Impulse breakdown of extruded cable insulation materials

A new test cell is designed for making breakdown test on solid insulation materials for DC cables. Different from a conventional method, the new system uses a specially designed test chamber instead of insulation oil as ambient medium in order to prevent experiment failure from external flashover. The measure eliminates the need of the liquid and has ability to create breakdown data closer to the nature of solid insulation. Lightning impulse breakdown tests were performed on selected insulation materials. Breakdown strength of 400 kV/mm has been obtained for films about 100 μm in thickness. The data was compared with those of model cables tested with a larger thickness of insulation. Breakdown volume theory and up-scale effect are discussed. Power curve fit was made between breakdown strength and the thickness of insulation. A good correlation has been found, which makes it possible to predict the low probability value of breakdown of extruded DC cables at any thickness of insulation. A hypothesis is proposed which indicates that the propagation velocity of breakdown made contributions to the lightning impulse breakdown strength of an insulation material. Higher breakdown strength was found with a lower propagation velocity of a breakdown. The breakdown results show good agreement with data reported by others for the similar materials, and for polyethylene with inorganic fillers the breakdown strength might be slightly decreased.