Wide-load operating characteristics and stability of fixed-speed pumped storage units under hydraulic short-circuit mode

To address the limited operating range and insufficient low-load regulation capability of fixed-speed pumped-storage power plants under high-penetration renewable energy integration, this study aims to quantify the wide-load regulation potential and transient stability characteristics of the hydraulic short-circuit (HSC) operating mode. A hydraulic system simulation model based on the method of characteristics is developed, incorporating a high-order bifurcated-pipe hydraulic modeling approach suitable for bidirectional-flow coupling and a weakly coupled boundary reconstruction strategy. Taking a one-tunnel two-unit pumped-storage power plant as the study object, the operating range, equivalent water inertia time constant, pressure response, and unit stability indicators are systematically evaluated. The results show that, under high-efficiency constraints, the HSC mode can form a continuous high-efficiency operating band in the low-load region. Compared with conventional operating modes, the net power regulation range of the plant is expanded by 37.34%. Under representative transient operating conditions, HSC operation effectively improves the extrema of key system stability indicators, thereby increasing the transient safety margin of the hydraulic system. These findings indicate that HSC operation can enhance the low-load operational flexibility of fixed-speed pumped-storage power plants without large-scale equipment retrofitting, and provide a reliable quantitative analysis method and theoretical basis for HSC feasibility assessment and flexible operation strategy optimization.