Preliminary Study on Frequency Stability Analysis for Active Power Regulation in Variable Speed Pumped Storage Plants

Pumped storage technology is crucial for supporting power systems towards de-carbonization and sustainability. Recently, variable-speed pumped storage plants (VSPSPs) have become a new developing direction because of the features of variable-speed units (VSUs) compared to traditional fixed-speed units (FSUs), including rapidity, high efficiency, flexibility, and reliability in operation and regulation. However, the operating stability of VSPSPs during active power regulation needs further investigations, because the VSUs with doubly fed induction machines (DFIMs) are de-coupled with power grids and the robustness of the rotational speed decreases. In this paper, the goal is to study the dynamic stability of VSPSPs in active power regulation, based on MATLAB and Simulink as the main tools. First, a numerical model integrating hydraulic-mechanical-electrical subsystems of VSPSPs with DFIM is adopted by using MATLAB/Simulink, and a thirteenth-order state matrix is established for theoretical eigen-analysis as the core approach. Then, a Chinese VSPSP is adopted as the engineering case. The dynamic stability of the system is studied by state matrix, and the time-domain simulations are conducted for further validation and illustration. The matching mechanism between the controller parameters and the featured parameters in hydraulic designs is discussed. The results show that, even under the grid-connected condition, the VSU could also lose stability during the active power regulation. These results could provide suggestions for the design and controller parameter tuning of VSPSPs for increasing the stability of operation and regulation.