Estimating the Parameters of a Superconducting Hybrid Transformer with a Spatial Magnetic System

Application of high-temperature superconductors (HTS) in power transformers is a promising development line in the electric power industry. The article addresses a study of an operating prototype of a three-phase 25 kV·A transformer that combines a hybrid design of superconducting windings, with a dielectric medium in the form of liquid nitrogen at a temperature of 77 K and a spatial magnetic system. The purpose of the study is to determine the transformer magnetic system that will make the transformer more insensitive to an increase of the HTS winding critical current value in the superconducting state. A theoretically substantiated procedure for calculating heat and mass transfer in the case of using liquid nitrogen as cryogenic dielectric medium is proposed. The procedure takes into account the nature of active power losses in the superconductor during its operation on alternating current, the design of the transformer active and passive elements, and the characteristics of the materials used. The efficiency of using a core with a spatial magnetic system is substantiated, which reduces the influence of leakage fluxes on the HTS windings and, together with the reduced mass of the magnetic circuit, reduces hysteresis losses. The use of a spatial core reduces the cryostat heat transfer and makes it possible to achieve uniform cooling of the HTS windings. The thermodynamic characteristics of a liquid nitrogen filled cryostat in a stationary mode of operation are presented. Owing to their being fire- and explosion safe, as well as having significantly smaller mass and overall dimensions, HTS transformers feature essential advantages over conventional transformers with a usual dielectric medium.