A Medium-Voltage Three-Phase AC-DC Converter Consisting of Cascaded Three-level Boost-type Rectifiers and an Open-End Winding Transformer

This paper proposes a three-phase AC-DC converter topology used for a front-end converter of medium-voltage (MV) motor drives suitable for inverter with a common DC-link bus, such as modular multilevel converters (MMCs) or others. A multilevel cascaded H-bridge inverter is traditionally used for the MV drives which has distributed DC-link buses. For the purpose of the reduction of harmonics and the production of the distributed DC-voltage sources, the multilevel cascaded H-bridge inverter is generally used with a phase-shifted multi-winding transformer and diode rectifiers. However, the phase-shifted multi-winding transformer has many secondary windings, resulting in complex structure, bulky and costly. The MMC is one of the most expected topologies as a next-generation inverter for the MV drives. However, the front-end converter topology still consists of the phase-shifted multi-winding transformer and the diode rectifiers. Besides, the diode rectifiers have no capabilities of boosting DC-link voltage and suppressing the influence of the voltage sags. By contrast, cascaded single-phase three-level boost-type rectifiers and an open-end winding transformer can provide less harmonics, high functionality and capabilities of boosting DC voltages and suppressing the influence of the voltage sags. This paper presents the circuit configuration, the operation principle, and the simulation results with the input voltage of 6.6 kVAC and the output voltage of 12 kVDC, where a novel control method by a third-order harmonic current injection to reduce the harmonics is proposed. The simulation results verify the validity of the proposed configuration and the proposed control method.