To achieve a preferable compatibility between liquid silicone rubber (LSR) and cable main insulation in a cable accessory, we developed SiC/LSR nanocomposites with a significantly higher conductivity nonlinearity than pure LSR, whilst representing a notable improvement in space charge characteristics. Space charge distributions in polarization/depolarization processes and surface potentials of SiC/LSR composites are analyzed to elucidate the percolation conductance and charge trapping mechanisms accounting for nonlinear conductivity and space charge suppression. It is verified that SiC/LSR composites with SiC content higher than 10 wt% represent an evident nonlinearity of electric conductivity as a function of the electric field strength. Space charge accumulations can be inhibited by filling SiC nanoparticles into LSR, as illustrated in both dielectric polarization and depolarization processes. Energy level and density of shallow traps increase significantly with SiC content, which accounts for expediting carrier hopping transport and surface charge decay. Finite-element multiphysics simulations demonstrate that nonlinear conductivity acquired by 20 wt% SiC/LSR nanocomposite could efficiently homogenize an electric field distributed in high-voltage direct current (HVDC) cable joints. Nonlinear conductivities and space charge characteristics of SiC/LSR composites discussed in this paper suggest a feasible modification strategy to improve insulation performances of direct current (DC) cable accessories.

In order to improve electrical matching between ethylene-propylene-diene misch-polymere (EPDM) reinforce insulation and crosslinked polyethylene (XLPE) main insulation in direct current (DC) cable accessories, the glyceryl monooleate (GMO) organic compound composed of several polar-groups and one long carbon chain is employed for chemical graft modification on EPDM to ameliorate DC electrical performances. Charge trap characteristics are analyzed by testing thermal stimulation current (TSC) and verified by calculating first-principles electronic properties to elucidate the GMO-graft-modified charge trapping mechanism accounting for DC electric conductance and dielectric breakdown. The grafted GMO molecules introduce substantial shallow charge traps that lead to nonlinear profiles of electric conduction versus electric field and cause hopping transports of percolation conductance. Electric conductance of EPDM is significantly improved by GMO graft for electrical matching with XLPE, while a high level of dielectric breakdown strength is retained sufficiently for reinforce insulation in cable accessories. Shallow charge traps introduced by GMO graft are capable of capturing charge carriers to form homocharge layers near electrodes which can scatter the transporting charge carriers and exclude further charge injections, thus to mitigate the dielectric breakdown strength reduction caused by electric conductivity improvement. Electric field finite-element simulations demonstrate that the electric field in DC cable terminals can be evidently homogenized by using GMO-grafted EPDM as reinforce insulation.

In order to improve electric-tree resistant performance and dielectric breakdown strength of ethylene-propylene-diene misch-polymere (EPDM) material used for cable accessory reinforce insulation, the two specific aromatic ketone compounds—vinylphenylacetone (VPE) and 4-propylene oxyxy-2-hydroxydibenzenone (AOHBP) are employed as two paradigms of voltage stabilizer for chemical-graft modifications. Electric-tree resistances and insulation performances of modified EPDM materials and their charge trapping mechanism of thermoelectron inhibitions are studied by the accelerated electric-tree aging experiments, alternating current (AC) dielectric breakdown tests, surface potential trap-level analyses and first-principles calculations. Both the two species of voltage stabilizers are effective for promoting electric-tree inception voltage and dielectric breakdown strength, leading to a high extension of electric-tree morphology and smaller dimension of electric-trees growth, in which AOHBP is more significant. The two species of voltage stabilizers have been successfully grafted onto EPDM molecular-chains in thermal-chemistry crosslinking reactions of EPDM, introducing multiple shallow levels of charge traps, which reduces the energy released by trapping charge carriers and thus alleviates electric-tree aging of EPDM. The AOHBP and VPE represent a high electron affinity and a small electronic energy gap, which is competent of assimilating the kinetic energies of hot charge carriers whilst restricting Auger electronic excitation. Especially, the benzene group in voltage stabilizer renders shallow level charge traps with a larger carrier capture cross-section than deep traps and simultaneously possesses the high atomic vibration frequencies similar as electronic-transition energies, which results in effective dissipation on the kinetic energies of hot charge carriers. This mechanism dominates to increase electric-tree resistance and insulation strength of EPDM. The present study proves the important role of voltage stabilizers in improving insulation performance of EPDM material, and reveals the refrigeration mechanism on hot charge carriers for restricting electric-tree growth, which provides a significant strategy of chemical modifications for developing high-insulation cable accessory materials.

Ethylene-Propylene-Diene Rubber (EPDM) is widely utilized as a high-performance insulation material in high-voltage direct current (HVDC) cable accessories, owing to its exceptional electrical and thermal properties. In this study, we have successfully synthesized and employed 4-vinyl oxyacetophenone (VPE) as a modification agent to develop the chemically grafted EPDM materials (EPDM-g-VPE) just through thermal crosslinking reaction and melt blending approach. Infrared spectroscopy results reveal that during thermal cross-linking process, VPE efficiently grafted onto EPDM molecular-chains through free radical addition reaction. Following VPE grafting, the DC dielectric breakdown strength and electrical conductivity of EPDM are significantly increased and noticeably decreased respectively. Theoretical electronic structure calculations corroborate that VPE’s electron-affinity and energy-gap enable it to efficiently absorb thermal electron energy without undergoing collision ionization, thereby enhancing EPDM’s breakdown resistance. Simultaneously, VPE molecules exhibit a high affinity for capturing electron charge carriers within EPDM polymer-molecules. Space charge and thermally stimulated current tests demonstrate that the stable and uniformly distributed charge traps can be effectively introduced into EPDM matrix by grafting VPE modification, thereby suppressing transport and injection of charge carriers. Consequently, this approach substantially improves DC electrical insulation performance of EPDM. This research not only successfully enhances the electrical insulation performance of EPDM but also showcases the wide-ranging potential of chemical modification technology in cable accessory materials.

This study explores and verifies the chemical modifications achieved by grafting 4-formylcyclohexyl heptanoate (FH) and 4-(2,5-dioxopyrrolidin-1-yl) cyclohexane-1-carbaldehyde (CC) onto ethylene propylene diene monomer (EPDM) elastomer, a prevalent dielectric material used for reinforced insulation in cable accessories. Employing a rigorous theoretical methodology combining first-principles calculations, molecular dynamics, and Monte Carlo molecular simulations, we elucidate the intricate effects of these chemical-graft modifications on the polymeric structure of EPDM to resist charge transport, moisture-aging, and thermal impact of partial discharge. Our investigation uncovers the emergence of both shallow and deep charge traps within the material, effectively mitigating electron avalanche breakdown. Additionally, we scrutinize the influence of two proposed organic species, acting as grafting agents, on several crucial properties of EPDM including water adsorption uptake, heat capacity, molecular thermal vibration, and polymer pyrolysis. These modifications substantially bolster EPDM’s resistance to high-temperature electrical breakdown and water thermodynamic adsorption, while also enhancing its thermal stability, rendering the proposed chemical-graft modifications an effective way and underling mechanisms for ameliorating electrical insulation performances of EPDM elastomer. Our findings highlight the significant potential of graft modification in molecular structures through comprehensive molecular simulations, offering valuable insights for advancing competent elastomeric polymers in cable accessory insulation.

Polypropylene (PP) composite materials with both high mechanical toughness and electrical insulation performance are prepared by incorporating styrene-ethylene/butylene-styrene (SEBS) block copolymer as a toughening agent and nanoscale silica (SiO₂) as a inorganic modifier to enhance electrical-tree and breakdown resistances. The effects and mechanisms of SEBS toughening agent and SiO₂ nanofiller on the thermal-mechanical properties and electrical insulation performances of PP material are investigated through the mechanical tests as well as the accelerated electrical-tree aging and alternative current (AC) breakdown experiments. The elastic modulus of SiO₂/SEBS/PP composite is slightly lower than that of pure PP material, while the thermal elongation rate remains superior to cross-linked polyethylene (XLPE), which is competent in mechanical performances for main insulation materials in high-voltage cables. The addition of styrene-ethylene/butylene-styrene (SEBS) facilitates electrical-tree growth in PP matrix and thus leads to the reduction in dielectric breakdown strength of PP material. In contrast, the incorporation of nano-SiO₂ can effectively improve the electrical-tree resistance and dielectric breakdown strength of PP material, making the SiO₂/SEBS/PP composite a promising candidate for high-voltage cable insulation. The tests and analyses of thermal stimulated depolarization current (TSDC) reveal that the SEBS toughing additive introduces the shallow charge traps in PP matrix, making it easier for the trapped charges to transition into charge carriers, thus leading to a considerable decrease in electrical-tree resistance and insulation strength of PP material. Meanwhile, the SiO₂ nanofiller can introduce deeper charge traps into PP matrix than the structural-defect intrinsic charge traps, resulting in a significant amelioration in the electrical-tree resistant and insulation performances for SEBS/PP composite. The present study demonstrates that SiO₂/SEBS/PP composite possesses sufficiently high electrical-tree resistance and dielectric breakdown strength as well as suitable thermal-mechanical properties, offering a potential application in main insulation of high-voltage cables and providing an effective pathway for developing novel recyclable AC high-voltage cables.