Study On The Transient Process Of The Tailrace System With Large-scale Hydroturbine Units

The hydraulic transient process is the consequence that the hydropower station meets the needs of the load change, the result of the change of the diversion and tailrace system flow, the rotational speed of the hydroelectric generating set and the regulation of the power grid, which is an important research content of the design, operation and management of hydropower stations. The underground caverns of the large-scale power station are more diversified by the shared surge chamber and the tailrace tunnel, which increases the complexity of the transient process of the tailrace system. Therefore, the research on the hydraulic transient process of the tailrace system needs to be more closely, accurate, to ensure the safety and stability of power station operation. The essence of the transient process is to study the change of the regulation guarantee parameters of the large fluctuation, the regulating quality of the small fluctuation and the output swing of the hydraulic disturbance.In this paper, based on the retrospective and summary of the previous research results, the basic mathematical model was introduced, and the boundary conditions such as import and export reservoir, series pipe, hydroturbine, surge chamber were established. Combined with the research object, the numerical simulation model was established by using the general software TOPsys-TP, and the water hammer pressure and the water level fluctuation of the surge chamber were calculated. The conclusions are as follows:(1) The results of the closing rule of the guide vane show that the two-step closure law can reduce the pressure of the volute end and the speed increase rate, which can reduce the water hammer pressure better. In the condition of large fluctuation, the selected power station parameters can meet the requirements of regulation guarantee calculation. When the restricted orifice flow characteristic is better, the unit speed and the vacuum of draft tube are improved, but the fluctuation of the surge level is intensified. The larger the surge chamber area is, the more safe for the surge chamber and the draft tube.(2) In the condition of small fluctuation, the surge level slowly converges, and the small fluctuation process of the system can be stabilized. As the restricted orifice area or flow coefficient increases, the speed attenuation decreases and the regulation quality deteriorates.While the surge chamber area increased, the regulation quality improves.(3) In the condition of hydraulic disturbance, when the unit works in the same power grid, the output swing of the disturbed unit by the other two units load rejection is more than one load rejection. The output swing of the disturbed unit working in the infinite grid is greater than and the isolated grid. The restricted orifice area or flow coefficient increases the negative impact of the disturbed unit swing, while the surge chamber area is conducive to the reduction of the unit output swing.