Partial discharges in power-cable joints: their propagation along a crossbonded circuit and methods for their detection
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A method of measuring the discharge activity on cable systems has been developed to assist in assessing and monitoring cable joints. Partial discharge activity in the cable joints is detected by measuring the currents produced in the links between bonding cables which are brought to the surface to achieve sheath transpositions. The high-frequency components of this current may be detected using current transformers around the links and a radio-frequency amplifier. The output of the latter may then be used to give an alarm indication of any significant rise in these components caused by discharges in the joints. To relate the measurements at a link position to the discharge activity in the cable joint an analytical model of the cable system has been developed to describe the propagation of pulses through a crossbonded-cable system. The results of this analysis have been compared with measurements made on two operational cable circuits, and its impact on the design of a monitoring system discussed. Instrumentation is being developed for installation on CEGB 275 and 400 kV cable circuits. Sensors and the detector would be fitted at each stop-joint bay with an alarm activated in a substation when the level exceeds a preset value.