Assessment of the Dielectric Type Test Currently Applied on Mass-Impregnated Non-Draining HVDC Cables

The technology and the operational patterns of HVDC links are changing, and therefore, the type test requirements for mass-impregnated non-draining HVDC cables are currently being reconsidered. A CIGRÉ working group is preparing a revised type test recommendation. Measurements and numerical modelling (without any connection to the CIGRÉ effort) of the of internal pressure, temperature and electric 􀂉eld distributions in mass-impregnated non-draining cables subjected to different environmental and loading conditions have recently been carried out. With basis in this work, it is discussed and assessed how well the loading cycle and polarity reversal test duties applied in the existing type test re􀂊ect the stresses such cables may experience in service. In particular, the potential risk associated with formation of shrinkage cavities after load reduction is considered. Most of the provisions in the existing type test are found to be appropriate and reasonable, but a few possible improvements are proposed. Including a preconditioning procedure prior to the load cycling duties could make the type test more realistic as it would also include the contributions to the internal pressure from slow deformations of the lead and polyethylene sheaths. Moreover, introducing load cycling duties with currents beyond the rated load current and with shorter loading times (e.g., 0.5, 1 or 2 h) and at an intermediate ambient temperature (e.g., 20–30 °C) as a part of the dielectric tests could qualify a mass-impregnated non-draining cable for temporary overloading.