Significantly Improved Electrical Properties of Photo-Initiated Auxiliary Crosslinking EPDM Used for Cable Termination

Abstract

In order to achieve high quality electrical materials for cable terminations, the crosslinked ethylene-propylene-diene monomer (EPDM) materials, with adequate breakdown strength, appropriately increased conductivity and are developed by employing auxiliary crosslinker and ultraviolet (UV) photoinitiated crosslinking technique. The characteristic cyclic anhydrides with coupled carbonyl groups are utilized as auxiliary crosslinkers to promote crosslinking efficiency and provide polar-groups to EPDM molecules in UV-initiated crosslinking processes, which can be effectively fulfilled in industrial cable production. The results of infrared spectroscopy show that the auxiliary crosslinkers have been successfully grated to EPDM molecules through UV initiation process. The conductivity of EPDM increases after individually utilizing three auxiliary crosslinkers to EPDM at various temperatures of cable operations, by which the highest conductivity has been acquired by grafting N.N-m-phenylene dimaleimide. The first-principles calculations demonstrate that some occupied local electronic-states have been introduced in the band-gap of the EPDM crosslinked by N.N-m-phenylene dimaleimide (EPDM-HAV2), which can be thermally excited from valence band to conduction band at lower temperature or in higher density, leading to augmentation in electrical conductivity. Meanwhile, the breakdown strength achieves a significant improvement in consistency with the theoretical estimation that deeper hole-traps can be introduced by auxiliary-crosslinking modification, and will consequently increase breakdown strength through the trapping mechanism of space charge suppression. in relation to the appropriately increased conductivity, in combination with persistent breakdown strength, the finite element simulations of the electric field distribution in EPDM cable terminations suggest that the effectively homogenized electric field at the root of stress cone will be realized for EPDM-HAV2. The present study offers a fundamental strategy to ameliorate EPDM materials in the application of insulated cable accessories.

Keywords: crosslinking reaction; electrical conductance; finite element simulation; first-principles calculation; ultraviolet irradiation.

Figure 1

Schematic cable termination model: 1-core; 2-cross-linked polyethylene (XLPE); 3-inner shield; 4-reinforced insulation; 5-stress cone; 6-outer shield.

Figure 2

Infrared transmission spectra of ethylene-propylene-diene monomer (EPDM) + Auxiliary + benzophenone (BP) mixture in UV-initiation process (Auxiliary = TMPTMA, TAIC or HAV2), together with EPDM + BP mixture in comparison.

Figure 3

(a–c) γ-E curves and (d) characteristic dielectric breakdown strength (DBS) of EPDM materials at 30–70 °C. The conductivity property of XLPE also being presented for analyzing electric field distribution in cable termination.

Figure 4

(a) Schematic molecular structure of EPDM-HAV2,( b) Density of states for EPDM molecules being chemically connected by different auxiliary crosslinkers in which the highest occupied molecular orbital (HOMO) is referenced as energy zero level indicated by vertical dashed lines.

Figure 5

Simulated electric field distribution in cable terminations being constructed with (a) EPDM, (b) EPDM-TAIC, (c) EPDM-TMPTMA, and (d) EPDM-HAV2 as the dielectric materials in reinforced insulation layer under 200 kV/mm at the cable core temperature of 70 °C.