principle of power system, Study notes of Power Electronics

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General

E

nergy is the basic necessity for the eco- nomic development of a country. Many functions necessary to present-day living grind to halt when the supply of energy stops. It is practically impossible to estimate the actual magnitude of the part that energy has played in the building up of present-day civilisation. The availability of huge amount of energy in the modern times has resulted in a shorter working day, higher agricultural and in- dustrial production, a healthier and more balanced diet and better transportation facilities. As a matter of fact, there is a close relationship be- tween the energy used per person and his stan- dard of living. The greater the per capita con- sumption of energy in a country, the higher is the standard of living of its people. Energy exists in different forms in nature but the most important form is the electrical energy. The modern society is so much dependent upon the use of electrical energy that it has become a part and parcel of our life. In this chapter, we shall focus our attention on the general aspects of elec- trical energy.

Introduction

C H A P T E R

1.1 Importance of Electrical Energy 1.2 Generation of Electrical Energy 1.3 Sources of Energy 1.4 Comparison of Energy Sources 1.5 Units of Energy 1.6 Relationship Among Energy Units 1.7 Efficiency 1.8 Calorific Value of Fuels 1.9 Advantages of Liquid Fuels Over Solid Fuels 1.10 Advantages of Solid Fuels Over Liquid Fuels

Introduction 33333

such as burning of fuel, pressure of water, force of wind etc. For example, chemical energy of a fuel ( e.g ., coal) can be used to produce steam at high temperature and pressure. The steam is fed to a prime mover which may be a steam engine or a steam turbine. The turbine converts heat energy of steam into mechanical energy which is further converted into electrical energy by the alternator. Similarly, other forms of energy can be converted into electrical energy by employing suitable machinery and equipment.

1.3. Sources of Energy

Since electrical energy is produced from energy available in various forms in nature, it is desirable to look into the various sources of energy. These sources of energy are :

( i ) The Sun ( ii ) The Wind ( iii ) Water ( iv ) Fuels ( v ) Nuclear energy. Out of these sources, the energy due to Sun and wind has not been utilised on large scale due to a number of limitations. At present, the other three sources viz. , water, fuels and nuclear energy are primarily used for the generation of electrical energy.

( i ) The Sun. The Sun is the primary source of energy. The heat energy radiated by the Sun can be focussed over a small area by means of reflectors. This heat can be used to raise steam and electrical energy can be produced with the help of turbine-alternator combination. However, this method has limited application because :

( a ) it requires a large area for the generation of even a small amount of electric power ( b ) it cannot be used in cloudy days or at night ( c ) it is an uneconomical method. Nevertheless, there are some locations in the world where strong solar radiation is received very regularly and the sources of mineral fuel are scanty or lacking. Such locations offer more interest to the solar plant builders.

( ii ) The Wind. This method can be used where wind flows for a considerable length of time. The wind energy is used to run the wind mill which drives a small generator. In order to obtain the electrical energy from a wind mill continuously, the generator is arranged to charge the batteries. These batteries supply the energy when the wind stops. This method has the advantages that maintenance and generation costs are negligible. However, the drawbacks of this method are ( a ) variable output, ( b ) unreliable because of uncertainty about wind pressure and ( c ) power generated is quite small.

( iii ) Water. When water is stored at a suitable place, it possesses potential energy because of the head created. This water energy can be converted into mechanical energy with the help of water turbines. The water turbine drives the alternator which converts mechanical energy into electrical energy. This method of generation of electrical energy has become very popular because it has low production and maintenance costs.

( iv ) Fuels. The main sources of energy are fuels viz ., solid fuel as coal, liquid fuel as oil and gas fuel as natural gas. The heat energy of these fuels is converted into mechanical energy by suitable prime movers such as steam engines, steam turbines, internal combustion engines etc. The prime mover drives the alternator which converts mechanical energy into electrical energy. Although fuels continue to enjoy the place of chief source for the generation of electrical energy, yet their reserves are diminishing day by day. Therefore, the present trend is to harness water power which is more or less a permanent source of power.

( v ) Nuclear energy. Towards the end of Second World War, it was discovered that large amount of heat energy is liberated by the fission of uranium and other fissionable materials. It is estimated that heat produced by 1 kg of nuclear fuel is equal to that produced by 4500 tonnes of coal. The heat produced due to nuclear fission can be utilised to raise steam with suitable arrangements. The steam

44444 Principles of Power System

can run the steam turbine which in turn can drive the alternator to produce electrical energy. However, there are some difficulties in the use of nuclear energy. The principal ones are ( a ) high cost of nuclear plant ( b ) problem of disposal of radioactive waste and dearth of trained personnel to handle the plant.

Energy Utilisation

1.4Comparison of Energy Sources

The chief sources of energy used for the generation of electrical energy are water, fuels and nuclear energy. Below is given their comparison in a tabular form :

S.No. Particular Water-power Fuels Nuclear energy

1. Initial cost High Low Highest
2. Running cost Less High Least
3. Reserves Permanent Exhaustable Inexhaustible
4. Cleanliness Cleanest Dirtiest Clean
5. Simplicity Simplest Complex Most complex
6. Reliability Most reliable Less reliable More reliable

1.5 Units of Energy

The capacity of an agent to do work is known as its energy. The most important forms of energy are mechanical energy, electrical energy and thermal energy. Different units have been assigned to various forms of energy. However, it must be realised that since mechanical, electrical and thermal energies are interchangeable, it is possible to assign the same unit to them. This point is clarified in Art 1.6. ( i ) Mechanical energy. The unit of mechanical energy is newton-metre or joule on the M.K.S. or SI system. The work done on a body is one newton-metre (or joule) if a force of one newton moves it through a distance of one metre i.e ., Mechanical energy in joules = Force in newton × distance in metres ( ii ) Electrical energy. The unit of electrical energy is watt - sec or joule and is defined as follows: One watt-second (or joule) energy is transferred between two points if a p.d. of 1 volt exists between them and 1 ampere current passes between them for 1 second i.e .,

Coal

Crude oil

Natural gas

Hydro-electric power

Nuclear power

Renewables

66666 Principles of Power System

( iii ) Electrical and Heat ( a ) 1 kWh = 1000 watts × 3600 seconds = 36 × 105 Joules

=

×^5

calories = 860 × 103 calories

∴ 1 kWh = 860 × 103 calories or 860 kcal ( b ) 1 kWh = 36 × 105 Joules = 36 × 105 /1896 C.H.U. = 1898 C.H.U. [Œ 1 C.H.U. = 1896 Joules] ∴ 1 kWh = 1898 C.H.U.

( c ) 1 kWh = 36 × 10 5 Joules = 36 10 1053

× 5

B.Th.U. = 3418 B.Th.U.

[Œ 1 B.Th.U. = 1053 Joules] ∴ 1 kWh = 3418 B.Th.U. The reader may note that units of electrical energy can be converted into heat and vice-versa. This is expected since electrical and thermal energies are interchangeable.

1.7 Efficiency

Energy is available in various forms from different natural sources such as pressure head of water, chemical energy of fuels, nuclear energy of radioactive substances etc. All these forms of energy can be converted into electrical energy by the use of suitable arrangement. In this process of conversion, some energy is lost in the sense that it is converted to a form different from electrical energy. Therefore, the output energy is less than the input energy. The output energy divided by the input energy is called energy efficiency or simply efficiency of the system.

Efficiency, η = Output energy Input energy As power is the rate of energy flow, therefore, efficiency may be expressed equally well as output power divided by input power i.e .,

Efficiency, η = Output power Input power Example 1.1. Mechanical energy is supplied to a d.c. generator at the rate of 4200 J/s. The generator delivers 32·2 A at 120 V. ( i ) What is the percentage efficiency of the generator? ( ii ) How much energy is lost per minute of operation?

Measuring efficiency of compressor.

Introduction 77777

Solution. ( i ) Input power, Pi = 4200 J/s = 4200 W Output power, Po = EI = 120 × 32·2 = 3864 W

∴ Efficiency, η =

P

P

o i

× 100 = 3864 ×

( ii ) Power lost, PL = Pi − Po = 4200 − 3864 = 336 W ∴ Energy lost per minute (= 60 s) of operation = PL × t = 336 × 60 = 20160 J Note that efficiency is always less than 1 ( or 100 % ). In other words, every system is less than 100 % efficient.

1.8 Calorific Value of Fuels

The amount of heat produced by the complete combustion of a unit weight of fuel is known as its calorific value. Calorific value indicates the amount of heat available from a fuel. The greater the calorific value of fuel, the larger is its ability to produce heat. In case of solid and liquid fuels, the calorific value is expressed in cal / gm or kcal / kg. However, in case of gaseous fuels, it is generally stated in cal / litre or kcal / litre. Below is given a table of various types of fuels and their calorific values along with composition.

S.No. Particular Calorific value Composition

1. Solid fuels ( i ) Lignite 5,000 kcal/kg C = 67%, H = 5%, O = 20%, ash = 8% ( ii ) Bituminous coal 7,600 kcal/kg C = 83%, H = 5·5%, O = 5%, ash = 6·5% ( iii ) Anthracite coal 8,500 kcal/kg C = 90%, H = 3%, O = 2%, ash = 5%
2. Liquid fuels ( i ) Heavy oil 11,000 kcal/kg C = 86%, H = 12%, S = 2% ( ii ) Diesel oil 11,000 kcal/kg C = 86·3%, H = 12·8%, S = 0·9% ( iii ) Petrol 11,110 kcal/kg C = 86%, H = 14%
3. Gaseous fuels ( i ) Natural gas 520 kcal/m^3 CH 4 = 84%, C 2 H 6 = 10% Other hydrocarbons = 5% ( ii ) Coal gas 7,600 kcal/m^3 CH 4 = 35%, H = 45%, CO= 8%, N = 6% CO 2 = 2%, Other hydrocarbons = 4%

1.9 Advantages of Liquid Fuels over Solid Fuels

The following are the advantages of liquid fuels over the solid fuels : ( i ) The handling of liquid fuels is easier and they require less storage space. ( ii ) The combustion of liquid fuels is uniform. ( iii ) The solid fuels have higher percentage of moisture and consequently they burn with great difficulty. However, liquid fuels can be burnt with a fair degree of ease and attain high temperature very quickly compared to solid fuels. ( iv ) The waste product of solid fuels is a large quantity of ash and its disposal becomes a problem. However, liquid fuels leave no or very little ash after burning. ( v ) The firing of liquid fuels can be easily controlled. This permits to meet the variation in load demand easily.

1.10 Advantages of Solid Fuels over Liquid Fuels

The following are the advantages of solid fuels over the liquid fuels :

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IntrIntrIntrIntrIntroductionoductionoductionoductionoduction

I

n this modern world, the dependence on electricity is so much that it has become a part and parcel of our life. The ever increas- ing use of electric power for domestic, commer- cial and industrial purposes necessitates to pro- vide bulk electric power economically. This is achieved with the help of suitable power produc- ing units, known as Power plants or Electric power generating stations. The design of a power plant should incorporate two important aspects. Firstly, the selection and placing of necessary power-generating equipment should be such so that a maximum of return will result from a mini- mum of expenditure over the working life of the plant. Secondly, the operation of the plant should be such so as to provide cheap, reliable and continuous service. In this chapter, we shall focus our attention on various types of generat- ing stations with special reference to their advan- tages and disadvantages.

2.12.12.12.12.1 Generating StationsGenerating StationsGenerating StationsGenerating StationsGenerating Stations

Bulk electric power is produced by special plants known as generating stations or power plants. A generating station essentially employs a

Generating Stations

C H A P T E RC H A P T E RC H A P T E R C H A P T E RC H A P T E R

2.1 Generating Stations 2.2 Steam Power Station (Thermal Station) 2.3 Schematic Arrangement of Steam Power Station 2.4 Choice of Site for Steam Power Stations 2.5 Efficiency of Steam Power Station 2.6 Equipment of Steam Power Station 2.7 Hydro-electric Power Station 2.8 Schematic Arrangement of Hydro- electric Power Station 2.9 Choice of Site for Hydro-electric Power Stations 2.10 Constituents of Hydro-electric Plant 2.11 Diesel Power Station 2.12 Schematic Arrangement of Diesel Power Station 2.13 Nuclear Power Station 2.14 Schematic Arrangement of Nuclear Power Station 2.15 Selection of Site for Nuclear Power Station 2.16 Gas Turbine Power Plant 2.17 Schematic Arrangement of Gas Turbine Power Plant 2.18 Comparison of the Various Power Plants

1010101010 Principles of Power System

prime mover coupled to an alternator for the production of electric power. The prime mover ( e.g ., steam turbine, water turbine etc.) converts energy from some other form into mechanical energy. The alternator converts mechanical energy of the prime mover into electrical energy. The electrical en- ergy produced by the generating station is transmitted and distributed with the help of conductors to various consumers. It may be emphasised here that apart from prime mover-alternator combination, a modern generating station employs several auxiliary equipment and instruments to ensure cheap, reliable and continuous service. Depending upon the form of energy converted into electrical energy, the generating stations are classified as under : ( i ) Steam power stations ( ii ) Hydroelectric power stations ( iii ) Diesel power stations ( iv ) Nuclear power stations

2.22.22.2 2.22.2 Steam Power Station (TherSteam Power Station (TherSteam Power Station (TherSteam Power Station (TherSteam Power Station (Thermal Station)mal Station)mal Station)mal Station)mal Station)

A generating station which converts heat energy of coal combustion into electrical energy is known as a steam power station. A steam power station basically works on the Rankine cycle. Steam is produced in the boiler by utilising the heat of coal combustion. The steam is then expanded in the prime mover ( i.e ., steam turbine) and is condensed in a condenser to be fed into the boiler again. The steam turbine drives the alternator which converts mechanical energy of the turbine into electrical energy. This type of power station is suitable where coal and water are available in abundance and a large amount of electric power is to be generated.

AdvantagesAdvantagesAdvantages AdvantagesAdvantages

( i ) The fuel ( i.e., coal) used is quite cheap. ( ii ) Less initial cost as compared to other generating stations. ( iii ) It can be installed at any place irrespective of the existence of coal. The coal can be trans- ported to the site of the plant by rail or road. ( iv ) It requires less space as compared to the hydroelectric power station. ( v ) The cost of generation is lesser than that of the diesel power station.

DisadvantagesDisadvantagesDisadvantages DisadvantagesDisadvantages

( i ) It pollutes the atmosphere due to the production of large amount of smoke and fumes. ( ii ) It is costlier in running cost as compared to hydroelectric plant.

2.32.32.3 2.32.3^ Schematic Arrangement of Steam Power StationSchematic Arrangement of Steam Power StationSchematic Arrangement of Steam Power StationSchematic Arrangement of Steam Power StationSchematic Arrangement of Steam Power Station

Although steam power station simply involves the conversion of heat of coal combustion into electri- cal energy, yet it embraces many arrangements for proper working and efficiency. The schematic arrangement of a modern steam power station is shown in Fig. 2.1. The whole arrangement can be divided into the following stages for the sake of simplicity :

1. Coal and ash handling arrangement 2. Steam generating plant 3. Steam turbine 4. Alternator 5. Feed water 6. Cooling arrangement 1. Coal and ash handling plant. The coal is transported to the power station by road or rail and is stored in the coal storage plant. Storage of coal is primarily a matter of protection against coal strikes, failure of transportation system and general coal shortages. From the coal storage plant, coal is delivered to the coal handling plant where it is pulverised ( i.e ., crushed into small pieces) in order to increase its surface exposure, thus promoting rapid combustion without using large quantity of

1212121212 Principles of Power System

2. Steam generating plant. The steam generating plant consists of a boiler for the production of steam and other auxiliary equipment for the utilisation of flue gases. ( i ) Boiler. The heat of combustion of coal in the boiler is utilised to convert water into steam at high temperature and pressure. The flue gases from the boiler make their journey through super- heater, economiser, air pre-heater and are finally exhausted to atmosphere through the chimney. ( ii ) Superheater. The steam produced in the boiler is wet and is passed through a superheater where it is dried and superheated ( i.e ., steam temperature increased above that of boiling point of water) by the flue gases on their way to chimney. Superheating provides two principal benefits. Firstly, the overall efficiency is increased. Secondly, too much condensation in the last stages of turbine (which would cause blade corrosion) is avoided. The superheated steam from the superheater is fed to steam turbine through the main valve. ( iii ) Economiser. An economiser is essentially a feed water heater and derives heat from the flue gases for this purpose. The feed water is fed to the economiser before supplying to the boiler. The economiser extracts a part of heat of flue gases to increase the feed water temperature. ( iv ) Air preheater. An air preheater increases the temperature of the air supplied for coal burn- ing by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draught fan and is passed through air preheater before supplying to the boiler furnace. The air preheater extracts heat from flue gases and increases the temperature of air used for coal combustion. The principal benefits of preheating the air are : increased thermal efficiency and increased steam capacity per square metre of boiler surface. 3. Steam turbine. The dry and superheated steam from the superheater is fed to the steam turbine through main valve. The heat energy of steam when passing over the blades of turbine is converted into mechanical energy. After giving heat energy to the turbine, the steam is exhausted to the condenser which condenses the exhausted steam by means of cold water circulation. 4. Alternator. The steam turbine is coupled to an alternator. The alternator converts mechanical energy of turbine into electrical energy. The electrical output from the alternator is delivered to the bus bars through transformer, circuit breakers and isolators. 5. Feed water. The condensate from the condenser is used as feed water to the boiler. Some water may be lost in the cycle which is suitably made up from external source. The feed water on its way to the boiler is heated by water heaters and economiser. This helps in raising the overall effi- ciency of the plant. 6. Cooling arrangement. In order to improve the efficiency of the plant, the steam exhausted from the turbine is condensed* by means of a condenser. Water is drawn from a natural source of supply such as a river, canal or lake and is circulated through the condenser. The circulating water takes up the heat of the exhausted steam and itself becomes hot. This hot water coming out from the condenser is discharged at a suitable location down the river. In case the availability of water from the source of supply is not assured throughout the year, cooling towers are used. During the scarcity of water in the river, hot water from the condenser is passed on to the cooling towers where it is cooled. The cold water from the cooling tower is reused in the condenser.

2.42.42.4 2.42.4 Choice of Site for Steam Power StationsChoice of Site for Steam Power StationsChoice of Site for Steam Power StationsChoice of Site for Steam Power StationsChoice of Site for Steam Power Stations

In order to achieve overall economy, the following points should be considered while selecting a site for a steam power station : ( i ) Supply of fuel. The steam power station should be located near the coal mines so that transportation cost of fuel is minimum. However, if such a plant is to be installed at a place

• Efficiency of the plant is increased by reducing turbine exhaust pressure. Low pressure at the exhaust can be achieved by condensing the steam at the turbine exhaust.

Generating Stations 1313131313

where coal is not available, then care should be taken that adequate facilities exist for the transportation of coal. ( ii ) Availability of water. As huge amount of water is required for the condenser, therefore, such a plant should be located at the bank of a river or near a canal to ensure the continuous supply of water. ( iii ) Transportation facilities. A modern steam power station often requires the transportation of material and machinery. Therefore, adequate transportation facilities must exist i.e ., the plant should be well connected to other parts of the country by rail, road. etc. ( iv ) Cost and type of land. The steam power station should be located at a place where land is cheap and further extension, if necessary, is possible. Moreover, the bearing capacity of the ground should be adequate so that heavy equipment could be installed. ( v ) Nearness to load centres. In order to reduce the transmission cost, the plant should be located near the centre of the load. This is particularly important if d.c. supply system is adopted. However, if a.c. supply system is adopted, this factor becomes relatively less important. It is because a.c. power can be transmitted at high voltages with consequent reduced transmission cost. Therefore, it is possible to install the plant away from the load centres, provided other conditions are favourable. ( vi ) Distance from populated area. As huge amount of coal is burnt in a steam power station, therefore, smoke and fumes pollute the surrounding area. This necessitates that the plant should be located at a considerable distance from the populated areas. Conclusion. It is clear that all the above factors cannot be favourable at one place. However, keeping in view the fact that now-a-days the supply system is a.c. and more importance is being given to generation than transmission, a site away from the towns may be selected. In particular, a site by river side where sufficient water is available, no pollution of atmosphere occurs and fuel can be transported economically, may perhaps be an ideal choice.

2.52.52.5 2.52.5 EfEfEfEfEfficiency of Steam Power Stationficiency of Steam Power Stationficiency of Steam Power Stationficiency of Steam Power Stationficiency of Steam Power Station

The overall efficiency of a steam power station is quite low (about 29%) due mainly to two reasons. Firstly, a huge amount of heat is lost in the condenser and secondly heat losses occur at various stages of the plant. The heat lost in the condenser cannot be avoided. It is because heat energy cannot be converted into mechanical energy without temperature difference. The greater the temperature dif- ference, the greater is the heat energy converted* into mechanical energy. This necessitates to keep the steam in the condenser at the lowest temperature. But we know that greater the temperature difference, greater is the amount of heat lost. This explains for the low efficiency of such plants. ( i ) Thermal efficiency. The ratio of heat equivalent of mechanical energy transmitted to the turbine shaft to the heat of combustion of coal is known as thermal efficiency of steam power station.

Thermal efficiency, η thermal =

Heat equivalent of mech. energy transmitted to turbine shaft Heat of coal combustion The thermal efficiency of a modern steam power station is about 30%. It means that if 100 calories of heat is supplied by coal combustion, then mechanical energy equivalent of 30 calories will be available at the turbine shaft and rest is lost. It may be important to note that more than 50% of total heat of combustion is lost in the condenser. The other heat losses occur in flue gases, radia- tion, ash etc. ( ii ) Overall efficiency. The ratio of heat equivalent of electrical output to the heat of combus- tion of coal is known as overall efficiency of steam power station i.e.

• Thermodynamic laws.

Generating Stations 1515151515

The steam produced in the boiler is led through the superheater where it is superheated by the heat of flue gases. Superheaters are mainly classified into two types according to the system of heat transfer from flue gases to steam viz.

( a ) Radiant superheater ( b ) Convection superheater The radiant superheater is placed in the furnace between the water walls and receives heat from the burning fuel through radiation process. It has two main disadvantages. Firstly, due to high furnace temperature, it may get overheated and, therefore, requires a careful design. Secondly, the temperature of superheater falls with increase in steam output. Due to these limitations, radiant superheater is not finding favour these days. On the other hand, a convection superheater is placed in the boiler tube bank and receives heat from flue gases entirely through the convection process. It has the advantage that temperature of superheater increases with the increase in steam output. For this reason, this type of superheater is commonly used these days. ( iv ) Economiser. It is a device which heats the feed water on its way to boiler by deriving heat from the flue gases. This results in raising boiler efficiency, saving in fuel and reduced stresses in the boiler due to higher temperature of feed water. An economiser consists of a large number of closely spaced parallel steel tubes connected by headers of drums. The feed water flows through these tubes and the flue gases flow outside. A part of the heat of flue gases is transferred to feed water, thus raising the temperature of the latter. ( v ) Air Pre-heater. Superheaters and economisers generally cannot fully extract the heat from flue gases. Therefore, pre-heaters are employed which recover some of the heat in the escaping gases. The function of an air pre-heater is to extract heat from the flue gases and give it to the air being supplied to furnace for coal combustion. This raises the furnace temperature and increases the thermal efficiency of the plant. Depending upon the method of transfer of heat from flue gases to air, air pre-heaters are divided into the following two classes : ( a ) Recuperative type ( b ) Regenerative type The recuperative type air-heater consists of a group of steel tubes. The flue gases are passed through the tubes while the air flows externally to the tubes. Thus heat of flue gases is transferred to air. The regenerative type air pre-heater consists of slowly moving drum made of corrugated metal plates. The flue gases flow continuously on one side of the drum and air on the other side. This action permits the transference of heat of flue gases to the air being supplied to the furnace for coal combustion.

2. Condensers. A condenser is a device which condenses the steam at the exhaust of turbine. It serves two important functions. Firstly, it creates a very low *pressure at the exhaust of turbine, thus permitting expansion of the steam in the prime mover to a very low pressure. This helps in converting heat energy of steam into mechanical energy in the prime mover. Secondly, the condensed steam can be used as feed water to the boiler. There are two types of condensers, namely :

( i ) Jet condenser ( ii ) Surface condenser In a jet condenser, cooling water and exhausted steam are mixed together. Therefore, the tem- perature of cooling water and condensate is the same when leaving the condenser. Advantages of this type of condenser are : low initial cost, less floor area required, less cooling water required and low maintenance charges. However, its disadvantages are : condensate is wasted and high power is re- quired for pumping water.

In a surface condenser, there is no direct contact between cooling water and exhausted steam. It consists of a bank of horizontal tubes enclosed in a cast iron shell. The cooling water flows through the tubes and exhausted steam over the surface of the tubes. The steam gives up its heat to water and is itself condensed. Advantages of this type of condenser are : condensate can be used as feed water, less pumping power required and creation of better vacuum at the turbine exhaust. However, disad-

• By liquidating steam at the exhaust of turbine, a region of emptiness is created. This results in a very low pressure at the exhaust of turbine.

1616161616 Principles of Power System

vantages of this type of condenser are : high initial cost, requires large floor area and high mainte- nance charges.

3. Prime movers. The prime mover converts steam energy into mechanical energy. There are two types of steam prime movers viz ., steam engines and steam turbines. A steam turbine has several advantages over a steam engine as a prime mover viz ., high efficiency, simple construction, higher speed, less floor area requirement and low maintenance cost. Therefore, all modern steam power stations employ steam turbines as prime movers. Steam turbines are generally classified into two types according to the action of steam on moving blades viz. ( i ) Impulse turbines ( ii ) Reactions turbines In an impulse turbine, the steam expands completely in the stationary nozzles (or fixed blades), the pressure over the moving blades remaining constant. In doing so, the steam attains a high velocity and impinges against the moving blades. This results in the impulsive force on the moving blades which sets the rotor rotating. In a reaction turbine, the steam is partially expanded in the stationary nozzles, the remaining expansion takes place during its flow over the moving blades. The result is that the momentum of the steam causes a reaction force on the moving blades which sets the rotor in motion. 4. Water treatment plant. Boilers require clean and soft water for longer life and better effi- ciency. However, the source of boiler feed water is generally a river or lake which may contain suspended and dissolved impurities, dissolved gases etc. Therefore, it is very important that water is first purified and softened by chemical treatment and then delivered to the boiler. The water from the source of supply is stored in storage tanks. The suspended impurities are removed through sedimentation, coagulation and filtration. Dissolved gases are removed by aeration and degasification. The water is then ‘softened’ by removing temporary and permanent hardness through different chemical processes. The pure and soft water thus available is fed to the boiler for steam generation. 5. Electrical equipment. A modern power station contains numerous electrical equipment. However, the most important items are : ( i ) Alternators. Each alternator is coupled to a steam turbine and converts mechanical energy of the turbine into electrical energy. The alternator may be hydrogen or air cooled. The necessary excitation is provided by means of main and pilot exciters directly coupled to the alternator shaft. ( ii ) Transformers. A generating station has different types of transformers, viz ., ( a ) main step-up transformers which step-up the generation voltage for transmission of power. ( b ) station transformers which are used for general service ( e.g ., lighting) in the power station. ( c ) auxiliary transformers which supply to individual unit-auxiliaries. ( iii ) Switchgear. It houses such equipment which locates the fault on the system and isolate the faulty part from the healthy section. It contains circuit breakers, relays, switches and other control devices. Example 2.1. A steam power station has an overall efficiency of 20% and 0·6 kg of coal is burnt per kWh of electrical energy generated. Calculate the calorific value of fuel.

18 Principles of Power System

W C

kWh kWh As the station output ( i. e ., kWh) increases towards infinity, the limiting value of W / C approaches 7·5/2·9 = 2·6. Therefore, the weight of water evaporated per kg of coal consumed approaches a limiting value of 2·6 kg as the kWh output increases. ( ii ) At no load, the station output is zero i.e., kWh = 0. Therefore, from expression ( ii ), we get, coal consumption at no load = 5000 + 2·9 × 0 = 5000 kg ∴ Coal consumption/hour = 5000/8 = 625 kg Example 2.5. A 100 MW steam station uses coal of calorific value 6400 kcal/kg. Thermal efficiency of the station is 30% and electrical efficiency is 92%. Calculate the coal consumption per hour when the station is delivering its full rated output. Solution. Overall efficiency of the power station is η overall = η thermal × η elect = 0·30 × 0·92 = 0· Units generated/hour = (100 × 103 ) × 1 = 10^5 kWh

Heat produced/hour, H = Electrical output in heat units η overall

=

5 × 6 ⋅ = ⋅ × kcal (∵ 1 kWh = 860 kcal)

∴ Coal consumption/hour =

Calorific value 6400

H ⋅ ×

= = 48687 kg

TUTORIAL PROBLEMSTUTORIAL PROBLEMSTUTORIAL PROBLEMSTUTORIAL PROBLEMSTUTORIAL PROBLEMS

1. A generating station has an overall efficiency of 15% and 0·75 kg of coal is burnt per kWh by the station. Determine the calorific value of coal in kilocalories per kilogram. [7644 kcal/kg] 2. A 75 MW steam power station uses coal of calorific value of 6400 kcal/kg. Thermal efficiency of the station is 30% while electrical efficiency is 80%. Calculate the coal consumption per hour when the station is delivering its full output. [42 tons] 3. A 65,000 kW steam power station uses coal of calorific value 15,000 kcal per kg. If the coal consump- tion per kWh is 0·5 kg and the load factor of the station is 40%, calculate ( i ) the overall efficiency ( ii ) coal consumption per day. [( i ) 28·7% ( ii ) 312 tons] 4. A 60 MW steam power station has a thermal efficiency of 30%. If the coal burnt has a calorific value of 6950 kcal/kg, calculate : ( i ) the coal consumption per kWh, ( ii )the coal consumption per day. [( i ) 0·413 kg ( ii ) 238 tons] 5. A 25 MVA turbo-alternator is working on full load at a power factor of 0·8 and efficiency of 97%. Find the quantity of cooling air required per minute at full load, assuming that 90% of the total losses are dissipated by the internally circulating air. The inlet air temperature is 20º C and the temperature rise is 30º C. Given that specific heat of air is 0·24 and that 1 kg of air occupies 0·8 m^3. [890 m^3 /minute] 6. A thermal station has an efficiency of 15% and 1·0 kg of coal burnt for every kWh generated. Determine the calorific value of coal. [5733 kcal/kg]

2.72.72.7 2.72.7 HydrHydrHydrHydrHydro-electro-electro-electro-electrico-electricicicic PPPPPooooowwwwwer Staer Staer Staer Staer Stationtiontiontiontion

A generating station which utilises the potential energy of water at a high level for the generation of electrical energy is known as a hydro-electric power station.

Generating Stations 19

Hydro-electric power stations are generally located in hilly areas where dams can be built conve- niently and large water reservoirs can be obtained. In a hydro-electric power station, water head is created 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 the alternator which converts mechanical energy into electrical energy. Hydro-electric power stations are becoming 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 drink- ing purposes.

AdvantagesAdvantagesAdvantages AdvantagesAdvantages

( 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 avail- able 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 construc- tion, yet for operation, a few experienced persons may do the job well.

DisadvantagesDisadvantagesDisadvantages DisadvantagesDisadvantages

( 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.

2.82.82.8 2.82.8 SchemaSchemaSchemaSchemaSchematictictictictic Arrangement of HydrArrangement of HydrArrangement of HydrArrangement of Hydro-electrArrangement of Hydro-electro-electro-electro-electricicicicic PPPPPooooowwwwwer Staer Staer Staer Staer Stationtiontiontiontion

Although a hydro-electric power station simply involves the conversion of hydraulic energy into electrical energy, yet it embraces many arrangements for proper working and efficiency. The sche- matic arrangement of a modern hydro-electric plant is shown in Fig. 2.2. 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 latter cuts off supply of water when the penstock bursts. From the valve house, water is taken to water turbine through a huge steel pipe known as penstock. The water turbine converts hydraulic energy into mechanical energy. The turbine drives the alternator which converts mechanical energy into electrical 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

• The governor opens or closes the turbine gates in accordance with the changes in electrical load. If the electrical load increases, the governor opens the turbine gates to allow more water and vice-versa.