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There are second-order harmonics in the dc voltage and current when the MMC–HVDC system is under unbalanced grid conditions, even if the negative-sequence current controller is employed. This paper presents a supplementary dc voltage ripple suppressing controller (DCVRSC) to eliminate the second-order harmonic in the dc voltage of the MMC-HVDC system. The instantaneous power of the converter arm and phase unit indicates that there are zero-sequence double-line frequency components in the three-phase unit voltages when the ac system is under an unbalanced fault. Since the zero-sequence components cannot be offset by each other, they lead to the second-order harmonic in the dc voltage and dc current. The DCVRSC is developed to compensate the zero-sequence components in three-phase unit voltages. Simulation results based on a detailed PSCAD/EMTDC model prove that the DCVRSC can eliminate the second-order harmonic in the dc voltage. Meanwhile, the ac currents are kept balanced under the unbalanced fault conditions. [ABSTRACT FROM AUTHOR]

This paper describes a modified phase-shifted carrier-based pulsewidth-modulation (PSC-PWM) scheme for modular multilevel converters (MMC). In order to reduce the average device switching frequency, a reduced switching-frequency (RSF) voltage balancing algorithm is developed. This paper also proposes a circulating current suppressing controller (CCSC) to minimize the inner circulating current in an MMC. Based on the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a pair of proportional integral controllers. Simulation results based on a detailed PSCAD/EMTDC model prove the effectiveness of the modified PSC-PWM method and the RSF voltage-balancing algorithm. The proposed CCSC not only eliminates the inner circulating current but also improves the quality of the converter ac output voltage. A simple loss evaluation demonstrates that the RSF voltage-balancing algorithm and the CCSC reduce the converter power losses. [ABSTRACT FROM AUTHOR]

This paper applies nearest level control (NLC) to the modular multilevel converter (MMC). Since there are a number of submodules (SMs) in high-voltage applications of MMC and all SM voltages are required to be measured and sorted, if the uniform sampling frequency is not high enough, the SM will not be triggered as quickly as possible. Thus, the converter-output voltage levels and voltage harmonics will be affected. A method to systematically analyze the voltage harmonics is presented, considering the uniform sampling frequency. In order to select a proper sampling frequency, two critical values, which significantly influence the output voltage levels and voltage total harmonic distortions, are calculated. Simulation results based on PSCAD/EMTDC proved the validity of the proposed modulation scheme and the importance of a properly selected sampling frequency. [ABSTRACT FROM AUTHOR]