| Induction heating has been applied to preheat metals especially for non-magnetic billets like aluminum,copper,or brass before extrusion operations.The conventional AC induction heating devices generally have an efficiency of 40~45%.The efficiency of superconducting DC induction heater can reach 85%.The high temperature superconducting(HTS)DC induction heater is featured with not only environment protection,energy conservation,but also with homogeneous heating,high quality heating advantages,and has has being a research focus in metal heating field.Prevous researches are foucused on the function design and optimization of the commercial HTS DC induction heater.There are new technical challenges in this design.Validity of magnet design and quench protection are carried out based on the 1# small dimension magnet prototype.The characteristics of peak load torque and adjustable temperature gradient are verified in 2# 10 k W laboratory-scale prototype of the HTS DC induction heater.In this paper,based on the verification of 1# and 2# prototype,the key technologies of 1 MW HTS DC induction heater prototype are analyzed.Four key technologies are analyzed in this paper:1)The first key technology is superconducting magnet design and optimization.An electromagnetic design for the high temperature superconducting magnet with iron core is presented.A numerical model is proposed to find out an optimal design.The dimension of supercondcuting coil is optimized to minimize the total comsuption/usage of tapes as well as the dimension of magnet.Finally,the specifications of our design is presented.In this technology,the accuracy of FEM model is the key of the optimization process.2)The second key technology is quench protection of the high temperature superconducting magnet with iron core.The inductance of magnet with iron is more than 100 H,which is much higher than that of conventional magnets.Due to the considerable amount of energy stored during persistent operation,the protection system becomes very important.A passive protection system is developed for the HTS magnet.The protection circuit is presented and the MATLAB/Simulink-based quench model is developed to optimize the design parameters of the protection circuit.Experiments with different magnet operating current and dump resistors are carried out based on the 1# small dimension magnet prototype.Due to the influence of HTS coated conductor,key parameters such as the threshold voltage of quench detection,the responding time of quench protection,the value of energy dissipation resistor and so on are calculated and analyzed by simulation.The results are of practical significance for setting the electrical parameters of the quench protection system.3)The third key technology is how to drive the billet economically.A hybrid driving system based on flywheel energy storage(FES)is designed to provide extra torque when the peak load torque occurs at a low rotation speed,which allows the successful start-up of the aluminum billet and the operation of the motor at its rated capacity.The mechanical structure of this hybrid driving system is introduced.A numerical model is constructed using MATLAB/Simulink,and the dynamic start-up process is analyzed.The influence of the flywheel’s inertia and minimum engagement speed are investigated.The optimization of the cost of this driving system is crucial,because it is in relation to the commercial potentical of this apparatus.4)The fourth key technology is how to achieve the control of temperature gradient along the axial direction.Higher than 100 °C adjustable temperature gradient along the axial direction of billets is required in the aluminum extrusion industry.In order to obtain an adjustable magnetic field distribution along the axial direction of billets,an adjustable air gap structure is proposed and fabricated in 2# 10 k W laboratory-scale prototype of the HTS DC induction heater.Heating experiments for uniform and gradient temperature distribution are performed with different air gap structures to verify the validality of adjustable air gap structure.In addition,the influence of gradient air gaps as well as the material of billets are discussed on basis of the experimental results.Finally,an economic feasibility study of a 1MW HTS DC induction heater in terms of electricity fee minimization is presented.Net present value,internal rate of return,and pay-back period methods are used to evaluate the investment returns.All indicators related to direct benefits are calculated and analyzed for finding economic feasibility.The analysis is compared including three power levels,three peak torque solutions and different operation conditions.The results will be helpful to make the final decision for the commercialization of the HTS DC induction heater.This paper presents the four key technologies during the study of DC HTS induction heater.The optimal total usage of tapes is 20.26 km.The results show that the hybrid driving system coupling the FES and the motor is able to start up the aluminum billet successfully.The adjustable magnetic field distribution along the axial direction of billets can meet the 100 °C adjustable temperature gradient design requirement.The result is very useful for its industrial application. |
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