Page 299 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Penstocks
Penstocks are open or closed conduits which carry water to the turbines. They are generally made of reinforced
concrete or steel. Concrete penstocks are suitable for low heads (< 30 m) as greater pressure causes rapid
deterioration of concrete. The steel penstock scan be designed for any head; the thickness of the penstock
increases with the head or working pressure. Various devices such as automatic butterfly valve, air valve and
surge tank (See Fig. 2.3) are provided for the protection of penstocks. Automatic butterfly valve shuts off water
flow through the penstock promptly if it ruptures. Air valve maintains the air pressure inside the penstock equal
to outside atmospheric pressure. When water runs out of a penstock faster than it enters, a vacuum is created
which may cause the penstock to collapse. Under such situations, air valve opens and admits air in the penstock
to maintain inside air pressure equal to the outside air pressure.
2 Water turbines
Water turbines are used to convert the energy of falling water into mechanical energy. The principal types of water
turbines are
i Impulse turbines
ii Reaction turbines
i Impulse turbines

Such turbines are used for high heads. In an impulse turbine, the entire pressure of water is converted into kinetic
energy in a nozzle and the velocity of the jet drives the wheel. The example of this type of turbine is the Pelton
wheel. It consists of a wheel fitted with elliptical buckets along its periphery. The force of water jet striking the
buckets on the wheel drives the turbine. The quantity of water jet falling on the turbine is controlled by means of
a needle or spear placed in the tip of the nozzle. The movement of the needle is controlled by the governor. If the
load on the turbine decreases, the governor pushes the needle into the nozzle, thereby reducing the quantity of
water striking the buckets. Reverse action takes place if the load on the turbine increases.
ii Reaction turbines
Reaction turbines are used for low and medium heads. In a reaction turbine, water enters the runner partly with
pressure energy and partly with velocity head. The important types of reaction turbines are:
a Francis turbines
b Kaplan turbines
A Francis turbine is used for low to medium heads. It consists of an outer ring of stationary guide blades fixed to
the turbine casing and an inner ring of rotating blades forming the runner. The guide blades control the flow of
water to the turbine. Water flows radially inwards and changes to a downward direction while passing through
the runner. As the water passes over the “rotating blades” of the runner, both pressure and velocity of water are
reduced. This causes a reaction force which drives the turbine.
A Kaplan turbine is used for low heads and large quantities of water. It is similar to Francis turbine except that the
runner of Kaplan turbine receives water axially. Water flows radially inwards through regulating gates all around
the sides, changing direction in the runner to axial flow. This causes a reaction force which drives the turbine.

3 Eectrical equipment
The electrical equipment of a hydro hydro-electric power station includes alternators, transformers, circuit breakers
and other switching and protective devices.

Diesel power station
A generating station in which diesel engine is used as the prime mover for the generation of electrical energy is
known as diesel power station. In a diesel power station, diesel engine is used as the prime mover. The diesel
burns inside the engine and the products of this combustion act as the “working fluid” to produce mechanical
energy. The diesel engine drives the alternator which converts mechanical energy into electrical energy. As the
generation cost is considerable due to high price of diesel, therefore, such power stations are only used to
produce small power. Although steam power stations and hydro-electric plants are invariably used to generate
bulk power at cheaper cost, yet diesel power stations are finding favour at places where demand of power is less,
sufficient quantity of coal and water is not available and the transportation facilities are inadequate. These plants
are also used as standby sets for continuity of supply to important points such as hospitals, radio stations, cinema
houses and telephone exchanges.

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CITS : Power - Electrician & Wireman - Lesson 106-116