| Modular multilevel converters(MMCs)have become the preferred option for voltage-source-converter based high-voltage direct-current(VSC-HVDC)transmission systems,thanks to their merits of modularity,scalability,flexible control capability and low harmonics.It is suitable for high-voltage and high-power VSC-HVDC,offshore wind power grid connection and other scenarios.However,MMCs’ weight,capital costs and power losses are higher than their counterpart line commutated converter.Submodule(SM)capacitors of MMCs account for more than 50%of the volume and about 40%of the cost.Suppressing the capacitor voltage ripples of SMs can reduce the capacitance and therefore,reduce the cost and volume/weight of MMCs.Thus,the research on the suppression strategy of MMC capacitor voltage ripples is of great theoretical and engineering significance.The research on the suppression strategy of MMC capacitor voltage ripples under steady-state operation has been very mature.However,when the grid voltages are unbalanced,due to the generation of negative sequence voltages and negative sequence currents,the capacitor voltage fluctuations of the SMs will also increase significantly,which will lead to the risk of overvoltage of the SMs and endanger the safe operation of the system.In view of the above problems,the paper has carried out research work on the capacitor overvoltage suppression under unbalanced grid voltages,and the contents are as follows:Firstly,a hybrid MMC modulation index design method is proposed to suppress the capacitor voltage ripples of SMs.Taking the single-line-to-ground(SLG)fault as an example,the characteristics of capacitor voltage fluctuation when the grid voltage is unbalanced are analyzed,and the optimal selection method of modulation index under unbalanced conditions is given.The selection range of modulation index is calculated,and the current stress before and after modulation index change is analyzed.Through the PSCAD simulation,it is verified that the proposed method has a good suppression effect on the capacitor voltage ripples of SMs.Secondly,a second-order harmonic current injection strategy suitable for hybrid MMC is proposed.The mathematical model of MMC under unbalanced grid voltage is established,the characteristics of capacitor voltage fluctuation of SMs are analyzed,and the relationship between capacitor voltage and arm power is revealed.Based on the arm instantaneous power model,the optimization strategy and calculation process of secondorder harmonic injection parameters are given.The effectiveness of the proposed strategy is verified by PSCAD simulation.Finally,a dual injection strategy of second-order harmonic current and third-order harmonic voltage for hybrid MMC is proposed,which takes into account the three-phase arm power fluctuation characteristics.The mathematical model of MMC after harmonic dual injection is established,and the calculation method of dual harmonic injection parameters is given.The optimal dual harmonic injection parameters can be obtained by optimizing the objective function.The MMC simulation model is built on the PSCAD simulation platform to verify the effectiveness of the proposed strategy.The simulation results show that the proposed strategy can greatly reduce the capacitor voltage ripples of SMs under the unbalanced grid voltage conditions,which is beneficial to the safe operation of semiconductor devices and capacitors. |
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