AC Loss Investigation Of High Temperature Superconducting Assembled Conductor And Superconducting Coil

Superconducting coil is one of the key components of HTS power equipment.The research of HTS coils is not only the basic work of HTS power devices,but also the important technical problems and key problems that need to be solved urgently in HTS power devices.AC loss is an important research aspect of HTS coils,which directly affects the efficiency of superconducting power equipment,manufacturing and operation costs,overall weight and stability of the system.Large capacity superconducting power equipment requires high current superconducting coils and conductors with high current carrying capability.Thus,it is of great importance to research HTS assembled conductor with large current carrying capacity.Based on numerical calculation and experimental measurement,in this work,we successively analyzed current distribution,magnetic field distribution and AC loss of superconducting single tape,stack/assembly and superconducting coil.Taking the application of superconducting coils in HTS transformer as an example,optical design of transformer winding structure was carried out,and AC loss characteristics as well as its quench detection method were studied.(1)A numerical model to analyse the effect of current distribution uniformity among the strands on AC losses of assembled conductor was established,and a circuit model of current evolution distribution among the strands was presented.By comparing the AC losses of bifilar stack compoased of Roebel cable and simple parallel stack without transposition,we concluded that AC losses are the lowest when current distributes uniformly among the strands of the stack;In high homogenized current region,AC loss of bifilar stack with non-uniform current distribution is twice that with uniform current distribution.It was proposed firstly that wind bifilar coil with large current using Roebel cable and that critical current and loss in Roebel cable can be measured from only one voltage loop.(2)An empirical formula to evaluate AC losses of HTS inductive stack composed of arbitrary strips,based on critical current and AC losses of a single strip,was proposed.It is found that the losses of ABAB type bifilar stack consisting of arbitrary even strips are basically the same as those of AB type bifilar stack.AC losses of the stacks/bifilar stacks composing 1,2,4 and 6 superconducting strips arranged in different ways are measured experimentally.Taking the example of a stack composed of two strips,it is denoted as AA,if the current flows in the same direction in the two conductors,otherwise it is denoted as AB.Results showed that compared with stacks of AAAA and ABAB,AABB not only improves the current carrying capacity,but also reduces AC losses.(3)Innovatively combine//-formulation,homogenization method,edge element and structured mesh,and apply this method to build finite element(FE)numerical modelling of AC losses in HTS transformer above MW class with large turn number.This modelling method solves the traditional problem of long computing time when simulating coils with large turn number,while maintaining high calculation accuracy.Existence of optimal turn spacing(dots)in low voltage(LV)winding,after fixing the height of high voltage(HV)winding,was found in terms of the lowest AC loss when analyzing loss characteristics in 1 MVA HTS transformer winding.When turn spacing(dts)is less than dots,loss of transformer windings decreases with the increase of dts;While,when dts is bigger than dots,loss of transformer windings increasess with the increase of dts.(4)Aiming at AC loss reduction,optimization design of 6.5 MVA HTS traction transformer winding structure was carried out.It is concluded that AC loss of transformer winding decreases with the increase of winding height,at given rated voltage and capacity of transformer;AC loss of transformer winding increases with the increase of short circuit impedance;The higher the critical current of strip and the smaller the attenuation of critical current in external magnetic field,the smaller the winding losses.AC loss in windings can be reduced by 20%-40%by installing flux diverters at the end of windings,since magnetic field distribution was changed at the end of the winding.(5)A new quench detection method for HTS coil was proposed,based on voltage subtraction and phase angle.The tap drawn from the middle of HTS coil divides the coil into two parts.Assume the current phase as a basement.According to the difference of terminal voltages At’from two parts of HTS coils and the phase angle Δφbetween △U and pure inductive signal phase,magnitude of quench voltage can be obtained by Ur = AU*sin(Δφ).This method enlarges the small phase angle between terminal voltage of coil and quench voltage,and it is advantageous to detect the quench signal in time.