| Due to the anisotropy of the high-temperature superconducting(HTS)tape,it is easily affected by the magnetic field perpendicular to the surface of the superconducting tape.The magnetic field is the largest at the end of the superconducting winding.Therefore,the end magnetic field of the superconducting transformer is optimized to reduce the vertical field on the surface of the strip.It is beneficial to increase the critical current of the superconducting winding and reduce the AC loss.There are few studies on the optimization of the end magnetic field of superconducting transformers using second-generation superconducting tapes in China.This thesis has a certain reference significance for the optimization design of superconducting transformers.Relying on the laboratory’s 6.6 MVA superconducting traction transformer project for high-speed trains,this thesis has scaled down and developed a superconducting transformer with a capacity of 5 kVA to provide support for the project.The main work of this thesis is as follows:(1)The overall electromagnetic scheme was designed with the short-circuit impedance of 43 %,efficiency of 99 %,and weight less than 50 kg as the goal,and the electromagnetic design is verified through empirical formula calculation,transformer simulation calculation and experiment.According to the iron core and winding size,the overall structure of the winding skeleton and the transformer is designed.(2)The electromagnetic calculation model of the superconducting double-cake coil with the Flux Diverter is established,and the change law of the overall and per-turn critical current and AC loss are explored when the single-ended and double-ended of superconducting double-cake coils are respectively installed with the Flux Diverter.The study found that both have the effect of reducing loss,but when the Flux Diverter is installed on one side,the critical current and electromagnetic stability are reduced.When Flux Diverters are installed on both sides,the AC loss distribution law of the superconducting coil is changed,so that the AC loss is concentrated on the outer turns of the superconducting coil,which improves the thermal stability of the superconducting coil;The influence of Flux Diverter on the electromagnetic performance of the double-cake coil under different loads and different frequencies are explored.The traditional Flux Diverters are suitable for heavy load and low frequency occasions.When the load is extremely low,the traditional Flux Diverter will slightly increase the AC loss of the superconducting winding.As the frequency increases,the optimization effect of the Flux Diverter is gradually weakened.(3)Based on the theory of interface conditions,when the interface of the medium is approximately perpendicular to the lines of magnetic force,the attraction to the lines of magnetic force is greatest.The structure of arc-shaped Flux Diverter is proposed and applied to the double-cake coil and the 5 kVA HTS transformer.The calculation formulas of structural parameters are deduced.The structure of arc-shaped Flux Diverter can embed the end of the superconducting winding into the arc-shaped slot,which further reduces the AC loss of the superconducting coil while occupying a smaller space.(4)The electromagnetic calculation model of superconducting transformer which is including iron core,Flux Diverter and superconducting winding is established,and the coupling law between the high and low voltage winding and the iron core is explored.The concentric winding distribution weakens the vertical field at the end of the winding and reduces the AC loss.The iron core has an attractive effect on the magnetic lines of force,which increases the vertical field around the low-voltage winding.Compared with the model without iron core,the AC loss of superconducting winding is increased by 26 %;The influence of the Flux Diverter of the high(low)voltage side on the low(high)voltage winding is explored.Compared with the near end,the Flux Diverter has a negligible effect on the AC loss of the remote winding.In the actual design of multi-winding equipped with Flux Diverter,the Flux Diverter can be individually optimized to reduce the amount of calculation;The influence of the Flux Diverter with width w,thickness h,distance d from the end of the winding,and relative permeability μ on the AC loss of superconducting windings is explored.With the increase of w and h and the decrease of d,the AC loss of the superconducting winding has been further reduced.According to the influence weight,the ranking is w>d>h>μ;Based on the existing law of the influence of the Flux Diverter on the superconducting winding,the structure parameters of Flux Diverter are optimized,and the structure of Flux Diverter adapted to the 5 kVA superconducting transformer is obtained.The AC loss of the superconducting winding is 14.78 W,which is 43.1 % lower than that of the HTS winging without Flux Diverters.The loss of Flux Diverter is 0.285 W,which only accounts for 0.7 % of the overall loss.which is negligible.After optimized design,the efficiency of the superconducting transformer is 99.152 %,which meets the design requirements.(5)Based on the theoretical analysis results under the complex magnetic field space inside the superconducting transformer,this thesis has developed a high-efficiency and lightweight single-phase HTS transformer test prototype with a capacity of 5 kVA and a safe current-limiting function.The critical current test platform of the superconducting transformer is built,and the critical characteristics of the prototype are evaluated.The average critical current of the high-voltage coil is 132 A,which is 41.3 % lower than the critical current of the tape after winding.The current decay rate is reduced by 6.2 % after adding the Flux Diverters.The critical current of the low voltage winding is 671 A,and the critical current decay rate is 39.1 %.The difference between the experiment and the simulation value is small,which verifies the accuracy of the simulation model. |
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