Dynamic ageing research of cross-linked polyethylene cable insulation based on temporal-spatial dimension under combined electro-thermal effect

Dynamic ageing process of cross-linked polyethylene (XLPE) cable insulation under combined electro-thermal effect is investigated by the construction of theoretical analysis model and the practice of operation-simulated ageing test. Firstly, theoretical analysis model is constructed from time and spatial dimensions. From the time dimension, in the light of the impeding trend of internal stress to external stress, the XLPE ageing process of Steady state -Transition state -Steady state (STS) is proposed to illustrate the dynamic ageing process of XLPE. From the spatial dimension, the insulation is divided into three scales to quantify the ageing. Secondly, to practice the theoretical analysis model, an operation-simulated ageing test was conducted on two high voltage alternating current cables with service years of 15 and 30 for 540 days. Subsequently, relevant diagnostic measurements were applied to analyse the influence of physicochemical characteristics on the dielectric and heat transfer performances of the cables. The results show that the degradation of the actual operating cable is an extremely slow process. On the one hand, the response in physical changes of morphology counterbalances the external stress, maintaining the stability. On the other hand, once the chemical changes occur, negative feedback mechanism tends to impede the occurrence of further degradation by regulating fields distribution.