Control of four-leg sinewave output inverter using flux vector modulation

The time-integral of the output voltage vector of a three-phase inverter is often termed as the inverter flux vector. This paper addresses the control of a three-phase four-leg sinewave output inverter having an LC filter at its output, by controlling the flux vector in three dimensions. Flux vector control has the property that the output filter resonance is actively damped by the output voltage control loop alone. Further, the inverter switching action inherently regulates the output voltage rapidly against dc bus voltage variations. Flux vector control of sinewave output inverters finds several applications in three-phase four-wire systems. This paper presents the flux modulation method for three-phase four-leg inverters feeding unbalanced and nonlinear loads. All the necessary steps for the digital implementation of the flux modulator are presented. To provide experimental validation, the modulator is implemented as part of the control system for a stand-alone three-phase four-leg inverter with an LC filter at its output. Control system details are also provided. Experimental results indicate the effectiveness of the modulator and the control system in providing balanced voltages at the output of the LC filter even under highly unbalanced conditions with nonlinear loads. The resonance damping and voltage regulation properties of the modulator are also apparent from the experimental results.