Maximum modulation index for modular multilevel converter with circulating current control

In a modular multilevel converter (MMC), the circulating current control is usually adopted. It can minimize the circulating current in order to reduce the converter power loss, and also provide an active damping which is beneficial for the converter control stability. The circulating current control is normally implemented by adding a compensating component into the modulation signal. Consequently, the maximum modulation index of the fundamental frequency component will be reduced so as to allow room for circulating current control, and the utilization of dc voltage is reduced. In this paper, the impact of circulating current control on the modulation signal in MMC is investigated. The maximum obtainable modulation index of MMC is theoretically derived. It shows that the modulation index reduction is related to the converter submodule capacitance design. If the capacitance is designed for a maximum 10% voltage ripple, the circulating current control could cause as large as a 5% decrease for the maximum modulation index, or 8% for the case with 3^rd harmonic component injection. Both simulation and experimental results verify the theoretical analysis.