Design, Fabrication, and Operation of the Cryogenic System for a 220 kV/300 MVA Saturated Iron-Core Superconducting Fault Current Limiter

High reliability is one of the key requirements for a power grid device. The reliability of a superconducting fault current limiter (SFCL) largely depends on the reliability of its cryogenic system. An open cryogenic system was designed and fabricated for a 220 kV/300 MVA saturated iron-core SFCL, which was composed of a Dewar, heat insulation pipelines, a liquid nitrogen tank, a control circuit, and a vacuum pump. In its configuration, a high-temperature superconducting (HTS) dc bias coil is immersed with liquid nitrogen inside the Dewar. The control circuit constantly monitors the liquid nitrogen level and controls the supply of liquid nitrogen in accordance with the liquid nitrogen level. Nitrogen vapor is directly released into the environment. The SFCL including the cryogenic system was installed in the Shigezhuang substation of Tianjin, China. There are three operation modes for the cryogenic system. The first is the ac coil and the HTS coil being loaded with current. The second is the HTS coil being loaded with current, while no current in the ac coil. The last mode is no current loading in both the coils. Operation data of the cryogenic system are analyzed to compare thermal load, pressure, and supply cycle of liquid nitrogen in the three operation modes.