Characterisation of surface partial discharge behaviour in liquid nitrogen

The advent of high temperature superconductors (HTS) caused resurgence in interest for the development of superconducting apparatus for use in the transmission and distribution of power. The potential benefits of reduced losses and the promise of greater power densities appear attractive to power utilities. For the technology to be implemented in operational circuits, however, requires that power utilities are convinced of their reliability. Many prototype HTS designs employ liquid nitrogen as the combined system coolant and dielectric fluid. It is therefore vital that liquid nitrogen be well characterised as a dielectric medium. Work has been performed to characterise partial discharge resulting from the application of a 50 Hz ac voltage to a two composite solid dielectric and liquid nitrogen (LN₂) sample arrangements. The first is for a solid dielectric immersed in LN₂ between parallel plate electrodes forming a triple-junction of electrode/solid/liquid. The second experiment examines point-plane discharge occurring onto a solid dielectric barrier used to cover the plane electrode face. A technique for time correlated measurement of partial discharge activity and density change streamer image recording has been employed investigating the partial discharge activity for multiple ac cycles. Example high-speed images are given showing the effects of streamer discharges experienced by the liquid both during and after the partial breakdown of the insulation. φqn analysis of ac results has been completed and characterisation of partial discharge behaviour for each system is presented and discussed. Pre-discharge results for point-plane sample geometries are considered and resulting failure of the solid dielectric in liquid and solid composite insulation systems are discussed.