2024-06-12 09:30:40
The Grand’Maison hydroelectric site is a pumped storage energy transfer station. Since last year, EDF has been carrying out simultaneous pumping and turbine operations there to provide services to the network.
In the magnificent landscape of the Olle water valley, near Grenoble, the Grand’Maison pumped storage energy transfer station (Step) stands in two places. At an altitude of 1,695 m, the dam contains the 140 million m3 of water from the upstream reservoir, mainly supplied by snowmelt. About ten kilometres downstream, at an altitude of 770 m, is the Verney reservoir containing a volume of water ten times smaller. Between the two, the hydroelectric plant managed by EDF can produce electricity when its turbines use the driving force of the waterfall. It can also pump water in the other direction, from the bottom to the top, when the grid needs to be relieved of excess electricity. Since 2023, it has been the only Step that can operate the turbines and pump at the same time. An asset for the electricity grid.
Commissioned between 1985 and 1987, the Grand’Maison installation is the largest Step in Europe, with a power of 1,800 MW. It far surpasses its smaller French sisters in Montézic (910 MW), Revin (800 MW), Super-Bissorte (730 MW), Cheylas (480 MW) and La Coche (380 MW). The plant has production facilities on two floors. On the upper level of this building located just before the Verney reservoir, four conventional groups, with Pelton turbines, are capable of generating water with a power of 157 MW each. On the lower level, eight Francis-type groups of 160 MW each can operate in turbine or pumping mode.
Evolution of the operating mode
Originally, this type of Step was designed, schematically, to produce electricity during the day during the week and to pump water up to the upstream dam at night and on weekends. By pumping during these night and Sunday periods, the Step can absorb the overproduction of the nuclear park during off-peak consumption hours and replenish part of the water storage of the upstream dam. Over time, with the rise of wind and solar photovoltaic parks, this weekly cycle has been supplemented by a daily cycle: when needed, the Step now preferentially operates at peak consumption times in the morning and late afternoon; and it pumps around midday when wind and solar production are at their maximum.
Two other developments complete this first change. First of all, EDF aims to increase the power of its hydroelectric facilities where possible. In the case of Grand’Maison, three of the four Pelton turbines were replaced with new wheels to increase their unit power to 170 MW. One of the eight reversible groups was also dismantled: it was found that the turbine-pump had successfully withstood the heavy loads it was subjected to, and its overhaul allowed it to return to its initial efficiency. At the beginning of June, this 19-metre-high group was being reassembled, an exercise that must be carried out with great precision despite the weight of the components (the shaft alone weighs 19 t, and the stator and rotor of the alternator weigh 180 and 160 t respectively!).
Support for frequency regulation
In addition to the power gain, there is a new operating mode. Studies conducted as part of a European project called XFlex Hydro validated the idea that the Step can pump and turbine at the same time. This is made possible by the fact that some of the pipes bringing water to the plant cross. At these “nodes”, the pumped water does not go back up to the upstream dam, because it branches off via a tapping point towards a turbine. The simulations carried out as part of XFlex Hydro showed that simultaneous pumping and turbine (PTS), with high flow rates and pressures, is supportable by the equipment (pipes, surge stack). Tests carried out in 2021 at Grand’Maison confirmed the operationality of this operating mode, which has since been integrated into the algorithms of the EDF control centre in Lyon.
But what is this PTS for? In fact, only the turbines have variable power, which allows the Step to provide frequency adjustment to the electricity grid. Previously, not all pumping periods allowed this service to be provided. Thanks to the PTS, Grand’Maison is in a position to regulate the frequency more often, both on primary reserves (in less than 30 seconds) and secondary reserves (in less than 3 minutes). In fact, pumping is now carried out 40% of the time in PTS. In addition to this essential support for the grid, the PTS often operates as a replacement for fossil fuel power plants, which avoids greenhouse gas emissions.
With this new asset, hydroelectricity remains more than ever a mode of production to be strengthened. Moreover, EDF is considering implementing the PTS in its Super-Bissorte Step.
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