Interface Charge Behaviors between LDPE and EPDM Filled with Carbon Black Nanoparticles

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. The charges are likely to accumulate at the interface between insulation materials with different conductivity and permittivity, which may induce the occurrence of partial discharge and breakdown. Nanoparticles can be applied to adjust the interface charge behaviors. In this paper, carbon black (CB) nanoparticles were dispersed into EPDM with 0, 0.5, 1, 3 and 5 wt% respectively. The effects of nanoparticle doping on the interface charge distribution of LDPE and EPDM composite insulation were measured under -15 kV/mm. Furthermore, dielectric properties and DC conduction were introduced to discuss the suppression mechanism of carbon black nanoparticle doping. Obtained results show that the nanoparitcles are well dispersed in the EPDM. The permittivity increases with the nanoparticle fillgrade while the ac conductivity and tan(delta) at lower frequencies are lower than the base EPDM. The interface charge density can be suppressed to 0.5 C/m(3) with 1 wt% carbon black nanoparticle doping, much less than the undoped and other doped groups. The mechanism of interface charge suppression is attributed to the decrease of conduction and permittivity mismatch by carbon black nanoparticle doping, which was proved by the dielectric spectroscopy and conduction current tests. The interface charge buildup and dissipation process depends on the time constant controlled by the doping proportion. As a consequence, the approximate 1 wt% carbon black nanoparticle doped EPDM can effectively suppress the accumulation of interface charge of cable accessories and will make a potential application for the dc cable accessories.