On-site experiences with multi-terminal IEC PD measurements, UHF PD measurements and acoustic PD localisation

The paper presents the experience using a combination of different partial discharge (PD) measurement methods both to detect PD in power transformers and to improve interpretation and localisation of their sources. The multi-terminal PD measurement is illustrated here using STAR diagrams for discrimination between external noise clusters and multiple internal PD sources. Several different PD sources in different phase windings during an off-line measurement on a power transformer were detected and the UHF method confirmed these results and conclusion. The UHF PD measurement method is usable as stand-alone measurement and as a supporting measurement for offand on-line PD detection. Fundamental knowledge of the PD phenomena is needed for interpretation of measuring results, comparable to knowledge of the lower frequency IEC 60270 measurements. UHF sensors are easily installable and useable. The sensitivity of UHF PD measurements is sufficient and normally is not affected by external disturbances. Especially in noisy surrounding it might be a very helpful method to support other PD measurement techniques for example dissolved gas analysis and acoustic localisation of PD. UHF oil filling valves are often electro magnetically shielded from the internal tank by tubes for directing the oil flow. But sensitive UHF measurements are still possible, and additionally broadband amplifiers can be used to improve detection sensitivity to UHF signals. Time of flights measured in the UHF range can be used for geometrical PD localisation. The accuracy seems to be adequate to determine the phase where the PD is located. Additionally, different measurable UHF amplitudes support an estimation of the PD location. However, since transformers rarely offer more than three UHF oil valves, an additional acoustic measurement method is usually required for localisation. Using the knowledge gained from the UHF sensors, acoustic sensors can be placed near to the PD source at the transformer tank. During an on-line measurement a PD source localisation was performed with the help of measured UHF time of flights and acoustic time of flights.