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. Index Terms-Active power control limits, cascaded H-bridge, capacitor voltage ripple, hybrid integration of energy sources, modular multilevel converter.

Grid-Connected Energy Storage Systems: State-of-the-Art and Emerging Technologies

“…The MMC topology with half-bridge SMs (see Fig. 11) provides an alternative cascaded topology for the integration of utilityscale BESS and has attracted research interest over recent years [115]- [117]. The structure of the MMC allows for connection of the energy storage elements either directly to the MV dc-link or in a distributed manner across the SMs that form the converters' arms [116], [118].…”

“…The control, however, needs to ensure that the generated power references are within the power limits of the ESS. Two types of limitations restrict the active power of the SMs, namely, hardware limits and control limits [110], [117].…”

Unbalanced Active Power Distribution of Cascaded Multilevel Converter-Based Battery Energy Storage Systems

“…where v * arm (t) represents the arm voltage reference, v n C represents the sum of capacitor voltages of the SMs in this subset, and V N C represents the sum of capacitor voltages of the N SMs in the arm. Similarly, for bipolar SMs, v n max (t) and v n min (t) are [24]:…”

Battery Fault Tolerance of Modular Multilevel Converter-Based Battery Energy Storage Systems with Redundant Submodules

“…The extended output voltage range enables the SMs with healthy batteries to absorb/provide more active power, according to (7) and (8). Thus, a reduced number of SMs will be required to absorb/provide the required arm power, which means an enhanced battery fault tolerance.…”

“…Since there are limits on the active power that can be processed by a certain number of SMs in each arm [6], [7], the remaining SMs may fail to meet the active power requirement if the batteries of too many SMs become faulty. There have been some investigations on the battery fault tolerance of the MMC-based BESSs.…”