Research Of Three Phase Four Legs Inverter Under Unbalanced Loads

As one of the three phase four wire converter topologies, three phase four legs converter can provide the channel for zero sequence current to deal with three phase unbalanced loads or unbalanced sources. Compared with traditional three phase four wire converters, especially the three phase four wire converter with 50Hz output transformer, three phase four legs converter has many merits such as simple structure, small volume and low cost. In this paper, the modulation techniques, closed loop control and imbalance burden of three phase four legs inverters are analyzed. In order to solve the problems mentioned above, a novel design which combines three phase four legs inverter and PIR double loop controller is proposed in this paper.In this paper a voltage modulation method of harmonic injection for the three-phase four-leg voltage source converter is described. The four-leg converter can produce three output voltages independently with one additional leg. The proposed modulation method for the four-leg converter can be implemented with a single carrier by a"offset voltage"concept. The method is equivalent to the three-dimensional space vector PWM method, but its implementation is much easier. The feasibility of the proposed modulation technique is verified by simulation and experimental results. These results show that this carrier-based pulse width modulation (PWM) technique can be easily implemented without conventional computational burden. The analysis and simulation of PIR double loop regulator are discussed. The simulation results verify the theory analysis.At last, a three phase four legs inverter is designed and the digital control based on DSP TMS320F2812 is realized. Experimental results are compared and analyzed.