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A hydro power plant utilizes the difference in height between two water levels (the head). Depending on the size of the head different turbine types are used. |
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If a power plant will be
profitable, a head of more than five metres is needed in most cases. At relatively small heads, 5-35 m, the
whole machine station is built into a dam building (see the picture to the left). In this
head range a kaplan turbine will be used. The water is led
into a spiral to increase its speed and distribute the pressure uniformly.The turbine
runner transformes the kinetic and potential energy of the water to mechanical energy
in the turbine shaft. The water flow is adjusted partly with so called guide vanes and
partly by changing the runner blade angle. Thanks to this double adjustment a high efficiency in a large flow area is reached with the
Kaplan turbine. The turbine is in most bigger machines directly connected to the generator, which transforms the mechanical energy in the shaft to electrical energy. The water is led out through a steel-lined outlet part called draft tube. The head is here practically the same as the difference in level between the water levels upstream and downstream the dam, as the flow losses in the short spiral intake are small. At higher heads Francis- or Pelton turbines are used. In Francis plants the machine station is usually blasted in the bedrock. The water is led to and from the turbines in rock tunnels. Here the head corresponds to the difference in height between the water levels at the inlet of the headrace tunnel and at the outlet of the tailrace tunnel. A conciderable part of this so caled geodetic head is lost as a rwsult of frictions in the rock tunnels. |
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higher heads (more than 300 m) do not exist
in our country. As the francis turbine can be used at considerably higher heads than those
found in Sweden, this turbine type is found in all Swedish high head power plants. There are often very high heads in for instance Norway and in the in the alpine coutries. Here we find many plants with Pelton turbines. In this type of turbine the whole water energy is transformed to kinetic energy in one or several water jets directed by means of nozzles towards the runner buckets located along the periphery of the runner. The water flow is adjusted with needle valves in the nozzles. In turbines with several jets the turbine flow can also be decreased by closing one or several jets completely. The geodetic head for Pelton turbines is the difference in level between the water level at the intake to the inlet tube and the water level in the drain basin below the runner. As the turbine, unlike earlier described turbine types, does not have a draft tube, the part of the head which corresponds to the difference in level between the runner and water level in the drain basin can not be used. As the head is very high (at least 400 m) this loss is of small importance. A dificulty of the francis turbine is that the water flow can not be closed as quickly as wanted (at emergency shutdown or load rejection), as then very strong pressure shocks (water hammer) will occur in the turbine spiral and inlet tube. The pelton turbine has not this problem, as each nozzle is supplied with a deflector which can quickly be led into the jet and deflect it. In this way the water jets and their energy are led away from the runner without having to decrease the water flow in theinlet pipe of the turbine.
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