Equalizing DC capacitor voltages in multimodule HVDC using SHE-PWM

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 pulse-width modulation (SHE-PWM) technique is employed to reduce the switching losses of the VSC modules, equalizing the voltages of the series-connected dc capacitors is required. 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 pre-set band, the realization of the multilevel waveform reallocates the task to another VSC module to ensure that voltage balance between all capacitors is achieved. Eight VSC modules are considered in this paper and each one is modulated by a pre-calculated multilevel SHE-PWM waveform. The time-domain waveforms and their line-to-line voltage spectra are verified by simulation using PSCAD/EMTDC. Selected results are presented to confirm the viability of the proposed method.