Ageing of an ADSS Cable Sheath on a 132kV Overhead Transmission Line

All-dielectric self-supporting (ADSS) optical cables are widely used in modern power utilities to provide communication infrastructure. These are able to provide a reliable high-bandwidth network because of the physical immunity of the embedded optic fibres from their electrical environment. However, the cable sheath and strength member are not immune to the electrical environment due to induced surface currents. These currents inevitably arise as a result of capacitive coupling to the phase conductors. These low level currents can degrade the cable sheath from a hydrophobic to a hydrophilic state, which increases the likelihood of surface electrical discharge phenomena such as corona or even dry-band arcing. In this paper, the sheath of cable which was installed for 15 years on a 132 kV transmission line, but has not failed, is analyzed. Models of the surface current experienced by the ADSS cable are used to reveal how the electrical environment influences the cable surface ageing. Characterization of the cable surface by visual inspection and by contact angle measurement along the cable span is reported. These are the first measurements of this sort and surprisingly, the predictions of leakage current magnitude correlate strongly with reductions in contact angle within a span. The measurements verify that long-term low surface current is one significant aspect attributing to the ageing of cable surface properties, even if the conditions are such that dry-band arcing does not occur.