Effect of Thermal Ageing on Space Charge Behavior of HVDC XLPE Materials

The aim of the paper is to present the influence of thermal ageing on the space charge characteristics, DC conductivities and trap energy levels of DC XLPE materials. A commercial compound for 320 kV HVDC XLPE cables was used for sample preparations. XLPE films were made by a heating press method in the laboratory. The samples were placed in a thermal oven and thermally aged at 130 □ up to 21 days. Space charges were measured by a pulsed electro-acoustic (PEA) method setup developed in the high voltage laboratory. The results showed that the reference samples had limited hetero charge accumulations near both electrodes in the samples. The homo-charge accumulation was appeared with an increase of the aged time. The electric field increased gradually with the appearance of the homo-charge accumulation in the samples. The conductivity of the sample aged for 21 day increased three orders of the magnitude from 30 °C to 60 °C. It was found that a turning point of the conductivity appeared for the aged sample at 40 °C, 50 °C and 60 °C, respectively. An increase of the trap energy density and a reduction of the trap level could facilitate the homo charge accumulation in the sample aged for 21 days. The bipolar homo-charge accumulation was elucidated as an essential characteristic in the thermally aged samples.