Research On Thermal Stability Of Resistive Type Superconducting DC Fault Current Limiter Magnet Unit Under DC Impact

In recent years,the development of flexible DC transmission technology has solved the problems of reactive power compensation and commutation failure.At the same time,the multi-terminal flexible DC transmission system has a reliable and flexible transmission mode,which has a very good application prospect in the future power grid.However,with the continuous improvement of power grid capacity,the short-circuit fault current of DC side of multi-terminal flexible system further increases,gradually exceeding the breaking capacity of DC circuit breaker.Therefore,in order to ensure the safe and stable operation of power system,it is urgent to develop DC short circuit fault current limiter.Compared with the traditional DC current limiter,the resistive type superconducting DC current limiter has many advantages,such as low impedance normal current carrying,high impedance current limiting,fast response speed,automatic reset,simple structure,little influence on power system,etc.It is suitable for the current development trend of multi-terminal flexible DC transmission system in China.However,the thermal stability under transient DC impact has always restricted the application of resistive type superconducting DC current limiter in higher voltage DC transmission system.In this thesis,the thermal stability of resistive type superconducting DC current limiter magnet unit is studied,and the main research contents are as follows:(1)The equivalent circuit model of REBCO coated conductor after quench is established,and the influence of coating interface resistance on current distribution between metal layers of conductor at different temperatures and the influence of long tape inhomogeneity on quench characteristics are analyzed.(2)A stacked parallel structure with insulation is proposed to improve the heat transfer efficiency and space utilization of YBCO superconducting tapes,and the heat transfer characteristics of different parallel modes are analyzed.The effects of different parallel modes on the quench recovery characteristics of YBCO superconducting tapes were studied by DC impact experiment.(3)The equivalent circuit of the parallel superconducting coil is established,and the influence of joint resistance on the current distribution characteristics of the parallel superconducting coil is analyzed.Two-dimensional magnetic field finite element simulation of parallel inductive and parallel non-inductive superconducting coils is carried out,and the law that the inductance of superconducting coils changes with coil spacing and turn spacing is given.What’s more,the influence of coil inductance on the current distribution characteristics of parallel superconducting coils under transient DC impact is analyzed.(4)Inductive and non-inductive pancake superconducting coils were designed and manufactured,and DC impact experiments were carried out.The effects of critical current and resistivity parameters on the uniformity of quench resistance of cake superconducting coils were studied under transient DC impact,as well as the effects of quench resistance and inductance on the current distribution characteristics.The above research on thermal stability of superconducting tapes,such as uniformity,quench recovery characteristics and current distribution characteristics,will eventually lay a theoretical and experimental foundation for the design and fabrication of resistive type superconducting DC current limiter.