Increasing power output from Francis turbines: Effektøkning i vannkraftverk med Francis turbiner
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The main objective in this thesis was to investigate a potential increase of power output from the existing turbines at Ruacana Hydropower Station. This was restricted to only consider a runner replacement, and only technical aspects of a rehabilitation project has been evaluated. This involved an investigation of hydraulic condtions, limited to surge oscilliation and water hammer calculations. The results was thereafter used in a calculation model to estimate potential increase in power output. Vital information about the plant and the condition of the waterway was obtained from a study trip to Ruacana. An increase in power output in a turbine can be obtained by increasing the volume flow and/or efficiency. The first criteria can be achieved through expanding the outlet diameter of the runner, and/or altering the flow angles. Therefore has the hydraulic conditions also been analysed with the aim of increasing volume flow. It was found that it can be increased to a maximum nominal volume flow of one unit to 85 m3/s. It was limited by the maximum upsurge in the Surge Headbay with the intake level at highest flood level. The restrictions are altered for the minimum water levels in the intake and maximum levels in the tailrace when operating 1 to 4 units. The advances in Francis runner design over the last deacades makes an increase in efficiency feasible. Finally a calculation method for estimating a potential increase in power output was developed. This was based on traditional turbine theory, and the aim was to find the maximum increase starting from the existing degree of submergence. In addition were the results compared to values obtained from a calculation method developed by Sintef. From a preliminary study of achievements in refurbishment projects was it found that on average has an increase of 18% been obtained. The method developed in this thesis concluded that an increase of 10 - 14% in nominal power output is possble. Assuming a turbine efficiency of 0.93 and only replacing the runner. Sintef's method estimated a maximum value of 15% for the same conditions. It is also found that the outlet diameter at Ruacana can be increased by 60 mm. The additional increase in power output is only estimated to be 1% from the method developed in this thesis. The Sintef method estimates additional 3% increase for the same scenario.