Experimental Validation of a Nested Control System to Balance the Cell Capacitor Voltages in Hybrid MMCs

In a hybrid modular multilevel converter (MMC), capacitor voltage balance between the
Full-Bridge Sub-Modules (FBSMs) and Half-Bridge Sub-Modules (HBSMs) is only possible when the
arm currents are bipolar. For a grid-connected MMC, operating at unity power factor, this is typically only
achievable when the modulation index is less than 2. Previous control methodologies, based on open-loop
feed-forward compensating currents, have been proposed to operate an MMC with a higher modulation
index. However, these solutions do not minimize the compensating currents; they cannot compensate
entirely for both the variations in the operating conditions and the parameters typically encountered in a
real implementation; and they do not consider the actual capacitor voltage imbalance between the FBSM
and HBSMs. In this paper, a new nested closed-loop control algorithm based on an outer voltage control
loop with an inner current loop is proposed and experimentally validated. Feed-forward currents are still
utilised in the inner loop, but they are calculated using a new optimising algorithm which minimises the
required compensating currents. Moreover, to the best of our knowledge, this is the first work where explicit
algebraic equations to calculate these compensating currents are provided. Experimental results to validate
the approach, obtained with an 18-cell hybrid MMC, are presented and discussed in the paper.