A new insertion index selection method to control modular multilevel converters

This paper introduces a new insertion index selection method to control a Back-to-Back Modular Multilevel Converter (MMC) laboratory platform. The test bench is based on a Power-Hardware-In-the-Loop (PHIL) setup constructed from two 6kVA, 3-phase, 10 cells per arm MMC systems connected in Back-to-Back configuration. The proposed insertion index selection method uses the arithmetic mean of available upper and lower arm voltages in a leg to generate insertion indices; unlike classical closed-loop methods which use the available arm voltage of each arm (i.e. sum of measured capacitor voltages in an arm). For this purpose, a detailed mathematical derivation of available arm voltage ripple equations is introduced. Furthermore, impact of the proposed insertion method on inserted arm voltages that drives input and output currents is thoroughly explored. No additional control loops for arm energy difference are required, as the proposed method inherently achieves arm energy stabilization. Nevertheless, the number of measured signals to be fed back to a high-level controller is reduced to half. The PHIL setup is formed of MMC-1 which emulates an AC grid and MMC-2 which is controlled as a grid tied converter. Analytical findings along with experimental results obtained from the Back-to-Back PHIL setup proves the effectiveness of the proposed method.