Potential risks in balancing flexibility and investment of pumped storage plants: Hydraulic disturbances during transient processes in parallel operation of fixed-speed and variable-speed units sharing a diversion tunnel

Pumped storage plants (PSPs) incorporate variable-speed units (VSUs) in parallel with fixed-speed units (FSUs) to balance investment and enhance system response flexibility. However, the parallel operation of FSU and VSU may lead to unfavorable hydraulic disturbance, posing a threat to transient stability of the units. This study aims to reveal the mechanism behind hydraulic disturbance and seeks to mitigate its impact on operational stability. Firstly, a refined simulation model of a PSP employing distinct units sharing a diversion tunnel (DUAT) layout is developed based on real unit data. Taking the load rejection as an example, the superiority of VSU during transient processes is quantified. Subsequently, the underlying causes of hydraulic disturbance generation in transient processes under DUAT layout are investigated. The research reveals that different steady-state operating points result in pressure changes initiated by the FSU being transmitted to VSU through the pipeline, thereby deteriorating the stability of VSU during load rejection process. To alleviate the impact of hydraulic interference, a comprehensive assessment framework is constructed using fuzzy entropy weighting method. This framework effectively balances stability indicators during transient processes and identifies optimization schemes to alleviate hydraulic disturbances caused by parallel operations. Compared with the original scheme, the maximum pressure in volute and the vacuum in draft tube have been reduced by 59.76 m and 11.28 m, respectively. Additionally, there has been an observed 8 % reduction in the maximum oscillation of rotational speed.