A Unidirectional Single-Phase AC-DC-AC Three-Level Three-Leg Converter

A unidirectional ac-dc-ac three-level three-leg converter is proposed in this paper. This converter is composed of a unidirectional neutral-point-clamped (NPC) leg and two bidirectional NPC legs. A bidirectional NPC leg is shared between rectifier and inverter sides. Compared with the two-level leg, the three-level leg presents voltage stress reduction on switches for the same dc-link voltage level and also allows a reduction in harmonic distortion for the same switching frequency. Suitable modeling, pulsewidth modulation, and control strategies of the system are presented. Also, a method for balancing dc-link capacitor voltages is shown. As the converter is unidirectional, the grid current and the generated voltage at the load side must be synchronized with the generated voltage at the grid side, which eliminates the zero-crossover distortion in the grid current caused by the use of diodes without the hysteresis control. Simulation and experimental results are also presented.