Suppressing interface charge between LDPE and EPDM for HVDC cable accessory insulation

Abstract

Space charges are likely to accumulate at the interface between insulation materials with different properties, which may lead to accelerated degradation of insulation systems. Cable accessories made of ethylene-propylene-diene terpolymer (EPDM) are considered to be the weakest part of HVDC cable system due to the existence of the interface between cable insulation and itself. Direct-fluorination can be applied to adjust the electrical properties of polymer insulation. Therefore, this paper intends to investigate the effects of direct-fluorination on the interface charge distribution between LDPE and EPDM composite insulation. The EPDM samples were fluorinated for 0, 15, 30 and 60 min, and LDPE for 0 and 30 min, respectively. The interface charge distribution were measured under 20 kV/mm. Obtained results show that the polarity of interface charge is the same with the voltage applied on EPDM sample, which can be verified by the Maxwell-Wagner-Sillars (MWS) theory. The fluorination for 30 min effectively suppresses the interface charge accumulation between LDPE and EPDM. However, excessive fluorination leads to significant accumulation of interface charge. Though fluorination of both dielectrics has the same effect with the single fluorinated EPDM on interface charge behaviors, more hetero-charges injection in the vicinity of cathode. The interface charge accumulation and dissipation process depends on not only the time constant but the surface state controlled by the fluorination time. As a consequence, the approximate direct fluorination can effectively suppress the interface charge accumulation of cable accessories and will make a potential application for the dc cable accessories.