Partial discharge characterisation in liquid nitrogen composite systems

Interest in superconductors for use in the transmission and distribution of power has grown since the advent of high temperature superconductors (HTS). The benefits of reduced ohmic losses leading to greater operational efficiency appear attractive to power transmission and distribution companies. Potentially this technology will increase power transfer for the same or lower voltage and for the same or smaller physical dimensions. For the successful employment of this technology in the field, power utilities must be convinced of the operational reliability of designs. Many of these designs make use of liquid nitrogen as the system coolant and also as a dielectric fluid. It is therefore vital that liquid nitrogen be well characterised as a dielectric medium. Point-plane sample geometries have been studied for ac applied electric fields to extend understanding of pre-breakdown phenomena and density change streamer effects in liquid nitrogen composite insulation systems. A technique for time correlated measurement of partial discharge activity and density change streamer image recording has been employed to investigate the partial discharge activity when a highly divergent electric field is applied for multiple ac cycles. Example highspeed images are given showing the effects of streamer discharges experienced by the liquid both during and after the partial breakdown of the insulation. Work has been performed to characterise the partial discharge activity in composite solid-liquid nitrogen insulation systems for different solid dielectrics resulting from the application of a 50 Hz highly divergent electric field. φqn analysis of ac results has been completed and characterisation of partial discharge behaviour for each system is presented and discussed.