Electrical and mechanical ageing behaviors of used heat-shrinkable insulation tubes

Ageing is strongly related to the external factors acting upon the polymer and to the manufacturing process. It mainly depends on the anti-ageing performance whether heat-shrinkable tubes for switchgear busbar can maintain excellent electrical and mechanical properties during the long term operation. In this paper, the collected samples were applied to the switchgear busbar of 10 and 35 kV in substations of Tianjin Power System. The 10 kV tubes have operated for one, two and nine years and 35 kV tubes for four and six years respectively. Besides, two more kinds of unused samples were chosen to compare with the aged ones. Dielectric permittivity, dissipation factor and AC impedance were tested to assess the extent of ageing. The breakdown characteristics under AC voltage and the trap distribution calculated by measuring the dynamic behavior of surface potential under DC voltage were obtained at room temperature. Moreover, the tests of tensile strength, including breaking strength and elongation at break, were performed to analyze the mechanical properties. Results show that dielectric permittivity and dissipation factor increase with the operating time while the tensile strength decreases. The decline of breakdown strength and AC impedance indicates that insulation capability is reduced by long time natural ageing. The results obtained from the investigation also show the correlation between the trap distribution and the dielectric strength with the operating time.