Capacitor Voltage Balancing in Semi-Full-Bridge Submodule with Differential-Mode Choke : (InvitedPaper)

This paper presents a semi-full-bridge submodule implementation with an integrated differential-mode choke for modular multilevel converters. The semifull-bridge submodule comprises two capacitors that are connected in either series or parallel to generate a positive voltage, while the capacitors are connected only in parallel to generate a negative voltage. A capacitor voltage difference appearing between two capacitors during the series connection period is inevitable due to the capacitance mismatch. This results in a large current spike during the parallel connection period. The paper evaluates the effect of the differential-mode choke, increased modulation index, and second-order harmonic injection. Numerical analysis indicate that the differential-mode choke can provide significant reduction in the peak current during parallel connection of the capacitors. The numerical results are validated by simulation results from a single semi-full-bridge module as well as a full converter. It is concluded that with proper control and dimensioning of the submodule capacitors and differential-mode choke, the current-spike can be reduced to levels in the same range as the peak arm current.