Adaptive multi-variable coordinated control and safe operating characteristics for MMC under unbalanced AC grid

In the grid-connected modular multilevel converter (MMC), grid faults not only cause the current to be higher than the rated current, but also bring the unbalanced issues to the inner unbalanced currents and arm voltages. In this paper, a multi-variable coordinated control method which is employed in the d-q coordinate is presented. First, a parameter is introduced in the calculation of the grid currents references. Afterwards, the relationships between the system operating parameters and the parameter are derived. Furthermore, the influences of parameter on the safe operation areas are analyzed in details. In addition, the P-Q diagrams of the MMC are deduced in terms of the safe operating areas under unbalanced conditions, and an adaptive multi-variable coordinated control is proposed. Finally, the simulating results from MATLAB/Simulink are presented to verify the theoretical analysis.