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Multimodule converter-based high-voltage dc (HVDC) power transmission incorporates numerous voltage-source converters (VSCs) connected in series/parallel to provide increased voltage/current and, hence, power ratings. When a selective harmonic elimination pulsewidth-modulation (SHE-PWM) technique is employed, the switching losses of the VSC modules are reduced but the voltages of the series-connected dc capacitors are not naturally balanced. In this paper, a voltage-balancing technique is reported. The technique relies on monitoring the voltage of each dc capacitor and when such voltage exceeds its preset band, the realization of the multilevel waveform reallocates the task to another VSC module to ensure that voltage balance between all capacitors is achieved. Three-phase bipolar VSC modules are considered in this paper and each one is modulated by a precalculated multilevel SHE-PWM waveform. The time-domain waveforms and their line-to-line voltage spectra are verified by simulation using PSCAD/EMTDC. The selected results are presented to confirm the viability of the proposed method. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) is a newly introduced switch-mode converter topology with the potential for high-voltage direct current (HVDC) transmission applications. This paper focuses on the dynamic performance of an MMC-based, back-to-back HVDC system. A phase-disposition (PD) sinusoidal pulsewidth modulation (SPWM) strategy, including a voltage balancing method, for the operation of an MMC is presented in this paper. Based on the proposed PD-SPWM switching strategy, a mathematical model for the MMC-HVDC system, under both balanced and unbalanced grid operation modes, is developed. Dynamic performance of the MMC-based back-to-back HVDC converter system, based on time-domain simulation studies in the PSCAD/EMTDC environment, is then evaluated. The reported time-domain simulation results show that based on the adopted PD-SPWM switching strategy, the MMC-HVDC station can respond satisfactorily to the system dynamics and control commands under balanced and unbalanced conditions while maintaining voltage balance of the dc capacitors. [ABSTRACT FROM PUBLISHER]

The three-phase series-connected modular multilevel converter (SC-MMC) is presented as one option for high-voltage direct current (HVDC) tap to reduce the cost of an HVDC station. Compared with conventional three-phase parallel-connected MMC, the number of semiconductor switches in SC-MMC is reduced by one-third to withstand the same dc voltage. First, the paper studies the operating principle of SC-MMC, and then presents an interspacing phase-shifted pulsewidth-modulated method which could improve the waveform of the output ac voltage for a given switching frequency. Next, the control strategy based on the identical real power input of each phase is proposed to balance the dc voltage of each phase under an unbalanced grid condition. The current limit of switching devices is also taken into consideration to protect SC-MMC from overcurrent. Furthermore, a universal zero-sequence current controller is developed as one part of the control strategy. In addition, an innovative method to suppress the third harmonics of ac current is presented. With the third harmonic suppression strategy, the SC-MMC can operate with much smaller SM capacitors without deteriorating the ac current, which contributes to the cost reduction further. Finally, simulation results obtained in PSCAD/EMTDC are provided to validate the proposed control strategies. [ABSTRACT FROM PUBLISHER]