Insulator leakage current monitoring: Challenges for high voltage direct current transmission lines

Power transmission using HVDC transmission technology has existed for several years. As part of the design specification for such schemes, transmission line and substation insulator selection is crucial to ensure good operational performance. Leakage current may occur on insulators due to contamination, ageing of insulation materials and aggressive weather conditions, amongst others. If not mitigated, these currents can lead to line faults which occur as flashover of the insulator and the degradation of the insulator material. Monitoring leakage current on alternating current transmission lines may be accomplished using current transformer devices. However, this cannot be used in direct current transmission lines due to static magnetic fields. Toughened glass insulators are generally preferred for HVDC schemes, but composite insulators are proving to be cost-effective alternatives. However, because of its relatively new application in HVDC schemes, little has been published relating to operational data on typical leakage current magnitudes, waveforms and consequential effects thereof. ESKOM currently operates and maintains the South African sections of the Cahora Bassa HVDC scheme. At present, the utility does not have an in-situ, non-intrusive solution for monitoring the leakage currents on this scheme. This paper presents an overview of leakage current and the options available for measuring it. In-situ and non-intrusive sensors have the benefit of being used for real time monitoring as well as for live line work.