| Popis: |
Due to the increased consumption of energy in the modern era, many new installations of different energy solutions are required. One of the more preferred is the renewable category, which governs vast technologies ranging from wind, solar, hydro, and nuclear power. One major issue is that they are usually located up to thousands of kilometers away from the load source. Thus, a reliable transmission system is a must. For such applications, a high-voltage direct current power transmission system (HVDC) is the favorable option, most of the modern types use a modular multilevel voltage source converter (MMC) to execute the AC/DC conversion and vice versa. The control of such devices is ultimately dependent on the modulation method, it serves as a key component of the stability of the whole system. Four different variants of description for such a control method are found in the published literature, as more methods can be found, but the focus is on the four introduced. Therefore, the aim of this master's thesis is to investigate how the highlighted standard modulation methods affect the total system's behavior. Note that the implementation by the company, referred to in this context as the benchmark method, is being compared to assess its efficacy against established standard modulation methods. Detailed comparison involving this method is intentionally omitted. First, the underlying system components are described using circuit topology, which serves as the first course before the analytical derivation using dynamic modeling. By obtaining the final dynamic expression and equation for the insertion index, a detailed control strategy is displayed. The total control system is separated into two parts, one governed by the higher-level controllers and one governed by the modulation methods. As the former is predefined and given, therefore, it will be referred to as the "black box." Then a comprehensive method regarding the implementation of modulation methods is provided, serving as the most crucial part of the work. Several different studies are conducted, including harmonic analysis, fault studies, impedance scanning, and screening studies of torsional interaction. The result indicated that some degree of differences can be concluded between the modulation methods, several cases showed that controlling done in a closed loop manner is the preferred option due to its accuracy and robustness. Some of the investigated modulation methods showed severe oscillations in circumstances where the connected grids were weak. Due to the complexity of the overall system, including higher-level controllers, no direct conclusion can be drawn to definitively say whether any of the modulation methods have a clear advantage in every tested aspect. But at least, based on the results obtained in this work, in this specific simulation set-up, the closed loop method is indeed the preferred option. |
