Abstract: In the grid-connected modular multilevel converters（MMCs） system, grid faults not only cause the quality deterioration of grid currents which may lead the MMC to shut down, but also bring unbalanced problems to the arm voltages. In this paper, a multi-variable protected control method which was employed in d-q coordinate was presented. Considering the permitted maximum of three-phase current, the permitted capacitor voltage ripple and the maximum reactive power set-point, the calculation of the parameter k was given respectively. The converter mathematical models which were based on the laws of Kirchhoff and energy balance were given first. Then the relationship between the system requirements, such as the maximum current and the permitted capacitor voltage ripple, and the parameter k was derived. Furthermore, the design principles of parameter k were deduced. Finally, the simulating results from real time digital simulator（RTDS）, and the experimental results from a scaled-down MMC prototype were presented to verify the theoretical analysis.