Start-up improvement in turbine mode for high head PSP machine

Pumped Storage Plants (PSP) using reversible pump-turbines are today the most efficient way of storing large amounts of electrical energy at acceptable costs. However, in order to suit high and sudden variations in power demand, a large amount of power must be available to be released quickly at the required time. When peak power production is required, then start-up in turbine mode must be fast, efficient and reliable. During start-up, the machine must operate at a stable speed close to the no-load condition (i.e. close to the runaway characteristic) while all synchronized guide vanes are open slightly. This is very challenging for high head units that can have an S-shaped characteristic in the turbine quadrant close to the no load condition. This paper focuses on a new hydraulic design that provides an improvement of the S-shaped characteristic and thus removes all speed instability sources during the start-up sequence of the turbine in a large operating head range while not involving a special stabilizing technique. Hydraulic design is fundamental for any PSP. Given the constraints in generating and pumping modes, S-shaped phenomena become even more important and then more difficult to be mastered for high head machines. Thanks to the pump-turbine design methodology, the use of CFD tools as well as model tests for optimization, a dedicated high head pump-turbine design was developed to achieve a significant improvement of the S-shaped characteristic using all synchronized guide vanes. This paper describes the S-shaped phenomena from a hydraulic point of view. We also detail the numerical study, the model tests results and the turbine coupling sequence simulation results for which a comparison is done between a typical high head design and a dedicated high head design to the S-shaped reduction.