Experience with UHF partial discharge detection and location in power transformers

Ultra-high frequency (UHF) methods for detecting and locating partial discharges (PD) are well established for gas insulated equipment. This paper provides an overview of their application to power transformers, discussing practicalities, capabilities and challenges. In recent years, UHF PD detection has proved helpful to manufacturers as a tool for enhancing quality control. The technique can now form a valuable addition to the suite of on-line monitoring technologies available for power transformers. A key aspect discussed in this paper is the installation of sensors, which must have an electromagnetic 'view' into the Faraday cage of the tank. Locating PD sources by time-of-flight methods in transformers requires accurate spatial resolution in three dimensions in the presence of large conducting components inside the tank that often block the line of sight between PD source and sensor. For this reason, instead of an 'empty box' representation, modeling techniques are used to account for the internal structure of the transformer so that observed time differences among PD signals received at different sensors can be interpreted more realistically. The PD location procedure is illustrated by means of practical results, which indicate that PD sources can be located typically to within 0.3 m in a power transformer.