Research Of Double-Mode Inverter Control Technology Based On Three-level T-type NPC

ABSTRACT:In recent years, shortages of conventional energy and aggravation of environmental pollution, so that the new energy development and utilization has become the focus of attention. At the same time, the power system have a lot of problems with distributed generation systems, if large-scale distributed power connect to grid, the power grid will have an impact, it may make large power grid uncontrollable, and even endanger the normal operation of the power grid. Prominent contradictions make microgrid systems become a hotspot of research in the field of new energy. Because the inverter is indispensable part of micro grid system, so the inverter has been more and more attention. In view of TNPC three-level double-mode-inverter as the research object, this thesis analyzes the control technology of TNPC three-level double-mode-inverter in detail.Double-mode-inverter is an important part of microgrid system, it can work in grid-connected mode when the microgrid energy enter into the grid and stand-alone mode to provide voltage and frequency support for local load. In order to make power supply uninterrupted for local load, it is seamless transfer between two modes. This thesis focuses on the grid-connected mode, stand-alone mode and seamless transfer of double-mode-inverter. Because this thesis is based on the research of TNPC three-level double-mode-inverter, therefore, the inherent midpoint potential problem of three-level inverter also will be the key and difficult point for this study.This thesis analyzes the cause of the double-mode-inverter with three-phase four-wire connection, and then by analyzing the unification of SPWM and SVPWM in in three phase four wire systems, this thesis indicate the reasons for selecting SPWM modulation algorithm. Then, this thesis by analysis and comparison of the midpoint potential control methods, finally, choice to use a fourth leg to control the midpoint potential, at the same time, put forward the corresponding control strategies and set up a simulation model to verify the control strategy. Then, this thesis builds the mathematical model for double-mode-inverter, studies the network voltage oriented grid-connected control strategy, voltage source stand-alone mode control strategy based on droop and seamless transfer control strategy. It sets up simulation models of grid-connected, stand-alone and seamless transfer in the MATLAB/Simulink to verify the effectiveness of the control strategy. Finally, with the experimental platform of100kW energy storage converter to verify the grid-connected control strategy; building a3kW TNPC three-level inverter semi-physical experimental platform with RT-LAB to verify stand-alone control strategy and the midpoint potential control strategy.