| The diversion type hydropower scheme has the merits of low dam height, small construction investment, small inundated area, small emigration and small environmental impact, which make it more popular scheme in the hydroelectric development of rivers with large slope in southwest ChinaMass oscillation in surge tanks of hydroelectric stations with long diversion tunnel is charactered by long undulation period, large amplitude and small damping factor which provide discommodities to the operation of power station. Differential surge tanks are better in damping surge than throttled surge tanks. Collapse of breast wall of gate shaft of Tianshenqiao-â…¡in hydraulic transients in 1993 makes people to be more careful about the structural risk of the differential surge tank. In this thesis, the lowest water level in surge tank and the maximal pressure head between the riser and main tank are chosen as two optimization objectives which represent hydraulic performance and structure risk respectively. Based on analysis of methods of multi-objective optimization, the non-dominated sorting genetic algorithm (NSGA-â…¡) is applied to optimize the design parameters including the height of overflow weir, the area of riser, the area of throttled orifice at the bottom of the riser, the area of interconnected orifice between riser and main tank. The Pareto front of the two objectives are obtain, tradeoff of the two objectives and sensitivity analysis of population and generation are performed.To damp the mass oscillation in throttled surge tank with a long diversion tunnel, small throttled orifices may be adopted, which may induce large water hammer pressure transmitting to upstream diversion tunnel. The existing theoretical study on instantaneous water hammer transmission through surge tank is not applicable to practical use as the valve is in slow closure to generate indirect water hammer in practical situation. This paper presents an analytical investigation of water hammer transmitting through a throttled surge tank subject to the linear and slow closure of valve (turbine), analytical formulas are derived of maximal pressure at the downstream end of the tunnel and maximal water hammer results are compared with numerical ones obtained using the method of characteristics, there is a good agreement between them. The formulas can be applied to estimating water hammer pressure at valve and transmission of water hammer pressure through the surge tank at the junction for hydraulic pipe systems with a surge tank.In condition of combined load changes of the turbines, the mass oscillation in surge tank may be superposed to induce a much bigger oscillation than a single load change. To the author's knowledge, there is no study on the analytical study of mass oscillation in a surge tank of hydroelectric station for combined load changes, the standard of surge tank design doesn't include the method to calculate the combined mass oscillation although it points out that check is needed to calculate the combined surge. In this paper, we use the asymptotic method to solve the nonlinear differential equation of mass oscillation of throttled surge tanks in two different conditions. The first condition is to increase load at an adverse time after load rejection to obtain the lowest water level in surge tank, and the second condition is to reject load at an adverse time after increasing load to obtain the highest water level in surge tank. It is shown that there is a good agreement between the analytical results and numerical results obtained by using the RK-4 method. The formula is convenient to be applied to engineering design.
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