Detection of bushing insulation defects by diagnostic monitoring

Comparative diagnostic tests were performed in the laboratory on oil-impregnated paper and resin-bonded paper bushings. The bushings have undergone long-term AC tests and insulation capacitance, dissipation factor and partial discharge were continuously monitored. A 110 kV oil impregnated paper bushing with artificially induced defects was tested. The effect of insulation contamination and oil leakage on the bushings' insulation condition were investigated with on-line monitoring. With concern to the resin bounded paper bushing, a new 52 kV unit with design flaws was tested. After the tests, the bushing was opened and the capacitive layers were inspected individually. The results are presented in the paper. A correlation between the monitored values and the bushing condition can be done based on the performed measurements. This information would support the field engineers to set realistic threshold limits of the monitored parameters and for asset managers to establish appropriate condition-based maintenance strategies.