Trap dependent interface charge behaviors of fluorinated HVDC cable accessories

Abstract

Interface charges are likely to accumulate between two different dielectric materials, which may lead to accelerated degradation of insulation systems. Ethylene-propylene-diene terpolymer (EPDM) is used mainly for HVDC cable joint, which is the most vulnerable part of the cable system because of there being the interface composed of the cable insulation and accessories. Direct-fluorination can be applied to adjust the electrical and chemical properties of polymer insulation. Therefore, this work intends to investigate the effects of direct-fluorination on the interface charge behaviors in the bilayer insulation system. The EPDM samples were fluorinated for 0, 15, 30 and 60 min, and LDPE for 0 and 30 min, respectively. The interface charge distribution was measured under 20 kV/mm. Obtained results show that the space charge polarity at the interface is consistent with the voltage applied on EPDM sample, which can be verified by the Maxwell-Wagner-Sillars (MWS) theory. The 30-min fluorination can suppress the interface charge accumulation more effectively. However, fluorination with longer time causes significant accumulation of interface charge. The accumulation process of interface charge depends on not only the time constant but the surface state and trap distribution controlled by the fluorination time. As a consequence, the approximate direct fluorination can availably suppress the interface charge accumulation and offers a possible method for the improvement of cable accessory performance.