Research On Control Technology Of Three Phase Four Legs Inverters

The three-phase voltage unbalance have a very important impact on the normal working of the electrical equipment and the electrical safety, with the rapid development of science and technology, the various unbalanced load put forward very high requirements on power quality. When the loads are unbalanced, the traditional three-phase inverter cannot output symmetrical three-phase voltage to meet the demand for electricity supply. The three-phase four-leg inverter can supply a midpoint for the four-wire outputs. It can control the zero sequence voltages and currents and have better output characteristics.This paper analyses the control strategies of three-phase four-leg inverter when the loads are unbalanced, such as the pulse width modulation (PWM), the hysteresis current control and the space vector modulation (SVM). The PWM control has a good waveform and switching frequency is low, but the DC BUS voltage utilization is low. The hysteresis current control hardware circuit can be easily designed, but the switching frequency is variable which makes us difficult to design the output filter, moreover the output waveform is not good. The space vector modulation has not only a high DC BUS voltage utilization, but also the control thoughts can be easily understand and easy to become digitized. There is no need for the SVM to have a transformer to be midpoint which the mass and volume are large, so it has become a popular control method. This paper researches the three-phase four-leg inverter based on the abc coordinate system SVM in detail.The SVM is used to the three-phase four-leg inverter in this paper which based on abc coordinates avoiding repeatedly transformation on αβγ coordinates, makes the selection of the switching vectors and duty cycles easier, reducing the complexity of the method. Firstly, This paper researches this method in detail and the method to obtain the reference voltage. Then, for the3kW specific system, it designs the main circuit and control circuit. Finally, the simulation model is bulit through MATLAB/SIMULINK. The simulation results illustrate the validity of the strategy and consistent with the theoretical analysis.