GIST Repository
College of Information and Computing
Department of Electrical Engineering and Computer Science
4. Theses(Ph.D)
Performance Improvement and Overcurrent Limiting of a Grid-Forming Inverter with Two Electromotive Forces
- Abstract
- Modern power systems are undergoing a significant transition with the increasing integration of inverter-based resources (IBRs). However, many challenges related to power system control and stability arise in the IBR-dominated power grid. Unlike traditional synchronous generators, conventional IBRs cannot provide inertial responses, resulting in more severe fluctuations in grid frequency. Additionally, low grid strength causes control instability issues, as IBRs are often connected to weak parts of the grid. In weak grid conditions, the sensitivity of the voltage at the point of connection (PoC) to the output current of IBRs is high, leading to reduced stability of conventional grid-following (GFL) control. To address these issues, grid-forming (GFM) control has emerged as a promising alternative to conventional GFL control. However, since GFM inverters are primarily designed for the responses to grid disturbances, their regulation performance according to the input references is compromised. In addition, GFM inverters can exhibit low damping characteristics depending on grid conditions and are vulnerable to overcurrent issues during large disturbances caused by grid faults. To overcome the limitations of GFM inverters, this dissertation proposes a method to enhance the performance of GFM inverters and limit overcurrent by utilizing a virtually emulated circuit structure with two electromotive forces (EMFs) connected in parallel. Fundamental control laws with active disturbance rejection control are developed to implement the control structure, which includes two EMFs produced by independent control algorithms in a single inverter. Each EMF operates as a voltage source and a current source, respectively. As a result, the proposed control structure represents a hybrid approach that integrates GFM and GFL functionalities into a single inverter. The dissertation is organized into three main parts. First, the control structure of the previously proposed method utilizing two EMFs, so-called current-referencing electrified synchronous machine (CURESYM), is reinterpreted based on the proposed parallel control structure. Through this reinterpretation, the conventional CURESYM is revised to a more straightforward structure, and its hybrid responses are clearly described. Second, a virtual impedance implementation method utilizing a disturbance observer (DOB), which has been adopted for improved control accuracy, is introduced. The DOB-based virtual impedance provides a more simple and robust approach to virtual impedance realization. Through a detailed analysis of the effects of virtual impedance applications, the usefulness of high output impedance in enhancing system damping is validated. Finally, a novel overcurrent limiting strategy for the GFM inverter based on the parallel EMF structure is proposed. The mechanism of the proposed method is highly intuitive and straightforward. Furthermore, the proposed method can preserve the original GFM responses as much as possible. The overall performances of the proposed methods are evaluated through simulations utilizing a practical 250 kVA inverter model and experimental results with a scaled-down inverter prototype.
- Issued Date
- 2025
- Type
- Thesis
- Alternative Author(s)
- Geon Heo
- Department
- 대학원 에너지융합대학원(학과)
- Advisor
- Park, Yongsoon
- Table Of Contents
- Abstract ............ i
Contents ............ ii
List of Tables ............ iv
List of Figures ............ v
Chapter 1. Introduction ............ 1
1. 1. Background ............ 1
1. 2. Research Objectives ............ 6
1. 3. Dissertation Outline ............ 8
Chapter 2. Parallel control structure with two EMFs ............ 9
2. 1. Introduction ............ 9
2. 1. 1. System description ............ 9
2. 1. 2. Conventional CURESYM ............ 11
2. 2. Revised CURESYM based on the parallel connection of two EMFs ............ 12
2. 2. 1. Fundamental control laws ............ 13
2. 2. 2. Virtual current regulator ............ 16
2. 2. 3. Virtual synchronous condenser ............ 18
2. 2. 4. Disturbance observer ............ 25
2. 3. Simulation results ............ 29
2. 3. 1. Active power control responses ............ 30
2. 3. 2. Frequency droop control ............ 34
2. 3. 3. Voltage droop control ............ 34
2. 4. Summary ............ 38
Chapter 3. DOB-Based Virtual Impedance ............ 39
3. 1. Necessity of virtual impedance ............ 39
3. 2. Conventional virtual impedance ............ 44
3. 3. DOB-based virtual impedance ............ 48
3. 4. Effects of high virtual impedance ............ 52
3. 5. Summary ............ 55
Chapter 4. Overcurrent Limiting in CURESYM ............ 56
4. 1. Introduction ............ 56
4. 2. Overcurrent situations in CURESYM ............ 57
4. 3. Virtual current regulator-aided overcurrent limiting ............ 59
4. 4. Simulation results ............ 62
4. 5. Summary ............ 68
Chapter 5. Experimental Results ............ 69
5. 1. Experimental setup ............ 69
5. 2. Experimental results ............ 71
5. 2. 1. Active power control responses ............ 71
5. 2. 2. Frequency droop control ............ 74
5. 2. 3. Voltage droop control ............ 74
5. 2. 4. Impedance reshaping ............ 77
5. 2. 5. Overcurrent limiting ............ 78
5. 3. Summary ............ 78
6. Conclusion ............ 81
6. 1. Summary ............ 81
6. 2. Future Works ............ 82
Bibliography ............ 83
Acknowledgement
Curriculum Vitae
- Degree
- Doctor
- Appears in Collections:
- Department of Electrical Engineering and Computer Science > 4. Theses(Ph.D)
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