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Modular Multilevel Converters (MMC) processing phase quantities at low frequency are vulnerable to high cell voltage ripples. MMC upper and lower arms processes pulsating power at fundamental and second harmonic phase components. As a consequence, arms are expected to buffer the energy difference exchanged between DC-link and each AC-phase. This buffered energy is translated into increased cell voltage ripple when operating at low phase frequency. Several remedies have been presented in literature, which can be classified under software and hardware methods. To address this short-come we introduced a plug-in hardware alternative that permit energy exchange between cells from upper and lower arms thus remarkably reducing capacitor voltage ripples caused by out of phase energy variations. The proposed method can improve MMC performance over the full operation region and is mainly attractive to low frequency operation such as in variable-speed motor drives where higher-than-nominal torques might be required for a wide speed range. Performed simulations over a synchronous machine using maximum torque per current control technique, conforms well with the analytical findings and shows the effectiveness of the proposed method.

This paper proposes a hybrid modulation method for a dual-output five-leg indirect matrix converter topology. The proposed control technique combines a space vector modulation scheme (SVPWM) applied to the rectifier stage so as to control the input currents and a new digital scalar modulation scheme (DSPWM) designed to control the two loads output voltages. A generalized expression of the output voltage references is also developed showing the capability of the proposed method to control more than two independent loads without requiring additional mathematical development. Furthermore, a new modified expression of the output voltages references is proposed to enable the correct operation of the converter under unbalanced grid voltages. Also, this paper proposes a detailed mathematical analysis that allows evaluating the effect of output loads unbalances on the input current harmonic content. Simulation and experimental results are provided to show the effectiveness of the proposed theoretical investigations and confirm the capability of the proposed method to control multiple drive systems as well as ac input currents.