Energy Storage Requirement and Low Capacitance Operation of Unidirectional Current H-Bridge Modular Multilevel Converters

Unidirectional current H-bridge modular multilevel converter (UCH-MMC) is a variant of full-bridge MMC and uses fewer semiconductor devices. The operating area and operation mode of UCH-MMC differ from those of conventional half-bridge MMC (HB-MMC) because of the unidirectional current and bipolar voltage of UCH-MMC arms. Thus, analyzing the energy storage requirement (ESR) of UCH-MMC differs from that of HB-MMC. This paper proposes a model for calculating the ESR of UCH-MMC. The ESR of the UCH-MMC under the entire operating area is analyzed. A modified operating scheme that dynamically trims the sub-module capacitor voltage to allow the low capacitance operation under the entire operating area is proposed. Results verify that the UCH-MMC has much lower ESR than HB-MMC. Evaluation results also show that the cost and volume of the UCH-MMC are much lower than those of HB-MMC because of the low ESR. The low ESR of UCH-MMC was verified by simulations and experiments.