UHF Sensors for Partial Discharge Diagnostics of Power Transformers

Abstract

Different diagnostic techniques are currently available for on-line insulation condition assessment of power transformers. Amongst these techniques, the UHF technique has been recently presented in the field of transformer diagnostics. This paper describes results from experimental investigations which have been carried out to study the partial discharge phenomena in power transformer insulation. The UHF technique has been applied to and investigated on two new identical 90 MVA, 150 kV power transformers. Both transformers are equipped with four external UHF sensors. Installation of external UHF sensors requires dielectric windows on the transformer tank for the placement of the sensors. On older transformers that have a higher risk of failure these windows might be added by replacing manhole covers, but that would require draining the oil. Therefore, a sensor which can be inserted in a transformer from one of its valves, without draining the oil, is studied as well. Both UHF sensor-techniques were compared during an induced voltage test on a 75 MVA, 50 kV shunt reactor. The practical experiences related in this paper provide further insight into the application of UHF PD monitoring techniques to power transformers.