Research And Design Of Three-phase Unilateral Three-level Inverter

With the high voltage power equipment widely used in the production and life, the requirements of the cost, efficiency and reliability of the inverter is higher and higher. Multi-level inverter has become a hot topic of many scholars, because the multilevel inverter output voltage closer to the sine wave, the harmonic distortion of its output voltage and current is low, the voltage stress of the switching device is small and the frequency and loss of the switch device is also low, etc.This paper firstly introduces the research significance, development course, research status of multilevel inverter and its application field, etc. It detailed analyzes the working principle and the advantages and disadvantages of several common inverters such as diode-clamped type, flying-capacitor type and cascade type with separated DC source inverters. Secondly, with the neutral point clamped type three-level inverter as an example, it analyzes the basic working principle, implementation process, and the applicable occasions of several kinds of commonly used pulse width modulation(PWM) control strategy such as the carrier disposition PWM control, the switching frequency optimal method, the seleeted harmonic elimination PWM and space vector pulse width modulation(SVPWM), etc,and several kinds of realization forms of the carrier disposition PWM control and switching frequency optimal method are analyzed in the simulation.Finally, this paper proposes a novel three-phase unilateral three-level inverter which uses Pre-Boost converter to implement three-level power supply and adopts three-level on one side and two-level on another side in the inverter part to reduce the number of switching devices and simplify the circuit structure. It analyzes the working principle and proposes a five segment work mode space vector pulse width modulation(SVPWM) strategy. At the same time, a switching frequency optimal method is presented which injectes specific zero sequence component into the unilateral three-level inverter to make its modulation wave a flat wave. Both of the modulation methods can implement normal modulation on the three-level side and low loss modulation on the two-level side. Under the two modulation strategys, the circuit has a relatively simple control algorithm, and can still achieve lower switching loss and less harmonic content of output voltage and current while reduce the volume of the inverter. Modeling in MATLAB/SIMULINK, the simulation results verify the effectiveness and superiority of the new type circuit and the validity and feasibility of its modulation methods.