Single-Phase AC–DC–AC Multilevel Converter Based on Parallel-/Series-Connected Three-Leg Modules

Abstract

A single-phase ac-dc-ac topology composed of two three-leg modules parallel-connected at the grid side and series-connected at the load side is proposed in this paper. This topology is appropriated for applications as uninterruptible power supply, unified power quality conditioner, as well as for conversion systems in which the output voltage required is higher than the input voltage (voltage step-up applications). Circuit model equations, dc-link voltage specifications, and a technique to minimize the current in the shared legs of the converter are presented. An overall control strategy to regulate the dc-link voltages and to maintain synchronized the grid voltage and current is discussed. An interleaved pulse width modulation technique (PWM) based on four carriers was adopted to improve the number of voltage levels generated by the converter and reduce the total power losses. The dead-time effect is also analyzed. Comparisons are performed with conventional topologies in a wide range of power in terms of rating of the semiconductor devices, harmonic distortion, semiconductor losses, transformer losses, and efficiency. Compared to conventional ac-dc-ac topologies, the proposed structure has achieved the following advantages: reduced harmonic distortions, lower power ratings, lower switching losses and lower semiconductor total losses per leg, and higher efficiency. Experimental results are shown to demonstrate the feasibility of the system operation in steady-state conditions, under load transients, and supplying non-linear loads.