An Improved Circulating Current Injection Method of Modular Multilevel Converter for Electric Machine Drives

Abstract

For the application of modular multilevel converter (MMC) in motor drives, the method of high-frequency common mode voltage (CMV) injection and circulating current (CC) control can well suppress the excessive voltage fluctuation of submodule (SM) capacitors when the motor is running at zero speed or low speed. However, the amplitude of the injected high-frequency CC is too large, resulting in increased system loss and reduced efficiency. This paper proposes an improved CMV injection and CC control strategy. First of all, the amplitude of CMV is adjusted with the motor speed according to the constant V/F control of motor operation, which can effectively avoid overmodulation and increase the CMV amplitude to decrease the CC. Secondly, the main differential mode power term (DMPT) that causes the voltage ripple of the SM capacitors in the MMC motor drives is analyzed, and an improved form of high-frequency CC is proposed. Compared with the conventional method, the CC expression is simpler, and the system complexity can be effectively reduced. In the end, a regulating factor is added to the injected high-frequency CC component to reduce the CC amplitude when the motor speed increases, so as to reduce the power losses and current ratings of the semiconductors. Simulation results show the effectiveness of the proposed control strategy.