A Control Strategy of Modular Multilevel Converters for Motor Operating at Low Frequency

As to the voltage fluctuation problem of capacitors in sub-modules when modular multilevel converters (MMC) operates at low frequency, a control strategy for low frequency is proposed, which mainly aims to increase floating capacitors' ripple frequency by the means of adding high frequency zero-sequence voltage and circulating current, thus restraining voltage fluctuation of floating capacitors. Combining voltage-ampere characteristics of capacitors with switching function, the voltage fluctuation of floating capacitors is analyzed, and the conclusions of the sub-module capacitor voltage fluctuation mainly containing the fundamental frequency component, double frequency component and triple frequency component are obtained. Based on the above conclusions, the control method of injecting high-frequency zero-sequence voltage into reference voltage and injecting high-frequency circulation into energy sharing control is detailed displayed, with the expression of high-frequency zero-sequence voltage and high-frequency circulation, the basis of frequency selecting being given. Combining with the control principle of modular multilevel converters, the control method is applied to the AC drive field and the algorithm of motor control, energy sharing control, voltage equalization control and reference voltage control are given. Finally, a seven-level motor control system simulation model is built in PSCAD/EMTDC based on modular multilevel converters, and the simulation researches on both the motor startup and operating at low frequency are done. The results show that the method is able to restrain voltage fluctuation of floating capacitors effectively, and make sure the system working at low frequency safely and reliably, with the bridge arm current corresponding to the modular multi-level converter having increased, it being mainly applicable to occasions where the output waveform has high requirements and the power of the bridge arm power device is large, which verifies its validity and feasibility.