Implications of on-line PD measurement on phase distributions for three-phase belted MV cables

Partial Discharge (PD) activity during on-line cable testing is due to the electric field stress related to the nominal voltages applied on the phase conductors. For a three-phase belted cable the corresponding field in the dielectrics is elliptical. This paper describes the impact of the elliptical nature of the electric field on PD patterns. The excitation of induced charges upon a PD in the phase-conductors and earth screen depend on the direction of the PD, and thereby on the phase angles of the applied voltages. Further, the signal propagation modes together with the signal decoupling at the cable end determine the detected wave-forms and their distribution. The consequences of these aspects are studied for a small spherical void serving as a simple model system. PD distributions are simulated assuming the field in the void completely vanishes by the charge redistribution upon a PD. The induced charges are calculated using Ramo Shockley considerations. The results show that for an elliptical field the patterns obtained from a sensor at one conductor (e.g. earth screen) can differ strongly from the linear field case, e.g. in case of off-line tests with only one phase energized.