Improving capacitor voltage ripples and power losses of modular multilevel converters through discontinuous modulation

The voltage ripple of the capacitors of the modular multilevel converter (MMC) increases when the converter output frequency decreases. This occurs in motor drive applications at low motor speeds, where the output frequency is reduced proportionally to the converter modulation index. In this paper, a discontinuous modulation technique for reducing the amplitude of the capacitor voltage ripples and the switching losses, especially at low modulation indices, is presented. The technique is based on adding a zero-sequence to the original modulation signals, clamping the MMC arms to the upper or lower rails of the fictitious dc-link bus. A current control strategy suitable for this modulation strategy is also proposed. Simulation results under various operating points are reported along with evaluation and comparison results against the conventional carrier-based pulse-width space-vector modulation.