Effects of ZnO particles on space charge of EVA copolymer for HVDC cable accessory insulation

The performance of cable accessories, as an essential part of the HVDC networks, is a great concern to the reliability of the system. Cable accessories made of ethylene-vinyl acetate copolymer (EVA) by incorporation of specific fillers have to face the problem of space charge accumulation and become more unpredictable considering the complex conditions. The effects of doping contents on the space charge behaviors of EVA/ZnO composite are not completely clear, which need further investigation. In this paper, EVA composites were prepared by incorporating ZnO nanoparticles into EVA matrix with fraction of 0, 1, 5 and 10 wt% respectively, with which 5 wt% nano-sized plus 5 wt% micro-sized ZnO doped samples were chosen for comparison. The dependence of dielectric properties and conduction current on the doping contents was measured. PEA method was applied to research the space charge behaviors of EVA composites under -25 kV/mm. Obtained results show that the particles in EVA composite are in homodisperse. The permittivity is increased by ZnO doping and the dissipation factor of EVA composites with 1 and 5 wt% nanoparticles is lower at the lower frequencies. The homocharges injection occurs in cathode instead of anode when ZnO nanoparticles are introduced and 5 wt% nanoparticles doping performs well in suppressing space charge injection. The electric field of the 5 wt% nanoparticles doped EVA distributes more uniformly under the high electric stress than that of others. During the depolarization procedure, the total remnants charges of 10 wt% doped samples are the least in the final. The above results are well explained by the DC conduction, apparent mobility and trap distribution characteristics. The micro-sized ZnO particles may break the chains and produce new defects, which will act as local deep traps.