Impact of Nearest-Level Modulation harmonics on the Small- Signal Dynamics of the Modular Multilevel Converter

Converter (MMC) in HVDC applications, due to its low switching frequency. In steady-state operation, its discrete submodule switching generates uncharacteristic harmonics, with the magnitude inversely proportional to the submodule count. However, it is still unclear whether the applicability of NLM is impacted by its influence on the overall dynamic response of the

MMC. Therefore, this paper investigates how the harmonics generated by NLM influence the small-signal dynamics of the MMC. The study compares the frequency-domain admittances, obtained through electromagnetic transient simulations, of an arm-averaged model and an

MMC model that includes NLM, highlighting the impact of the number of submodules.

Mitigation strategies such as increasing perturbation magnitude or extending the measurement window are evaluated, with results showing that only the perturbation magnitude significantly reduces variability. A time-domain example further demonstrates that frequency-domain stability assessments may yield incorrect conclusions when the influence of these harmonics is not properly considered.