High-Voltage Time-Domain Dielectric Response Characteristics Analysis of 72.5 kV Epoxy-Impregnated Dry-Type Bushings Under Different Moisture Contents

Epoxy-impregnated dry-type bushings are widely employed in high-voltage equipment, such as transformers, due to their excellent electric field regulation capability, oil-free design, and ease of maintenance. During actual operation, the crepe paper in the core of the bushing is susceptible to moisture absorption and dampness during production and service life, leading to a degradation of its insulation performance. Therefore, it is of critical importance to diagnose the moisture condition of dry-type bushings in a rapid, accurate, and nondestructive manner. In this study, three dry-type bushings with varying moisture contents were prepared, and their polarization and depolarization current (PDC) characteristics un-der different voltage levels were measured. The results indicate that the PDC characteristics of the bushing with varying moisture contents exhibit significant differences. Under the same voltage, as the moisture content increases, both the polarization current and depolarization current of the bushing increase markedly. Furthermore, the bushing with high moisture content demonstrates electrical conductivity comparable to that of an electrolyte, with the steady-state polarization current reaching up to $1.35 \times 10^{-6} \mathrm{~A}$ at the maximum. This study provides an experimental basis for assessing the moisture condition of epoxy-impregnated dry-type bushings.