Impedance Analysis of an RMU for On-line PD Measurement

Abstract A new system to monitor partial discharges (PD) online in medium-voltage power cables is being developed. On-line measurement implies that the equipment can be applied when the cable is in operation, i.e. without disrupting service. A PD gives a high frequency pulse, propagating through the MV power cable. At the termination cable, the PD pulses partially reflect and partially propagate into the connected Ring Main Unit (RMU) due to the impedance transition. For correct interpretation of PD signals from the cable under test, it is essential to know the involved impedances up to several megahertz. An analysis of the impedances of the RMU elements is performed. A theoretical analysis based on lumped impedances is verified using various measurements. The measurements have been carried out on a test-grid consisting of two RMUs and several cables and show good agreement with predictions. This method of analysis will be used in the on-line PD monitoring system

Directional Sensing for Online PD Monitoring of MV Cables Citation for published version (APA): Directional Sensing for Online PD Monitoring of MV Cables

Abstract A new system to monitor partial discharges (PDs) online in MV power cables is being developed. For this system sensors are placed in the Ring-Main-Units (RMUs) at each end of the cable. In an RMU, the cable under test is not the only possible source of PDs. Also other MV cables and the MV/LV transformer may produce PDs. From the direction of each measured signal, it is known whether the PD comes from the cable under test or elsewhere. For the proposed solution, two current probes are installed at two locations in the RMU. The two measured currents are combined into one signal. PDs from other sources are removed from the resulting signal, while PDs from the cable under test remain unaffected. Measurements on a test set-up with two powered RMUs showed that this method works well