| Abstract [eng] |
The objective of this master’s thesis is to investigate the response of the 5th unit of the Kruonis Pumped Storage Hydropower Plant, connected to the electrical grid through a full-scale power converter, to various disturbances occurring in the power system. The thesis analyzes the dynamic behavior of a variable-speed pumped storage hydropower unit and evaluates its influence on overall power system stability. The relevance of the topic is related to the increasing integration of renewable energy sources into modern power systems, the decreasing overall system inertia, and the growing need for fast and flexible generation sources capable of maintaining frequency and voltage stability. The thesis includes an analysis of scientific literature related to hydropower plant modeling methods, frequency control principles, reactive power regulation techniques, and the application of full-scale power converters in pumped storage hydropower plants. The operating principles of variable-speed hydropower units, synthetic inertia functions, and converter-based frequency and voltage control methods are also reviewed. Attention is given to the role of converter-connected hydropower units in maintaining power system stability under rapidly changing operating conditions. During the research, a mathematical model of the 5th unit of the Kruonis Pumped Storage Hydropower Plant was developed. The model includes the hydraulic system, turbine and generator mechanical parts, a full-scale MMC-based static frequency converter, and an equivalent model of the electrical grid. The model was implemented in the MATLAB/Simulink environment and adapted for the analysis of transient processes and dynamic operating modes. Frequency control, reactive power control, and grid-side voltage regulation algorithms were implemented in the model to reproduce realistic converter and hydropower unit operation. Simulation studies of the hydropower unit operation under different disturbances were performed. The response of the unit to frequency deviations, rapidly changing system frequency, voltage dips, operating mode transitions between pumping and generating modes, and symmetrical and asymmetrical short circuits were investigated. During the simulations, active and reactive power variations, generator angular speed changes, grid-side converter currents, and voltage transients were analyzed. The ability of the converter to limit fault currents and maintain stable operation during transient and fault conditions was also evaluated. The obtained results demonstrated that a variable-speed hydropower unit connected through a full-scale converter can effectively participate in power system frequency regulation and voltage support. The simulations showed that the unit is capable of rapidly responding to system frequency deviations, providing synthetic inertia support, and smoothly changing operating modes between pumping and generation. Furthermore, the converter control system was found to effectively regulate reactive power, suppress transient oscillations, and limit fault currents during symmetrical and asymmetrical short circuits. The results confirmed that variable-speed hydropower units equipped with full-scale converters can significantly contribute to power system stability, flexibility, and secure operation under modern power system conditions with a high penetration of renewable energy sources. |
