A generating station which utilizes the potential energy of water at a high level for the generation ofElectrical energy is known as aHydro-electric power station.
Hydro-electric power stations are generally located in hilly areas where dams can be built convenientlyand large water reservoirs can be obtained. In a hydro-electric power station, water head iscreated by constructing a dam across a river or lake. From the dam, water is led to a water turbine.The water turbine captures the energy in the falling water and changes the hydraulic energy (i.e.,product of head and flow of water) into mechanical energy at the turbine shaft. The turbine drives thealternator which converts mechanical energy into electrical energy. Hydro-electric power stations arebecoming very popular because the reserves of fuels (i.e., coal and oil) are depleting day by day.They have the added importance for flood control, storage of water for irrigation and water for drinkingpurposes.
Advantages
(i) It requires no fuel as water is used for the generation of electrical energy.
(ii) It is quite neat and clean as no smoke or ash is produced.
(iii) It requires very small running charges because water is the source of energy which is available
free of cost.
(iv) It is comparatively simple in construction and requires less maintenance.
(v) It does not require a long starting time like a steam power station. In fact, such plants can be
put into service instantly.
(vi) It is robust and has a longer life.
(vii) Such plants serve many purposes. In addition to the generation of electrical energy, they
also help in irrigation and controlling floods.
(viii) Although such plants require the attention of highly skilled persons at the time of construction,yet for operation, a few experienced persons may do the job well.
Disadvantages
(i) It involves high capital cost due to construction of dam.
(ii) There is uncertainty about the availability of huge amount of water due to dependence on
weather conditions.
(iii) Skilled and experienced hands are required to build the plant.
(iv) It requires high cost of transmission lines as the plant is located in hilly areas which are quite
away from the consumers.
Schematic Arrangement of Hydro-electric Power Station
Although a hydro-electric power station simply involves the conversion of hydraulic energy intoelectrical energy, yet it embraces many arrangements for proper working and efficiency. The schematicarrangement of a modern hydro-electric plant is shown in Fig below.
The dam is constructed across a river or lake and water from the catchment area collects at the
back of the dam to form a reservoir. A pressure tunnel is taken off from the reservoir and water
brought to the valve house at the start of the penstock. The valve house contains main sluice valves
and automatic isolating valves. The former controls the water flow to the power house and the lattercuts off supply of water when the penstock bursts. From the valve house, water is taken to waterturbine through a huge steel pipe known as penstock. The water turbine converts hydraulic energyinto mechanical energy. The turbine drives the alternator which converts mechanical energy intoelectrical energy.
A surge tank (open from top) is built just before the valve house and protects the penstock from
bursting in case the turbine gates suddenly close* due to electrical load being thrown off. When the gates close, there is a sudden stopping of water at the lower end of the penstock and consequently the penstock can burst like a paper log. The surge tank absorbs this pressure swing by increase in its level of water.
Choice of Site for Hydro-electric Power Stations
The following points should be taken into account while selecting the site for a hydro-electric powerstation:
(i)Availability of water-Since the primary requirement of a hydro-electric power station is the
availability of huge quantity of water, such plants should be built at a place (e.g., river,
canal) where adequate water is available at a good head.
(ii) Storage of water-There are wide variations in water supply from a river or canal during the
year. This makes it necessary to store water by constructing a dam in order to ensure the
generation of power throughout the year. The storage helps in equalizing the flow of water
so that any excess quantity of water at a certain period of the year can be made available
during times of very low flow in the river. This leads to the conclusion that site selected for
a hydro-electric plant should provide adequate facilities for erecting a dam and storage of
water.
(iii) Cost and type of land-The land for the construction of the plant should be available at a
reasonable price. Further, the bearing capacity of the ground should be adequate to withstand
the weight of heavy equipment to be installed.
(iv) Transportation facilities-The site selected for a hydro-electric plant should be accessible
by rail and road so that necessary equipment and machinery could be easily transported.
It is clear from the above mentioned factors that ideal choice of site for such a plant is near a river
in hilly areas where dam can be conveniently built and large reservoirs can be obtained.
Constituents of Hydro-electric Plant
The constituents of a hydro-electric plant are (1) Hydraulic structures (2) Water turbines and
(3) Electrical equipment. We shall discuss these items in turn.
- Hydraulic structures.Hydraulic structures in a hydro-electric power station include dam,
spillways, headwork’s, surge tank, penstock and accessory works.
(i) Dam– A dam is a barrier which stores water and creates water head. Dams are built of
concrete or stone masonry, earth or rock fill. The type and arrangement depends upon the
topography of the site. A masonry dam may be built in a narrow canyon. An earth dam
may be best suited for a wide valley. The type of dam also depends upon the foundation
conditions, local materials and transportation available, occurrence of earthquakes and other
hazards. At most of sites, more than one type of dam may be suitable and the one which is
most economical is chosen.
(ii) Spillways–There are times when the river flow exceeds the storage capacity of the reservoir.
Such a situation arises during heavy rainfall in the catchment area. In order to discharge the
surplus water from the storage reservoir into the river on the down-stream side of the dam,
spillways are used. Spillways are constructed of concrete piers on the top of the dam. Gates
are provided between these piers and surplus water is discharged over the crest of the dam
by opening these gates.
(iii) Headwork’s–The headwork’s consists of the diversion structures at the head of an intake.
They generally include booms and racks for diverting floating debris, sluices for by-passing
debris and sediments and valves for controlling the flow of water to the turbine. The flow of
water into and through headwork’s should be as smooth as possible to avoid head loss and
cavitation. For this purpose, it is necessary to avoid sharp corners and abrupt contractions
or enlargements.
(iv) Surge tank–Open conduitsleading water to the turbinerequire noprotection. However, when closed conduitsare used, protectionbecomes necessary to limitthe abnormal pressure in theconduit. For this reason,closed conduits are alwaysprovided with a surge tank.A surge tank is a small reservoiror tank (open at thetop) in which water levelrises or falls to reduce thepressure swings in the conduit.A surge tank is located nearthe beginning of the conduit.When the turbine is running at a steady load, there are no surges in the flow of water throughthe conduit i.e., the quantity of water flowing in the conduit is just sufficient to meet theturbine requirements. However, when the load on the turbine decreases, the governor closesthe gates of turbine, reducing water supply to the turbine. The excess water at the lower endof the conduit rushes back to the surge tank and increases its water level. Thus the conduitis prevented from bursting. On the other hand, when load on the turbine increases, additionalwater is drawn from the surge tank to meet the increased load requirement. Hence, asurge tank overcomes the abnormal pressure in the conduit when load on the turbine fallsand acts as a reservoir during increase of load on the turbine.
(v) Penstocks-Penstocks are open or closed conduits which carry water to the turbines. Theyare 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 penstockscan be designed for any head; the thickness of the penstock increases with the head orworking pressure.
Various devices such as automatic butterfly valve, air valve and surge tank areprovided for the protection of penstocks. Automatic butterfly valve shuts off water flow through thepenstock promptly if it ruptures. Air valve maintains the air pressure inside the penstock equal tooutside atmospheric pressure. When water runs out of a penstock faster than it enters, a vacuum iscreated which may cause the penstock to collapse. Under such situations, air valve opens and admitsair in the penstock to maintain inside air pressure equal to the outside air pressure.
- 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 entirepressure of water is converted into kinetic energy in anozzle and the velocity of the jet drives the wheel. Theexample of this type of turbine is the Pelton wheel (SeeFig. 2.4). It consists of a wheel fitted with ellipticalbuckets along its periphery. The force of water jet strikingthe buckets on the wheel drives the turbine. Thequantity of water jet falling on the turbine is controlledby means of a needle or spear (not shown in the figure)placed in the tip of the nozzle. The movement ofthe needle is controlled by the governor. If the load onthe turbine decreases, the governor pushes the needleinto the nozzle, thereby reducing the quantity of waterstriking the buckets. Reverse action takes place if theload 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 importanttypes 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 guideblades control the flow of water tothe turbine. Water flows radiallyinwards and changes to a downwarddirection while passing through therunner. As the water passes overthe “rotating blades” of the runner,both pressure and velocity of waterare reduced. This causes a reactionforce which drives the turbine.A Kaplan turbine is used forlow heads and large quantities ofwater. It is similar to Francis turbineexcept that the runner ofKaplan turbine receives water axially.Water flows radially inwards Bhakra Damthrough regulating gates all around the sides, changing direction in the runner to axial flow. Thiscauses a reaction force which drives the turbine.
- Electrical equipment. The electrical equipment of a hydro-electric power station includes
alternators, transformers, circuit breakers and other switching and protective devices.