Optimized Circulating Current Injection Control Scheme for Modular Multilevel Converters

Modular multilevel converter (MMC) is the most preferred topology for high voltage dc (HVDC) transmission. The circulating current (CC) in MMC is a natural phenomenon, which is conventionally suppressed to enhance efficiency. Nonetheless, proper control of circulating current in MMC can help in reducing the peak value of arm current and/or size of the SMs capacitor. However, minimizing SMs capacitor size using the CC control leads to an increase in the peak and rms values of the arm current. Similarly, minimizing the peak and rms values of the arm current using CC control can lead to an increased SMs capacitor size. In this paper, an optimized circulating current injection control scheme is proposed to reduce the peak value of arm current while keeping the size of SMs capacitor the same. A generalized expression of the magnitude of circulating current to be injected is derived. Using this expression, a comparative analysis is presented to show the merits of the proposed control scheme over the existing schemes. Two circulating current controllers that can be used to control the second harmonic circulating current to have magnitude and phase-angle as per the derived expressions are discussed. The efficacy of the proposed scheme is verified using a 9-level grid-connected simulation model developed in PSCAD/EMTDC. The principles are further verified using a 7-level grid-connected scaled-down lab prototype.