Development of the p-factor in an accelerated ageing experiment of the MV PILC cables

Insulation materials deteriorate with time due to numerous operational and environmental influences. Owing to thermal and electrical ageing, moistening, etc., several chemical and physical deterioration processes in the insulation materials arise, which result in the decrease of the electrical strength and influence dielectric processes and characteristics like conduction processes and the polarization of the insulation. By using the return voltage method (RVM) it is possible to determine probable decrease in electrical strength without damaging. This method has been shown as very appropriate particularly in the case of cables with impregnated paper insulation. MV PILC (medium voltage paper insulated lead covered) cables and its remaining time of life are in the focus of an international research project for which purpose an automated ageing system called ICAAS (Integrated Cable Accelerated Ageing System) was developed. The RVM method was applied on specially selected samples of the entire test field with an average operation age of 20, 40 and 60 years. All Cables in the test field will be artificially and continuously aged in the ICAAS for around two years. In this article the results of the initial return voltage measurements are presented and discussed. The measurements were repeated in defined time intervals, and the influences of the ambient temperature and the air humidity were monitored. The return voltage curves under different conditions and corresponding p-factors were determined and shown.