Coordinated Control Of Front And Rear Stages Of AC-DC Isolated Solid State Transformer

In recent years,with the development of energy Internet,more and more renewable energy and energy storage devices are connected to the power grid.Because the traditional power transformer can not adapt to the flexible transmission and efficient utilization of energy,and the single interface can not meet the diversification of user needs,it is gradually eliminated.As a new type of connection equipment,solid state transformer(SST)plays a vital role in the future energy Internet because of its high and low voltage interfaces,flexible transmission and efficient utilization of energy.This paper takes the two-stage cascade SST as the research object,and analyzes how to strengthen the consistency and coordination of power transmission between rectifier stage and isolation stage when SST is disturbed by the outside world,so as to reduce the voltage fluctuation between the two stages and improve the overall dynamic performance of the system.The research contents are as follows.Firstly,this paper introduces the research background and significance of solid-state transformer,summarizes the research status of typical topology of solid-state transformer at home and abroad,including the detailed introduction of single-stage,two-stage and three-stage topology,and leads to the topology structure of this paper.By summarizing the control strategies studied by scholars at home and abroad,the front and rear stages of solid-state transformer are coordinated with the idea of nonlinear control.The SST circuit topology with two cascaded front and rear stages consists of rectifier stage and bidirectional intermediate frequency DC/DC isolation stage.The working principle and four quadrant operation characteristics of rectifier stage are analyzed in detail;The state space model in the dq coordinate system of the rectifier stage is established.Then the topology and working state of the bi-directional if DC/DC isolation stage are analyzed in detail.Finally,the mathematical model of bi-directional power transfer is derived to provide help for the next control.When the power grid is unbalanced,if only SST integrated control under balanced conditions is adopted,negative sequence current will be generated on the AC side,and different characteristic harmonic voltages and currents will be generated on the DC side,which will affect the normal operation and dynamic performance of solid-state transformer.Therefore,taking the front and rear two-stage cascaded SST as the research object,a two-stage integrated average model is established under the condition of three-phase power grid imbalance,and accurate feedback linearization is used to eliminate the nonlinear coupling between rectifier stage current loop,interstage voltage and DC side voltage.On this basis,the integral sliding mode controller is designed for each control loop,and the parameter configuration is designed.This control method not only ensures the quality of AC side current,but also improves the coordination of front and rear power transmission and the robustness of system parameter perturbation.When the power grid is unbalanced,if only the three-phase VSR control strategy under the condition of three-phase balance is adopted as the basis,it will lead to negative sequence current on the AC side and characteristic and non characteristic harmonic voltage and current on the DC side,so as to respond to the normal operation of solid-state transformer.Therefore,the former and latter two-stage cascaded SST is the research object,Under the condition of unbalanced three-phase power grid,a two-stage integrated average model is established,and the input-output accurate feedback linearization method is introduced.At the same time,the nonlinear coupling between RS current loop,interstage voltage and DC side voltage is eliminated,and the accurate feedback linearization of SST is realized;The decoupling of the front stage current loop,the voltage between the front and rear stages and the voltage on the DC side of SST is realized,the consistency of power transmission between the front and rear stages is strengthened,and the quality of grid connected current on the AC side of SST is guaranteed.The sliding mode controller is designed and the control parameters are designed in detail.Finally,the experimental platform of solid-state transformer is built,and the design process of hardware system is introduced in detail,including device selection and hardware circuit design.Secondly,the design of the software system is given,including the main program design,sampling circuit design,control algorithm program design,protection program design,configuration screen interface design,etc.,and the detailed control flow chart is given.Finally,the correctness and effectiveness of the above methods are verified by simulation and experimental results.