Improved Decoupled Control of Modular Multilevel Converter under Constaint of Nearest Level Modulation via Disturbance Observer Design
Nearest level modulation (NLM) is an attractive modulation method for its implementation simplicity in modular multilevel converter (MMC). However, it introduces significant voltage and current distortion when the number of submodules (SMs) per arm is small, as in medium-voltage applications. While indirect modulation offers fully decoupled control of ac-side current, dc-side current and SM capacitor energy, its performance is fundamentally reliant on accurate arm voltage synthesis, making it incompatible with the large quantization error inherent in NLM. To resolve this conflict, this paper proposes a new control strategy based on a disturbance observer (DOB). The key idea is to estimate and actively compensate for the inevitable arm voltage synthesis error induced by NLM, thereby enabling fully decoupled control of indirect-modulated MMC even under NLM operation with a small number of SMs. A key advantage is its ease of implementation, as it requires no modifications to the conventional NLM and decoupled control structure. The validity and effectiveness of the proposed method in improving current quality and decoupled SM energy control are verified through both simulation and experimental results.