Abstract: Dynamic characteristics of the grid connected operation of a multi-units-shared tailwater system in hydropower stations are complicated, which is unfavorable to high-quality regulation of the units. This paper develops a mathematical model paradigm for the multi-units-shared tailwater surge chamber and tailwater tunnel connected to the power grid, and examines the stability and dynamic characteristics of such a coupled system. First, the Hopf bifurcation theory is applied to an analysis of the stability domain and bifurcation characteristics of each unit operating in the coupled multi-unit system. Then, the dynamics of the system operating at typical control parameters are simulated and examined. The results indicate that this coupled system is prone to supercritical Hopf bifurcation, and the oscillation characteristics of its unstable units will be transmitted to the stable ones, leading to low-frequency oscillations occurring on all its units. The influence of the inertia time constant and load ratio on its stability region is the greatest. For each unit, the greater the inertia time constant of the flow in its pilot pipe, the smaller the stability region. A critical point of load ratios exists－on its left side, the larger a load ratio, the larger the stable region, while on its right, the larger a load ratio, the smaller the stable region. The coupled system is stable only when the control parameters of each unit stay in the stability region. During dynamic regulation, water level in the surge chamber generates ultra-ultra-low frequency oscillation spectrum by its own water surface wave. The frequencies of the unit and the power grid include ultra-ultra-low frequency oscillations generated by the surge chamber and low frequency ones generated by the speed regulating system and the power grid.

Key words: hydropower station, surge tank, power grid, hydro-turbine governing system, stability, dynamic characteristic