| Author: | Wei, Boyang |
| Title: | Control strategies of modular multilevel converters |
| Advisors: | Liang, Huiwen Rebecca (EEE) |
| Degree: | M.Sc. |
| Year: | 2024 |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | xiii, 129 pages : color illustrations |
| Language: | English |
| Abstract: | With the rapid development of power electronics technology and related industries, today's new generation of power electronic equipment is moving towards higher voltage levels and capacity, finer, high-quality voltage and current waveforms, more intelligent and flexible operation and control capabilities. Modular multilevel converter (MMC) is precisely in this trend should come into being under the power electronic conversion topology. Compared with the traditional multilevel converter topology, MMC shows a lot of excellent performance, such as its voltage and power level can reach ±500 kV/3000 MW or more, the conversion efficiency is as high as 99%, the quality of the output voltage and current waveform is good and even no filter is required, the modular structure is easy to expand, the installation and maintenance is simple, and the active and reactive power can be controlled independently and quickly. These advantages make MMC become one of the hottest research topics in the field of international power electronics in recent years, and present a very broad application prospect. According to the literature review, MMC can be used in high-voltage direct current (HVDC) transmission systems, high-voltage inverter motor drive systems and many other power conversion applications in power industry. However, this also creates higher requirements for the control strategy of MMC. Without a good control system design, MMC will generate a series of problems during operation. Therefore, control strategies for MMC will be discussed in detail in the thesis. In this thesis, firstly, the submodules of full-bridge structure and half-bridge structure of MMC and their operation principles are presented, the output voltage waveforms of MMC are analyzed, and the parameter selection is introduced. By analyzing the topology of the main circuit of MMC, the mathematical model is established in the rotating coordinate system. This thesis presents modulation methods for three main types of MMC, including phase-disposition pulse width modulation (PD-PWM), phase-shifted carrier pulse width modulation (PSC-PWM), and nearest level modulation (NLM), where the NLM is used as the basis for subsequent simulation modelling. The inner-loop current controller and the outer-loop power controller in the MMC double closed-loop control system are analyzed in detail, and the theoretical analyses of the submodule capacitor voltage balancing control as well as the circulating current suppression are accomplished to design the double closed-loop control system of the MMC. For optimizing the control system, the thesis introduces the Euler-Lagrange (EL) model and passive theory, proves that the MMC is a passive system, and designs the passive controller of the MMC as the inner-loop current controller. Comparative simulation is used to confirm the advantages of the passive controller and complete the system optimization. Finally, in order to verify the above control theory, the system simulation model of MMC is built in MATLAB/Simulink to deeply analyze the design of the control system of MMC. |
| Rights: | All rights reserved |
| Access: | restricted access |
Files in This Item:
| File | Description | Size | Format | |
|---|---|---|---|---|
| 8562.pdf | For All Users (off-campus access for PolyU Staff & Students only) | 20.76 MB | Adobe PDF | View/Open |
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