Insulation Design of the 160 kV DC Superconducting Fault Current Limiting Winding

Superconducting fault current limiter (SFCL) is a promising device for limiting the short-circuit current of the power system. A 160-kV SFCL is developed for protecting the voltage source converter-based high-voltage direct current (VSC-HVdc) transmission system in Nanao of China. The main insulation and longitudinal insulation design of the 160-kV superconducting fault current-limiting winding are presented, where the liquid nitrogen (LN2) is used as the cooling and insulation medium. The main insulation simulation results show that the maximum electric field strength of the SFCL winding could be restricted to 1.2 kV/mm during the 252-kV dc voltage withstand test by using optimized grading and shielding ring. The SFCL winding is composed of 96 noninductive-type superconducting bifilar coils, and the turn-to-turn insulation of the bifilar coil is a combination of LN2 and polytetrafluoroethylene (PTFE). The turn-to-turn distance of the bifilar coil is optimized based on the longitudinal insulation analysis under the 550-kV lightning impulse withstand voltage test, 450-kV switching impulse withstand voltage test, and 16-kA power switching test. According to the simulation and experimental results, the maximum voltage drop occurs on the first turn of the bifilar coil.