Voltage modulation strategies and performance limitations of multilevel PWM rectifiers

A multilevel unidirectional PWM rectifier requires less switches compared with a bi-directional multilevel converter. With levels utilization decrease, semiconductor devices count is further reduced at the expense of compromising performance in terms of maximum modulation index without over modulation vs. voltage-current phase shift angle. After introducing multilevel PWM rectifiers classification according to levels utilization, this paper presents carrier based voltage modulation strategies for rectifiers with maximal and minimal possible level utilization. Suggested modulation strategies are based on appropriate discontinuous voltage commands transformations that keep track of available switching states dependent on phase current directions. Performance limitations are easily derived using simple trigonometry without making use of space vector calculus.