The Design And Research Of Power Supply And Quench Protection System For High-Field MRI Superconducting Magnet

Ultra-high magnetic field nuclear-magnetic images have ultra-high resolution and signal-to-noise ratio,and have become an important means of exploring life sciences and material sciences.Superconducting magnets are the core equipment of magneticresonance-imaging(MRI)equipment,to provide the main magnetic field for imaging.The magnet power supply and the quench protection system are important components of the superconducting magnet,and work cooperatively to ensure the stable operation of the superconducting magnet.In response to this,a magnet power supply and quench protection system have been designed and developed,as well as the dual-active-bridge(DAB)converter in the magnet power supply and the key components in the quench protection are deeply studied in this thesis.The main research work are as follows:The design and development of power supply prototype for 7 T MRI superconducting magnet have been carried out.Based on the performance requirements of the power supply(i.e.output adjustable controllability,high power density,and energy can be fed back to the grid),the design criteria and technical solutions of the magnet power supply are constructed.The focuses are on the design of the middle-stage DAB converter for the power supply,and the experimental prototype.The overall prototype of the magnet power supply was developed,and its performance was tested under load,which meets the design requirements.The developed power supply is a 10 kW power level superconducting magnet power supply with high power density and energy fed back to the power grid,which provides a reference for the development of power supply for MRI magnet.When the output power of power supply is suddenly changed,there will be a bias current in the DAB converter.And it will lead to the bias of the transformer,may cause the magnetic core to saturate,and thus a failure in power supply.In view of this hidden danger,the analysis and optimization of the bias current of the magnet power DAB converter are carried out.The formation mechanism of bias current was explored,the mapping relationship between bias current and power mutation have been quantitatively analyzed.Additionally an optimal control strategy was proposed.The theoretical analysis of the bias current has been verified,and the optimization strategy to suppress the bias magnetism is shown feasibility referring to the experiments.The magnetic core saturation margin is checked and calculated by the bias current theory,showing that the magnetic core still has a considerable margin.As a result,the converter can operate safely,and thus the power supply.The magnet power supply operates in the low-power range,and the MOS tube of the DAB converter is turned on hard mode,which will produce switching losses.Furthermore,there are an increase in temperature and a decrease in efficiency.To solve this problem,the research on the soft switching strategy of magnet power DAB converters has been carried out.The minimum inductive energy required for zerovoltage switch(soft switching)is quantified.For the goal of the minimum current peak value combining with the operating mode and zero-voltage switch constraints,the optimal control strategy is analyzed and solved.The experimental results show that the MOS tube can realize zero-voltage switch in the low power range.The optimization strategy has been integrated into the power supply prototype,showing that the temperature rise in the power supply is reduced and the efficiency is improved(when the output is 20 V/40 A,the temperature rise is reduced by 29℃,and the efficiency is increased by 3%).The design and testing of MRI superconducting magnet quench protection system have been carried out.Coil segmentation is a commonly used method for superconducting magnet quench protection,which can effectively limit the terminal voltage,inhibit heat accumulation and avoid damage to the coil.The MRI quench protection system is initially designed.As the MRI superconducting magnet is still being assembled,a quench protection system is designed,manufactured and validated by the 15 T/78 mm aperture solenoid superconducting magnet with similar structure as the research object.16 quench exercise tests have been carried out and the results showed the effectiveness of the quench protection system.Besides,the quench time sequence is analyzed;the discharge amount of energy stored in the component is calculated,and the reliability of the calculation results is checked with the measured temperature rise of the coil.The research on key components of quench protection system have been conducted.The diode and superconducting switch components work in the 4.2 K temperature region and 0-3 T stray magnetic field environment.So the characteristics are directly related to the reliability of the quench protection system.A diode performance test platform was built to test and characterize the characteristics of the diode in the low temperature and magnetic field environment.The NbTi superconducting switch was developed,and the relationship between quench resistance and heating power was studied.The research work provides key performance parameters for the analysis and design of the quench protection system,which further ensures the reliability of the superconducting magnet operation.The magnet power supply and quench protection system are guarantees for the reliable operation of high-field superconducting magnets.In this paper,power supply and quench protection system are regarded as the object to carry out related design and development work.In addition,the research on key components to improve the operation of the magnet reliability.The research results of this paper provide important support for the stable and reliable operation of superconducting magnets,and reference for future performance optimization of MRI magnets.