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 ageing and – eventually – failure of the insulation. Diagnostics of their conditions – often performed by means of PD measurement – can be tricky, as PD activity depend on many parameters.

In this work, a castresin 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 to measure the current bled to ground by the transformer shield, whose results are compared to the apparent charge measured by an IEC 60270-compliant commercial system installed on the MV winding.

The effect of temperature and ageing (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 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, 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 extent (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 ageing rate of different defects.