushing defects, providing theoretical support for defect mechanism analysis and insulation condition assessment.