Research On Steady-State Operation Characteristics Of HTS Fault Current Limiting Transformer

High temperature superconducting fault current limiting transformer(SFCLT)has the dual functions of changing voltage in normal operation and limiting current in case of short-circuit fault,so it has a good application prospect.Because the current mainly flows through the superconducting layer in the superconducting state,and the current flows through the parallel metal layer after the short-circuit fault,and the resistivity of stainless steel is much higher than that of copper,it is considered to use stainless steel reinforced REBCO tape to wind the winding of the SFCLT.After setting up the experimental platform,not only the critical current of stainless steel reinforced REBCO strip was measured,but also the maximum short-circuit impulse current that it can withstand for 100 ms was obtained.When the impact current is less than this critical value,the strip has a limiting effect on the short-circuit current,and the greater the short-circuit current is,the stronger the limiting effect is.Next,a 125 kVA(6 kV/400 V)single-phase SFCLT is designed according to the experimental parameters,including the core structure and size,winding type and size,as well as the optimization of core column section.Then the impedance voltage value is obtained according to the design parameters.In order to verify the economy and safety of transformer winding in long-term operation,this paper establishes the simulation model of transformer winding based on H-formulation.By comparing the current waveform of this transformer and a conventional transformer with the same structure in 100 ms short-circuit simulation,it is proved that this SFCLT has good current limiting ability.Next,the characteristics of the magnetic field and current distribution of the winding are analyzed,and the AC losses of the winding are calculated by the analytical method and the finite element method,which are both verified and compared with each other.Because the analytical method only has high accuracy under the premise of uniform distribution of current and magnetic field,but the distribution of current and magnetic field in each turn of the winding is not uniform,so although the analytical method adopts the treatment of dividing each turn of the strip into smaller units and calculating and then accumulating,it is still different from the AC loss value obtained by the finite element method.Considering that the core is outside the cryogenic vessel,the electromagnetic-thermal coupling simulation is carried out with the AC loss of the winding as the heat source,and the temperature distribution of the winding,as well as the electric force distribution,electric field distribution and the corresponding maximum value are obtained.No matter the temperature of the hottest spot or the maximum stress of the winding are less than the critical value of the strip,and the maximum electric field strength of the strip is less than the withstand voltage strength of the insulating material.In order to reduce the operation cost of the transformer,this paper also optimizes the design of the gas cooled current lead,which is an important part of the superconducting current limiting transformer.If the structure and size of the transformer are not properly selected,it will produce great heat leakage and may damage the stability of the transformer operation.Due to the different current flowing through the current leads at the high and low voltage sides,the preliminary optimization calculation is carried out by using the analytical method.On this basis,the simulation model is established,and its size and structure are analyzed and calculated in more detail.It is determined that the conduction cooling structure is adopted for the high voltage side lead,and the gas cooling structure is adopted for the low voltage side lead,as well as their respective sizes.At the same time,the total heat leakage of the lead under the optimal scheme is calculated.The above research on the steady-state operation characteristics of the superconducting current limiting transformer shows that the transformer has the ability of long-term stable operation,which has important reference value for the practical application of the superconducting current limiting transformer.