A Study On Insulation Material And Structure Problems In Pancake Structure Superconducting Fault Current Limiter

A resistive-type superconducting fault current limiter(SFCL) is one of the most promising superconducting devices in an electrical power system. However, an SFCL has the very obvious disadvantage of a large space requirement when applied in high-voltage-level power networks. Considering the space utilization efficiency, a pancake structure in which superconducting tapes are wound into coils should be adopted. When certain faults occur in a power grid, a relatively high voltage difference will be present in the tape coil. The inner and outer layers are in direct contact; therefore, special measures have to be carried out to overcome high-voltage insulation safety issues. Two commonly used materials are considered in this study: Nomex and Kapton. The basic AC breakdown characteristics in air and liquid nitrogen are explored. A new breakdown-strength testing platform is manufactured to obtain the breakdown voltage. Considering that the edges of the superconducting tape are very thin, the impact of the superconducting-tape thickness on the breakdown strength is explored. Two methods, parallel and vertical lapping, are used to lap the insulation layer onto the tape and are subsequently compared. Finally, a face-to-face electrode structure breakdown test is conducted to simulate the actual working conditions in a tape coil. This paper proposes an innovative method to add insulation layer: coating a very thin layer of polyimide on the surface of superconducting tapes. A comparison is made between the two methods. The surface flashover voltage is tested. Considering that the edges of superconducting tapes are very sharp, the influence of pre-breakdown partial discharge on insulation is explored.At last, this paper deals with the bushing problems of the 220 kV SFCL. The condenser cone method is adopted to make the electric field distribution at both ends of the bushing uniform. The important parameters of the condenser cone, the length and radius of each condenser plate included, are calculated.The experimental results of this paper will be used to design and optimize the geometrical structure of the SFCL pancake component. Make the SFCL more space-saving and economic while ensuring its insulation safety.