Research on voltage balance and power balance control for three-phase cascaded multilevel converter based power electronic transformer

As one of the most important parts of the future smart grid system, the three-phase cascaded multilevel converter based power electronic transformer (PET) mainly consists of three parts, the three-phase input rectifier, middle dual-active bridges (DAB) dc-dc converter and output inverter. For cascaded multilevel converter based PET, due to unbalanced issue of DC output voltage of each rectifier module or transmitted power in each DAB module, switches in PET often suffers uneven voltage or current stresses. This paper presents a voltage balance control strategy in multilevel rectifier stage and a sensorless power balance control strategy for the modular DAB converter to solve voltage unbalance issue and the power unbalance issue caused by mismatch of the parameters of each DAB module. The voltage balance of the three-phase rectifier is achieved by the combination of a primary voltage loop and an auxiliary voltage loop. The active power component of the duty cycle in the rectifier stage is used as power feedback to adjust the duty cycle of each DAB module to realize power balance. Furthermore, there is no current sensor used in this power balance control algorithm. Finally the proposed strategy is verified through simulation.