Grand'Maison Dam

The Grand'Maison Dam is an embankment dam on L'Eau d'Olle, a tributary of the Romanche River. It is located in Vaujany of Isère within the French Alps. The primary purpose of the dam is to serve as the upper reservoir for a pumped-storage hydroelectric scheme where Lac du Verney located lower in the valley is the lower reservoir. The dam was constructed between 1978 and 1985 with its power station being commissioned in 1987. With an installed capacity of 1,800 MW, it is the largest hydroelectric power station in France.

Design and operation
The Grand'Maison is an embankment dam with a height of 140 m from the riverbed and 160 m from foundation. It is 550 m long and has a fill volume of 12000000 m3. The reservoir withheld by the dam, Lac de Grand Maison, has a storage capacity of 140000000 m3. The power generation process begins with water stored in its reservoir at an altitude of 1698 m. By means of a 7100 m long head-race tunnel which splits into three 1450 m long penstocks, water is sent down to the power station. It is located on the rear bank of Lac du Verney at 45.14528°N, 6.05083°W which lies at an elevation of 770 m. The power station has above-ground and below-ground levels. On the above-ground level, there are four 150 MW Pelton turbine-generators which are used for normal conventional hydroelectric power generation. The below-ground level contains eight 150 MW Francis pump turbines which can be used for both power generation and pumping. After electricity is generated, the water is discharged into Lac du Verney, the lower reservoir. When storage in Grand'Maison needs to be replenished, the turbines reverse into pumps and move water from Lac du Verney back to the Grand'Maison Reservoir. The change in elevation between the reservoirs afford the above-ground station a maximum hydraulic head of 920 m and the below-ground station 955 m.

The power station repeats the pumped-storage process as needed and acts as a peaking power plant. Power generation or pumping can be initiated within minutes. On an annual basis, the power station generates 1,420 GWh of electricity and consumes 1,720 GWh in pumping mode. Because pumping occurs during periods of low demand when electricity is cheaper than power generation during those of high demand, the power station is profitable.