Modulation, efficiency and lifetime of two-level and multilevel converters for a hydropower application

Licentiate thesis, 2022

Along with the integration of the renewable energy in the electrical grid, the pumped-storage hydropower has gained more and more attention due to its fast response and energy storage ability. To have a higher overall efficiency and more flexibility of the system, variable speed is preferred in the operation of the pumped-storage hydropower applications. The key component for the variable speed pumped-storage hydropower application is the full-size power converter, which is the main study object in this work.

Different converter topologies, such as the two-level converter, the neutral point clamped converter, and the modular multilevel converter, have been investigated in this study. The simulation and experimental results verify the feasibility of the studied modulation and control methods for different converter topologies. The nine-level modular multilevel converter needs four times the amount of the power modules compared with the two-level converter, not to mention the extra submodule capacitors and arm inductors in the nine-level modular multilevel converter. However, the nine-level modular multilevel converter shows the best efficiency of 99.37% at nominal power in the loss study, while the classical two-level converter shows an efficiency of 98.44%. At the end, a lifetime study is conducted for power switches inside a modular multilevel converter, and it is found that with the used semiconductors design, i.e., the semiconductors have an RMS current value that of half of its stated maximum value, the lifetime requirement of 30 years can always be fulfilled.