Advantage analysis of variable-speed pumped storage units in renewable energy power grid: Mechanism of avoiding S-shaped region.

• A coupling model of VSU with nonlinear pump-turbine is established for analyzing. • The advantages of VSU avoiding running to S-shaped region are revealed and analyzed. • Quantitative comparisons are carried out to show the advantages of VSU. Pumped storage units (PSU) as the most efficient energy storage device (ESD) play an important role in absorbing renewable energy (RE) sources. Nevertheless, pump-turbines have inherent S-shaped characteristics caused by their reversible design which seriously threaten the operation stability of PSU. In recent years, the variable-speed pumped storage units (VSU) have been proposed as an alternative to the fixed-speed pumped storage units (FSU). Therefore, it is a meaningful subject to evaluate the operation advantage of VSU, which has not been adequately quantified. This paper aims to analyse the advantages of VSU in operation stability by revealing the mechanism of avoiding trapping in the S-shaped region. Firstly, a precise coupling model for VSU with nonlinear pump-turbine characteristics is established and the operation areas in turbine mode are analysed. Then, the mechanism of how the trap of S-shaped region could be avoided is revealed and the regulation performance of the control system is quantified by comparing with FSU. Based on a real Chinese VSU, three scenarios are studied, including turbine start-up process, power regulation and turbine closing process. The results indicate that the S-shaped region could be escaped effectively by operating the VSU. The periodic oscillations times and setting time index of rotational speed in turbine start-up process are 0.5 and 20.74 s respectively, which are much smaller than the values of FSU. VSU outperforms the FSU in the aspect of power regulation performance: much smaller average and standard deviation of power difference for wind and load disturbance. Moreover, VSU can also obtain a significantly higher assessment in closure process: the maximum water pressure deviation is only 40% of FSU. [ABSTRACT FROM AUTHOR]