Thermal ageing and its impact on charge trapping parameters in LDPE

With the riding use of HVDC transmission systems, the impact of space charge on insulation has attracted attention for several decades and the traps in the dielectrics are considered as the dominated factor of ageing. In our research, we aim to establish a model to connect ageing and lifetime for the insulation materials in HVDC systems, and the space charge characteristics are used to extract parameters associated with traps. In this paper, the trapping parameters of ageing samples estimated by an improved model will be described, and a comparative analysis based on experiments data and simulation results will be made. Commercially available additive-free LDPE films were thermally aged in fan oven with different ageing degrees. Space charge dynamics were measured using the pulsed electroacoustic (PEA) technique and further trapping parameters including injection barrier, the depth of deep and shallow traps and trap density were estimated. Both polarization and depolarization stages and the neutralization process were taken into considered in this model. It has been found that at the early stage of thermal ageing in the air, a small number of deep traps was introduced near the surface of LDPE films, which can hinder further charge injected. Further ageing will generate more shallow and deep traps, and deep traps dominate the impact on electrical properties.