液氮环境下电老化对聚丙烯层压纸交流击穿及陷阱分布特性的影响

Different degrees of electrical aging test for PPLP insulation is carried out to study the aging mechanisms of polypropylene laminated paper (PPLP), which is the major insulation of high temperature superconducting (HTS) cables. Effects of electrical aging on the breakdown performance and trap distribution of PPLP insulation are studied by AC breakdown and isothermal surface potential decay (ISPD) measurements. And the aged samples are further analyzed by Fourier transform infrared spectrum, morphology observation and thermogravimetric analysis. Results show that with the increase of electrical agingdegree, AC breakdown field strength of PPLP insulation decreases from 113.3 kV/mm to 92.3 kV/mm. The density of shallow traps increases gradually while that of deep traps decreases gradually during aging process. It is observed that the partial cellulose in the insulating paper of the PPLP out layer is broken, and a great number of voids are generated, resulting in loose structure and reduced strength. The IR absorption peak intensities of hydrogen bond and glycoside bond in the insulating paper decrease. Meanwhile, the temperature of fastest thermal weight loss of insulating paper decreases, while the thermal weight loss of polypropylene keeps stable. The experimental results reveal that the degradation of PPLP insulation performances during aging is mainly attributed to the deterioration of insulating paper. The analysis shows that the glycoside bonds in the main chain of cellulose and the hydrogen bonds between cellulose molecules break under the impact of high energy electrons. As a result, the interaction force among molecular chains weakens and the structural compactness decreases. The internal free volume of insulating paper increases, and the binding ability of cellulose molecular chains to charges weakens, so the electric charges migrate more easily between molecular chains, resulting in the decrease of deep trap density and AC breakdown field strength of PPLP insulation.