Physicochemical origin of space charge dynamics for aged XLPE cable insulation

The occurrence of ageing of cross-linked polyethylene (XLPE) cable insulation greatly affects the safe operation of cable system. The influence of different ageing modes on the space charge dynamics and physicochemical properties of insulating materials in highvoltage alternating current (HVAC) XLPE cables was investigated. The XLPE specimens were obtained from un-aged cables, 1-year laboratory accelerated aged cables, and 22- year practical cables. Pulsed Electro-Acoustic (PEA) technique was used to measure the space charge dynamics and accumulation properties in the aged XLPE insulation. Differential Scanning Calorimetry (DSC) and Fourier-Transform Infrared (FTIR) were used to reveal the details of their physicochemical properties. It was found that the homocharge accumulated at the aluminum (Al) electrode for the un-aged cables, while the hetero-charge for aged cable insulations. The total amount of homo-charge near anode decreases with temperature for un-aged specimens, while the amount of hetero-charge increases with temperature for aged specimens. It was suggested that, the homo-charge results from charge injection of electrodes, and the hetero-charge appear to be intimately related with the ionization of ageing products in XLPE insulation. Additionally, the total charge of un-aged cables and 22-year practical cables increased along the radial direction from inner to outer, whereas the total charge of 1-year accelerated aged cables decreased. It was suggested that accelerated ageing of cables starts from the inside of the insulating materials, while ageing of 22-year practical cables starts from the outside. It was also found that two new absorption peaks appeared in FTIR spectrums of aged specimens around 1720 cm-1 and 3500 cm-1, and the carbonyl groups increase with ageing time. The crystallization morphology, impurity particles ionization, injection and transportation of charges show great effect on the space charge dynamics.