Modeling Of Modular Multilevel Converter Based On Harmonic State Space

In recent years,the modular multilevel converter(MMC)has attracted extensive research efforts due to its modularity,expansibility,high voltage level,etc.,and many MMC-based high-voltage direct current(HVDC)projects have also been commissioned around the world.MMC has now become the mainstream topology in this field.It is necessary to derive an appropriate model to analyze stability and design controllers for MMC.Several MMC models have been discussed in the present literatures which have derived the models in single-phase form to simplify the modeling mathematics.However,very little attention is given to the zero-sequence voltage,which introduces couplings in the single-phase model and leads to a significant error.In this paper,after revealing the mechanism of causing zero-sequence voltage,a more accurate generalized single-phase model of MMC is proposed.The proposed model eliminates the coupling effect introduced by the zero-sequence voltage,and is linearized based on harmonic state space(HSS)theory to precisely characterize the internal harmonic features of MMC.A systematic HSS modeling process is presented for both open-loop and closed-loop conditions.This model is generalized as it can incorporate any control strategy by replacing the transfer function of the controller hence is valuable to analyze the stability and dynamics of MMC.Finally,the effectiveness of the proposed HSS-MMC model is verified by MATLAB/Simulink simulations as well as experimental results.