The Dimensional Approach in the Design and Qualification Tests of AC and DC HV Cables: The Occhini Approach Revisited

The expression of the dimensional effect proposed by Enrico Occhini in the 70's is still commonly used to transform cable insulation breakdown and life data, obtained through laboratory accelerated testing on cable models to those relevant to full-size cables. However, the Occhini model does not take into account the dependence of dielectric strength on insulation thickness. Indeed, the balance between the insulation losses generated in the volume and the dissipated energy which occurs through cable insulation surface affect dielectric strength and this can have an impact in cable characterization and testing. This paper presents a modification of the statistical expression of dimensional effect, through the introduction of the dependence of dielectric strength on insulation thickness due to thermal instability. The modelling study carried out in the paper, and the preliminary validation of the revised model, show that the derived correction terms can influence both the design field and the life line estimation for cables having different insulation thickness and lengths. Eventually, it is highlighted how much and in which way the dimensional effect issues can impact on the results of prequalification and type tests made according to the present IEC and CIGRE specifications