Evaluation of possible penstock fatigue resulting from secondary control for the grid

Due to electricity market deregulation, the contribution of hydroelectric power plants to secondary control becomes of major interest regarding the electrical grid stability. Besides the grid stability improvements obtained with secondary control, the power utilities are more and more concerned by the influence of the constantly changing power set point on the lifetime of the power plant components. The pumped storage power plant of Moiry-Mottec owned by Forces Motrices de la Gougra SA in Switzerland of 69MW, was originally commissioned in 1960. The power plant featuring complex hydraulic layout comprises 3 Pelton units of 23MW, one storage pump of 24MW and one siphon pump of 7.5MW. Since 2009, 2 units of this power plant provide secondary control services to the grid in generating mode. If inspections and related maintenance can be done rather easily for turbines, pumps and generators parts, these tasks are more difficult to undertake for the penstock. This paper addresses the solicitations of the penstock resulting from the new control mode analyzed by means of numerical simulation for predicting possible fatigue of the penstock materials. To carry out this analysis, a simulation model is elaborated using the simulation software SIMSEN and includes the upstream reservoir, the gallery, the surge tank, the penstock, the Pelton turbines and the control loops for secondary control of active power. First, the simulation model is presented and validated with in-situ measurements carried out in case of both prequalification and secondary control. Then, time domain simulations are carried out considering 3 different types of active power set point signals: (i) Pseudo-Random signal, (ii) triangle signals and (iii) real secondary control set point history. Finally, the pressure amplitudes resulting from the simulation are converted into penstock materials stress to carry out fatigue analysis. The fatigue analysis accounts for welding details and enables defining allowable number of cycle along the penstock.