Two plants, one above the other

Published on 31 January 2008 (updated on 1 February 2008)
Spherical tap in an underground powerstation
Spherical tap in an underground powerstation

The two power plants have a total output of 1820 MW : the external polant houses the generator sets and the other (underground) plant houses the 8 8 pumpt-turbines.

At Grand’Maison there is one external power plant and one underground power plant which are separated by thirty metres of rock. This arrangement, which is a result of the type of machinery installed, means the Grand’Maison dam has a total of twelve 150MW turbine sets, giving a total output of about 1,820MW.

The four generator sets in the upper power plant have Pelton turbines which are driven by five jets and turn at a rate of 426.6 rpm. For them to function, their wheels must be above the highest level of Verney pondage. In reality, this power plant is partially underground, with its floor at natural ground level. It also houses all the monitoring and control devices for both power plants.

The 8 reversible pump-turbines in the underground power station had to be at least 40 metres below the minimum level of Verney pondage. The turn at 600 rpm in each direction and are located in an enormous underground cavern.

Each of the turbines in both power plants is coupled to an alternator which outputs 153,000 kW at 15,500 volts. Power leaves the plant after passing through six transformers (15.5 - 15.5/405kV) each of which handles the output from 2 generator sets.

To the left of the Verney dam the Oz power plant returns the water to the river via two differently rated Kaplan and Francis turbine sets.

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