Comparative investigations using UHF and conventional PD-measuring equipment for the detection of artificial failures in a 110-kV-XLPE-cable termination

The assessment of the correct installation of 110-kV-Cable accessories is becoming more and more important as the number and length of 110 kV-cable installations is growing significantly. Therefore sound on-site commissioning tests, applying a withstand voltage test in combination with partial discharge (PD)-measurement are very useful and therefore often required by the Utilities. Especially in case of critical terminations, where standard PD measurement shows suspicious behavior, an additional PD detection via ultra-high frequency (UHF) sensors provide essential additional information. Also, UHF equipment can be used for online measurement and monitoring - without interruption of service power. Monitoring the long-term condition of the power cables provides grid operators with information on aging and possible defects at the critical system components. Consequently, it has to be proven, that the results of UHF test method is in principle comparable with the standard PD measurement according to IEC 60270. Therefore, this paper reports about comparative investigations of standard PD measurements and UHF PD detection applying 50Hz High Voltage to a 110kV-XLPE-Cable system with an artificial defect in one of the cable terminations. This artificial defect can represent a metal object on high voltage. It can cause field strength problems that can manifest themselves as sliding discharges or other electrical partial discharges like corona effects under oil. This paper compares the UHF PD detection methods, applicable for off- and online-measuring of PD on site with the standardized PD measurement according to IEC60270. It must be mentioned that the UHF detection is limited to the vicinity of the sensors - meaning approximately 1 m distance. Furthermore, previous investigations have shown that no shielding ground object should be between the sensor and the fault. The Ultra High Frequency Sensors (UHF) work in a frequency range higher than 300 MHz.