Arrangement of hydropower plants

Hydro power machine is the designation used for a machine that directly convert the hydraulic power in a water fall to mechanical power on the machine shaft. This power conversion involves losses that arise partly in the machine itself and partly in the water conduits to and from the machine.

The discharge operating a water turbine, is conveyed from a river or a water course through an intake and a conduit to the turbine. From the turbine the discharge is conducted through a so-called tailrase canal to a downstream river course. A brief review of the main details in a hydro power plant is given in the following sections.

Fig. 1 shows schematically an example of a plant arrangement with indication of the localization of the details to be mentioned.

Fig. 1 - Arrangement of a hydro power plant

Fig.1 shows in principle the water conduits of a traditional Norwegian power plant with a high head Francis turbine. Downstream from the upstream reservoir the coarse trash rack, intake gate, head race tunnel, surge shaft, sand trap, fine trash rack, pen-stock isolating valve with air valve, pressure shaft, spherical valve, turbine, draft tube, draft tube gate, outlet surge shaft and tale race tunnel.

Water intake

The water intake is normally constructed in connection with an accumulation dam (1) Fig.1, in the river course. The shallow water intake is equipped with a coarse trash rack (3) which prevents trees, branches, debris and stones from entering the conduit system to the turbine. An intake gate (2) is arranged to shut off the water delivery when the conduit system has to be emptied. In addition a small gate (4) may be arranged for drainage of the leakage through the main gate.

Fig. 2 - Sketch showing in principle the water conduits of a traditional Norwegian high head power plant

A deep water intake takes the water directly from the reservoir. It has no trash rack. There is a sump below the intake. Its main function is to collect blasting stones from the piercing of the the head race tunnel into the reservoir. It also traps stones sliding into the reservoir close to the intake. Deep water intakes allows for very strong regulation of the reservoirs. An intake gate is installed with the same function as described for the shallow water intake.

Conduit system

From the water intake to the turbine it is a conduit system constructed as open canal, tunnel, penstock or pressure shaft or a combination of these. Open canals are usually digged in the ground, blasted in rock or built up as a chute of wood or concrete.

In high head power plants it is normally a so-called head race tunnel between the water intake and the pressure shaft. It may either be drilled and blasted or bored with a tunnel boring machine (TBM). The latter method leaves a much smoother wall surface than the first one, and consequently the head loss is significantly smaller for the same cross section. At the end of the head race tunnel there is a sand trap. Beside the sump in the tunnel floor the cross section of the tunnel is gradually increased to reduce the water velocity and allow for a better sedimentation of suspended particles.

At the downstream end of the head race tunnel there is also a surge chamber system. The function of the surge chamber is briefly to reduce water hammer pressure variations and keep the mass oscillations, caused by load changes, within acceptable limits and decrease the oscillations to stable operation as soon as possible. At the end of long head race tunnels it is also normally istalled a gate. This makes it possible to empty the pressure shaft and penstock upstream of the turbine, for inspection and maintenance without emptying the head race tunnel. Before the water enters the pressure shaft it passes a fine trash rack. It is the last protection of the valve and the turbine against floating debris or smaller stones if the sand trap is full or omitted.

The pressure shaft may either be lined or unlined. Where the rock is of sufficiently high quality the shafts are normally unlined. The excavation of the rock masses may be done either by drilling and blasting or by boring with TBM-machines. Shafts in lower quality rock is being lined either by concrete or by steel plate lining embedded in concrete. Lining of the shafts reduces the losses but increases the costs.

A steel penstock connects the shaft with the valve in the machine hall. Inside the rock the penstock is embedded in a concrete plug. Penstocks are normally welded pipe constructions of steel plates. A flange connects the penstock with the valve. Penstocks above ground are mounted on foundation concrete blocks where the penstock may slide according to thermal expansion. In certain positions the penstocks are fixed in reinforced concrete anchoring blocks (8) on Fig. 1. Between these anchoring blocks the penstocks are equipped with expansion stuffing boxes (7) in Fig. 1.

At the upstream end of a penstock an automatic isolating valve is normally installed. This valve closes automatically if a pipe rupture should occur.

Turbine

The main parts of the turbine, with reference to Fig. 1, are:

• The guide vane cascade, usually adjustable, gives the water flow the velocity and the direction required for the inlet to
• The runner where the hydraulic power is transferred to mechanical power on
• The turbine shaft (9) to which the runner is fixed. The turbine shaft is guided in a
• Radial bearing and an
• Axial bearing that is loaded with the axial force from the runner, caused by the water pressure and impulses from the flow, and the weight of the rotating parts.
• The scroll case (10) conducts the water flow to the guide vane cascade.
• The draft tube (11) conducts the water flow from the turbine outlet into the tale race canal.

Closing valve

Upstream of the turbine a closing device (12) on Fig. 1, is installed. Depending on water head and capasity it may be a gate, butterfly valve, gate valve or a spherical valve. By submerged turbines a closing device, normally a gate, is installed also at the outlet from the draft tube.