Abstract-This paper studies different circulating current references for the modular multilevel converter (MMC). The circulating current references are obtained from the instantaneous values of the output current and modulation signal of the phase-leg. Therefore, determination of the amplitude and phase of the output current is not needed, which is a significant improvement compared to other methods such as those based on injecting specific harmonics in the circulating currents. Among the different methods studied in this paper, a new method is introduced, which is able to reduce the capacitor voltage ripples compared to the other methods. A closed-loop control is also proposed which is able to track the circulating current references. With the discussed methods the average values of the capacitor voltages are maintained at their reference while the voltage ripples are kept low. Experimental results are presented to demonstrate the effectiveness of the proposed and discussed methods.

This paper introduces a low complexity implementation of the voltage balancing algorithm aiming to reduce the switching frequency of the power devices in modular multilevel converters (MMCs). The proposed algorithm features a relatively simple implementation without any conditional execution requirements and is easily expandable regardless of the number of submodules (SMs). Two modulation techniques are evaluated, namely the staircase modulation and the phase-disposition pulse width modulation (PD-PWM) under the conventional and the proposed algorithm. Using a circulating current controller in an MMC with 12 SMs per arm, PD-PWM yields better results compared to the staircase modulation technique. The test condition for this comparison is such that the power devices operate at a similar switching frequency and produce similar amplitudes to the capacitor voltage ripples in both modulation techniques. The results are verified through extensive simulations and experiments on a low power phase-leg MMC laboratory prototype.

This paper proposes an algorithm to calculate the optimal amplitude and phase of the harmonic current components that can be injected in the circulating currents of a modular multilevel converter (MMC) to minimize the capacitor voltage fluctuations. An optimal second harmonic component and an\ud optimal set of second and fourth harmonic components are proposed. Simulation results are obtained in MATLAB/Simulink environment to study the effectiveness of the calculated optimal currents. Selected experimental results have been obtained from an MMC laboratory prototype, testing the effects of the circulating currents. The reported results demonstrate the effectiveness of using a fourth harmonic component in the\ud circulating current, which improves the effect of the second harmonic on reducing the capacitor voltage fluctuations.Postprint (published version

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.

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