Sensor response characteristics for UHF location of PD sources

Ultra-high frequency (UHF) systems are well established for monitoring partial discharges (PD) in gas-insulated substations (GIS) and are increasingly being used on power transformers. Different sensor designs such as disk sensors, spiral sensors, monopoles and dipoles have been used depending on the application. A primary concern is to ensure that the sensitivity of these different sensors can meet the CIGRE requirement for detecting 5 pC partial discharges GIS. Recent work on locating PD in power transformers has focused attention on recording the arrival time of the received UHF signal, which is important for locating insulation defects in three dimensions. An accuracy of 1 ns or better is desirable for time-of-flight measurements to locate a PD source. In some situations, a clear arrival time for the signal can be identified but in other cases the arrival time can be uncertain within a tolerance of several nanoseconds. Therefore, a number of sensors were tested for their speed of response using a transient calibration system. Results suggest that more complex sensor structures have a slower build-up of energy at their output, while simple sensors exhibit less of a dasiablurringpsila effect on the leading edge of the signal.