Effect of capacitor voltage ripples on submodule active power control limits of cascaded multilevel converters

Abstract

In the operation of cascaded H-bridge (CHB) converters and modular multilevel converters (MMCs) with energy storage or renewable power resources, unbalanced active power distribution among the submodules (SMs) is unavoidable. Depending on the operating conditions, there are certain upper and lower limits on the active power that can be processed by a single SM or a subset of SMs. The control system needs to restrict the SM power references to these limits, hence, accurate knowledge of the power limits is important. In existing methods to derive the power limits, the SM capacitor voltages are assumed to have negligible ripples, whereas in practice the ripples can be considerable. This paper analyzes the effect of capacitor voltage ripples on the SM active power control limits and highlights the importance of considering the ripple effect. A methodology is proposed to accurately incorporate capacitor voltage ripples in the derivation of SM active power control limits. Simulation and experimental results are provided to evaluate the effectiveness of the proposed methodology.