Research On Optimization Of MMC Capacitance Based On Derived Model Of Modulation Ratio

The modular multilevel converter based high voltage direct current,which is also called as MMC-HVDC,has been paid with wide attention with its typical technological advantages in research and engineering industries,and shares great potentiality in future development.The analysis and suppression for the capacitance voltage fluctuation of the MMC submodules are key parts for MMC operation.This paper first starts its analysis from the structure of MMC in comparison between various MMC structures and their operating principles.On the basis,this paper analyzes the principle of capacitance voltage fluctuation of the MMC sub-modules and comes up with the analysis method for the properties of capacitance voltage fluctuation of the MMC sub-modules based on proposed modulation derivation model.This paper analyzes the application conditions of the second-order harmonic circulating current in the capacitance voltage fluctuation suppression based on third-order harmonic wave modulation and sine wave modulation by discussing the relationship between the capacitance voltage fluctuation,modulation and the injection percentage of second-order harmonic circulating current.Furthermore,this paper establishes an automatic optimum modulation exploration method to require the optimum modulation,which can suppress the capacitance voltage fluctuation to its largest extent.Moreover,this paper conducts the comprehensive analysis for the capacitance value of the sub-modules and the MMC converter loss with the suppression analysis,and comes up with the economy enhancement design for the hybrid MMC system.In the end,the paper proves the correctness and the effectiveness for the paper results by simulation.First of all,based on operation principle of the common topological structure of the MMC sub-modules(half-bridge and full-bridge submodules),this paper illustrates the working principle of the DC fault current suppression for full-bridge submodules,and conducts the research on DC fault current suppression of full-bridge,and full and half bridge hybrid MMC submodules from economic prospective.On the basis,this paper analyzes the mathematic model of the capacitance voltage fluctuation of the MMC sub-modules,and comes up with the capacitance voltage fluctuation derivation model of the MMC submodules based on modulation ratio.The capacitance voltage fluctuation can be suppressed by controlling the modulation ratio.Then,the application situation of the second-order harmonic circulating current injection based on third-order harmonic wave modulation and sine wave modulation to suppress the capacitance voltage fluctuation of the submodules is analyzed and de-duced based on the modulation ratio derivation model of the capacitance voltage fluctuation of the MMC submodules.Also,this paper comes up with the effect that the certain value of modulation ratio brings to the capacitance voltage fluctuation suppression.The automatic exploration of optimum modulation ratio method is then proposed on the basis.The derivation model of the capacitance voltage fluctuation based on third-order harmonic wave modulation and sine wave modulation are respectively proposed when second-order harmonic circulating current is injected,and the optimum modulation ratio is finally established.Under the optimum modulation ratio,the capacitance voltage fluctuation during the third-order harmonic wave modulation and sine wave modulation can be suppressed to the greatest extent.Finally,this paper conducts the comprehensive analysis for the capacitance value for submodules and the losses for MMC converter station,and the determinant factor for the MMC comprehensive losses is analyzed and proposed.By controlling and suppressing the determinant factor,this paper has effectively lowered the capacitance voltage fluctuation and the arm bridge current,which can further lower the capacitance value of the MMC submodules and the losses for converter station.The simulation results have proved the effectiveness for the factor and the suppression method.The decrease of capacitance value of the submodules can not only lower the volume of the converter station and land occupation,but also can lighten the weight of the converter station,which can help with the structure design in hanging style,and has great economic value for the Flexible HVDC MMC projects.