Three-phase three-level one-cycle controlled bidirectional AC-to-DC NPC converter

A one-cycle control based controller for three-phase three-level bidirectional ac-dc NPC converter with unity power factor is proposed in this paper. One-cycle control (OCC) based schemes applied to three-phase three-level rectifiers with reduced number of switches (VIENNA rectifier) have been reported in literature. But, they cannot operate in inverting mode of operation due to the absence of essential power electronic switches and inherent instability of OCC based systems during inverting mode. The controller proposed in this paper addresses this limitation of instability and enables the said NPC converter to operate stably even when the load on the converter is suddenly removed or it is made to operate in inverting mode of operation. The proposed controller is simple to implement as it can be realised using an integrator-with-reset, few flip-flops and comparators. Simulation studies are carried out to validate the proposed scheme.