Charge trapping characterisation and electrical performance of thermally aged polymeric cable insulation

Abstract

With the growing interest in high voltage direct current (HVDC) transmission systems, the impact of space charge on insulation has attracted more attention recently and the traps in the materials are considered to be one of the dominant factors that affects the electrical performance of the insulation. Meanwhile, in past decades, significant effort has been made to establish the relationship between ageing and the lifetime of electrical insulating materials so that the degree of ageing of insulation can be used to predict its lifetime. Generally, the chemical structure of dielectrics could be affected by ageing, and further electrical performance such as breakdown strength, dielectric loss, and conductivity will change. However, we still cannot use a simple and clear model with few characteristics to connect ageing with chemical and electrical changes to predict insulation lifetime. So, in my research, space charge behaviour is used to explain the ageing process in a HVDC system and the trapping parameters are the key to linking ageing with other electrical properties. In this case, how to use space charge dynamics to estimate the trap parameters, the impact of ageing on trapping, and the relation between trapping and electrical performance are tough problems. The relationship between ageing and trapping parameters will be explored. Hence, the initial trapping parameters and breakdown strength could be used to infer breakdown behaviour after ageing, which means trapping parameters can be used to assess the degree of ageing of the insulation in a HVDC system and predict dielectrics lifetime. In this thesis, the structure, morphology, and impact on the properties of polyethylene (PE) will be introduced and the thermal ageing mechanisms will be explored. In order to study the innate character of space charge, the mechanism of charge injection and transport in dielectric materials will be presented. Moreover, previous research on space charge behaviours in polymeric materials is reviewed for clear research direction. The relative space charge and trapping parameters model will also be introduced. The experimental work focuses on the low‐density PE (LDPE) films with different degrees of thermal ageing, its impact on charge trap density, and changes in electrical breakdown strength. The samples were aged in two environments, nitrogen and air, at various temperatures over different lengths of time. The degree of ageing of the samples was characterised using Fourier‐transform infrared (FTIR), and differential scanning calorimetry (DSC) was used to investigate the impact of thermal ageing on the internal structure and morphology of PE. Meanwhile, the Raman technique was used to detect the degree of thermal ageing in different layers inside of the films. Space charge dynamics for ageing LDPE were measured using the pulsed electroacoustic (PEA) technique. In addition, the electrical breakdown strength of the aged samples was measured, and breakdown data was processed using the Weibull distribution. DC electrical conductivity was measured to verify the hypothesis about the substantial growth of conductivity in seriously aged LDPE and is the reason a rapid charge decay rate was observed in these samples. The results of a chemical analysis show that thermal ageing in the air leads to a gradual increase in the thermo‐oxidative‐degree of LDPE. However, the crystallinity of aged films cannot be impacted by the degree of oxidation but the ageing temperature. Nitrogen‐aged LDPE, with a notably lower carbonyl index, shows similar trends in DSC tests with air‐aged specimens. As for space charge dynamics, traps are introduced into the samples during thermal ageing in the fan oven and deep traps dominate the process. At the early stage of the ageing process, a small number of deep traps in the vicinity of the electrodes can suppress further charge injection, which results in the increase in breakdown strength and the reduction of DC conductivity. During the sustained rise of the degree of ageing, more deep and shallow traps are introduced into the sample, thus higher DC conductivity and lower breakdown strength are detected due to the large amount of charge injection. The space charge behaviours of nitrogen‐aged LDPE suggest that deep traps are mainly introduced by oxidation and shallow traps are generated by the thermal effect. Trapping parameters (injection barrier, trap density, and trap energy level) are evaluated using an improved model. The injection barrier is enhanced by thermal ageing due to surface oxidation. Thermal ageing can introduce traps into LDPE, but the test method for deep ageing samples needs to be modified. The traps introduced by thermal ageing have a higher trap energy level

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