Optimal control of water level oscillations in surge tank of hydropower station with long headrace tunnel under combined operating conditions

This paper aims to realize the optimal control of water level oscillations in surge tank of hydropower station with long headrace tunnel under combined operating conditions. Firstly, the definitions of four typical combined operating conditions are presented, and the optimal control objectives of water level oscillations in surge tank under combined operating conditions are proposed. Then, the concept of the most favorable superimposition time of water level oscillation is proposed, and the analytical solutions of the most favorable superimposition time are derived and verified. Finally, for the superimposition operating condition, the attenuation of the water level oscillation under the most favorable superimposition time is analyzed, a novel control strategy of the load adjustments of the units is proposed, and an application example for the proposed load adjustment mode is presented. The results indicate that the optimal control of the water level oscillations in surge tank under combined operating conditions depends on the selection of the superimposition time and the optimization of the mode of the load adjustments under the superimposition operating condition. The most favorable superimposition time can minimize the extreme water levels in surge tank under the superimposition operating condition, and the analytical solutions under the four typical combined operating conditions are highly accurate. The cascaded load adjustment mode proposed in this paper can realize the ideal attenuation of water level oscillations under superimposition operating conditions, and can obviously improve the flexibility of the unit operations.