Abstract
A novel transformerless three-phase unidirectional bridgeless flying
capacitor (UBFC) rectifier for medium-voltage and high-voltage
applications is presented by this paper. The rectifier consists of three
single-phase rectifiers in wye connection, the DC outputs of the three
single-phase rectifiers are connected in parallel to provide a high
common DC-link bus which makes it convenient to connect with NPC or FC
multilevel inverters. To avoid phase to phase current circulation, an
additional inductor and diode are added to each single-phase rectifier.
Compared with conventional bidirectional multilevel converters, only
half IGBTs are employed with lower voltage stress, line-frequency
transformers are no longer be required, which makes the proposed
rectifier has a series of advantages such as reduced cost, simplified
control complexity, increased reliability and improved overall
efficiency. The basic operating principle of the proposed rectifier in
continuous conduction mode is discussed, an improved carrier-based
level-shifted PWM modulation method integrated with voltage balancing
and the double closed-loop control strategy are proposed to achieve more
voltage levels and keep the flying capacitors in balance. The
feasibility of proposed rectifier is verified by the simulation as well
as experimental results.