Speed-no-load stability analysis and instability mitigation measure of pumped storage powerplant

Stability of the speed-no-load (SNL) is significant to the synchronization of pumped storage powerplants (PSPs). However, unstable hydraulic vibration phenomena always happen in SNL due to the pump turbine’s S-shaped characteristics. This article proposes a stability criterion that considers the hydraulic vibration effect, elucidates the instability mechanism of SNL, and provides mitigation measures to stabilize the system under SNL. First, by using the hydraulic vibration theory, the relationship between the pipeline’s hydraulic vibration characteristics and the water hammer reflection coefficient (WHRC) at both ends of the pipeline is obtained, and thus, the stability criteria based on the WHRC for the hydraulic turbine governor system (HTGS) is derived. The criteria’s physical meaning is clear: If the product of the WHRC moduli at both ends of the pipeline in HTGS is greater than 1, the system is destabilized. Combining the criteria with the derived WHRC for quantifying the contribution of various hydraulic components to system stability can evaluate the PSP’s stability. Ultimately, the instability mechanism of SNL is elucidated through the derived stability criteria. Inspired by the stability criteria, the ball valve in front of the turbine is utilized to control hydraulic vibration and change the system WHRC, solving the instability under the SNL. In addition, the algorithm for calculating the critical value of the ball valve flow resistance coefficient required to stabilize the system is presented. The results indicate that using the ball valve for hydraulic control can solve SNL instability. The critical valve flow resistance increases with the pipeline’s length; however, since the valve position change affects the system’s main frequency position order, there is no specific pattern for critical valve flow resistance to change with the valve position.