Capacitor voltage reduction in modular multilevel converters under grid voltages unbalances

Jinke Li, Georgios Konstantinou, Harith R. Wickramasinghe, Christopher D. Townsend, Josep Pou

Research output: Contribution to journalArticlepeer-review

14 Citations (Scopus)


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.

Original languageEnglish
Article number8725606
Pages (from-to)160-170
Number of pages11
JournalIEEE Transactions on Power Delivery
Issue number1
Publication statusPublished - Feb 2020


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