Thermal Stability Analysis For High Temperature Superconducting Transformer

ABSTRACT:In the field of power transmission and transformation, the application technology based on HTS material has gradually attracted people’s attention and become one of the hot topics for scholars at home and abroad. HTS transformer has a number of benefits over conventional transformer, such as lighter and smaller units, higher efficiency, no contamination, lower life-cycle costs, substantial energy savings and strong overload capability. Therefore, HTS transformer plays an important role in the reform of power equipment in the future. If the heat is not transferred in a timely manner during the transformer load operation, it will accumulate on the winding gradually. As a result, the winding is quenched, speed of insulation aging is accelerated, overload capacity is limited, and efficiency and life-cycle are reduced, even the transformer is damaged because of temperature of winding is higher and higher. Thus, it is of great significance to study of HTS transformer thermal stability.In the heat load of HTS transformer, the AC loss of the winding is the key factor to affect the transformer temperature. The sub-cooled liquid nitrogen is used to cool the winding in order to ensure the normal operation of the transformer, which has a direct influence on the stability of the transformer. In this paper, the mathematical model of the AC loss of YBCO HTS strip is deduced and analysed. And the mathematical model of the AC loss are gat for the HTS transformer in the self-field and out-field in consideration of the structure, current characteristics and the magnetic field distribution of the composite conductors. Based on this theoretical value of the AC loss of the transformer, the3D physical model of cryostat for the25MVA HTS transformer is built and the internal temperature distribution is simulated by using FLUENT software. As to the parameters which has an effect on the HTS transformer temperature, it is mainly analysed that the influence of the inlet and outlet location, load factor, and heat leakage of the bushing and liquid nitrogen flow on the HV and LV winding and the temperature of liquid nitrogen. On the one hand, the cryostat structure is optimized; On the other hand, it shows that the temperature of winding and liquid nitrogen rise gradually with the increase of load and heat leakage of the bushing and drop slowly with the quicker flow of liquid nitrogen. The calculation result can provide a certain theoretical reference basis for the design and optimization of HTS transformer cooling system. In addition, the AC loss of composite conductors is measured by setting up the AC loss experiment platform. The experimental result verifies the correctness of the calculation method and the effectiveness of the electrical method to measure the composite conductors AC loss by comparing the theoretical value with experimental results.