Constant power start-up control strategy for modular multilevel converter

HVDC system based on modular multilevel converter(MMC) usually adopts fixed DC bus voltage to realize the start-up control of converter, which will have a great impact on AC network during charging process. This paper presents a constant power start-up control strategy based on MMC arm current control, which can effectively control active power, reactive power, output current and submodule(SM) capacitor voltage. Based on the existing two-stage start-up control mode, according to the output characteristics of arm voltage in controlled charging process, the circuit topology of DC side open circuit of converter is proposed, and the mathematical model is established. Based on the derived instantaneous arm power calculation method and the average sliding filter of SM capacitor voltage, the double closed-loop fast response controller of fixed AC power in controlled precharging stage is given. The safety of the power electronic equipment in the converter is effectively guaranteed and the impact of converter on ac power grid is greatly reduced. Finally, simulation models of conventional start-up method and proposed start-up method are built on MATLAB/Simulink platform to verify the validity and correctness of the proposed start-up control strategy.