Coordinated Optimization Control For Power Electronic Transformer Based Hybrid AC/DC Microgrid

As the core equipment of hybrid microgrid,power electronic transformer is a new type of multifunctional transformer which integrates power electronic technology,information communication technology and control protection technology.On the background of the Beijing Chongli Low-Carbon Winter Olympics Smart Grid Demonstration Project,this paper proposed an AC/DC hybrid microgrid topology with dual-end power supply based on power electronic transformer.Moreover,a coordinated optimization control for AC/DC hybrid microgrid based on power electronic transformer is studied.The content of the paper mainly includes:First of all,the common topology of power electronic transformer is studied,and a threestage four-port power electronic transformer is proposed according to the demand of demonstration project.The interface converter topology structure and control method of distributed generation and energy storage in hybrid microgrid are analyzed,and the system model of hybrid microgrid is established.Simulation results show that MPPT control can maximize the power output of distributed generation,and piecewise droop control can realize seamless switching between power control and voltage control in the grid-tied or autonomous mode.Secondly,the coordinated control method of power electronic transformer under grid-tied or autonomous mode is studied.In grid-tied operation mode,the power electronic transformer operates in voltage control state based on master-slave control.In autonomous mode,the power electronic transformer operates in the power control state based on peer control.In order to solve the problem of frequency and voltage deviation of AC/DC bus under autonomous mode,a coordinated AC/DC power control method based on residual power is proposed,which can not only avoid excessive coupling,but also support each other to a certain extent.Furthermore,the rotor motion equation is introduced into the coordinated AC/DC power control method,so that the power electronic transformer can not only balance the power of the AC/DC bus,but also have a certain inertia,so that its output power changes relatively gently,so as to improve the power quality of the hybrid microgrid.Finally,the operation mode switching method of AC/DC hybrid microgrid based on power electronic transformer is studied,including seamless operation mode switching strategy and adaptive operation mode switching strategy.In order to realize seamless mode switching,the transient process of mode switching is analyzed,and different modes are fused together by different input from the outer ring.In order to realize the adaptive switching of operating modes,the steady-state criterion and switching conditions of each operating mode are studied on the basis of the semi-tensor product.Simulation results under PSCAD/EMTDC environment show that the proposed control strategy can accurately identify the operation mode of the microgrid,and can automatically switch the operation mode under the fault condition.The switching process has less impact on the microgrid,which improves the power supply security and reliability of the microgrid.