Evaluation of different types of sensors and their positioning for on-line PD detection and localisation in distribution cables

Different types of sensors can be used for on-line detection and localisation of PDs in medium voltage cables. These sensors can be placed on different locations in the substa-tions where the cable under test is terminated. Both aspects have a significant influence on the measured signals. In this paper both capacitive and inductive sensors are briefly evaluated, especially concerning their applicability for on-line measurements. Further-more, when the sensors are applied on-line, it is essential to realize which signals are being measured. Especially when three-phase belted cables are being measured, the situation differs considerably from off-line situations. When the cable is on-line, i.e. powered by the network itself, all three the phases are energized simultaneously. This means the PD pulses are propagating through two distinctive propagation modes: the Phase-to-Phase (PP) mode and the Shield-to-Phase (SP) mode. In this paper the differ-ent possible locations in a substation for placing sensors are discussed, with respect to the measured propagation modes, signal- and interference sensitivity, safety and practi-cal applicability. Field measurements confirm the findings