Hydraulic short circuit (HSC), corresponding to the simultaneous operation of the pumps and turbines, enhances the power flexibility of a pumped storage power plant (PSPP). However, comprehensive analyses are imperative to guarantee a secure and reliable operation within this novel operational mode. Unforeseen engaging dynamics may perturb the safe operation of the groups. The existing literature still lacks comprehensive analysis concerning the water flow structures and tubomachines performances during HSC operations across the entire hydraulic circuit. This paper evaluates the fluid -dynamic interactions between the units during HSC operations using 3D unsteady -state CFD simulations of an existing PSPP not designed for enduring such conditions. The total pressure evolution in the upstream trifurcation and consequent effects on the turbine performance are discussed by the selected HSC configurations. In the trifurcation, between the pumping unit and the turbine, the relative head loss reaches 1,78% and, eventually, anomalies of the velocity field are documented on a case -by -case basis. Moreover, the pump operations are investigated in HSC operations with the matter of efficiency detriments, cavitation and air entrainment risk. The results highlight that the pumping unit in HSC compared to the baseline operation undergoes a reduction in hydraulic efficiency up to 0.51% due to the operation of adjacent turbine. The documented observations led to the application of additional criteria for the safe and stable governing control of the PSPP in HSC.