Insertion Index Generation Method Using Available Leg-Average Voltage to Control Modular Multilevel Converters

This paper presents a new insertion index generation method to control modular multilevel converter's (MMC) topology. Unlike classical closed-loop methods, where the sum of all measured capacitor voltages per arm is used in the generation process of insertion indices, the proposed method uses available leg average voltage, i.e., the arithmetic mean of per leg available upper and lower arm voltages. Full mathematical development of available arm voltage ripple expressions is presented. Impact of the insertion method on arm voltages driving input and output currents is thoroughly evaluated and discussed. The proposed method inherently stabilizes arm stored energy; thus, no additional control loops are required to achieve regulation of arms energy difference. Moreover, the number of measured signals to be fed back to a high-level controller is reduced to half. Analytical findings along with experimental results obtained on a 6-kVA, 10-cells-per-arm MMC proved the effectiveness of the proposed method.