Design And Simulation Verification Of Isolation Step-up Transformer For CFETR N-NBI Prototype Accelerator Grid Power Supply

In order to develop the technology of accelerator grid power supply for the negative-ion-based neutral beam injectors(N-NBI)of China fusion engineering test reactor(CFETR),the national key research and development project Research and development of CFETR N-NBI negative ion accelerator and accelerating power supply technology funds the research of the 200 k V / 25 A AGPS.The prototype is planned to adopt inverter DC high-voltage power supply scheme,in which the isolation step-up transformer undertakes voltage isolation and energy conversion between the primary and secondary sides.The transformer is the key equipment to determine the high-voltage power supply voltage level as well as safety and reliability.In addition to the conventional characteristics such as large capacity and high voltage level,the transformer also needs to meet the operation requirements under non-power frequency and high harmonic input,but China has no relevant research and development experience at present.According to the requirements of CFETR N-NBI prototype,this article analyzes and calculates the parameters and the operation characteristics of the isolation step-up transformer,also proposes corresponding solutions for special operating conditions,then completes the main and accessory electromagnetic structural design of the 6 MVA/141 k V/150 Hz isolation step-up transformer,furthermore,uses simulation to carry out verification work.This artical starts with the calculation of key parameters and the design of electromagnetic structure.According to relevant requirements of the AGPS,the connection of transformer winding is determined to be △ / Y,and the operating frequency is 150 Hz.Afterthe numerical calculation,the design input parameter step-up ratio and the short-circuit impedance are obtained.Corresponding measures are taken to solve the special operation situation,so the low-loss lamination materials and the working magnetic density are determined,therefore,the electromagnetic structure design of the core and windings is completied and the transient characteristics are fully improved.Finally,the basic parameters of the transformer are checked to verify the overall rationality of the scheme.Since the design plan is clarified,the following aspects of research are carried out according to the electromagnetic,thermal technical specifications in the national standard.First,verification of main and longitudinal insulation of the isolation step-up transformer.Using the professional electromagnetic simulation calculation software VLN and ELAX-2D,the lightning surge process and the optimization of insulation structure design are studied.The electric field calculation results of insulation test method are compared and analyzed to ensurethe safety of transformer main longitudinal insulation design in various high voltage insulation tests.Secondly,magnetic field verification with loss and temperature rise calculation of isolation step-up transformer.The main flux of transformer is calculated by Maxwell.The maximum magnetic density and the corresponding loss value are obtained by calculating the distribution of the leakage magnetic field in the structural part with Elmag-3D,and the temperature rise value is obtained.Focusing on the non-standard voltage input of this project,the winding loss and temperature rise caused by high-order harmonics are studied to verify the reliability of transformer thermal design.Thirdly,the inrush current calculation of isolation step-up transformer.This part begins with the empirical formula based on the engineering experience to clarify the severity of the inrush current of the project,then uses the MATLAB Simulink software to study the simple no-load switch-in situation.This artical discusses the method of using control switch to restrain inrush current,and finally decides to adopt the way of zero rise voltage to close.Fourth,DC bias analysis of isolation step-up transformer.The algorithm of DC bias withstand capacity of three-phase three-limb transformer in the engineering experience is adopted to analyze the relevant characteristics of the transformer.The magnetic circuit coupling algorithm is used to study the influence of DC bias on the transformer through ANSYS electronics,and the change rules of excitation current,output voltage and core magnetic flux density under different DC biases are summarized.According to the topology structure of the AGPS,sliding window algorithm is selected to detect and suppress the DC component of the inverter from the source.Fifth,seismic verification of isolation step-up transformer.According to the seismic vulnerability and specific destroy type of each component of transformer,ANSYS 3D finite element modeling and simulation analysis are used to solve the natural vibration frequency and mode shape,consequently,the dangerous parts in structural design are found out by response spectrum analysis method,which provides the basis for subsequent stress monitoring.Combining the above theoretical analysis and simulation calculation,it can be shown that the non-power frequency and high-power square wave isolation step-up transformer designed in this paper is safe and reliable.The research and design process can provide reference for the development of similar special transformers.