Improved Decoupled Control of Modular Multilevel Converter under Constaint of Nearest Level Modulation via Disturbance Observer Design

TL;DR

This work tackles arm-voltage synthesis error in indirect-modulated MMCs operating with Nearest Level Modulation (NLM) when the per-arm SM count is small. It introduces a disturbance observer (DOB) as an add-on to estimate and compensate NLM-induced disturbances in the dc-side, ac-side, and circulating current loops, preserving decoupled control. The proposed design uses simple, realizable Q-filters and either a principal or simplified implementation, with stability guaranteed under standard DOB conditions. Simulation and hardware experiments demonstrate improved current quality, capacitor-voltage balancing, and flexible decoupled energy control, while revealing a trade-off between DOB bandwidth and switching frequency. The approach enables practical deployment of indirect-modulated MMCs with NLM in MVDC/MV applications, reducing distortion without altering the existing NLM or control architecture.

Abstract

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.

Figures (21)

• Figure 1: Modular multilevel converter with ac grid and dc bus.
• Figure 2: Decoupled equivalent models of MMC. (a) dc-side current, (b) circulating current, (c) ac-side current. $v_{xg}$ is the $x$-phase ac grid voltage.
• Figure 3: Overview of the decoupled control strategy for inverter mode MMC. $\mathbf{E}_{abc}^\Sigma$ and $\mathbf{E}_{abc}^\Delta$ are defined as $E_{a}^\SigmaE_{b}^\SigmaE_{c}^\Sigma^\top$ and $E_{a}^\DeltaE_{b}^\DeltaE_{c}^\Delta^\top$, where $E_{x}^\Sigma$ is the sum and $E_{x}^\Delta$ is the difference of the upper and lower arm capacitor energies for the $x$-phase leg. $D_{xu}$ and $D_{xl}$ are the duty cycles of the SMs in the $x$-phase upper and lower arms, respectively.
• Figure 4: Block diagrams of decoupled current controls for indirect-modulated MMC and plant transfer functions for each current component. (a) dc-side current, (b) circulating current, (c) ac-side current control loops.
• Figure 5: Operation flowchart of NLM for indirect-modulated MMC to determine duty cycles of SMs in $x$-phase upper arm. Duty cycles of SMs in lower arm can be determined in the same way. Half-bridge SM is assumed to be used in this flowchart.