Influence of thermal ageing on high-field polarisation characteristics and conductivity behaviour of submarine polymeric cables insulation

Diagnosing insulation intrinsic ageing is essential for preventing insulation failure of submarine cables, and in case of failure, helping in any decision about maintenance actions. In this study, a novel aged cable diagnosis approach based on high-voltage frequency domain spectroscopy (HV-FDS) with broader frequency width is proposed to analyse the high-field polarisation characteristics and conductivity behaviour of 500 kV submarine polymeric cables insulation, aiming to support the evaluation of overall insulation ageing. The measurement principle of HV-FDS and relaxation mechanism are stated, and then focus is on studying the extent of change of polarisation and conduction properties of submarine cables insulation as a function of ageing stress and time. Moreover, the high-field polarisation characteristics and conductivity behaviour are discussed in-depth based on the carrier transport and trap properties. Finally, the novel diagnostic indicator based on the high-field characteristics is extracted to quantify the thermal ageing state of submarine cables insulation. Results show that the high-field polarisation characteristics and conductivity behaviour seem to become sensitive diagnostic quantities if measured at reasonably high voltage, which are able to reveal mechanisms associated with thermo-oxidative ageing as that generated in the testing procedure considered in this paper. Sensitivity is considerably higher than that experienced in typical dielectric spectroscopy approaches.