Research of Conductive Characteristics and Space Charge on Polypropylene Insulation Material

In order to evaluate the feasibility of electric performance in polypropylene(PP) as an insulation material for DC power cable, isotatic polypropylene was chosen to be prepared for investigating. The melting temperature(165 ℃) and crystallization temperature(111 ℃) were tested through differential scanning calorimetry(DSC), and the electric performances such as breakdown strengths, electrical conductivity, distribution of space charge, and dissipate of space charge were tested for PP materials. It is revealed in the experiments of breakdown strengths that the AC breakdown strength(BDS) is lower than DC BDS, as well as the data dispersion of AC BDS is better than DC BDS. Through the measurement of high field conductivity, it is found that when the temperature increases to 90 ℃, the high field conductivity of PP has an apparent increase and reaches twice of that at room temperature with the electrical field of 3.3 kV/mm. Space charge experiments with voltage show that no obvious space charge appears in PP under low electrical field strength(10 kV/mm and 20 kV/mm). A lot of hetero-charges appear in sample when the electrical field strength increases to 50 kV/mm. During the process of short circuit in space charge experiments, it is found that space charge in samples can be released completely in 0.5 h after applying an electrical field strength of 10 kV/mm and 20 kV/mm for 0.5 h. However, when the process of short circuit lasts for 1.5 h, a lot of space charges still remain inside after applying the electrical field strengths of 50 kV/mm for 0.5 h.