A Robust Flywheel Energy Storage System Discharge Strategy for Wide Speed Range Operation

Wide speed range operation in discharge mode is essential for ensuring discharge depth and energy storage capacity of a flywheel energy storage system (FESS). However, for a permanent magnet synchronous motor/generator-based FESS, the wide-range speed variation in a short discharge period causes consecutive decreases in ac voltage frequency and amplitude. As a result, operation point shift leads to performance deterioration of the conventional local linearization based dc-link voltage control strategies. This study aims to realize a consistent robust discharge performance within the entire available operation range for FESS. We propose a robust discharge strategy that incorporates the speed variation to the dc-link voltage controller. A speed-dependent extended state observer is designed to realize global linearization and enhance the robustness. A speed adaptive feedback control law is designed to ensure consistent dynamic performance within the entire available operation range. Finally, the discharge strategy is validated at different speeds on a high-speed FESS test bench.