Research On Control Algorithm For The Active Clamped Three-level Grid-connected Inverter

This paper takes the active clamped three-level inverter as the research object. Mathematical models are established according to each specific problem and appropriate control algorithms are selected to achieve the control purpose. Finally the overall design of the inverter system is completed.At first, the paper analyzes T type topology structure of the main circuit and its working principle to summarize its control rules and discusses several common modulation methods in detail. The paper takes SPWM control method based on the in-phase triangular carrier waves and sets aside certain dead time.There is a neutral point potential unbalance problem in three-level inverter. The paper points out the reason and harm. On the basis of defining switch function, it studies the close relationship between the neutral point potential and the neutral point current. It deducts the neutral point potential model of three-level inverter and takes the method of superposing zero sequence voltage component which is calculated exactly. The neutral point potential unbalance is effectively controlled.Before connecting the grid the inverter needs to use phase-locked loop control technology. The paper analyzes the working principle of PLL to establish the dynamic model. The classical PI controller is used and control parameters are reasonably designed in order to make the output current of the inverter to track the grid voltage. Thus they can work in the same phase and frequency.In order to improve the dynamic and static performance of the system, double closed-loop control strategy is adopted in this paper. The dynamic model of current loop controller is established. PI control strategy with voltage feed-forward and decoupling method are used. Control parameters are properly designed so that the grid current tracks the given current command to make the inverter work in unit power factor and it is convenient to control the active power and reactive power. The dynamic model of the voltage loop controller is also established. Applying proper PI control strategy and choosing proper control parameters, it can stabilize DC bus voltage and reduce the fluctuation.At last, in order to verify all of the control algorithms, on the basis of theoretical analysis the simulation model of the whole inverter system is built on the Matlab/Simulink platform. All above issues are simulated and the simulation results are analyzed. The results verify the correctness and feasibility of the control algorithms.