Abstract
Hydraulic turbines exhibit total load rejection during operation because of high fluctuations in the grid parameters. The generator reaches no-load instantly. Consequently, the turbine runner accelerates to high speed, runaway speed, in seconds. Under common conditions, stable runaway is only reached if after a load rejection, the control and protection mechanisms both fail and the guide vanes cannot be closed. The runner life is affected by the high amplitude pressure loading at the runaway speed. A model Francis turbine was used to investigate the consequences at the runaway condition. Measurements and simulations were performed at three operating points. The numerical simulations were performed using standard k-ε, k-ω shear stress transport (SST) and scale-adaptive simulation (SAS) models. A total of 12.8 million hexahedral mesh elements were created in the complete turbine, from the spiral casing inlet to the draft tube outlet. The experimental and numerical analysis showed that the runner was subjected to an unsteady pressure loading up to three-times the pressure loading observed at the best efficiency point. Investigates of unsteady pressure pulsations at the vaneless space, runner and draft tube are discussed in the paper. Further, unsteady swirling flow in the blade passages was observed that was rotating at a frequency of 4.8-times the runaway runner angular speed. Apart from the unsteady pressure loading, the development pattern of the swirling flow in the runner is discussed in the paper.
Highlights
Hydraulic turbines are used extensively to stabilize power grids, because they can restart rapidly and/or change the power output according to the real-time demand
We primarily focus on experimental and numerical studies of the flow field and its effects on the runner blades at the runaway condition
The unsteady pressure loading in the turbine during the runaway operating condition more than doubles relative to normal operating conditions
Summary
Hydraulic turbines are used extensively to stabilize power grids, because they can restart rapidly and/or change the power output according to the real-time demand. Studies [2,6,7,9,10] on hydraulic turbines during total load rejection have shown that the runner was subjected to an unsteady pressure loading with an amplitude that was more than twice that at the normal operating condition, i.e., the best efficiency point (BEP). The amplitudes and frequency of the pressure fluctuations are primarily attributed to the rotor stator interaction (RSI), which increases with the runner angular speed This speed rise condition may be observed for a few seconds because the guide vanes close rapidly after total load rejection [2]. Unsteady swirling flow develops for which the discharge is extremely low, and the runner rotates at high speed This flow results in high-amplitude unsteady pressure fluctuations on the blade surfaces. The experimental results were used to validate the numerical model, which were used to analyze the flow further
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