Closed-loop discontinuous modulation technique for capacitor voltage ripples and switching losses reduction in modular multilevel converters

In this paper, a new discontinuous modulation technique is presented for the operation of the modular multilevel converter (MMC). The modulation technique is based on adding a zero sequence to the original modulation signals so that the MMC arms are clamped to the upper or lower terminals of the dc-link bus. The clamping intervals are controlled according to the absolute value of the output current to minimize the switching losses of the MMC. A significant reduction in the capacitor voltage ripples is achieved, especially when operating with low modulation indices. Furthermore, a circulating current control strategy suitable for this modulation technique is also proposed. Simulation and experimental results under various operating points are reported along with evaluation and comparison results against a conventional carrier-based pulse width modulation method.