Effect of thermal ageing on electrical and mechanical properties of tubular bus model insulation

Thanks to its large carrying capacity, excellent insulation property and high mechanical strength, insulated tubular bus has been widely used in power stations. Polytetrafluorethylene (PTFE) film which has been utilized as insulating material for the tubular bus is subjected to a great risk of thermal ageing that may degrade its electrical and mechanical properties. From the viewpoint of safety, it is essential to gain a firm understanding of thermal ageing mechanisms of PTFE film. In this work, PTFE film with thickness of 100μm was employed as test sample. Tubular bus models were established and the models were thermally aged at 150°C for a certain time period after which dielectric spectroscopy, AC breakdown and thermal expansion tests of the film samples were performed to estimate the effect of ageing time on the electrical and mechanical properties of the PTFE film. Obtained results indicated that with the increase of the ageing time, the relative permittivity decreased but the dielectric loss is basically unchanged. The electrical strength of PTFE film decreases slightly with the ageing time as well. Furthermore, the thermal expansion properties of PTFE changes sharply as the ageing time increases up to 540h, but remains almost the same if the time is shorter than 360h.