Abstract: In order to determine the steady-state operation range of the modular multilevel converter（ MMC）, the determining method of the rated operating point of MMC is studied. The process of designing the converter transformer second-side voltage is described.Based on an analysis of the phasor diagram, the relationship between the electrical phasor diagram and the power capability curve of MMC is explained. And the relationship between the magnitude of the electrical quantity and the steady-state power operation range of MMC is intuitively revealed. The possible ranges of the inner electromotive force and phase angle are analyzed, since they are the dominating factors to determine the reactive power limits. The calculation and verification processes of determining the active and reactive power ranges are displayed by an actual case study.