Research And Design Of The Acceleration Grid Power Supply Of CFETR N-NBI Prototype

The planned China fusion engineering test reactor(CFETR)needs one or more negative-ion-based neutral beam injectors(N-NBIs)with a beam energy of 500 keV to heat plasma and drive current.In order to prepare for CFETR,a prototype of CFETR N-NBI is being designed to accelerate hydrogen negative ions up to 200 keV with a beam current as high as 20 A for 3600 s @ max,and an acceleration grid power supply(AGPS)rated at 200kV/25 A is needed.However,China has no experience in the development of such high-voltage power supply(HVPS).As the first inverter-type HVPS used in fusion plasma auxiliary heating field in China,the development of the AGPS for CFETR N-NBI prototype is extremely difficult and full of challenges.Reasonable and careful design is the fundamental guarantee for successful development.According to the requirements of CFETR N-NBI prototype,this paper completes the overall design of the AGPS.As the key components of the AGPS,the three-level neutral point clamped(NPC)inverter and the isolated step-up transformer are designed and studied carefully.What's more,some key technologies such as output voltage ripple of the AGPS and DC component of the inverter are also studied.Through investigation and comparison,this paper decides to adopt single-stage inverter-type HVPS as the whole scheme of the AGPS for CFETR N-NBI prototype.The AGPS mainly includes a 12 pulse rectifier based on thyristor,a dual-dc-link system,a three-phase three-level NPC inverter,a isolated set-up transformer,a high voltage(HV)uncontrolled rectifier and a HV R-C dc-filter.The two key parameters of dc-link voltage and inverter frequency are determined as follows,Vdc =5000V and finv =150Hz.By means of theoretical analysis and numerical calculation,the schemes and basic parameters of each part are determined.Also,a circuit simulation model of the AGPS is established using MATLAB/Simulink,and the rationality of the scheme design is verified by the simulation analysis of various working conditions.In this paper,the three-level NPC inverter based on press pack IEGT is designed and studied,focusing on the phase leg assembly.For three-level applications,IEGT has been tested extensively and its characteristics have been clarified.Based on the test results of single transistor,the design of three-level NPC phase leg is completed.The reliability of the electrical,magnetic and thermal design of the inverter is preliminarily verified and studied by simulation.In addition,a three-level NPC phase leg assembly is developed and tested by a novel four-switch action double-pulse test method,which further verifies the rationality of the design of the three-level NPC inverter.Based on the research and design of this paper,the fault switch-off time of the AGPS for CFETR N-NBI prototype can be reduced to less than 100?s,which is greatly improved compared with the similar parameter of ITER(150?s).The isolated step-up transformer is designed and studied in this paper.The transformer is the first high-voltage high-power square-wave transformer with non-power frequency in China.This paper summarizes its characteristics and puts forward some corresponding solutions.With emphasis on core and winding,the design of main electromagnetic structure of the transformer is completed.The basic parameters such as no-load loss,no-load current,load loss and short-circuit impedance are calculated.The loss and the temperature rise caused by the leakage magnetic field are studied and calculated by means of numerical calculation and finite element simulation.The main and longitudinal insulation of the isolated step-up transformer is studied and calculated by special electromagnetic simulation software,and the reliability of the design is verified.In this paper,the output voltage ripple of the AGPS and DC component of the inverter are studied,which are related to the performance and safety of the accelerator power supply.By means of simulation and theoretical analysis,the output voltage and ripple characteristics of the AGPS are studied,and the condition for limiting the ripple of the output voltage is found.Based on the analysis results of ripple characteristics,a output voltage ripple restriction measure is proposed.The cause,influence and precise extraction method of the DC component of the inverter are studied,and a DC component control strategy suitable for the AGPS of CFETR N-NBI prototype is proposed.The analysis results and the proposed control strategy are verified by circuit simulation and prototype experiment.The design and research work of this paper lay a solid foundation for the development of AGPS for CFETR N-NBI prototype,and have a certain reference value for the design,development and operation of the same kind of inverter-type HVPS.