Closed-Loop Discontinuous Modulation Technique for Capacitor Voltage Ripples and Switching Losses Reduction in Modular Multilevel Converters

Abstract

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.

Discontinuous modulation of modular multilevel converters without the need for extra submodules

“…In this figure, in addition to the N basic SMs of each arm, M additional SMs have been included. These additional SMs are needed when operating the MMC with the discontinuous modulation introduced in [9], as explained in Section III.…”

“…When applying the discontinuous modulation to the MMC [8], [9], all the SMs of one arm of a particular phase-leg are deactivated (bypassed) for some intervals. Therefore, the phase-leg is clamped to the upper or lower dc-bus terminals.…”

“…However, if the differential control signal is negative, the reference of the opposite arm presents a value higher than 1 or lower than -1. In order to avoid this overmodulation situation [9], a number M of additional SMs can be added to the N basic ones, allowing to work with a modulation signal out of the bounds [1, −1].…”

“…Another method that can be used for reducing the capacitor voltage ripples is discontinuous modulation [8], [9]. This technique is based on clamping one arm of the converter to a nonswitching position.…”

“…Instead of by-passing the arm of the clamped phase-leg with less SMs activated, the arm with the highest or lowest modulation signal is clamped, without taking into account if it is the upper or lower arm. Since there is no requirement for increasing the number of SMs in the MMC with this approach, the conduction losses of the converter are reduced compared to the case of implementing the original discontinuous modulation [9].…”