Improved Decoupled Control of Modular Multilevel Converter Under Constraint of Nearest Level Modulation via Disturbance Observer Design

Nearest level modulation is widely used in modular multilevel converters (MMCs) for its simplicity, but it introduces significant quantization effects on arm-voltage synthesis when the number of submodules (SMs) is small, as in medium-voltage applications. Although the indirect modulation method enables fully decoupled control of ac-side current, dc-side current, and SM capacitor energy, its performance relies on accurate arm voltage synthesis, making it incompatible with the large quantization errors induced by NLM. To address this challenge, this paper proposes a disturbance observer (DOB)-based compensation scheme, which is integrated into the indirect modulation framework. The proposed method compensates for the arm voltage synthesis error as a disturbance, thereby restoring the decoupled control performance of the indirect-modulated MMC without modifying the NLM and decoupled contro…