Circulating Current Suppression And Modeling Of Modular Multilevel Converter

Owing to the low resistance loss,high voltage direct current transmission(HVDC)is more conducive to facilitate the integration of clean energy utilization and long-distance transmission applications.The thyristor current source converter(CSC)has occupied a dominant position in HVDC projects.However,CSC is hard to achieve power reversal in the complex multi-terminal direct current transmission system(MTDC)and has constrained the development of MTDC.The emergence of voltage source converter(VSC)has so lved the problem of power reversal,and becomes the trend of MTDC in the future.Due to its high degree of modularization,no AC harmonics,low losses and no DC filter capacitor,modular multilevel converter(MMC)has already given rise to an upsurge in the application of the voltage source converter high voltage direct current transmission system(VSC-HVDC).On the other hand,because of its own particular structure,there are many problems to be solved in the aspects of circulating current suppression,control strategy and off-line simulation model.Firstly,this paper has stated that active circulating current suppression can consume certain amount MMC power capacity and cause power penalty.Based on the mathematical formula of MMC,the penalty of output power was figured out.Moreover,parameters dimensioning of the passive circulating current filter was also presented.The results can be used to weigh the power penalty of active control against the additional hardware cost of passive filters.Simu lations and experimental results prove d validity of the analysis and parameters dimension ing.Next,this paper proposed a modified MMC control strategy based on energy decoupling by adding a DC control loop to directly control the DC current.The proposed method preserved the function of conventional method and it embodied the energy decoupling characteristic of the MMC.The proposed control strategy was proved by simulations and experiments.Finally,in order to accelerate the off-line simulation speed of MTDC system,this paper proposed a MMC off-line simu lation model based on the MMC energy decoupling control strategy.The proposed model realized accurate simulation of the steady state,loss,power reversal and fault latchin g of the MMC.Meanw hile,the DC current could be directly controlled by this model.Besides,this model was applied to the MMC-base isolated DCDC converter.The accuracy and feasibility of the proposed model were verified by comparing with the reference model in the PSCAD software.