High voltage transmission line design requires careful insulator selection to ensure good operational performance. This paper reports on the in-situ measurements of leakage current (LC) on composite and glass insulators of the Cahora Bassa high voltage direct current (HVDC) transmission line in South Africa over a 6month period. The influence of temperature, humidity, dew, rain and the HVDC line's voltage and current on LC are investigated. The results show that the composite and glass insulator LC behaviour is similar, except in cases of high humidity or rain. At the commencement of rainfall and humidity (>90%), elevated LC levels are observed on glass insulators, while composite insulators demonstrate lower LC levels under these conditions. Under nominal weather conditions of no rain and low humidity, the LC measurements exhibit an almost square-wave behaviour with LC switching between lower (≈20 µA) and higher values (≈60 µA) with relatively short transitions on a daily basis. This phenomenon can be ascribed to condensation on the insulators, which is a primary determinant of the LC levels on contaminated insulators. The line current and voltage fluctuations do not influence the LC level.

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.

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