The Upper Gesso Plant

The hydroelectric development of the valley of the Gesso Torrent, which was started a few years ago with the construction of the Piastra-Andonno Plant, is completed, upstream of the Piastra basin, with the construction of two new plants which utilize the Chiotas-Piastra and Rovina-Piastra heads.

The geographic location of these plants is favourable, both for its proximity to consumption centres in Piemonte Region’s industrial areas and to the 380 kV lines which link the Vado Ligure thermal power station to Turin.

The two plants, Chiotas and Rovina, have a common power station, the Entracque station, where a total capacity of 1500 MVA is installed.

The Chiotas-Piastra Plant

The Chiotas-Piastra plant is practically a pure pumping plant between the two reservoirs of Chiotas (effective capacity 27,300,000 m3 – catchment area 11.6 km2) and Piastra (effective capacity 9,000,000 m3) with a maximum geodetic head of 1048 m and annual productibility of 40 GWh from natural flows and 1000 GWh from pumped flows. The Chiotas reservoir has the maximum storage at E1.1978 m.a.s.l.

The Rovina Piastra Plant

The Rovina Piastra plant of generation and pumping is located parallel to the first at an E1400m lower. The head reservoir is Rovina, a natural lake in which some modest waterproofing works have been carried out, in the lake, having an effective capacity of 1,200,000 m3, will run, besides the small flows from the direct catchment area, also the waters which are collected by two channels on the right and left bank, from a total 77.2km2 catchment area. Annual productivity is estimated at 150GWh from natural flows and 70 GWh from pumped flows.

The Entracque Power Station

The station, common to both plants, is located underground on the left bank at about 600m from the Piastra Reservoir. The machinery of the station is arranged as follows:

  • The spherical valves are arranged in an independent hall, 170m long;
  • The rotating machines some electrical equipment and control devices are arranged in the main hall 195m long, 15.20m wide and about 40m high.
  • The transformers are installed in a third hall which houses also circuit breakers and unit isolators.

There are also galleries for penstocks branches, tailrace, downstream gates, 17kV busducts; a long drain tunnel conveys the drainage water in the Gesso Torrent.

The machines utilising the Chiotas head are eight of the “multistage reversible” type. Upstream of each of them there are two spherical valves, one of them is a safety double seal valve; downstream there is a sliding gate. The reversible machines have four stages; only full capacity operation is admitted. Provision is made for starting up the units in pumping with the back to back system with any of the other ones including the Rovina unit, when the valve is closed, hot water is discharged in an appropriate circuit. The main features of each of these machines are.

Generating

H max – 1048m

O max - 16m3/s

Pr max – 148MW

N – 600rpm

Pumping

H min – 955m

O max – 15m3/s

Pa – 155MW

n – 600rpm

The electrical machines are 500rpm synchronous motor-generators with an apparent capacity of 170 M.V.A

They are provided with static exciters made up by tyristors bridge bars and are connected with the transformers, one every two machines, by 17 KV bars made up by metal enclosed ducts in welded aluminium.

The unit breakers are air blast type. The back to back operation is carried out through appropriate isolators arrangements in a metal enclosed duct.

On Rovina head there is only a triplex unit, that is the motor-generator the turbine and the pump are arranged on a single shaft. The control devices are: a double seal spherical valve upstream of the turbine, two spherical valves on the high pressure pipe of the pump, one of which is a safety double seal valve, a sliding gate on the suction pipe of the pump, a sliding gate and a flap gate on the turbine outlet.

The hydraulic machines consist of a “Francis” turbine with a conventional guide vane and a two stages pump connected with the former with a joint which can be clutched in a standstill position. The main features are:

Turbine

H max – 598m

O max – 26.95m3/s

Pr max – 133.67 MW

N – 600rpm

Pump

H min – 561m

O max – 19.44m3/s

Pe – 124.92 MW

N – 600rpm

The electrical machine consists of a 600rpm synchronous motor generator with an apparent capacity of 150 M.V.A.

All transformers are installed in the hall cavern. They are arranged in appropriate cells with an anti-fire device, connected with a water and oil collecting system in case of failure of the tank. The ancillary rooms and exciters transformers have solid non-flammable insulations.

The electrical link between the underground power station and the open air substation is carried out by means of five 400KV oil cable connections, one for each transformer. These cables are placed in trenches filled up with sand, housed in reinforced concrete structure running over the roof of the access gallery.

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