Aging Phenomenon of Composite Insulators Based on Chemical Traps’ Change of Silicone Rubber Molecular Chain

Some strong polar groups such as vinyl, carbonyl and hydroxyl can be produced or elevated after the composite insulators’ aging. These groups may further aggravate insulators’ aging and cause insulation accidents in the field. So, this paper discusses the effects of different chemical groups on the trap characteristics of aged silicone rubber. The polydimethylsiloxane (PDMS) chain was used as the matrix. The defect models containing carbonyl group, hydroxyl group and vinyl group were constructed with molecular simulation technology. Then, the stable models were obtained after the dynamic simulation to the defect models. Firstly, based on the density functional theory, the microscopic structure parameters of stable defect models were calculated with first principles. The changes of bond length, bond angle, atomic layout and dipole moment under the action of electric field were obtained. Also, the corresponding trap energy levels and density distribution of states were derived. The internal relationships between the micro-structure of defect models and the trap characteristics were analyzed. The trap characteristics variety of defect models with the electric field was explored from three aspects. They are the trap depths, the variety rate of trap depth and density with the electric field and the trap types. The results show that the bond lengths and bond angles of PDMS containing vinyl and carbonyl groups decrease with the electric field. The PDMS containing carbonyl groups exhibits the greatest changes. This results in a greater degree of molecular chain curling and contraction, providing favorable conditions for charging accumulation. However, the PDMS containing hydroxyl groups presents the opposite trends. This is not conducive to the accumulation of charges and the formation of deeper traps. On the other hand, the PDMS containing carbonyl groups has the maximum molecular dipole moment and the smallest molecular energy gap. This brings stronger molecular chemical activity. So the charges were easy to be captured and the traps are deeper. Therefore, the trap depth of PDMS containing carbonyl groups is the largest. The variation of trap depths with the electric field indicates the trap depth of the defect models all increases with the electric field. The change rates in descending order was carbonyl, vinyl, and hydroxyl. A possible explanation is that the molecular activity is more related to its corresponding energy gap. The PDMS containing carbonyl groups has a stronger ability to capture charges under the electric field, and the corresponding trap depth changes is the most. As for the trap density, the results show it is linearly corresponding to the trend of trap depth variation. The highest energy level corresponds to the highest density, which is consistent with relevant experimental results. The trap types could transform with the electric field. However, the three defect models take different change trends. The electron and hole traps of PDMS containing hydroxyl groups do not change significantly, which were all shallow traps. The hole traps appeared in the structure of PDMS containing vinyl group. And the number of electron traps was much greater than that of hole traps. The electron traps of PDMS containing carbonyl groups dissipated with the electric field. The following conclusions can be drawn from the simulation analysis: (1) The trap deepening of PDMS containing carbonyl groups’ may mainly be attributed to the hole traps. (2) The dissipation and recombination of charges in electron traps are important factors causing aging and surface discharge. (3) The role of PDMS containing carbonyl groups in the aging process of silicone rubber cannot be ignored. It is expected to be a new reference standard for silicone rubber aging evaluation. © 2024 China Machine Press. All rights reserved.