Passivity-based sliding mode current control for grid-following modular multilevel converter with system disturbances

This paper proposes a passivity-based sliding mode current control (PBSMCC) for addressing challenges in stable grid-following operation of the modular multilevel converter (MMC) with system disturbances such as grid voltage sag caused by load variation or faults in the main grid or mismatches in system parameters caused by impedance uncertainty or equipment aging. An energy storage function in passivity theory is constructed based on the MMC Euler-Lagrange (EL) model for preserving the system passivity property. Then, the additional inputs are designed for energy shaping and guaranteeing the global asymptotical convergence to the sliding surface with the passivation process. The external disturbances would be dissipated rapidly under a designed sliding regime with extra damping terms. Hence the proposed control scheme in the grid-following MMC provides stable operation across a wide range with enhanced robustness with reduced control efforts. The system dynamics with active power step changes, grid voltage sag, power factor plunge, and model parameter variation are investigated in case studies. The simulation results can validate the effectiveness and superiority of the proposed control scheme, which owes a broad stable operation domain with minimal control efforts, even if it operates at an extremely low power factor. The limitation of the narrow operating range of the traditional proportional-integral (PI) control with local linearization is overcome in the grid-following MMC output currents regulation and circulating currents suppression.