Research On Rectification Technologies For Motor Drives Based On Modular Multilevel Converters

The modular multilevel converter(MMC)has advantages of high modularity,low harmonics,low losses and high expansibility.As a result,MMC has the potential to become the most favorable topology in high-voltage,high-power applications such as high voltage direct current(HVDC)transmission systems and motor drives.However,MMC still suffers from the inherent problem of capacitor voltage fluctuation,especially under low operating frequenc ies in motor drives.In recent studies,it appears that the most efficient solution is based on the hybrid topology with a serie-connected switch on the DC side.But the suitable rectification technologies have not been analyzed so far.To begin with,this paper comprehensively summarizes the typical solutions that effectively suppress the capacitor voltage fluctuations and specially focuses on the predominant hybrid MMC topology.On the condition of connecting with multi-pulse rectifier,this paper gives detailed analysis on grid current harmonics caused by the switching actions,and the design of proper filters.The validity of the proposed design is verified by simulation results.Based on the forementioned hybrid MMC topology,this paper further considers including MMC as the rectifier,which can eliminate the grid-side filter and allow a feed-back operation.For the grid side MMC,this paper proposes an energy-balancing based control strategy as well as a decoupled control strategy.Concentrating on the effectiveness of DC voltage control with the specific hybrid topology under different duty cycles,the two strategies are compared and analyzed by simulation results.In the last part,this paper proposes a back-to-back hybrid MMC topology,which can actively regulate the DC voltage thus no extra filter is required in this topology.The mathematical analysis and the control strategies are detailed ly introduced.And the ability of reducing the capacitor voltage fluctuation along with the effectiveness of the control strategies are verified both by simulation and experiment results.