AC Breakdown and Trap Distribution Characteristics of Polypropylene Laminated Paper after Electrical Ageing

The failures of polypropylene laminated paper (PPLP), the major insulation of high temperature superconducting (HTS) cables, are mainly rooted in the electrical ageing. In this paper, effects of electrical ageing on AC breakdown and trap distribution characteristics of PPLP are investigated. Electrical ageing under AC voltage of PPLP in liquid nitrogen is carried out. The breakdown performance and trap distribution of PPLP are characterized by AC breakdown and isothermal surface potential decay (ISPD) measurements. In addition, the aged samples are further analyzed by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscope (SEM). Results demonstrate that the breakdown strength of PPLP decreases from 113.32 kV/mm to 92.36 kV/mm after ageing for 60 min under 30 kV. The energy level and trap density of deep trap calculated by ISPD results both decrease with ageing. FTIR and SEM indicate that the partial cellulose of insulating paper on the out-layer of PPLP is broken under the impact of high-energy electrons. As a result, the free volume of insulating paper is expanded, and the binding ability of cellulose molecular chains to charge carriers is weakened, which leads to the decrease of energy level and trap density of deep trap and further promotes the charge transportation. Therefore, a decreased breakdown strength of PPLP can be observed after electrical ageing. In conclusion, it is proposed that the destruction of molecular chain structure in the aged PPLP can lead the change of trap distribution, further resulting in the decline of breakdown strength.