The role and measurement of DC conductivity for HVDC cable insulation materials

This paper discusses the importance of DC conductivity for insulation materials to be used in extruded HVDC cables. It is one of the key electrical properties, especially for cables operating at very high voltage and stress levels when heating from dielectric losses generated by the leakage current might potentially lead to thermal runaway. Therefore, in material development and quality control it is vital to ensure that the DC conductivity measurements are carried out in an accurate and reproducible way. DC conductivity measurements have been done on XLPE pressmoulded plaques under different temperature and electric field conditions. The test set-up has been designed to limit the diffusion of peroxide decomposition products during the 23 hours of measurement time that are needed to reach a quasi-steadystate. This includes the use of thick plaques, in this case with a thickness of 1 mm, as such samples are expected to retain the decomposition products better than the significantly thinner plaques/films that are commonly discussed in the scientific literature. Full-sized crosslinked HVDC cables will always contain peroxide decomposition products giving an increase of the DC conductivity. For this reason the most relevant results from plaque measurements will be obtained from samples containing a controlled level of peroxide decomposition products. The conclusion is that care needs to be taken when the temperature and stress dependence parameters are used as a cable construction design tool and it is important that the data used are generated on samples as representative as possible, for example with a controlled level of peroxide decomposition products.