Research On Control Stratege Of Three-Level T-Type Inverter For Micro Grid System

Micro grid can run in the normal operation of power grid, and supplies the whole load system with independent power. As an important unit of power electronics in micro grid applications, the status of invertorwill directly determine the quality of the whole operation of microgrid. This paper uses T type NPC inverter, and on this basis, the main contents are as follows:(1) With regard to the large amount of calculation and low efficiency in traditional SVPWM strategy, it adopts g-h coordinate system to improve the efficiency of this modulation strategy. In-depth analysis of coordinates SVPWM vector synthesis based on g-h coordinate system and the odd function characteristics, Chapter two proposes a improved modulation strategy to rearrange vector synthesis order on each switching cycle. Compard the improved modulation strategy with the traditional one through MATLAB simulation, the results show that the improved modulation strategy is easy to achieve, and can reduce the harmonic content of the output voltage in the process of modulation with improved efficiency.(2) According to the operation characteristics of the micro grid system, this paper uses droop control and feed forward decoupling control to form three loop control of the inverter. To study the characteristics of droop control in low voltage operation, the virtual impedance strategy is adopted to realize power decoupling control in order to calculate the power Accurately. In order to achieve smooth switching of dual mode in micro grid, it puts forward a kind of dual mode switching strategy by analyzing characteristics of the droop control of active and reactive power ring. This strategy is able to satify seamless switching between two modes with simpledesign and debugging. Through the MATLAB simulation experiments, the results show that this switching strategy can achieve fast and dual mode switching between grid connection and island mode.(3) To detect harmonic information in the output voltage and current of inverter, Chapter four proposes a harmonic measurement algorithm based on state observer theory. By separate observation of the harmonic information in mixed signals through multi module design and generation of a quadrature signals corresponding to the harmonic information, Observer applies the obtained information to calculate the virtual impedance. Stimulating the harmonic estimator algorithm through MATLAB, the results show that for any periodic signal, the algorithm can quickly observe the harmonic information and orthogonal signal.(4) Aiming at the harmonic in output voltage of the inverter in the islanding operation,and presents a feedforward algorithm by using harmonic observer algorithm according to feedforward control theory. Through the harmonic in output voltage and current, and produces new modulation waveform concerning the modulation instruction in three loop control. A compound control algorithm is produced by the combination of harmonic feedforward algorithm with modulation strategies and control algorithm. Through MATLAB simulation, the results demonstrates that T type NPC inverter based on compound control algorithm can improve the output quality and realize fast response and stable operation in microgrid system.(5)Based on TMS28377 S, FPGA and STM32 design of all digital control platform, it builds the experimental device of T type NPC inverter, and conducts hardware experiment about a part of the strategies and algorithm in this paper. Finally, the experimental results verify the feasibility and effectiveness of the algorithm.