Study and analysis of conduction mechanisms and space charge accumulation phenomena under high applied DC electric field in XLPE for HVDC cable application

The development of High Voltage Direct Current (HVDC) cables requires design according to specific criteria and materials with appropriate properties. Cross-linked polyethylene (XLPE) has established itself over the past 20 years as the most used insulation material for HVAC cables, but also more recently for HVDC cables. If the electrical properties of this polymer have been widely studied under AC stress, the behavior of these materials under high DC stress is less known and needs thorough investigation. It is well known that, in DC conditions, the electric field distribution is highly dependent on operating conditions (thermal gradient and electric field) and can be affected by electric charges trapped in the insulation. The resulting space charge accumulation is able to increase significantly the local electric field, thus accelerating ageing and increasing the risk of breakdown. Consequently, the influence of electrical and thermal stresses on the material properties could be a key parameter on the ageing law for HVDC insulating material. The purpose of the present work is to investigate the dielectric behavior of XLPE insulation under different combined thermal and DC electrical stresses. The first step is to evaluate the volume electrical conduction and interface injection mechanisms by using current versus voltage (I-V) measurements. The second step is to investigate the development of space charges by using a non-destructive space charge measurement technique (the Thermal Step Method).