Discrimination between internal PD and other pulses using directional coupling sensors on HV cable systems

On-site partial discharge (PD) measurement is required to ensure proper installation of extra high voltage (EHV) cable systems accessories. To achieve high sensitivity and good localization, two problems have to be overcome. First, the strong high frequency in long XLPE cables requires that the sensors be located along the cable, preferably directly at the accessories. Secondly, the detection system must be able to distinguish internal PD from other pulses. This paper describes a solution based on directional coupling sensors and a data visualization system, which displays phase-amplitude diagrams for individual PD sources which are identified by the direction of pulse propagation. It has been applied to on-site measurements, type and routine testing of HV cable joints and stress cones. Due to the reliable discrimination between internal PD from the accessory measured and from other pulses, testing can be done in unshielded rooms even using terminations with internal PD and corona. The method works independently well on line voltage, resonance sources, oscillating voltages and 0.1 Hz cosine-square voltage. It has been used to verify the cable accessories installed in the 6.3 km long 380 kV cable system in Berlin, Germany.