Voltage-balancing controller in cascaded multilevel converters has been discussed extensively in previous literatures where several effective methods have been proposed. The coupling effect between a voltage-balancing controller and the original system controller is however not addressed comprehensively. This paper proposes a new voltage-balancing controller for single-phase cascaded multilevel converters in a d-q coordinate. The theoretical finding shows that the proposed method can effectively eliminate the coupling between two controllers in both steady and dynamic state. Simulation and experimental results validate the proposed method.