Application of UHF diagnostics to detect PD during power transformer acceptance tests

Different diagnostic techniques are currently available for on-line insulation condition assessment of power transformers. Amongst these techniques a relatively new technique for detecting partial discharges (PD) in the Ultra High Frequency range of the spectrum is also introduced. The UHF technique has been recently presented in the field of transformer diagnostics, and although it is under development it shows great potential for on-line detection, recognition and location of PD activity. This paper describes the practical application of the UHF technique on two new identical 90 MVA, 150 kV power transformers. Both transformers are equipped with four external UHF sensors. Initially, injection measurements were performed to investigate the effects experienced by UHF signals when propagating through the transformer. Moreover, software developed to model the propagation of UHF signals inside the transformer tank was tested in its application to these specific transformers. The results obtained during the both the induced voltage test, as well as during the PD acceptance test in the factory are described. The practical experiences related in this paper provide further insight into the application of UHF PD monitoring techniques to power transformers.