A Control Scheme of Modular Multilevel Converter Operating at Low Frequency

By now the modular multilevel converter(MMC) has been mainly applied to constant frequency applications(50 or 60 Hz) and not been used in the AC drive field.It is because that the sub-module capacitor of MMC does not have clamp circuit and a fluctuation of sub-module capacitor voltage would emerge.Moreover the lower the output frequency is,the more serious the fluctuation is.On one hand,the fluctuation of the sub-module capacitor voltage of MMC would threaten the security operation of the switching devices and sub-module capacitors;On the other hand the fluctuation also would cause distortion on the output voltage of MMC.In order to suppress the fluctuation of capacitor voltage,this paper presented a control method that high frequency component is added simultaneously to common-mode of arm current and output voltage.And also this paper proposed an actual value modulation algorithm to eliminate the distortion of output voltage caused by the fluctuation of capacitor voltage.At last experiment results demonstrated the effectiveness of the proposed control scheme.