Energy Storage Requirement Reduction Using Negative-Voltage States of a Full-Bridge Modular Multilevel Converter

Utilization of negative-voltage states substantially reduces the energy storage requirements of a full-bridge submodule-based modular multilevel converter (FB-MMC). This study provides a detailed analysis of the capacitor voltage ripple and energy storage requirement reduction effects of the FB-MMC by utilizing negative-voltage states. The analytical expressions of capacitor voltage ripple following the change in negative-voltage state factor (NVSF) are derived. Results show that the capacitor voltage ripple reduction effects using negative-voltage states are strongly correlated with the operated power factor angle. In addition, the second-order ripple component must be considered to increase the accuracy in evaluating the voltage ripple reduction effects. This study defines the energy storage reduction factor (ESRF) to indicate the level at which the energy storage requirement can be minimized by selecting an appropriate NVSF for a given permitted operating range of the power factor angle. The curve of the ESRF following the change in the permitted operating range of the power factor angle was calculated. The analytical method and the evaluation results for the capacitor voltage and energy storage reduction effects of the FB-MMC using negative-voltage states are verified on the basis of the simulation and experimental results.