Identification of partial discharge defects in transformer oil

Partial discharges in (power) transformers are often a predecessor of a serious fault. For this reason partial discharge measurements are an important diagnostic tool to monitor the insulation condition of a transformer. For on-line measurements, a detection method based on the high-frequency (UHF) signals emitted by the discharging source is used and can be detected. Moreover, the UHF technique offers the possibility to locate and identify the PD source. To investigate the ability to accurately identify and discriminate between different sources, a research with modeled defects has been recently started. Up to now, three different partial discharge sources using three different defect models were tested in transformer oil: HV corona, floating part and surface discharge. Simultaneous measurements on these defects have been performed using 1) an detection circuit based on the IEC 60270 recommendations and 2) wide-band UHF system. With the IEC 60270 based measuring system the PD magnitude and phase resolved PD pattern is acquired. With the UHF system measurements are performed in frequency domain and in time domain. By using the spectrum analyzer as a band pass filter, a phase resolved PD pattern can be obtained as well. The results of the two measurement systems are compared with each other and conclusions are drawn.