The DC-link capacitor is a key component of modular multi-level converter（MMC） in flexible HVDC

provides DC voltage support for the sub-module and determines its voltage fluctuation range.The investment cost of sub-module capacitors accounts for a large proportion in the entire system

therefore the choice of the capacitance has a great impact on the capacitor cost.In this paper

the parameter design of the sub-module capacitor is studied with consideration of such two aspects as voltage steady-state fluctuation of the sub-module and the dynamic response speed of the DC system.In addition

when capacitance of submodule decreases

both the capacitor voltage fluctuation rate of the sub-module and the total harmonic coefficient of the DC side voltage will change.In this paper

considering the two factors

the end-of-life criterion of DC support capacitor is proposed through PSCAD simulation research.