Transient instability elimination during turbine runaway process of an ultra-high head pump-turbine

Pumped storage power plant plays a vital important role in absorbing renewable energy in power grid while the instability in transient processes of a pump-turbine threaten the security of the hydraulic system. In present paper, based on one- and three-dimensional (1D-3D) coupled computational approach and Detached Eddy Simulation (DES) model, the transient instability elimination during turbine runaway process of an ultrahigh head prototype pump-turbine is investigated. It is found that during turbine runaway process, compared to the original runner, the runner with optimized high-pressure side can effectively reduce the maximum runaway speed by 0.8 %, reducing the maximum pressure in spiral casing by 0.5 % and increasing the minimum pressure in draft tube by 23.5 %. Moreover, the flow control mechanism is revealed from the view of attack and backflow. During turbine runaway process, when deviating from design point to some extent, the runner with optimized high-pressure side can contribute to strong rotating stall phenomenon in the vaneless region through sudden increasing the average angle of attack at runner inlet. With the help of the additional rotating stall cells, the water ring in vanless region is broken and the flow capacity of the runner is largely increased, weakening the backflow near high-pressure side. In summary, the additional rotating stall cells in the optimized runner can be analogized to the flywheel in machinery field that increases the runner inherent inertia in the form of equivalent hydraulic inertia, improving the hydraulic stability during turbine runaway process.