Three-Dimensional Flux Vector Modulation of Four-Leg Sine-Wave Output Inverters

The time integral of the output voltage vector of a three-phase inverter is often termed the inverter flux vector. This paper addresses the control of a three-phase four-leg sine-wave output inverter having an LC filter at its output, by controlling the flux vector in three dimensions. Flux vector control has the property that an output filter resonance is actively damped by an output voltage control loop alone. Furthermore, an inverter switching action inherently regulates the output voltage rapidly against dc-bus voltage variations. The flux vector control of sine-wave output inverters finds several applications in three-phase four-wire systems. This paper presents a 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. The switching behavior of the modulator has been evaluated, which is useful for the variable fundamental frequency applications of the inverters. To provide experimental validation, the modulator is implemented as a 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 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.