Abstract
Negative- and zero-sequence internal currents of a modular multilevel converter (MMC) under unbalanced ac-grid conditions can increase the peak voltage of the submodule (SM) capacitors, which should be kept below a maximum voltage limit. This paper investigates the impact of offset pulsewidth modulation (OPWM) and zero-sequence voltage injection on reducing peak SM capacitor voltages of the MMC during unbalanced ac-grid conditions. It is demonstrated that OPWM extends the operating range of the MMC for a given set of restrictions, as it leads to a reduction of capacitor voltage in every phase by utilizing all the SMs in the arms. Zero-sequence voltage injection further reduces the maximum SM capacitor voltage among three phases under certain conditions. Simulation results from an 800-MVA MMC-based high-voltage direct current transmission system demonstrate the effect of OPWM in the operation of the converters and the efficacy of the method compared to standard modulation methods.
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