Circulating Current Suppression Control for Modular Multilevel Converter without Arm Current Sensors = 암전류 센서리스 모듈화 멀티레벨 컨버터의 순환전류 저감제어

다국어 초록 (Multilingual Abstract)

Grid-side faults can lead to voltage imbalance and distortion on the AC side, making the robust operation of modular multilevel converters (MMCs) increasingly critical for reliable power systems. Under unbalanced grid conditions, controllers based on the a–b–c stationary reference frame offer structural simplicity by avoiding the decomposition of positive- and negative-sequence components typically required in the d–q frame. Meanwhile, several arm current sensorless control methods have been developed to improve the reliability of MMCs by reducing sensor count and sampling complexity.
This paper proposes a novel arm current sensorless circulating current suppression control (CCSC) method for MMCs, implemented in the a–b–c stationary reference frame. Unlike conventional methods that rely on circulating current references, the proposed approach utilizes only the common-mode component extracted from the instantaneous phase inductor voltage. As a result, the proposed CCSC method eliminates the need for arm current sensors while maintaining all essential MMC functionalities, including circulating current suppression, average SM capacitor voltage regulation, and arm energy balancing. A practical algorithm for its implementation is also presented.
The effectiveness and reliability of the proposed CCSC method are validated through comprehensive simulation and hardware experiments, confirming its suitability for MMC systems operating under both balanced and unbalanced grid conditions.

목차 (Table of Contents)

• LIST OF FIGURES ⅲ
• LIST OF TABLES ⅵ
• ABSTRACT ⅶ
• 1. Introduction 1
• 1.1. Modular Multilevel Converter 1
• 1.2. Hierarchical Control Structure of MMC 2
• 1.3. Arm Current Sensorless Control Methods in MMCs 3
• 1.4 Motivation for Sensorless CCSC Development 4
• 1.5. Objectives of Research 5
• 2. Background 10
• 2.1. MMC Structure and Operating Principle 10
• 2.2. Submodule Topologies of MMC 12
• 2.3. Modulation Techniques for MMC 15
• 2.4. Mathematical Analysis of Circulating Current 18
• 2.5. Conventional Circulating Current Suppression Control Methods 21
• 2.5.1. Conventional d-q frame based CCSC method 23
• 2.5.2. Conventional a-b-c frame based CCSC method 25
• 2.5.3. Existing d-q frame based arm current sensorless CCSC method 28
• 2.5.4. Overview and Research Gap 30
• 3. Proposed Arm Current Sensorless CCSC Method 32
• 3.1. Motivation and Key Features 32
• 3.2. Control Algorithm 35
• 3.2.1. AC Circulating Current Control 36
• 3.2.2. DC Circulating Current and Arm Energy Balancing Controls 38
• 3.2.3. Overall Control Strategy 40
• 3.3. Closed-Loop Design 43
• 4. Implementation of Proposed CCSC Method 46
• 4.1. Challenges in Phase Inductor Voltage Measurement 46
• 4.2. Overall Procedure for Implementation 51
• 4.2.1. Selection of Measuring Point of Phase Inductor Voltage 52
• 4.2.2. Calculation of Required Number of SMs 53
• 5. Simulation Results 56
• 5.1. Three-Phase Simulation 56
• 5.2. Single-Phase Simulation 66
• 6. Experimental Results 74
• 6.1. Hardware and Experiment Design 74
• 6.2. Results 76
• 7. Conclusion 80
• Reference 82
• Appendix. C Code Used for Hardware Experiment 85
• Abstract in Korean 133