Unit II Hydro Electric Power Plants

7/28/2019 Unit II Hydro Electric Power Plants

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UNIT II HYDRO ELECTRIC POWER PLANTS

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Unit II Course material

N.KARTHIKEYAN

UNIT II

EE1252-POWER PLANT ENGINEERING


Layout

Dams

Selection of water turbines

Types

Pumped storage hydel plants.

Hydraulic Power Plants

Rainwater which falls over the earths surface possesses potential energy in relation to

mean sea level. This energy can be converted into mechanical energy and finally to electric

energy. The development of power from flowing water depends on the volume of flow and

the differential pressure. The branch of science which discusses these aspects is known as

hydrology. The rainfall, flow of water and evaporation forms a cycle known as hydrological

cycle.

Depending on the capacity, hydel power plants are divided into the following categories:

Hydrological Cycle

Hydel power is a renewable source of energy which is the cheapest form of energy

available in the world. Hydel power is superior to any other power source, because of their

long plant life, low operating cost and pollution free nature. Other factors like renewability

and utilization of discharge for irrigation purpose made it a superior energy source where it isenvironmentally feasible.

Power = W.Q.H. watts.

Where W = Specific weight of water, N/m3Q = rate of water flow, m3/sec.H = Height of fall or head, m= efficiency of conversion of potential energy intomechanical energy.


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Hydro projects are developed for the following purposes:

1. To control the floods in the rivers.2. Generation of power.3. Storage of irrigation water.4.

Storage of the drinking water supply.

Advantages of hydraulic power plants1.Operating cost is very low,

2.Less Maintenance cost and less manpower required,

3.Pollution free

4.Quick to start and easy to synchronize,

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Part load efficiency is very high,6.Can be used for irrigation and flood control,

7.Long plant life.

Disadvantages of Hydraulic Power Plants1. Initial cost of total plant is comparatively high

2. Power generation depends on availability of water

3. Cost of transmission is high since most of the plants are in remote areas

4. Project duration is long.

General layout of Hydro-Power Plant

A hydro power plant the energy of water is utilized for power generation may Kinetic or

potential. The general layout of a hydro electric power plant is given in figure. The main


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components of a hydro plant include the storage reservoir, dam, penstock, surge tank,

turbines, generator and transformers.

a) Reservoir

Reservoirs ensure supply of water throughout the year, by storing water during rainy season

and supplying the same during dry season.

b) Dam

The function of the dam is to increase the reservoir capacity and to increase the working head

of the turbine.

c) Penstock

A pipe between dam and turbine is known as penstock. It will carry the water from dam to

turbine. Penstock is commonly made of steel pipes covered with RCC.

d) Surge tank

When the rate of water flow through the penstock is suddenly decreased, the pressure inside

the penstock will increase suddenly due to water hammer and thereby damage the penstock.

Surge tank is constructed between the dam and turbine. It will act as a pressure regulator

during variable loads.

e) Turbine/Prime Mover

Turbines convert the kinetic and potential energy of water into mechanical energy to produce

electric power. Major types of turbines used in hydro power stations are

a. Pelton wheel - Low discharge, high head

b. Francis turbine - Medium discharge, Medium head

c. Kaplan turbine - High discharge, Low head.

(Head is the elevation difference of reservoir water level and downstream water level, and

discharge is the volume flow of water per unit time)

f) Generator and Transformer

Electric generator converts mechanical energy into electrical energy. A step up transformer

will increase the voltage for loss free transmission.


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Turbine

The turbine is a device which converts the hydraulic power from the water to rotating mechanicalenergy. There are several turbines that have been developed for specific purposes. Turbines areclassified into

1. Impulse turbines for high heads.

2. Reaction turbines for low heads.

3. Submersible propeller turbines.

Some of the turbines are Elton, cross flow, turbo, Francis, Harris etc.

Generator

Generally, induction generators and synchronous generators are used to produce the power.Induction generators are especially suited for providing electricity in remote areas becausethey are rugged and reliable. However, synchronous generators and dc generators have also

been used for some applications.

The three phase induction motors can be used as generators. These are easily available and

quite inexpensive. For most home applications, a single phase supply is required. It isrequired to produce a single phase supply from a three phase motor. This is done byconnecting unequal amounts of excitation capacitance across the windings of the machine.

Load Control

The speed of the turbine changes when the load connected to the generator changes.Since this change of speed affects the voltage and frequency, the load on the generator must

be kept constant or the flow of water through the nozzle must be adjusted. The most reliablemethod of controlling the load and keeping the voltage and frequency constant is by using an

electronic load controller. The unused power of the induction generator is sent to a ballast or


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dummy load so that the total load on the generator remains constant. For example if thegenerator produces 1000W and the total load connected is 700W, then the remaining 300Wwill be dissipated in the ballast load.

RUN OFF

Rain fall (used in a general sense) or precipitation may be defined as the totalcondensation of moisture that reaches the earth in any form. It includes all forms of rains, ice,snow, hail or sleet etc. Evaporation represents practically that entire portion of the rainfallthat does not reach the point of ultimate use as stream flow. So, evaporation includes all therainfall that is returned to the atmosphere from land and water surfaces.

Thus total evaporation is:1. Evaporation from land and water surfaces.2. Evaporation by transpiration which is the vaporization of water from the breathing pores

of vegetable matter.3.

Atmospheric evaporation (evaporation while precipitation is falling).

Rain-fall is measured in terms of centimetres of water over a given area and over a givenperiod (usually one year). The portion of the total precipitation that flows through thecatchment area is known as Run-off. The catchment area of a hydro site is the total area

behind the dam, draining water into the reservoir.

Thus,

Run-off = Total precipitation Total evaporation

Part of the precipitation is absorbed by the soil and seeps or percolates into ground and willultimately reach the catchment area through the underground channels.Thus.

Total run-off = Direct run off over the land surface T Run-off through seepage.

The unit of run-off are m3/s or day-second meter.

Day-second meter = Discharge collected in the catchment area at the rate of 1 in 3/S for oneday = 1 24 3600 = 86400 m3/day.

The flow of run-off can also be expressed in cms. of water on the drainage area feeding theriver site for a stated period, or km, cm of water per unit of time.

Factors Affecting Runoff

1. Nature of Precipitation. Short, hard showers may produce relatively little run-off. Rainslasting a longer time results in larger run-off. The soil tends to become saturated and the rateof seepage decreases. Also, the humid atmosphere lowers evaporation, resulting in increasedrun-off.2. Topography of Catchments Area. Steep, impervious areas will produce large percentageof total run-off. The water will flow quickly and absorption and evaporation losses will be

small.


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3. Geology of Area. The run-off is very much affected by the types of surface soil and sub-soil, type of rocks etc. Rocky areas will give more run-off while pervious soil and sub-soiland soft and sandy area will give lesser run-off.4. Meteorology. Evaporation varies with temperature, wind velocity and relative humidity.Runoff increases with low temperature, low wind velocity and high relative humidity andvice versa.

5. Vegetation. Evaporation and seepage are increased by cultivation. Cultivation opens androughens the hard, smooth surface and promotes seepage. Thick vegetation like forestsconsumes a portion of the rain fall and also acts as obstruction for run-off.6. Size and Shape of Area. Large areas will give more run-off. A wide area like a fan willgive greater run-off; whereas, a narrow area like a leaf will give lesser run-off. In an areawhose length is more than its width, the flow along its width will give more run-off than ifthe flow is along its length, since in the former case, seepage and evaporation will be less.

SELECTION OF SITE FOR A HYDRO-ELECTRIC POWER PLANT

While selecting a suitable site, if a good system of natural storage lakes at high

altitudes and with large catchment areas can be located, the plant will be comparativelyeconomical. Anyhow the essential characteristics of a good site are: large catchment areas,high average rainfall and a favourable place for constructing the storage or reservoir. Thefollowing factors should be given careful consideration while selecting a site for a hydro-electric power plant:

1. Water Available. To know the available energy from a given stream or river, thedischarge flowing and its variation with time over a number of years must be known.Preferably, the estimates of the average quantity of water available should be prepared on the

basis of actual measurements of stream or river flow. The recorded observation should betaken over a number of years to know within reasonable, limits the maximum and minimum

variations from the average discharge. the river flow data should be based on daily, weekly,monthly and yearly flow ever a number of years. Then the curves or graphs can be plotted

between tile river flow and time. These are known as hygrographs and flow duration curves.The plant capacity and the estimated output as well as the need for storage will be governed

by the average flow. The primary or dependable power which is available at all times whenenergy is needed will depend upon the minimum flow. Such conditions may also fix thecapacity of the standby plant. The, maximum of flood flow governs the size of the headwordsand dam to be built with adequate spillway.

2. Water-Storage. As already discussed, the output of a hydropower plant is not uniform due

to wide variations of rain fall. To have a uniform power output, a water storage is needed sothat excess flow at certain times may be stored to make it available at the times of low flow.To select the site of the dam ; careful study should be made of the geology and topography ofthe catchment area to see if the natural foundations could be found and put to the best use.3. Head of Water. The level of water in the reservoir for a proposed plant should always bewithin limits throughout the year.4. Distance from Load Centre. Most of the time the electric power generated in a hydro-electric power plant has to be used some considerable distance from the site of plant. For thisreason, to be economical on transmission of electric power, the routes and the distancesshould be carefully considered since the cost of erection of transmission lines and theirmaintenance will depend upon the route selected.


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5. Access to Site. It is always a desirable factor to have a good access to the site of the plant.This factor is very important if the electric power generated is to be utilized at or near the

plant site. The transport facilities must also be given due consideration.

RUN-OF-RIVER POWER PLANTS

These plants can be classified as either without pondage or with pondage. A run-of-river plant without pondage has no control over river flow and uses the water as it comes.These plants usually supply peak load. During floods, the tail water level may becomeexcessive rendering the plant inoperative. A run-of-river plant with pondage may supply baseload or peak load power. At times of high water flow it may be base loaded and during dryseasons it may be peak loaded.

PUMPED STORAGE POWER PLANTS

1. These plants supply the peak load for the base load power plants and pump all or aportion of their own water supply.

2. The usual construction would be a tail water pond and a head water pond connectedthrough a penstock. The generating pumping plant is at the lower end.

PUMPED STORAGE POWER PLANTS

3. During off peak hours, some of the surplus electric energy being generated by the baseload plant is utilized to pump the water from tail water pond into the head water pond andthis energy will be stored there.

4. During times of peak load, this energy will be released by allowing the water to flowfrom the head water pond through the water turbine of the pumped storage plant.


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5. These plants can be used with hydro, steam and i.e. engine plants. This plant is nothingbut a hydraulic accumulator system and is shown in Fig.

6. These plants can have either vertical shaft arrangement or horizontal shaft arrangement.In the older plants, there were separate motor driven pumps and turbine driven generators.

7. The improvement was the pump and turbine on the same shaft with the electrical elementacting as either generator or motor. The latest design is to use a Francis turbine which is

just the reverse of centrifugal pump.8. When the water flows through it from the head water pond it will act as a turbine and

rotate the generator.9. When rotated in the reverse direction by means of an electric motor, it will act as a pump

to shunt the water from the tail water pond to the head water pond.10.The efficiency of such a plant is never 100 per cent. Some water may evaporate from the

head water pond resulting in the reduction in the stored energy or there might be run offthrough the soil.

PRIME-MOVERS OR TURBINES, HYDRAULIC TURBINES

The prime-mover in the hydraulic power plant converts the energy of water into mechanicalenergy and further into electrical energy. These machines are classified on the basis of the action of waterand moving blades.As per the action of water on the prime-mover, they are classified as impulse turbine and reaction turbine.

IMPULSE TURBINE

1. In impulse type turbine, the pressure energy of the water is converted into kinetic energy when passedthrough the nozzle and forms the high velocity jet of water.

2. The formed water jet is used for driving the wheel.3. The casing of the impulse turbine operates at atmospheric pressure.

REACTION TURBINE

1. In case of reaction turbine, the water pressure combined with the velocity works on the runner.2. The power in this turbine is developed from the combined action of pressure and velocity of water that

completely fills the runner and water passage.3. The casing of the reaction turbine operates under high pressure.

Types of turbines

1. Pelton Turbine.2. Francis Turbine.3. Kaplan Turbine.

1. PELTON TURBINE.

1. Figure shows the layout of the Pelton turbine. This was discovered by Pelton in 1880.2. This is a special type of axial flow impulse turbine generally mounted on horizontal shaft. A number

of buckets are mounted round the periphery of the wheel as shown in Fig.3. The water is directed towards the wheel through a nozzle or nozzles. The flow of water through the

nozzle is generally controlled by special regulating system.4. The water jet after impinging on the buckets is deflected through an angle of 160 and flows axially in

both directions thus avoiding the axial thrust on the wheel.5. The hydraulic efficiency of Peltan wheel lies between 85 to 95%. Now-a-days, Pelton wheels are used

for very high heads upto 2000 meters.


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Arrangement of jets.

1. In most of the Pelton wheel plants, single jet with horizontal shaft is used. The number of the jetsadopted depends upon the specific speed required.

2.

Any impulse turbine achieves its maximum efficiency when the velocity of the bucket at the centreline of the jet is slightly under half the jet velocity.

3. Hence, for maximum speed of rotation, the mean diameter of the runner should be as small as possible.There is a limit to the size of the jet which can be applied to any impulse turbine runner withoutseriously reducing the efficiency.

4. In early twenties, a normal ratio of D/dwas about 10: I. In a modern Turbo impulse turbine, it isreduced up to 4.5: I.

5. The basic advantage of Turbo impulse turbine is that a much larger jet could be applied to a runner ofa given mean diameter.

6. The jet of pelton turbine strikes the splitter edge of the bucket, bifurcates and is discharged at eitherside.

7. With the turbo impulse turbine, the jet is set at an angle to face the runner, strikes the buckets at the

front arid discharges at opposite side.8. The basic difference between the two is shown in Fig. The Turbo impulse turbine bridges the gap ofspecific speed between the Pelton wheel and Francis turbine.

9. Two turbo impulse turbines are used in a power house at Poonch which is 320 km from Jalnmu. Thereaction turbines are further divided into two general types as Francis and Propeller Type.

10.The propeller turbines are further subdivided into fixed blade propeller type and the adjustable bladetype as Kaplan Turbine.

2. FRANCIS TURBINE

In Francis turbine, the water enters into a casing with a relatively low velocity, passesthrough guide vanes located around the circumference and flows through the runner andfinally discharges into a draft tube sealed below the tail water level. The water passage fromthe headrace to tail race is completely filled with water which acts upon the wholecircumference of the runner.

Two types of Francis turbines

i.

open flume type


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ii. closed type

Open flume type

In open flume type, the turbine is immersed under water of the headrace in a concretechamber and discharges into the tailrace through the draft tube. The main disadvantage of this

type is that runner and guide-vane mechanism is under the water and they are not open eitherfor inspection or repair without draining the chamber.

Closed type

In the closed type, the water is led to the turbine through the penstock whose end is

connected to the spiral casing of the turbine. The open flume type is used for the plants of 10meters head whereas; closed type is preferred above 30 meters head. The guide vanes areprovided around the runner to regulate the water flowing through the turbine The guide vanesprovide gradually decreasing area of flow for all gate openings, so that no eddies are formed,and efficiency does not suffer much even at part load conditions.

The majority of the Francis turbines are inward radial flow type and most preferred formedium heads. The inward flow turbine has many advantages over outward flow turbine aslisted below:

1. The chances of eddy formation and pressure loss are reduced as the area of flow becomes

gradually convergent.


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2. The runaway speed of the turbine is automatically checked as the centrifugal force actsoutwards while the flow is inward.

3. The guide vanes can be located on the outer periphery of the runner; therefore, betterregulation is possible.

4. The frictional losses are less as the water velocity over the vanes is reduced.5. The inward flow turbine can be used for fairly high heads without increasing the speed of

the turbine as centrifugal head supports considerable part of supply head.

3. KAPLAN TURBINE.

Great strides are made in last few decades to improve the performance of propellerturbine at part load conditions. The Kaplan turbine is a propeller type having a movable bladeinstead of fixed one. This turbine was introduced by Dr. Vitkor Kaplan. This turbine hasattained popularity and rapid progress has been made in recent years in the design andconstruction of this turbine:

1. The rotor of the Kaplan turbine is shown in Fig. The blades are rotated to the mostefficient angle by a hydraulic servo-motor.

2. A cam on the governor is used to change the blade angle with the gate position so thathigh efficiency is always obtained at almost any percentage of full loads.

3. These turbines are constructed to run at speeds varying from 60 to 220 r.p.m. and to workunder varying head from 2 to 60 meters.

4. These are particularly suitable for variable heads and for variable flows and where the

ample quantity of water is available.


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5. The specific speed of Kaplan lies in the range of 400 to 1500 so that the speed of the rotoris

6. much higher than that of Francis Turbine for the same output and head or Kaplan turbinehaving the same size as Francis develops more power under the same head and flowquantity.

7. The velocity of water flowing through Kaplan turbine is high as the flow is large and,

therefore, the cavitation is more serious problem in Kaplan than Francis Turbine.8. The propeller type turbines have an outstanding advantage of higher speed which results

in lower cost of runner, generator and smaller power house substructure andsuperstructure.

9. The capital and maintenance cost of Kaplan turbine is much higher than fixed bladepropeller type units operated at a point of maximum efficiency.

10.For a low head development with fairly constant head and requiring a number of units, itis always advisable to install fixed blade propeller type runners for most of them andKaplan type for only one or two units.

11.With this combination, the fixed blade units could be operated at point of maximumefficiency and Kaplan units could take the required variations in load.

12.Such combination is particularly suitable to a large power system containing amultiplicity of the units.

Cavitation and Limitation of Turbine Height above Tailrace

Level.

1. The formation of water vapour and air bubbles on the water surface due to the reductionof pressure is known as "Capitation".

2. When the pressure on the water reduces below the saturation pressure corresponding tothe temperature of the water, the rapid formation of water vapour and air bubbles starts.

3.

The bubbles suddenly collapse with the violent action and collapsing pressure will bevery high.

4. The rapid formation and collapsing of the bubbles causes the pitting of the metallicsurface.

5. It also reduces the efficiency of the hydraulic prime mover causing honeycombing ofrunner and blade contours which reduces the power output.

METHODS TO AVOID CAVITATION

Installation of Turbine below Tailrace Level.

1.

The danger of cavitations increases in case of low head and high speed propeller runneras the value of (V02 Vd2)/g is considerably large as mentioned earlier.

2. In order to keep the value ofpc within the cavitation limit, the value of h is made negativekeeping the runner below tailrace level.

3. For such installations, the turbines remain always under water. It is riot advisable as theinspection and repair of the turbine is difficult.

4. The other method to avoid cavitation zone without keeping the runner under water is touse the runner of low specific speed as mentioned earlier.

Cavitation Free Runner.


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1. The cavitation free runner can be designed to fulfill the given conditions with extensiveresearch.

2. The shape of the blade, the angle of the blade, the thickness of the blade can be changedand experiments can be conducted to find out the best dimensions of the blade (shape,size, angle. etc.

Use of Material.1. The cavitation effect can be reduced by selecting materials which can resist better the

cavitation effect.2. The cast steel is better than cast iron and stainless steel or alloy steel is still3. better than cast steel.4. The pitting effect of cavitation on cast steel can be repaired more economically by

ordinary welding.5. It has been observed that the welded parts are more resistant to cavitation than ordinary

ones.

Polishing.The cavitation effect is less on polished surfaces than ordinary one. Mat, is why the cast steel

runners and blades are coated with stainless steel.

UNDERGROUND POWER HOUSE.

The conventional hydro-electric power stations are usually located over-ground at thefoot of a dam or a hill slope on the banks of a river. The first underground power stationNerayaz was built in 1897 in Switzerland. The high capacity underground power plants werebuilt only after Second World War. The idea of locating powerhouse underground wassuggested not only with the intention of protecting them against air attack but also technicaland economical considerations were mainly considered. After Second World War, theimmunity against air attacks was unquestionably regarded as an important. Advantage-of

underground power stations, a large number of underground power stations have beeninstalled in U.K., U.S.A., Russia, Canada, and Japan after Second World War and recently inIndia also.

The considerations supporting the construction of underground power stations are stated below :

1. Non-availability of a suitable site for a conventional surface station and good slope for penstock.2. Danger of falling rocks and snow avalanches particularly in narrow valleys.3.

Availability of underground sound rock and avoidance of a long pressure tunnel and facility


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for a convenient tail-race outlet.4. Possibility of elimination of surge tank required for surface station due to long pressure tunnel5. The rugged topographical features and the difficulties in finding a suitable short and steep slope

for pipe lines make it more economical to install the water conduit, the machine, and transformerhall and tailrace system underground.

6. Foundation costs for over ground power house become excessive in case of poor quality surfacelayers.

ADVANTAGES

1. The reduced length of the pressure conduit reduced the pressures developed due to water hammer.Therefore, smaller surge tank is also sufficient.

2. For the economical arrangement, the ratio of the pressure conduit to the tail-race tunnel is alsosignificant. The overall cost of the system is lower if the tail-race tunnel length is relatively large.

3. The construction work at underground power station can continue uninterrupted even underseverest winter conditions. The overall construction cost and period of construction is reduced due

to continuity of work.4. The construction of underground conduit instead of penstock results in considerable saving insteel, the internal pressure is carried partly by the rock if it is of good quality.

5. Much care is devoted today in many countries to preserve landscape features such as picturesquerock walls, canyons, valleys and river banks in their original beauty against spoiling by exposedpenstocks, canal basins and machine halls.

6. The regular maintenance and repair costs are lower for underground stations as the maintenancerequired for rock tunnels is less.

7. The power plant is free from landslides, avalanches, heavy snow and rainfall.8. The useful life of the structures excavated in rock is considerably longer than that of concrete and

reinforced concrete structures.

9. The construction period is reduced mainly due to the possibility of full-scale construction work in

winter.DISADVANTAGES1. The operational cost of the power plant increases due to following reasons :

a) The lighting cost.b) The running cost of air-conditioned plant.

c) The removal of water seeping may be more costly than for the surface arrangement.

2. The construction cost of the underground power house is more compared with the over ground powerhouse:a) The excavation of the caverns required for housing the turbine generator units and auxiliary

equipments (machine hall of Koyna project is $00 120 60in dimensions) is veryexpensive.

b)

The costs of access tunnels are considerable.c) The separate gallery excavated for the inlet valves adds the extra cost.d) The construction of air ducts and bus galleries also adds in total construction costs.

e) Special ventilation and air-conditioning equipment required for underground adds in theconstructional costs.

Documents

Unit II Hydro Electric Power Plants