Abstract
The main objective of this study is to investigate the effect of hydropower plant operation on the long-term stability of unlined pressure tunnels of hydropower plants in Norway. The authors analyzed the past production data of some hydropower plants to find out the number of starts/stops and the frequency and magnitude of load changes. The study demonstrates that an average of 200–400 start/stop events are occurring per turbine per year for the analyzed period, with an increasing trend. Currently, 150–200 large load changes per turbine smaller than 50 MW are occurring every year, and this is expected to increase by 30–45% between 2025 and 2040 for one of the studied power plants. Most importantly, the monitored pressure transients and pore pressure response in the rock mass during real-time operation at Roskrepp power plant are presented. A new method is proposed to calculate and quantify the hydraulic impact (HI) of pressure transients on rock joints and the effect of duration of shutdown/opening, which is found to be the most dominant parameter affecting the magnitude. The results show that faster shutdown sequences cause unnecessary stress in rock mass surrounding pressure tunnel. The hydraulic impact (HI) can be more than 10 times higher when the shutdown duration is reduced by 50 percent. The study recommends that duration of normal shutdowns/openings in hydropower plants should be slower so that hydraulic impacts on the rock joints are reduced and cyclic hydraulic fatigue is delayed, prolonging the lifetime of unlined pressure tunnels and shafts.
Highlights
The world energy market is going through a monumental shift from a fossil-fuel-based system to a renewable energy-based system, following the Paris agreement signed in 2015
The main objective of this study is to investigate the effect of hydropower plant operation on the long-term stability of unlined pressure tunnels of hydropower plants in Norway
It can be seen that Transient 1 is a result of a gradual de-loading and shutdown, whereas Transient 2 is a faster shutdown, before the main inlet valve (MIV) closure which occurs almost at similar durations indicated by the largest water-hammer pulses
Summary
The world energy market is going through a monumental shift from a fossil-fuel-based system to a renewable energy-based system, following the Paris agreement signed in 2015. The inclusion of larger amount of wind and solar power causes price volatility due to surplus or deficit of energy at any given time This applies for Norway because the share of its regulated production is falling, as the growth in consumption is primarily covered by wind power [3]. Flexibility is the ability to make quick changes in operation at any time such that the balance between production and consumption can always be maintained, with lowest possible cost for carrying out such changes The need for such flexibility can be both short-term, where changes are needed to balance the system within hours, minutes, and seconds, or long-term, in order to balance the system for days or weeks. Among these solutions, regulated hydropower can provide both short- and long-term flexibility
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