I/O-Linearization Based Current Decoupling Control of Modular Multilevel Cascade Converters

A generic current control system for modular multilevel cascade converters (MMCCs) is proposed. It is based on (i) input/output-linearization (I/O-linearization), (ii) feed-forward disturbance compensation and (iii) parallelized proportional-integral-resonant controllers to compensate for arbitrarily many harmonics (including positive and negative sequence). The controller design is performed in state space. It is generic and flexible and combines pole placement and LQR tuning. Moreover, it allows taking all system parameters such as cluster or output (grid) inductances or resistances individually into account during controller design and tuning. Moreover, the typically imposed simplifying assumptions on system dynamics (e.g., identical parameters) or operating conditions (e.g., balanced loading) can be overcome. The proposed control system is validated for a dual-star bridge-cell MMCC. The simulation results illustrate its excellent control performance under ideal but also non-ideal conditions, where conventional approaches usually fail.