Dielectric response of aged transformer bushings utilizing power system transients

The reliability of power transformer bushings is of great importance. Failures may lead to severe consequences. Frequent diagnostics by off-line methods is not feasible as the interruption is costly. Therefore it is of interest to have the possibility to perform on-line diagnostics. Transformer bushings can become aged during their operation periods. Several phenomena such as partial discharges, thermal effects and humidity may change the strength of the insulation in power transformer bushings. There are several methods which provide diagnostics of such components, but some of them can only be done offline in maintenance periods and some online methods would lack useful information about the insulation characteristics. In this paper, a new method is investigated, which uses natural transients such as switching and lightning impulses as stimuli to get the on-line dielectric response of the insulation system. Also, laboratory investigations are reported on three in-service aged 132 kV power-transformer bushings. The dielectric response measured under excitation by transient voltage is compared with results from low-voltage dielectric spectroscopy.