Experimental Analysis of Aging of a Dry Transformer: Comparison of Two Methods for PD Measurements

Dry-type transformers require little maintenance and are more environmentally friendly with respect to traditional oil-and-paper design. However, partial discharges (PDs) and thermal stress may lead to aging and—eventually—failure of the insulation. Diagnostics of their conditions—often performed by means of PD measurement—can be tricky, as PD activity depends on many parameters. In this work, a cast-resin power transformer is aged by means of 28 heating/cooling thermal cycles, to reproduce the thermal–electrical stress it may experience in operation. PD activity is monitored during the cooling branch of each cycle using a high-frequency current transformer (HFCT) to measure the current bled to the ground by the transformer shield, whose results are compared to the apparent charge measured by an IEC 60270-compliant commercial system installed on the medium-voltage (MV) winding. The effect of temperature and aging (number of cycles) is then investigated. The results show good agreement between the two measuring systems, in terms of both the phase-resolved partial discharge (PRPD) patterns and the average intensity trend over time. According to the intensity dependence on temperature, two PD series were identified: one series shows a loose dependence on temperature, and the other one a strong dependence. It was speculated that two types of defects may exist, whose geometry (and hence PD activity) may be affected by temperature to different extents (e.g., due to their position). Finally, a decrease in PD rate in time was observed, without a significant reduction of the average discharge intensity, which is supposedly due to different aging rates of different defects.