142 / 2018-08-25 14:10:45

Study on Nonlinear Conductivity and Breakdown Characteristics of Zinc Oxide Nano Sheets/EPDM Composites

EPDM, ZnO sheets, non-linear conductivity, breakdown strength.

With the development of cost-effective, low loss, manageable, and flexible for connect into the power systems merits, high-voltage direct current (HVDC) has been widely used in long-distance transmission. When the DC cable system operates in steady state, the problems of space charge accumulation and uneven distribution of electric field arise, which are always caused by the difference in conductivity gradient for different materials used in the cables, leading to the partial discharge, electric breakdown or permanent failure, which may seriously shorten the service life of the cable accessories. Numerous studies have found that the nonlinear conductivity insulating materials are attracted much more attentions, which exhibit excellent ability to uniform the distribution of electric field, and can be prepared by adding inorganic fillers with non-linear conductive characteristics into the polymer matrix, and the conductivity of the obtained non-linear composite materials is changed with the applied electric filed, therefore, the accumulated space charge can be dissipated easily and the uneven distribution of electric field in cable accessories are effectively alleviated.
In this paper, the two-dimensional shape ZnO Nano Sheets (abbreviated as ZnO NS) was prepared by coprecipitation method. The obtained ZnO NS were added into EPDM matrix during the plastic process, and undergoing the curing and vulcanizer process, the ZnO NS/EPDM composite materials were prepared. At the same time, Zinc oxide particles (ZnO NPs) were introduced into the EPDM for constructing ZnO NPs/EPDM composite materials as a contrast experiment. The study found that when the concentration of ZnO NS was 4.1vol %, the threshold field strength represented the non-linear conductivity was 16.23kV/mm, and the nonlinear coefficient increased from 0.11 of pure EPDM to 1.5, and the breakdown field strength was 70.81 kV/mm. However, the non-linearity conductivity was not observed in the ZnO NPs/EPDM composite dielectrics doped with the same volume concentration of ZnO NPs.
The results in this study indicate that the doping of ZnO NS leads to the fantastic non-linear conductivity of the composite dielectrics. As the two-dimensional structure of ZnO NS is more beneficial to form a conductive path than that of one-dimensional structure of particles, giving rise to the ZnO NS/EPDM composite dielectrics have higher conductivity and more obvious conductivity nonlinearity than ZnO NPs/EPDM composite dielectrics with the same filler concentration. In addition, the threshold field strength gradually decreases with the increase of filler concentration, and the conductivity nonlinearity of the composite media becomes more obvious. When the filler volume concentration is 6.8 vol %, the threshold field strength is 10.95 kV/mm, the non-linear coefficient β increases to 2.69, and the breakdown field strength is 54.05 kV/mm. The composite dielectrics exhibits excellent conductivity nonlinearity and still maintains excellent insulation performance, which is of great significance for realizing homogenized electric field better and enhancing the insulation performance of cable accessories at low filler concentration. The results in this study provides a scientific basis for further exploring the conductive mechanism of polymer composite insulation materials doped with inorganic fillers and a new idea for the design of insulating materials used in cable accessory.