Three-phase inverters-analysis of ability to maintain symmetrical output voltages

One of the important performance issues for three-phase PWM inverter technology is the maintenance of symmetrical output voltages. In this paper an equivalent circuit model for a three-phase PWM inverter is proposed. Based on this model the mechanism which distorts the symmetrical voltage output is analysed. It is revealed that the nonzero neutral current resulting from unbalanced AC outputs intrudes into the DC-link of the inverter. A proper path must be provided in the DC-link for this intruding current to circulate, otherwise its effect could destroy the symmetrical output voltage performance. A configuration, which has high inherent stability in maintaining symmetry, is suggested. Laboratory experiments are carried out on a 20 kW prototype three-phase PWM inverter and the results confirm the analysis well