Experimental comparison of UHF sensor types for PD location applications

In this work, transient response characteristics obtained from different types of UHF sensors were investigated by means of laboratory experiments. Three typical UHF sensors (i.e., disc-, monopole- and spiral-type) were tested using an artificial PD source freely moved around a tank that was used to represent a small power transformer. The aim was to identify the UHF sensor characteristics that are most effective for locating PD sources. A threshold-crossing method based on the signal power waveform was chosen to determine the arrival times of UHF signals because it has proved to be effective in many practical applications. Locating PD sources in the tank depends on time differences obtained from the arrival times. Our experiments compared the measured time differences between UHF signals with those expected based purely on geometrical calculations. Results suggest that disc-type would allow high ability for determining arrival times accurately, while the spiral-type illustrates a potential problem of slower energy build-up with broadband antennas that have a complex and electrically long structure. This experimental work highlights an important issue for UHF PD monitoring installations, especially for sensor design and selection.