Design and Control of E-STATCOM Systems Based on Multilevel Converter with Centralized Energy Storage = 멀티레벨 컨버터와 중앙 집중형 에너지 저장 기반의 E-STATCOM 시스템의 설계 및 제어

다국어 초록 (Multilingual Abstract)

With the increasing integration of inverter-based resources (IBRs) and the decreasing share of synchronous generators utilizing fossil fuels, power systems are facing greater challenges in maintaining voltage and frequency stability. As a promising solution to enhance system stability, the energy-storage-enhanced STATCOM (E-STATCOM), which integrates an energy storage (ES) with a static synchronous compensator (STATCOM), has gained significant attention. Conventional multilevel converter-based E-STATCOM systems are typically classified into two configurations: one where the energy storage elements are distributed within the converter, and another where they are centrally installed at the DC link. Among these, the centralized ES configuration offers advantages in terms of ES management and maintenance. However, the single-star multilevel converter poses structural limitations that hinder the implementation of a centralized ES. To address this issue, this thesis proposes novel E-STATCOM systems that incorporate a zigzag transformer, enabling the centralized installation of energy storage elements independently from the single-star multilevel converter. Furthermore, control strategies are developed to ensure stable and reliable operation of the proposed E- STATCOM systems. The feasibility and effectiveness of the proposed E-STATCOM systems and their associated control strategies are validated through full-scale simulations and scaled down experiments.

국문 초록 (Abstract)

인버터 기반 에너지원의 계통 투입 증가와 화석 연료를 사용한 동기 발전기의 발전량 감소로 인해 계통 전압 및 주파수의 불안정성이 증가하고 있다. 따라서 계통 안정성 향상을 위해서 에너지 저장 장치와 정지형 무효전력 보상 장치를 결합한 E-STATCOM이 각광받고 있다. 기존의 멀티레벨 컨버터 기반의 ESTATCOM 시스템은 에너지 저장 요소를 멀티레벨 컨버터 내부에 분산하여 설치하는 방식과 DC 링크에 집중적으로 설치하는 방식 2가지로 나뉠 수 있다. 둘 중에서 에너지 저장 요소를 집중적으로 설치하는 방식은 에너지 저장 요소의 관리와 유지보수 측면에서 더욱 효과적이다. 하지만 싱글 스타 멀티레벨 컨버터는 구조 상 에너지 저장 요소를 집중적으로 설치하기가 어렵다. 따라서 본 논문에서는 에너지 저장 요소를 싱글 스타 멀티레벨 컨버터와 독립적으로 구성하기 위해 지그재그 변압기를 사용한 E-STATCOM 시스템을 제안한다. 또한, 제안한 E-STATCOM 토폴로지의 안정적이고 신뢰성 있는 운전을 위한 제어 전략을 제시한다. 제안한 토폴로지의 실현가능성 및 제어 전략은 시뮬레이션과 실험을 통해서 검증한다.

목차 (Table of Contents)

• 1. Introduction 1
• 1.1. Background 1
• 1.1.1 Inverter Based Resources and E-STATCOM 1
• 1.1.2 Multilevel Converter 3
• 1.1.3 Centralized and Distributed Energy Storage 5
• 1.2. Purpose of thesis 7
• 1.3. Outline 9
• 2. Configuration and Operation Principle 10
• 2.1. Zigzag Transformer 15
• 2.2. Operation Principle 17
• 2.2.1 Proposed E-STATCOM System I 17
• 2.2.1.1. Modeling and Analysis from Perspective of AC Grid Side 17
• 2.2.1.2. Modeling and Analysis from Perspective of Energy Storage Side 19
• 2.2.2 Proposed E-STATCOM System II 21
• 3. Control Method and Strategy 25
• 3.1. Multilevel Converter Control 25
• 3.1.1 Differential Current Control 26
• 3.1.2 Total Energy Control 28
• 3.1.3 Three-Phase Cluster Capacitor Energy Balancing Control 30
• 3.1.3.1. Zero-Sequence Current-Based Balancing Control. 31
• 3.1.3.2. Zero-Sequence Voltage-Based Balancing Control. 34
• 3.2. Bidirectional DC/DC Converter Control 37
• 3.3. Neutral Cluster Control 40
• 4. Design Comparison 41
• 4.1. Modules and Power Semiconductor Switches 44
• 4.2. Total Device Rating 48
• 4.3. Arm and Cluster Inductors 55
• 4.4. Capacitors. 59
• 4.4.1 Capacitance Required for Double-Star Multilevel Converter 60
• 4.4.2 Capacitance Required for Single-Star Multilevel Converter 62
• 5. Simulation Verification 65
• 5.1. Active and Reactive Power Compensation 68
• 5.2. Load Imbalance Condition 74
• 5.3. Grid and Load Imbalance Condition 78
• 6. Experimental Verification 81
• 6.1. Active and Reactive Power Compensation 83
• 6.2. Compensation of Negative Sequence Current 86
• 6.3. Grid Fault Condition 90
• 6.4. Grid Frequency Drop Condition 94
• 6.5. Grid-Forming (Virtual Synchronous Generator) Method 98
• 6.5.1 Swing Equation-based PLC 104
• 6.5.1.1. Active and Reactive Power Compensation 104
• 6.5.1.2. Grid Frequency Drop Condition 107
• 6.5.2 IP Controller-based PLC 113
• 7. Conclusion 116
• Bibliography 118