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
