The Control Methods And Simulations Of Four-leg Inverter

Generally, the three phase inverters are composed of three legs. However, when it is applied to the three phase unbalanced loads, its three phase voltage outputs will become dissymmetrical. Recently three-phase four-legged inverter is proposed to solve unbalanced three-phase loads. The new topology is known to produce balanced voltages even under unbalanced load conditions. Compared to the other three-phase four-wire inverters, its size and weight are smaller.Firstly, mathematics model of four-leg three-phase inverter is analyzed for which the relationship between variables, such as output voltage and inductance current, can be established by the model.Secondly, main control strategies for three phase four leg inverter include SPWM, hysteresis current control and space vector modulation. The SPWM control has a good waveform but the DC BUS voltage utilization is low. Based on the SPWM, a control method for four-leg inverter is proposed. The designed controller based on the idea of entire compensation by the fourth leg is very simple and effective, which realizes the independent modulations for four legs.Thirdly, the hysteresis current control hardware circuit is easily designed. However the switching frequency is variable which makes us difficult the design the output filter, moreover the output waveform is not good. The space vector modulation techniques, by selecting the zero vectors properly, can reduce the harmonic content of the output voltage, so it has become a popular control method. The modified voltage regulator on the basis of PI regulator is proposed to eliminate output voltage static error under any load condition。The two-dimensional SVM control strategy can still be available in four-legthree phase inverter. This method is featured with the fact that it could increase voltage utilization, lower total harmonic distortion and reduce the switching frequency in a practical way.Finally, the simulation of system control validated consistency with theory analysis using MATLAB, and the effectiveness of the controller is verified by simulation results.