A decoupling offset-based PWM control for a multilevel inverter under DC voltage unbalance

In this paper, a novel offset-based two-level pulse width modulation (PWM) scheme for a multilevel inverter with unbalanced dc sources is presented. The offset voltage is introduced as an indispensable variable in the control model for a multilevel voltage source inverter. An expression for the inverter parameters in abc coordinates can enable the determination of the operating ranges of related voltages for the unbalanced dc source condition. The PWM performance is controlled through the design of two offset components in a subsequence. One offset defines the operating points of the three-phase leg voltages among the dc levels, while the second offset restricts its effect on the quality of PWM control in related dc levels. A nominal switching time diagram and voltage vector diagram for a virtual two-level inverter reveal the typical characteristics of offset-based two-level modulation. With regards to the actual application of the scheme, the voltage outputs remain balanced even with abnormal dc sources. An improving of the PWM quality as reduction of the switching loss in Discontinuous PWM mode can be obtained by setting the local offset related to the load currents. The validity of the proposed algorithm was verified by experimental results.