Dynamic characteristics of load rejection process in a reversible pump-turbine

The dynamic behavior during load rejection process of a reversible pump-turbine is numerically investigated based on the dynamic grid method. For the dynamic characteristics, the external features like runner rotation speed, torque and flow rate are analyzed. Then the time-frequency characteristics of pressure fluctuations at different positions (like guide vane, vaneless space, runner inlet and draft tube) are discussed in details. During the unsteady process of load rejection, four peculiar frequency bands, are identified and their sources are assessed. These frequency bands are 2 low frequency-high amplitude components, marked as zones a and b, which occur around the zero-torque condition. Another 2 high frequency-high amplitude components, marked as zones c and d, appear near the maximum reversed flow condition. Besides, the rotor-stator interaction plays an important role in the pressure pulsation bands distribution, showing components multiples of runner rotational frequency. They could propagate towards both upstream and downstream. For reversed flow condition, it is also found that the entrance flow from draft tube into runner contributes most energy to the pressure fluctuations. (C) 2019 Published by Elsevier Ltd.