Research On Bus Voltage Control Strategy For Bidirectional AC/DC Converter Of Solid State Transformer

In order to cope with the crisis of global fossil energy shortage,natural environment deterioration,and reduce the excessive use of non-renewable energy,energy Internet has become a hot research topic all over the world.It is an energy network that connects a large number of distributed energy storage devices,distributed energy collection devices and various types of energy loads to form two-way flow of energy and information,which can make full and effective use of renewable clean energy.Solid state transformer(SST)is the core of electric energy conversion and transmission.In addition to achieving electrical isolation,energy transfer and voltage conversion,its biggest feature is a high degree of controllability,and it can flexibly control voltage amplitude and phase.In addition,the solid-state transformer can also realize the functions of harmonic optimization,reactive power compensation,power grid interconnection and distributed energy grid connection.In this thesis,the control strategy of bidirectional AC/DC converter in rectifier stage of solid-state transformer is designed.After defining the topology of the system,the control system is modeled in different coordinate systems,the current loop is decoupled by feedforward,the mathematical model is simplified,and a double closed-loop controller is designed to deeply study the control strategy of the rectifier stage.In order to solve the problem of poor disturbance rejection performance of traditional control strategy,linear active disturbance rejection control(LADRC)is introduced.Using linear extended state observer(LESO),the core part of LADRC,to observe the state variables of the system,and then the controller composed of sliding mode control(SMC)theory can greatly improve the anti-disturbance performance of the system.In view of the serious problems of observation error and phase lag in traditional LESO,in order to improve the dynamic response speed of the system,the LESO was improved based on the principle of error improvement.By introducing new errors as the adjustment basis of each state variable in LESO,the observation accuracy of LESO was improved.Through frequency domain analysis,it can be compared that the improved LESO has better observation performance and disturbance rejection performance,and the stability of the observer is analyzed by Lyapunov function,which proves that the new controller has better performance.Finally,the model is modeled by Matlab/Simulink simulation platform,and different kinds of working conditions are designed in the model.The effectiveness of the designed controller is verified by simulation results.