Abstract: Power electronic transformer is an important interface device for connecting AC/DC power distribution systems in the renewable energy generation grid. However, the DC voltage of the front-stage single-phase cascaded H-bridge rectifier will deviate because of the back-stage unbalanced load. In this paper, the voltage balancing boundary based on the optimal vector selection strategy is derived to solve the problem that space vector pulse width modulation (SVPWM) fails to balance the voltage. According to the relationships between the load unbalance index boundary and the number of cascade cells, modulation index, and power factors, a control strategy is proposed. The voltage balancing capability can be improved by changing the modulation index or power factors. Therefore, to ensure stable operation under severe unbalanced load conditions, a wider voltage balancing range is established by changing the DC voltage reference or reducing power factors. The balancing boundary and the effectiveness of the proposed expansion method, which guarantees equalization under extremely unbalanced conditions, are verified by simulation and experimental results with a three-cell cascaded rectifier.