Flexibility of variable-speed pumped-storage unit during primary frequency control and corresponding assessment method

Abstract

As a new type of large-scale energy storage facility, variable-speed pumped-storage unit (VSPSU) has unknown potential in providing flexibility for power system to help solve the frequency control problem caused by variable renewable energy. To assess VSPSU’s flexibility during primary frequency control (PFC), in this paper, VSPSU’s complicated dynamic model is simplified first with internal currents retained for operating constraint evaluation, based on which a concise assessment model is then established by combining the necessary dynamics and the PFC metric. For the assessment model, the solution algorithm is discussed and a pattern-search-based algorithm is proposed for the parameter optimization of VSPSU’s PFC strategy. Finally, a case study based on IEEE 24-bus and 57-bus systems is conducted to verify the accuracy and adequacy of the proposed assessment method under various scenarios. The results show that the average assessment error is 2.92% and the bottleneck constraints of VSPSU are as close to be violated as expected. In the case study, VSPSU is able to provide 33.97% more flexibility than traditional fixed-speed pumped-storage unit on average, while the variability of VSPSU’s flexibility is also revealed.