Influence of the flow phenomena on pump-turbine stability at part-load in pump mode

Pumped storage power plants can play an important role in solving the grid stability problems due to the increasing production share of the intermittent and, to some extent, unpredictable renewable energy sources. Even though reversible pump turbines are widely regarded as the most cost effective solution, their working range is still limited by problematic hysteresis/instability regions in their machine characteristics that could lead to problems such as severe self-excited oscillations in the hydro-mechanical system. In order to understand the underlying physical mechanism of pump-mode instability of a two stages reversible-pump turbine, pressure fluctuations were monitored at different flow rates by flush mounted micro pressure transducers in the guide vanes, in the bladed return channel and in the inflow. Moreover, unsteady numerical flow fields were analyzed by the commercial code ANSYS CFX14.0 to highlight the fluid-dynamical characteristic of the instabilities, and investigate their origin. Large-scale instabilities due to the dynamic interaction between rotor and stator were identified and characterized in the frequency and in the time-frequency domains. The analysis of the pressure signals highlighted the presence at part loads of a pulsating phenomenon at a frequency of St= 0.33 both in the interaction zone between the diffuser and the return channel and along the blades of the return channel (Fig. 1). An unsteady reverse flow was identified inside the impeller on the blade suction side that moved along the blade, interacted with the wake zone near the impeller blade trailing edge and the diffuser flow field and even forced the instability in the return channel.