Optimization Design And Experimental Research Towards The Excitation Performance Of The Selfdriving Hts Flux Pump

Compared with the resistive magnet,superconducting magnet has the advantages of compact structure,high energy density and low energy consumption.So,it possesses a wide application prospect in the fields of high-end medical equipment,high-speed transportation,national defense industry,etc.With the discovery of high-temperature superconducting(HTS)material reaching higher critical magnetic field and higher critical temperature,it creates an opportunity for the research of superconducting magnet technology in higher magnetic field level and simpler cooling mode.However,due to the joint resistance of high-temperature superconductor and AC losses,the magnet can't work in the persistent current mode.To maintain the stability of magnetic field,the magnet requires continuous excitation.In the meanwhile,the existing contact excitation method where magnet is directly charged by DC power has the defect of current leads.Actually,the current leads which undergo an extremely large temperature difference generate a mass of heat loss.Then the loss reduces the cooling efficiency of the cryogenic container and further influences the thermal stability of the magnet,which even leads to magnet quenching.Thus,excitation method becomes the bottleneck technology for HTS magnet technology further development.In order to get rid of the adverse effect of contact excitation method,it is necessary to develop an efficient non-contact excitation method for HTS magnet.Flux pump technology can pump the magnetic flux into the superconducting magnet without contact,which is an ideal excitation method for HTS magnet.Among them,the self-driving transformer-rectifier type flux pump only needs to input a small electric energy to excite the magnet.Besides,it doesn't need additional excitation source and has a simple control strategy,which owns a potential in application.Therefore,this thesis conducts the basic research of self-driving transformer-rectifier type flux pump,focuses on the analysis of its working principle,and proposes an effective means to improve the excitation efficiency of the system.Firstly,based on the circuit topology of the self-driving HTS flux pump,the theoretical analysis has been carried out to obtain the key parameters of driving the system effectively,which lays the foundation for the subsequent research on excitation performance.Next,the finite element model was set up with full consideration of the strong nonlinear characteristics and anisotropy of superconductor.Based on the simulation model,we have established the clearer cognition of system's driving parameters and find that the charging loop will induce DC current during the excitation process,which will influence the reliability of excitation.To improve excitation performance,we have proposed that a “consumption resistance" could be added into the charging loop.Moreover,it is found that,with the ratio of the positive and negative peak increasing,the saturation value of the load current first increases and then becomes constant,while the excitation time continues increasing.As a result,the ratio of the positive and negative peak of the asymmetric triangular waveform should be considered according to the practical application requirements.Then,we utilized MATLAB Simulink to build the self-driving flux pump calculation model for overcoming the limitation where the finite element model can't take the consumption resistance into account.At first,we have discussed the effects of the frequency and the consumption resistance on the excitation performance respectively and find that 1)within the range of 1?100 Hz,with the frequency increasing,the saturation value of the load current increases first and then decreases;2)compared with the condition without consumption resistance,adding consumption resistance can significantly increase the saturation value of load current,but the excitation time will be prolonged;3)at a fixed working frequency,there is an optimal consumption resistance value which maximizes the saturation value of the load current,and the excitation time is relatively short.At the same time,by studying the influence of transformer parameters on excitation performance,it is found that 1)with the coupling coefficient k decreasing from 0.99 to 0.6,the saturation value of load current decreases significantly,and the excitation time is shortened;2)there exists a primary selfinductance which maximizes the saturated load current,but the advantage of excitation time is weakened;3)there exists a secondary self-inductance which maximizes the saturated load current but doesn't minimize the excitation time,and another secondary self-inductance which minimizes the excitation time but doesn't maximize the saturated load current.To sum up,since the maximum saturated load current and the shortest excitation time can't be obtained simultaneously,it should be reasonable to set the working frequency,consumption resistance value and transformer parameters according to the actual application requirements of the system for the sake of obtaining the target excitation performance.Finally,in order to verify the effectiveness of the above optimization method,a prototype of the self-driving HTS flux pump was built.The experimental results show that the addition of consumption resistance can effectively improve the excitation performance of the selfdriving HTS flux pump.Meanwhile,there is an optimal consumption resistance value to maximize the saturation value of the load current and shorten the excitation time.Further,the transformer iron core was removed and the non-contact excitation experiment was carried out.The experimental results show that 1)increasing the working frequency can offset part of the adverse effect(reduction of the saturated load current)caused by the introduction of the hollow coupler,and 2)the saturation value of the load current of this system can also be effectively increased by adding a consumption resistance with the appropriate resistance value.