Study on Insulation Characteristics of Oil Impregnated Paper Bushings under Long-term Electrothermal Combined Test

Oil Impregnated Paper (OIP) bushing is widely used in power transformers, and its performance can directly affect the safety of transformer. In order to study the defect development process and the rule of insulation degradation of OIP bushing, some typical defects such as bubbles in oil, edge tip on plate, plate deviation, moisture intrusion and diffusion are simulated in 72.5kV OIP bushing samples. The samples are installed in tanks filled with 60°C oil and energed by voltage and current simultaneously for a long-term in a combined test platform. Insulation resistance(R), the dielectric loss factor (tanδ), capacitance(C), Frequency-domain Dielectric Spectroscopy (FDS) and Partial Discharge (PD) of these bushings are tested and analyzed. Test results indicated that the tan δ and capacitance are sensitive to the damp defects of the bushing. The process of moisture diffusion inside the bushing can be analyzed by tan δ-f curve and C-f curve. The frequency band in T-F diagram of partial discharge can be used to diagnose defects such as tips on the condenser plate and bubbles in oil. With the electro thermal test time increases, the defect of bushing will cause insulation degradation such as R decreasing and tanδ increasing. And the degradation rate of bushing with bubbles in oil is relatively gentle. The insulation performance of the bushing with edge tip on plate will stabilize for a certain period of time before the cumulative effect reaches to a certain level, and the degree of degradation increases step by step. To the plate deviation defect bushing, the early stabilization time is longer, but the degradation trend is similar to that of the edge tip on plate defect once degradation occurs. The results are useful to formulate more rational evaluation index and effective maintenance strategy for OIP bushings.