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The latest UHF PD detectors are small, easy to carry devices that can operate either from internal batteries or from a mains supply
The most sensitive method of detecting UHF PD is with a permanently installed sensor, although this approach requires preplanning and does not therefore lend itself to ad-hoc PD surveys. A typical sensor will be UV resistant and will have an IP67 ingress protection rating, as well as a wide operating temperature range, making it suitable for use indoors or outdoors. It will be maintenance free, and will comply with IEC 60229. Sensors of this type are readily available for use on systems up to 500 kV. In practice, UHF PD measurements are best carried out in three steps: spectrum analysis, to determine the frequencies of interest for further measurement; time domain measurement for PRPD pattern recognition, to help distinguish between true PD and noise; and level measurement, to localise the PD source and to determine how stable the discharge is over time. The last step is only needed if the first two steps show that PD activity is present. The results obtained will, of course, vary according to the application and the type of defect – if any – that’s present. In one recent case, PD activity was detected but was producing emissions with frequency content only up to around 300 MHz. PRPD pattern analysis, with the detector synchronised to power frequency, showed that maximum PD activity coincided with the voltage peaks. These results are indicative of corona discharge and this diagnosis was ultimately confirmed.
In another instance, the spectral distribution of the PD emissions covered a wide frequency range and extended into the gigahertz region. PRPD pattern analysis showed that in this case, the maximum PD activity coincided not with the peak voltage, but with the maximum rate of change of voltage. These results are consistent with an internal fault, and this was confirmed when a small void was found in the insulation of the test specimen. The latest UHF PD detectors are small, easy to carry devices that can operate either from internal batteries or from a mains supply. They offer a range of power frequency synchronisation options via direct or wireless connections. The best types have two UHF input channels to facilitate comparisons of sensors and sources, and incorporate intuitive touchscreen operation. These detectors are easy and convenient to use, they promote safety as no connections to the equipment under test are needed, and they also eliminate the need for equipment to be taken out of service for testing. Finally, it’s worth noting UHF PD detectors represent only a very modest investment but for utility companies and other organisations that operate power networks, if such a detector averts even a single failure, it will undoubtedly deliver savings that are orders of magnitude greater than its cost.
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