Types of Turbines

Turbines can be classified according to the direction of the water flow through the blades, e.g. radial, axial or combinedflow turbines, or as reaction, impulse or mixed-flow turbines. In reaction turbines there is a change of pressure across the turbine rotor, while impulse turbines use a high velocity jet impinging on hemispherical buckets to cause rotation. There are three basic types of turbine broadly related to low, medium or high heads.
Propeller or axial flow turbines are used for low heads in the range from 3 to 30 metres. They can have relatively inexpensive fixed blades, which have a high conversion efficiency at the rated design conditions but a poorer par€-load efficiency, typically 50%, at one third of full rated output.
Alternatively, the more expensive Kaplan turbine has variable-pitch blades which can be altered to give much better part-load efficiency, perhaps 90% at one third of full rated output. The Francis turbine is a mixed-flow radial turbine and is used for medium heads in the range from 5 to 400 m. It has broadly similar performance characteristics to the fixed-blade propeller type and its speed is controlled by adjusting the guide vane angle. The best-known impulse turbine is the Pelton wheel.
Each bucket on the wheel has a centrally placed divider to deflect half the flow to each side of the wheel. It is normally used for heads greater than 50 m and has good performance characteristics over the whole range, very similar to the Kaplan turbine, reaching 60% efficiency at one-tenth of full rated output.

The speed is controlled by a variable inlet nozzle, so that with a constant head, the delivered torque to the generator is proportional to the flowrate and the turbine speed can be held at that required for synchronous generation at the particular grid frequency. This type of installation is known as a constantspeedkonstant- frequency system and optimization of the power output is relatively easy. I1O In smaller installations, optimum power cannot be obtained at constant speed where the hydraulic head is both relatively low and variable over a wide range.
A detailed description of methods which can be used for optimizing electric power from small-scale plant has been given by Levy.”’ He points out that small hydroelectric systems will become more financially attractive through developments of low-cost power converters (from 100 W upwards), special variable-speedkonstant-frequency generators and cheap computing units for on-line power measurement and optimizing control. This means that many run-of-the-river sites that were considered in the past to be unsuitable for electricity generation can now be used