A hydroelectric plant consists of a reservoir for storage of water, a diversion dam, an intake structure for controlling and regulating the flow of water, a conduit system to carry the water from the intake to the waterwheel, the turbines coupled with generators, the draft tube for conveying water from waterwheel to the tailrace, the tailrace and a power house i.e., the building to contain the turbines, generators, the accessories and other miscellaneous items.
The size, location, and type of each of these essential elements depend upon the topography and geological conditions and the amount of water to be used. The height to which the dam may be built is usually limited by the extent of flowage damage. Pondage may have great value, particularly for peak load power plants, warranting the purchase of extensive flowage rights. The spillway section of the dam must be long enough to pass safely the maximum amount of water to be expected. Likewise the abutments and other short structures must be built to withstand successfully the greatest freshet conceivable on the river.
The elements of hydroelectric power plant are as follows:
Element # 1. Storage Reservoir:
It is the basic requirement of a hydroelectric plant. Its purpose is to store water during excess flow periods (i.e., rainy season) and supply the same during lean flow periods (i.e., dry season) and thus it helps in supplying water to the turbines according to the load on the power plant.
A reservoir can be either natural or artificial. A natural reservoir is a lake in high mountains and an artificial reservoir is made by constructing a dam across the river. Low head plants require very large storage reservoir. The capacity of reservoir depends on the difference between runoffs during high and lean flows.
Element # 2. Dam:
The function of dam is not only to raise the water surface of the stream to create an artificial head but also to provide the pondage, storage or the facility of diversion into conduits. A dam is the most expensive and important part of a hydro-project. 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 choice of dam also depends upon the foundation conditions, local materials and transportation available, occurrence of earth quakes and other hazards.
Mansory dams are of three major classes’ viz., solid gravity, buttress and the arch dams.
Solid gravity dam shown in Fig. 2.8 (a) is made of concrete and is suitable for most sites. The height of the dam, which cannot be very high, depends on the strength of subsoil strata. Arch dam, shown in Fig. 2.8 (b) is a curved dam and transmits a major portion of its water pressure horizontally to the abutments by arch action.
An arch dam is preferred where a narrow canyon width is available. This dam has the inherent stability against sliding. The buttress or deck dam has an inclined upstream face, so that water pressure creates a large downward force which provides stability against over-turning or sliding. Such a dam is more suitable for weak foundations and earth quake prone sites.
An earth dam has a very wide base as compared to its height. Such dams are quite suitable for a pervious foundation because the wide base provides a long seepage path.
The earth dams have got the following advantages:
1. They are cheaper than masonry dams.
2. They fit best in natural surroundings.
3. Such a dam provides the most permanent type of structure if protected against erosion.
However, earth dams have the following disadvantages:
i. The seepage of water is more than that in case of masonry dams.
ii. They are subject to erosion by water.
iii. They are not suitable for a spillway; therefore, a supplementary spillway of adequate capacity is required.
Element # 3. Forebay:
The forebay serves as a regulating reservoir storing water temporarily during light load period and providing the same for initial increase on account of increasing load during which water in the canal is being accelerated. In short, a forebay may be considered as an enlarged body of water just above the intake to store water temporarily to meet the hourly load fluctuations. This may either be a pond behind the diversion dam or an enlarged section of a canal spread out to accommodate the required widths of intake.
Where the hydroelectric plants are located just at the base of the dam, no forebay is required because the reservoir itself serves the purpose of the forebay. However, where the plants are situated away from the storage reservoir a forebay is provided.
Element # 4. Spillway:
This is constructed to act as a safety valve. It discharges the overflow water to the down-stream side when the reservoir is full, a condition mainly arising during flood periods. These are generally constructed of concrete and provided with water discharge opening shut off by metal control gates. By changing the degree to which the gates are opened, the discharge of the head water to the tailrace can be regulated in order to maintain the water level in the reservoir.
Element # 5. Intake:
The intake includes the head-works which are the structures at the intake of conduits, tunnels, or flumes. These structures include booms, screens or trash racks, sluices to divert and prevent entry of debris and ice into the turbines.
Booms prevent the ice and floating logs from going into the intake by diverting them to a bypass chute. Screens or trash racks are fitted directly at the intake to prevent the debris from going into the take. Debris cleaning devices should also be fitted on the trash racks.
Intake structures can be classified into high pressure intakes used in case of large storage reservoirs and low pressure intakes used in case of small ponds provided for storing small amount of water for daily or weekly load variations.
Element # 6. Surge Tank:
A reduction in load on the generator causes the governor to close the turbine gates and thus create an increased pressure in the penstock. This may result in water hammer phenomenon and may need pipe of extraordinary strength to withstand it otherwise the penstock may burst. To avoid this positive water hammer pressure, some means are required to be provided for taking the rejected flow.
This may be accomplished by providing a small storage reservoir or tank (open at the top) for receiving the rejected flow and thus relieving the conduit pipe of excessive water hammer pressure. This storage reservoir, called the surge tank is usually located as close to the power station as possible, preferably on ground to reduce the height of the tower.
A decrease in load demand causes a rise in water level in the surge tank. This produces a retarding head and reduces the velocity of water in the penstock. The reduction in flow velocity to the desired level makes the water in the tank to fall and rise until damped out by friction.
Increase in load on the plant causes the governor to open the turbine gates in order to allow more water to flow through the penstock to supply the increased load and there is a tendency to cause a vacuum or a negative pressure in the penstock. This negative pressure in the penstock provides the necessary accelerating force and is objectionable for very long conduits due to difficult turbine regulation.
Again under such conditions, the additional water flows out of the surge tank. As a result the water level in the surge tank falls, an accelerating head is created and flow of water in the penstock is increased. Thus surge tank helps in stabilising the velocity and pressure in the penstock and reduces water hammer and negative pressure or vacuum.
Though by providing a relief valve at the turbine inlet rejected flow can be dealt in a better manner, but it cannot provide excess water required by the turbine when the load demand increases. Open conduits leading water to the turbine require no protection but when closed conduits are employed, protection becomes necessary to limit the abnormal pressure in the conduit. For this reason, close conduits are always provided with a surge tank. A forebay also serves the function of a surge tank.
The ideal location of a surge tank is at the turbine inlet but in the case of medium and high head power plants, the height of the surge tank will become excessive. Because of this reason, the surge tanks are usually provided at the junction of the pressure tunnel and the penstock.
Several designs of surge tanks have been adopted in hydroelectric power plants, the important considerations, being the amount of water to be stored, the magnitude of pressure to be relieved of and the availability of space at the construction site.
Surge tanks may be simple surge tank (Fig. 2.10), restricted orifice surge tank or differential surge tank. Simple surge tank is very sluggish in action and needs the largest volume. So this is the most expensive and is seldom used, except in special cases.
The restricted orifice surge tank is more efficient and economical than the first one, but its main drawback is that the desirable sudden creation of accelerating and retarding heads in the conduits also results in correspondingly sudden fluctuations of head on the turbine, which the governors may have difficulty to accommodate. Differential surge tank is the compromise between the simple surge tank and the restricted orifice surge tank.
Element # 7. Penstock:
It is a closed conduit which connects the forebay or surge tank to the scroll case of the turbine. In case of medium head power plants each unit is usually provided with its own penstock. In case of high head plants, a single penstock is frequently used, and branch connections are provided at the lower end to supply two or more units. Penstocks are built of steel or reinforced concrete. Steel penstocks are almost always welded on the longitudinal seam.
The circumferential seam may be welded also. In long penstocks great care must be taken to protect the conduit against water hammer. The thickness must be adequate to withstand both the normal hydrostatic pressure and also the sudden surges both above and below normal caused by fluctuations in load and by emergency conditions.
Element # 8. Valves and Gates:
In low head plants gates at the entrance to the turbine casing are usually all that is needed to shut off the flow and provide for unwatering the turbine for inspection and repairs. Individual hoist-operated gates are provided in cases where frequent shutdowns may be called for and where the time available for inspection is limited.
Other plants employ stop gates or stop logs which are placed in sections by means of travelling crane. For installations employing medium or longer length penstocks or employing a common penstock for more than one unit, it is necessary to install valves at or near the entrance to the turbine casing. These are usually of the butterfly or pivot type for low and medium heads.
Element # 9. Trash Racks:
These are built up from long, flat bars set vertically or nearly so and spaced in accordance with the minimum width of water passage through the turbine. The clear space between the bars varies from 25 mm or 40 mm to 150 or 200 mm on very large installations. These are to prevent the ingress of floating and other material to the turbine. In some cases where large diameter turbines are employed, the racks are omitted, but provision is usually made for skimmer walls or booms to prevent ice and other material from entering the unit.
Element # 10. Tailrace:
The water after having done its useful work in the turbine is discharged to the tailrace which may lead it to the same stream or to another one. The design and size of tailrace should be such that water has a free exit and the jet of water, after it leaves the turbine, has unimpeded passage.
Element # 11. Draft Tubes:
An airtight pipe of suitable diameter attached to the runner outlet and conducting water down from the wheel and discharging it under the surface of the water in the tailrace is known as draft tube.
If there is no draft tube and the water discharges freely from the turbine exit, then the turbine operates under a head equal to the height of the headrace water level above the runner exit. By installing draft tube, the operating head is increased by an amount equal to the height of the runner outlet above the tailrace. This creates a negative pressure head at the runner exit. This makes it possible to install the turbine above the tailrace without loss of head.
By installing the draft tube and increasing its section from runner exit to the tailrace, some of the kinetic energy possessed by the water leaving the runner outlet is converted into pressure energy and the water leaves at the tailrace at a much reduced velocity. This results again in the kinetic head which increases the negative pressure at the runner exit. This in turn increases the operating head on the turbine increasing its output and efficiency.
The height and type of tube used depends upon two factors. The pressure at the turbine exit or inlet of the draft tube should not be less than one-third of the atmospheric pressure. This is essential to avoid cavitation. Also to maintain continuity of flow without vaporisation, the pressure at any point in the tube should not fall below the vapour pressure of water. Further, to avoid separation of flow, the included angle should not exceed 10°.
Various types of draft tubes are shown in Fig. 2.11. The straight conical type draft tube, shown in Fig. 2.11 (a), has an efficiency of about 90% and is employed for low specific speed, vertical shaft Francis turbine. Vertical bell shaped draft tube is shown in Fig. 2.11 (b). Where there is a little head room available, the bent draft tubes, shown in Figs. 2.11 (c) and 2.11 (d) are used. In Fig. 2.11 (d), the horizontal portion of the tube is gradually bent upwards to lead the water gradually to the tailrace and to prevent entry of air from the outlet end. The exit end of the tube must always be immersed in water.
Element # 12. Prime Movers or Water Turbines:
In hydroelectric power plants, water turbines are used as prime movers and their function is to convert the kinetic energy of water into mechanical energy which is further utilised to drive the alternators generating electrical energy.