Reactive Power Distribution in the Cascaded H-Bridge Converter With Unbalanced Submodule Active Power for Capacitance Reduction

In the operation of the cascaded H-bridge converter-based battery energy storage system, the active power of each phase can be unevenly distributed among the submodules (SMs) for various reasons. In such conditions, conventional control methods yield unbalanced capacitor voltage ripple magnitudes among the SMs due to the active power disparity. Consequently, the capacitor voltage ripples will be relatively large in some SMs, increasing the capacitance requirement of the system. This article develops a method to balance the capacitor voltage ripples by coordinating the reactive power distribution with the active power distribution among the SMs. Accordingly, the proposed method can effectively reduce the capacitance requirement, as verified in both simulations and experiments. In one case study, the proposed methods can reduce the capacitance requirement by more than 50% compared with the conventional methods.