Modeling and stability investigation on the governor-turbine-hydraulic system with a ceiling-sloping tail tunnel.

For a hydropower system with a relatively short tail tunnel and wide variation of tail water level, the ceiling-sloping tail tunnel (CSTT) is often recommended. Considering various flow patterns and traveling free-surface-pressurized interface in the CSTT, stability modeling and analysis of the governor-turbine-hydraulic system shows more challenging. Based on state equations analysis, an improved nonlinear model considering transition part, and a new linear model for downstream free-surface flow are derived. Further comparative analysis indicates that, by introducing these models, water flow characteristics in transition part have positive effect on operation stability with an around 4.0% increase of attenuation rate, while that of downstream free-surface flow has a negative effect with a 21.8% decrease rate on the absolute value of negative attenuation factor. For stability optimization of the CSTT, to properly increase its bottom width is preferable, herein with a 6.2% increase of attenuation rate for a 2.0 m increase of bottom width, while a reasonable ceiling slope should be optimized considering the counteraction effect of upstream pressurized flow and downstream free-surface flow. The new developed models and further discussion for the hydropower systems with a CSTT will greatly improve system's stability evaluation and advance the development of renewable energy. • A new nonlinear model considering the effect of flow transition part is developed. • A linear stability model for the downstream free-surface flow is derived. • Positive effect of water flow in transition part on operation stability is revealed. • Negative effect of downstream free-surface flow on operation stability is clarified. • Optimization criterion of the ceiling-sloping tail tunnel is presented. [ABSTRACT FROM AUTHOR]