Use of capacitive couplers for partial discharge measurements in power cables and joints

Partial discharge (PD) detection and measurement is one of the main nondestructive techniques in high voltage cable systems to verify the quality of the manufactured bulk insulation product and to check the integrity of installed cable accessories. To have confidence in the installation and to ensure system reliability, more sensitive on-site testing and on-line pd monitoring is required particularly for transmission class cables. Over the last ten years significant developments have been made and several techniques, including capacitive, dual capacitive, directional coupler, sheath interruption and inductive methods, have been proposed and partially put into practice Due to advances in material processing, manufacturing and factory test procedures, it is unlikely that the cable itself will normally exhibit pd activity. It is the cable accessories (the, joints and terminations) that are the most likely source of PD. Placing a capacitive coupler inside or close to the cable accessory is an attractive PD monitoring option However, a, full understanding of the technique is essential in order to maximise its potential application. This paper reports on some aspects of the theoretical characteristics of the capacitive coupler. Based on an equivalent circuit model, simulations to investigate the influence of different parameters on the performance of the coupler have been undertaken. Experimental results show good agreement with theoretical predictions.