Identification and localization of PD-sources in power-transformers and power-generators

Partial discharge (PD) measurements are in use since more than 50 years with the aim to assess the condition of the insulation systems of high voltage (HV) apparatus and components. In large power transformers and power generators, PD-sources can be situated well hidden inside complex insulation systems. For PD-sources inside the active parts of such systems, measured values of the apparent charge in pC/nC do not sufficiently reflect the real risk of these defects. This is mainly due to the strong attenuation and deterioration of the PD-signals in such insulation systems. Therefore, continuous PD-activity detected at or below nominal voltage during a factory acceptance test, or recorded on-site by a monitoring system, may be harmful for a HV apparatus in service. In case of confirmed internal PD-activity, further investigation and localization should follow. Beside the analysis of simultaneously recorded PD-patterns at all terminals, the evaluation of PD-signals in the time-domain and frequency-domain offers a considerable potential to localize internal PD-sources in extended insulation systems. PD-signals detected at the terminals of a transformer or generator contain information about their propagation paths from the unknown location of the PD-source to the specific sensor. This contribution describes both, the advanced PD-measuring techniques and procedures for identification and localization of PD-sources. The advantages and limits of PD-signal analysis in the time- and frequency-domain are discussed using practical examples.