In this article we will discuss about:- 1. Arrangement of Hydroelectric Power Plants 2. Selection of Site for Hydroelectric Power Plants 3. Types 4. Hydro Potential in India 5. Automatic and Remote Control 6. Advantages and Disadvantages 7. Generators Employed 8. Plant Layout 9. Auxiliaries 10. Environmental Impacts.
Introduction to Hydroelectric Power Plants in India:
Hydroelectric power is the power obtained from the energy of falling water whereas hydroelectric power plant is the power plant utilising the potential energy of water at a high level for the generation of electrical energy.
The chief source of energy is the sun. Solar energy in the form of heat is absorbed by the oceans, lakes and rivers resulting in the formation of clouds eventual precipitation over the land. Thus the work done on water during the hydrologic cycle, by the energies of the sun, wind and the cooling forces places it upland, impregnating it with the huge potential energy, which may be extracted from it, during its descent back to the oceans? Most of this energy, if not harnessed, dissipates itself into heat before reaching the ocean.
If harnessed, on the other hand, this energy is converted into kinetic energy and this kinetic energy is converted into the mechanical energy by allowing the water to flow through the hydraulic turbine runner. This mechanical energy is utilized to run an electric generator which is coupled to the turbine shaft.
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The electrical power, P developed in this way is given as:
P = wQHη × 9.81 × 10-3 kW ….( 2.1 )
Where w is the specific weight of water in kg/m3, Q is the rate of flow of water in m3/s, H is the height of fall or head in metres and η is the overall efficiency of operation.
In a hydroelectric power station, water head is created by constructing a dam across a river or lake. The pressure head of water or kinetic energy of water is utilized to drive the water turbines coupled to alternators and, therefore, generation of electrical power.
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There are two reasons for the extensive development of water power. One is the tremendous increase in demand of electric power for industrial, agricultural, commercial and domestic purposes and the other is high cost of fuels (coal and oils) and the limited resources. A water power site is usually developed to supply electric power to a newly and a specially established industry or town or to provide additional power to an already existing or proposed interconnected electric power system.
Hydroelectric power stations are usually located in high hilly areas, where the dam can be built easily and large reservoirs can be obtained. Generally such projects are multipurpose i.e., in addition to generation of electric power they are used for irrigation, flood control and navigation etc., Irrigation can be remarkably improved by routing the canals in the downstream of hydro plants, draught and flood both can be effectively controlled utilising the dam water and discharge water of hydro plants.
According to the Central Electricity Authority (CEA), the viable hydro potential in India is 84,000 MW at 60 per cent load factor which is equivalent to 148,700 MW installed capacity. In addition to it, around 10,000 MW from about 2,000 small hydro schemes can also be economically exploited.
Most of the high head power sites naturally lie in hilly areas such as the sub-Himalayan region of North, the Western Ghats and the Nilgiries etc. The medium and low head site developments, combining power and irrigation together, usually occupy the steep plateaus and the plains.
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However, before a water power site is considered for development, the following factors are required to be thoroughly analysed:
1. Capital cost of the plant.
2. Capital cost of erecting and maintaining the transmission lines and the annual energy loss in transformation and transmission of electric power. This is because the hydroelectric power plants are usually located in hilly areas, far away from the load centre.
3. Energy generation cost compared with those in case of steam, oil or gas plants which can be conveniently set up near the load centre.
Arrangement of Hydroelectric Power Plants:
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The chief requirement for hydroelectric power plant is the availability of water in huge quantity at sufficient head and this requirement can be met by constructing a dam across a river or lake.
An artificial storage reservoir is formed by constructing a dam across a river (or lake) and a pressure tunnel is taken off from the reservoir to the valve house at the start of the penstock. The valve house contains main sluice valves for controlling water flow to the power station and automatic isolating valves for cutting off water supply in case the penstock bursts.
A surge tank is also provided just before the valve house for better regulation of water pressure in the system. From the reservoir the water is carried to valve house through pressure tunnel and from valve house to the water turbine through pipes of large diameter made of steel or reinforced concrete, called the penstock. The water turbine converts hydraulic energy into mechanical energy and the alternator coupled to the water turbine converts mechanical energy into electrical energy. Water after doing useful work is discharged to the tailrace.
Selection of Site for Hydroelectric Power Plants:
While selecting a suitable site for a hydroelectric power plant, if a good system of natural storage—lakes at high altitudes and with large catchment areas can be located; the plant will be comparatively economical. Anyhow the essential characteristics of a good site are – large catchment area, high average rainfall, steep gradients and a favourable place for constructing the storage or reservoir. For this purpose, the geological, geographical and meteorological conditions of a site need careful investigation.
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The following factors should be considered in the selection of site for hydroelectric power plants:
1. Availability of Water:
Since in such power stations potential energy of waterfall or kinetic energy of flowing stream is utilized for generation of electric power, therefore such stations should be built where there is adequate water available at good head or huge quantity of water is flowing across a given point.
The plant capacity and the estimated output as well as the need for storage are governed by the average flow. The primary or dependable power which is available at all times depends upon the minimum flow. These conditions may also fix the capacity of the standby plant. The maximum of flood flow governs the size of the dam to be built with adequate spillway.
For estimation of energy available from a given stream or river, the discharge flowing and its variation with time over a number of years must be known. The estimates of the average quantity of water available is prepared on the basis of actual measurements of stream flow over as long a period as possible.
Previous records of rainfall must be studied and minimum and maximum quantity of water available during the year should be estimated. After allowing for losses due to evaporation and seepage the net quantity of water available for power generation can be estimated.
2. Water Storage:
Since storage of water in a suitable reservoir at a height or building of dam across the river is essential in order to have continuous and perennial supply during the dry season, therefore, convenient accommodation for the erection of a dam or reservoir must be available.
The storage capacity can be determined from the hydrograph or from mass curve or by using analytical methods. For selection of site for the dam a careful study of the geology and topography of the catchment area is required to see whether the natural foundations can be used to the best advantage.
3. Water Head:
The available water head depends upon the topography of the area. Availability of head of water has considerable effect on the cost and economy of power generation. An increase in effective head reduces the quantity of water to be stored and handled by penstocks, screens and turbines and, therefore, capital cost of the plant is reduced. In order to determine the most effective and economical head it is necessary to consider all possible factors which affect it.
4. Distance from Load Centre:
Hydroelectric power plant is usually located far away from the load centre. Hence for economical transmission of electric power, the routes and the distances need active considerations.
5. Accessibility of the Site:
Adequate transportation facilities must be available or there should be possibility of providing the same so that the necessary equipment and machinery could be easily transported.
6. Water Pollution:
Polluted water may cause excessive corrosion and damage to the metallic structures. Hence availability of good quality water is essential.
7. Sedimentation:
Gradual deposition of silt may reduce the capacity of the storage reservoir and may also cause damage to the turbine blades. Silting from forest areas is negligible but the regions subject to violent storms and not protected by vegetation contribute lot of silt to the run-off. In some cases, this factor alone may render an otherwise suitable site unsuitable.
8. Large Catchment Area:
The reservoir must have a large catchment area so that level of water in the reservoir may not fall below the minimum required in dry season.
9. Availability of Land:
The land available should be cheap in cost and rocky in order to withstand the weight of the large building and heavy machinery.
10. There should be possibility of stream diversion during period of construction.
Types of Hydroelectric Power Plants:
1. Underground Hydro Power Plant:
An underground hydro power plant is one in which whole of the generating equipment is placed in an underground chamber. An underground layout of hydro power plant may be favoured due to technical and economic considerations.
The underground layout of hydro power plant have the advantages of better rocking bearing properties, greater security in war time, lower maintenance costs, shorter conduit lengths and therefore smaller surge tanks, minimum problem of land acquisition, easier design of tailrace tunnel in comparison to design of headrace pressure tunnel, no interference of surface topography in planning, design and location of the principal works, lower initial cost in some cases, no risk of avalanches and land slips, that may cause difficulties in site selection in steep and rugged countryside, no risk of forest fire hazard, protection to the staff as well as to plant against severe winter climate, no effect of bad weather on the progress of construction work, and maintenance of temperature control inside the plant with adequate control of ventilation irrespective of outside weather conditions.
However, underground hydro power plants have some shortcomings also such as increased cost of construction of power house and accessories, excessive cost of lighting, special ventilation and air-conditioning, additional cost of underground location of transformers and switchgears, costly tailrace tunnels and additional surge chambers.
There are two types of layouts for underground power plants viz., head development and tail development. In first case, the plant is situated near the intake and has a long tailrace tunnel. So the advantages of lower cost of intake structure and possibility of quicker control of turbines are available in the former case. In the second case, the plant is located at the end of a long pressure tunnel and has a short tailrace. The decision is influenced by the topography and character of the rock.
In case of high head power plants, consideration of access dictates the location of power house and it should be located at the down-stream end of the aqueduct system. On the other hand, if the head is low or medium and the rock condition is sound, the power house is situated close under the reservoir and the tail water is led to the stream by long unlined tailrace tunnel. The water from the head pond is led to the turbines by a vertical or nearly a vertical shaft.
860 MW Koyna underground hydro power plants in Maharashtra and 105 MW Salal semi-underground hydro power plants are important hydro power plants in India.
2. Small Hydroelectric Power Plants:
Mini hydro plants of capacity around 101-2,000 kW and micro hydro plants of capacity up to 100 kW fall under this category. Such plants operate under heads of a few metres. Such plants are becoming more and more popular due to rising fuel cost of thermal plants on one side and long construction period and heavy cost of civil works for large hydroelectric power plants on the other side. Small hydro power plants are simple in operation, reliable, need minimal maintenance and more effective than large plants in saving fuel.
In mini hydro and micro hydro schemes, the civil works are simple, and can be built with the local labour in a very short period. High equipment cost was the only hindrance in the development of small hydro power plants. But now due to newly developed ranges of hydroelectric units and their availability in market in standardized packaged units easily adoptable to different site conditions this problem has also been solved.
Small hydro schemes are very useful for small isolated groups of consumers and are likely to be technically and economically feasible at any site where adequate flow and head are available.
Global installed capacity of small hydro is about 47,000 MW against the estimated potential of 200,000 MW. India has a history of more than 100 years in small hydro. An estimated potential of 15,000 MW of small hydro exists in India. However, nearly 5,000 MW have been actually identified through more than 2,000 sites in 13 states of India and most of the sites are located in Himalayan region. Himalayan region is endowed with one of the world’s largest water resources mostly rain and snow fed rivers, river-lets and streams with perennial flows.
A turbine unit, used in small hydro plants, consists of a runner connected to a shaft that converts the potential energy in falling water into mechanical or shaft power. A turbine and generator unit is shown in Fig. 2.24.
3. Pumped Storage Power Plants:
This is a unique design of peak load plant in which the plant pumps back all or a portion of its water supply during low load period. The usual construction is a tail water pond and a head water pond connected through a penstock. The generating pumping plant is at the lower end. The plant utilises some of the surplus energy generated by the base load plant to pump the water from the tail water pond into the head water pond during off-peak hours.
During peak load period this water is used to generate power by allowing it to flow from the head water pond through the water turbine of this plant to the tail water pond. Thus the same water is used again and again and extra water is required only to take care of evaporation and seepage.
The capacity of the pond should be such that the plant can supply peak load for 4 to 10 hours. A general arrangement of a pumped storage power plant is shown in Fig. 2.25. The plants can be used in conjunction with hydro, steam and IC engine plants. This plant is also called a hydraulic accumulator system.
Such plants can have either vertical shaft arrangement or horizontal shaft arrangement. In the older plants, there used to be separate motor driven pumps and turbine driven generators. The improvement was the pump and turbine on the same shaft with electrical element acting as either generator or motor. The recent development in this field is to use a Francis turbine which is just the reverse of centrifugal pump.
During peak hours the turbine drives the generator and the plant generates electrical energy while during off-peak hours the generator operates as a motor and drives the turbine which now works as a pump raising the water from the tail water pond to the head water pond. The power for driving the generator as a motor is taken from the system. This arrangement reduces the capital cost of the plant and improves the operating efficiency and thus results in economical operation.
The efficiency of such plants is around 60-70 per cent. Some water may evaporate from the head water pond resulting in the reduction in the stored energy or there might be run off through the soil. Also there will be some energy loss in the generating and pumping equipment and in power transmission.
However, the energy loss to the system (the difference of energy input to the pumped storage plants and their output) is quite negligible in comparison to the substantial savings in fuel when these plants are operated in a mixed system for supplying peak loads.
The most obvious economies are in the replacement of standby thermal plants. When an old steam power plant is employed for peak load supply as an alternative to a pumped storage plant, it is a costly affair and, moreover, it needs large starting time and shutdown time. Hence these are not at all convenient for supplying peak loads. These steam power plants cannot be operated in the reversible mode as is possible with pumped storage plants.
Such plants can be operated only in interconnected systems where other generating plants (such as steam, nuclear, hydro power plants) are available. The steam and nuclear plants operate most economically when operated continuously at or near full load. The major problem with these plants is the peak load.
Covering of short peak load demands by steam power plant is economically undesirable and is not adoptable due to slow pick-up. Pumped storage plants for peak load supply in interconnected systems are most suitable where the quantity of water available for power generation is insufficient but natural site for construction of high dam exists.
Pumped storage power plants have some very important advantages. Some of these are given below:
1. Peak loads can be supplied at a lower cost than that when supplied by steam and nuclear power plants.
2. The steam and nuclear power plants can be operated at almost unity load factor which ensures their most efficient and economic operation.
3. Because of their ability to take up loads in a very short time (pumped storage plants need a starting time of only 2-3 seconds and can be loaded fully in about 15 seconds), the spinning reserve requirement of the system is reduced.
4. In the event of an extra demand coming up suddenly on the system, such plants can be immediately switched on to meet this extra demand.
5. They can be employed for load frequency control.
It has been estimated that a pumped storage plant can be set up at half the cost of a nuclear power plant of the same size. The pumped storage schemes, because of their inherent benefits, are likely to play an important role.
The pumped storage plant essentially consists of a head water pond and a tail water pond. The site selection for such plant should, therefore, take into consideration of natural pond facilities in hilly area.
56 number of pumped storage projects have been identified with probable installed capacity of 94,000 MW. The first scheme was commissioned about 30 years back. At present five schemes (Paithon on Godavari river with installed capacity of 12 MW in Maharashtra, Kadamparai on Kadamparai river with installed capacity of 400 MW in Tamilnadu, Nagarjunsagar on Krishna river with installed capacity of 700 MW in AP, Panchet hill unit on Damodar river with installed capacity of 40 MW and Kadana stage I on Mahi river with installed capacity of 120 MW in Gujarat) are in operation. Out of these only first two schemes (Paithon and Kadamparai) are being operated as pumped storage plants and the rest three are being operated only in generating mode due to non-construction of tail-water dams.
A number of other pumped schemes (such as 900 MW Srisailam in AP, 120 MW Kadana stage II, 1200 MW Sardar Sarover, both in Gujarat, 1000 MW Tehri Stage I in UP, 90 MW Bhivpuri, 150 MW Bhira, 250 MW Ghatgash, and 12 MW Ujjaini, all in Maharashtra) are under planning.
Hydro Potential in India:
India is blessed with immense amount of hydroelectric potential and ranks 5th in terms of exploitable hydro potential on global scenario. According to the assessment made by CEA, India is endowed with economically exploitable hydro power potential to the tune of 148,700 MW of installed capacity.
In addition, 56 number of pumped storage projects have also been identified with probable installed capacity of 94,000 MW. In addition to this, hydro potential from small, mini and micro schemes has been estimated as 6,782 MW from 1512 sites. Thus, in totality India is endowed with hydro potential of about 250,000 MW. However, exploitation of hydro potential has not been up to the desired level due to various constraints confronting the sector.
In 1998, Government of India announced “Policy on Hydro Power Development” under which impetus is given to development of hydropower in the country. This was a welcome step towards effective utilization of our water resources in the direction of hydropower development.
During October 2001, Central Electricity Authority (CEA) came out with a ranking study which prioritized and ranked the future executable projects. As per the study, 399 hydro schemes with an aggregate installed capacity of 106,910 MW were ranked in A, B and C categories depending upon their inter-se attractiveness.
During May 2003, Govt., of India launched 50,000 MW hydro initiatives in which preparation of Pre-Feasibility Reports of 162 projects totalling to 50,000 MW was taken up by CEA through various agencies. The PFRs for all these projects have already been prepared and projects with low tariff (first year tariff less than Rs 2.50/kWh) have been identified for preparation of DPR.
Automatic and Remote Control of Hydroelectric Power Plants:
Control, automation and monitoring system in a hydro power plant is associated with start and stop sequence for units and optimum running control of power (true and reactive), voltage and frequency. In addition, many protective devices (such as overload relays, reverse power relays, negative phase sequence relays, generator short-circuit protection relays, generator winding temperature relays, bearing temperature relays etc.), are also required. For efficient and fast operation of protective devices, their automatic operation is essential so as to detect the abnormal conditions which call for their operation and shut down the unit as and when required.
Control functions of control in a hydro power plant may be categorized into the following:
1. Turbine Control:
This is the speed/load control of turbine in which the governor adjusts the flow of water through the turbine to balance the input power with the load (output power). With an isolated system, the governor controls the frequency. In interconnected system, the governor may be used to regulate the unit load and may contribute to the system frequency control. In case of micro hydro plants in the range of micro hydel (100 kW unit size), load control is also used, where excess load is diverted to the dummy load to maintain constant speed.
2. Generator Control:
This is the excitation control of synchronous generator. The excitation is an integral part of a synchronous generator which is used to regulate the operation of the generator.
3. Plant Control:
Plant control deals with the operation of the plant. It includes sequential operations like start-up of the machine, excitation control, synchronisation, loading of unit under specified operating conditions, normal and emergency shutdown, etc. The mode of control may be manual, semi-automatic or automatic and may be controlled locally or from remote location. Plant control usually includes monitoring and display of the plant conditions.
Control of different units in a power station is done from a centrally situated control room, from where all the operations including starting, synchronizing, loading and shutdown of units are carried out.
Modern hydro plants are always operated as a part of an integrated power system consisting of many power plants. The whole system is remote controlled from an area control centre. The control engineer, also called the power controller, (in the case of manual load dispatching) or an on line computer (in the case of automatic load dispatching) determines the generation needs of various plants and transmits the information to the engineers in-charge of the plants for taking necessary steps. The communication between the control centre and power plants is through carrier current communication.
The control of the hydro plants is very much less complicated.
The various methods of starting and stopping of machines can be grouped under three headings:
1. Fully manual control,
2. Semi-automatic, and
3. Fully automatic.
The fully manual control is still common. In this system, every operation from starting to stopping is performed by hand. This may be achieved either mechanically or by push buttons. In semi-automatic control system, only a single impulse is required for starting the machine and then the machine is brought to the condition where it is ready to synchronize on the system.
Similarly, the machine can be stopped by single impulse. In fully automatic control, sequence of starting the machine, synchronizing and loading to some predetermined extent are all done automatically. Periodic cleaning, inspection and usual routine maintenance are the only needs to be met by the plant staff. The machines can be started in least possible time. There will also be savings in the salaries and wages of the staff. The extra cost of equipment is fully met by other savings.
A fully automatic plant may be controlled by means of either of three method- time switch, float switch and a load sensitive device. In time switch technique, the clock timings preset and the plant is started and stopped accordingly. In the float switch technique, the output is dependent upon the water level in the reservoir. In load sensitive device, the device is operated by the load or power demand of the area and the output of the generators, varies automatically.
If a hydro plant is controlled from a distance either from a control centre or a large hydro plant it is said to be remote controlled. The plant is controlled with the help of signals sent from the remote control centre through the automatic telephone equipment. Small hydro plants which are not justified to be developed for manual control can usually be justified by the application of remote automatic control.
Advantages and Disadvantages of Hydroelectric Power Plants:
Hydroelectric power plants offer many distinct advantages over other power plants.
These advantages can be summarised as under:
(i) No fuel is required by such plants as water is the source of energy. Hence operating costs are low and there are no problems of handling and storage of fuel and disposal of ash.
(ii) The plant is highly reliable and it is cheapest in operation and maintenance.
(iii) The plant can be run up and synchronized in a few minutes.
(iv) The load can be varied quickly and the rapidly changing load demands can be met without any difficulty.
(v) Very accurate governing is possible with water turbines so such power plants have constant speed and hence constant frequency.
(vi) There are no standby losses in such plants.
(vi) Such plants are robust and have got longer life (around 50 years).
(viii) The efficiency of such plants does not fall with the age.
(ix) It is very neat and clean plant because no smoke or ash is produced.
(x) Highly skilled engineers are required only at the time of construction but later on only a few experienced persons will be required.
(xi) Such plants, in addition to generation of electric power, also serve other purposes such as irrigation, flood control and navigation.
(xii) Hydroelectric plants are usually located in remote areas where land is available at cheaper rates.
However, the hydroelectric power plants have the following disadvantages also:
(i) It requires large area.
(ii) Its construction cost is enormously high and takes a long time for erection (owing to involvement of huge civil engineering works).
(iii) Long transmission lines are required as the plants are located in hilly areas which are quite away from the load centre.
(iv) The output of such plants is never constant owing to vagaries of monsoons and their dependence on the rate of water flow in a river. Long dry season may affect the power supply.
(v) The firm capacity of hydroelectric plants is low and so backup by steam plants is essential.
(vi) Hydroelectric power plant reservoir submerges huge areas, uproots large population and creates social and other problems.
Generators Employed in Hydroelectric Power Plants:
The generators employed in hydroelectric power plants are three phase alternating current synchronous generators, called the alternators. Alternator consists essentially of two parts namely armature and field magnet system.
The armature of an alternator is an iron ring, formed of laminations of special magnetic iron or steel alloy (silicon steel) having slots on its inner periphery to accommodate armature conductors and is known as stator. The whole structure is held in a frame which may be of cast iron or welded steel plates. For minimising eddy current losses due to rotation of field structure in between the stator, the stator core is laminated.
The laminations (usually of thickness 0.5 mm or less) are stamped out in complete rings (for small machines or in segments (for larger machines) and insulated from each other with paper or varnish. The stampings also have openings which make axial and radial ventilating ducts to provide efficient cooling. The open slots are more commonly used because the coils can be form wound and insulated prior to being placed in the slots giving least expenditure and more satisfactory winding method. Such slots also facilitates in removal and replacement of defective coils.
The field structure is the largest and heaviest component of alternator (in large machines, it may be 15 m in diameter and 1,000 ton in weight), and is called the rotor. The rotor houses the dc excitation winding and the exciting current is supplied to the rotor through two slip rings and brushes.
The field windings are connected in series to form the excitation winding which is supplied with dc at 110/220/300 V. Earlier, the excitation was usually supplied by small dc generators (shunt or compound wound type) driven through pilot exciters from the turbo-generator shaft. The recent generators make use of static excitation systems.
The alternators employed in hydroelectric power plants have the following features:
The machines may have horizontal or vertical configuration. The alternators employed in conjunction with impulse turbines are usually of horizontal configuration while those employed with Francis and Kaplan turbines are of vertical configuration. Low speeds (about 50—500 rpm in case of vertical machines and 100—1,000 rpm for the horizontal machines).
Protections against runaway speeds are to be provided. The machines are of large diameter and small lengths. The machines are usually of salient pole type and the number of poles, they consist of, varies from 6— 120. These machines should be capable of supplying heavy line charging currents since hydroelectric power plants are usually located at considerable distance from load end. A higher value of short-circuit ratio (around unity) is therefore required.
The specifications of the generator for a hydro power plant includes MVA rating, number of phases (always 3), frequency (50 Hz in India), connections of stator windings (nearly always star), voltage rating (3.3/6.6/11 kV), power factor, current rating, speed, method of cooling, temperature rise, type of excitation, excitation voltage, different reactance’s, short-circuit ratio, position of shaft, type of voltage regulator and its response time, efficiency, field current at full load and rated power factor, regulation at full load, mechanical details like runaway speed, direction of rotation, overall size, flywheel effect, number and location of bearings etc. and details of accessories.
The machines are usually air cooled. The air is diverted through the machine by different methods (radial, axial and circumferential). Air is taken in at one or both ends, forced through ducts between the windings and sections of core and discharged again at the periphery. An intake may be taken from points over the tailrace at a safe distance above the highest water level in it.
The amount of air requirement is about 3 m3/minute/kW of loss in the generator, and air velocity should be restricted to 475 m/ minute. The larger machines are completely enclosed and the air is recirculated in the housing after being cooled by means of water-cooled heat exchangers.
Closed-circuit cooling has the advantage of contamination free air permitting the machine to run for much longer periods without cleaning. The moisture exclusion is also advantageous. The restricted amount of air available gives a natural protection against extensive damage by fire, in the event of electrical breakdown.
Choice of Size and Number of Generating Units:
The load on the power station is never constant but varies at different timings of the day owing to varying demands and the generating plant should have the capacity to meet the maximum demand. In case one unit is taken of such a size as to meet the maximum demand of the power station, the plant will be operating on full load only for a short duration and will be running light or even practically on no load for rest of the day.
The generating unit would, therefore, not be running at all times, under conditions best suited for its operation giving maximum efficiency. In case of an isolated station, in order to maintain reliability and continuity of power supply at all times, another one unit of equal capacity will be required.
Thus the capital cost would be for two units, each of capacity corresponding to the maximum load on the power station. Alternatively number of small sized generating units can be chosen so as to fit the load curve as closely as possible i.e., the generating units be taken of such sizes and in such a number that they work on suitable portions of the load curve in such a way that each unit will operate on full load or at the load giving maximum efficiency.
In this case the reserve required would only be one unit of the largest size chosen and this unit would be much smaller than the maximum load capacity required in the former case. This alternative will require a large number of generating units and, therefore, the area required for the building will also increase and so the cost of the building. It will also require more frequent starting, stopping and parallel operation of the equipment. The maintenance cost will also increase.
Hence a compromise is to be made in the selection of the size and the number of generating units in the generating station in order to avoid both the extremes mentioned above. The aim should be to have a small number of units and to fit them as well as possible on the load curve. Neither should we go for a single generating unit of larger capacity nor for a large number of generating units of smaller sizes.
Plan Layout of Hydroelectric Power Plant:
The general layout of the hydro power plant is determined by its type. For plants employing vertical turbines, the most convenient and economical layout will be with turbines installed in a line parallel to the length of the turbine house, as illustrated in Fig. 2.26. The spacing between the machines will depend upon the size of scroll case, width of flume, or by the overall diameters of the alternators.
In case of turbines with horizontal shaft arrangement, the most suitable layout will be placement of turbines at right angles to the length of turbine house. The horizontal machines can also be placed parallel to the longitudinal axis of the turbine house. A repair bay should be provided at one end of the turbine house near the workshop with ample space for dismantling and re- erection of machines.
Hydro Plant Auxiliaries:
The auxiliaries essentially required for hydroelectric plant are governor, cranes, lubricating oil pumps, air compressors, high pressure oil pumps for generator rotor jacking system, fans, cooling water pumps, drainage and dewatering pumps, gate hoists, valves, battery charging units, CO2 cylinders etc.
These auxiliaries are generally electrically driven. Water may be used to cool the bearings of the turbines and generators and the transformers and is circulated through water pumps. Air compressors maintain a supply of air under pressure for operation of generator brakes and other uses in the power station.
Fans are required for ventilation of the turbine and switchgear room or for cooling of transformers. Oil pumps handle transformer oil through the cleaning and cooling system. Cranes are required to lift heavy parts or place them in position during repairs. Water pumps are required for unwatering of turbine pits during repairs or inspection. Storage batteries are required to supply low voltage dc power for switchgear control.
These batteries are constantly charged through a battery charging equipment using a rectifier or motor-generator set. Carbon dioxide cylinders and other fire extinguishing equipment are required in case the fire breaks out. The supply for the above auxiliaries is usually obtained from the station transformer which is installed solely for this purpose.
Environmental Impacts of Hydroelectric Power Plants:
Though the hydro power plants are considered clean and harmless but they have the following environmental impacts.
Hydro power plants need huge amount of water storage and therefore, large dams are to be constructed. This leads to displacement of the inhabitants of the area. The displacement of a large number of inhabitants may create a great variety of social and economic problems. The dam’s lakes change markedly the local ecological conditions; vary the pressure applied to the land and the groundwater level, which adversely affects plant and animal life in the nearby region.
The construction of dams for hydroelectric power plants slows down the flow of rivers and thus causes the pollution of water, growth of deleterious blue- green algae, encourages the reproduction of epidemic-carrying bacteria, checks the flood water with the result that water meadows cease to exist and sometimes salinization of soil occurs.
Large area acquisition means destruction of forest cover which is harmful for environments. In addition to environmental degradation and disturbing ecological balance, the hydro power plants disturb the demographic balance also. Large numbers of workers required for construction are brought into the area and they disturb the very nature of local population.