Loads On Electric Power System/Station

Function & Structure Of Electric Power System:

The function of an electric power system is to connect the power station to the consumers’ loads.The power demanded by the consumers is supplied by the power station through the transmission and distribution networks.As the consumers’ load demand changes, the power supply by the power station changes accordingly.

The function of a power station is to deliver power to a large number of consumers. However, the power demands of different consumers vary in accordance with their activities. The result of this variation in demand is that load on a power station is never constant,rather it varies from time to time. Most of the complexities of modern power plant operation arise from the inherent variability of the load demanded by the users.Unfortunately, electrical power cannot be stored and, therefore, the power station must produce power as and when demanded to meet the requirements of the consumers.On one hand, the power engineer would like that the alternators in the power station should run at their rated capacity for maximum efficiency and on the other hand, the demands of the consumers have wide variations.This makes the design of a power station highly complex. 

Variable Loads On Power Station:
The load on a power station varies from time to time due to uncertain demands of the consumers and is known as variable load on the station.
A power station is designed to meet the load requirements of the consumers.An ideal load on the station, from stand point of equipment needed and operating routine, would be one
of constant magnitude and steady duration.However, such a steady load on the station is never realized in actual practice.The consumers require their small or large block of power in accordance with the demands of their activities.Thus the load demand of one consumer at any time may be different from that of the other consumer.The result is that load on the power station varies from time to time.

Effects of variable load. The variable load on a power station introduces many perplexities in its operation. Some of the important effects of variable load on a power station are :
(i) Need of Additional Equipment.   The variable load on a power station necessitates to have
additional equipment.By way of illustration, consider a steam power station. Air, coal and water are the raw materials for this plant.In order to produce variable power, the supply of these materials will be required to be varied correspondingly. For instance, if the power demand on the plant increases, it must be followed by the increased flow of coal, air and water to the boiler in order to meet the increased demand.Therefore, additional equipment has to be installed to accomplish this job.  As a matter of fact, in a modern power plant, there is much equipment devoted entirely to adjust the rates of supply of raw materials in accordance with the power demand made on the plant.

(ii) Increase in Production Cost.   The variable load on the plant increases the cost of the production of electrical energy.An alternator operates at maximum efficiency near its rated capacity. If a single alternator is used, it will have poor efficiency during periods of light loads on the plant. Therefore, in actual practice, a number of alternators of different capacities are installed so that most of the alternators can be operated at nearly full load capacity.However, the use of a number of generating units increases the initial cost per kW of the plant capacity as well as floor area required. This leads to the increase in production cost of energy.

Schematic Arrangement of Gas Turbine Power Plant

Schematic Arrangement of Gas Turbine Power Plant:

The schematic arrangement of a gas turbine power plant is shown in Figure.The main components of the plant are :

(i) Compressor 

(ii) Regenerator

(iii) Combustion chamber 

(iv ) Gas turbine

(v) Alternator 

(vi ) Starting motor

Schematic Arrangement of Gas Turbine Power Plant

(i) Compressor. The compressor used in the plant is generally of rotatory type. The air at atmospheric pressure is drawn by the compressor  via the filter which removes the dust from air.The rotatory blades of the compressor push the air between stationary blades to raise its pressure.Thus air at high pressure is available at the output of the compressor.

(ii) Regenerator. A regenerator is a device which recovers heat from the exhaust gases of the turbine. The exhaust is passed through the regenerator before wasting to atmosphere. A regenerator consists of a nest of tubes contained in a shell.The compressed air from the compressor passes through the tubes on its way to the combustion chamber. In this way, compressed air is heated by the hot exhaust gases.

(iii) Combustion Chamber. The air at high pressure from the compressor is led to the combustion chamber  via the regenerator.  In the combustion chamber, heat is added to the air by burning oil.The oil is injected through the burner into the chamber at high pressure to ensure atomization of oil and its thorough mixing with air.The result is that the chamber attains a very high temperature (about 3000F). The combustion gases are suitably cooled to 1300F to 1500F and then delivered to the gas turbine.

(iv) Gas Turbine. The products of combustion consisting of a mixture of gases at high temperature and pressure are passed to the gas turbine. These gases in passing over the turbine blades expand and thus do the mechanical work.  The temperature of the exhaust gases from the turbine is about 900oF.

(v ) Alternator. The gas turbine is coupled to the alternator.The alternator converts mechanical energy of the turbine into electrical energy.The output from the alternator is given to the bus-bars through transformer, circuit breakers and isolators.

(vi ) Starting Motor.  Before starting the turbine, compressor has to be started.  For this purpose, an electric motor is mounted on the same shaft as that of the turbine.The  motor is energized by the batteries. Once the unit starts, a part of mechanical power of the turbine drives the compressor and there is no need of motor now.

Gas Turbine Power Plant

Gas Turbine Power Plant:

A generating station which employs gas turbine as the prime mover for the generation of electrical energy is  known as a  gas turbine power plant

In a gas turbine power plant, air is used as the working fluid.The air is compressed by the compressor and is led to the combustion chamber where heat is added to air, thus raising its temperature.Heat is added to the compressed air either by burning fuel in the chamber or by the use of air heaters.The hot and high pressure air from the combustion chamber is then passed to the gas turbine where it expands and does the mechanical work.The gas turbine drives the alternator which converts mechanical energy into electrical energy.It may be mentioned here that compressor, gas turbine and the alternator are mounted on the same shaft so that a part of mechanical power of the turbine can be utilized for the operation of the compressor. Gas turbine power plants are being used as standby plants for hydro electric stations, as a starting plant for driving auxiliaries in power plants etc.

Advantages:

(i) It is simple in design as compared to steam power station since no boilers and their auxiliaries are required.

(ii) It is much smaller in size as compared to steam power station of the same capacity.This is expected since gas turbine power plant does not require boiler, feed water arrangement etc.

(iii) The initial and operating costs are much lower than that of equivalent steam power station.

(iv) It requires comparatively less water as no condenser is used.

(v) The maintenance charges are quite small.

(vi) Gas turbines are much simpler in construction and operation than steam turbines.

(vii) It can be started quickly form cold conditions.

(viii) There are no standby losses.However, in a steam power station, these losses occur because boiler is kept in operation even when the steam turbine is supplying no load.

Disadvantages:

(i) There is a problem for starting the unit.It is because before starting the turbine, the compressor has to be operated for which power is required from some external source.However, once the unit starts, the external power is not needed as the turbine itself supplies the necessary power to the compressor.

(ii) Since a greater part of power developed by the turbine is used in driving the compressor, the net output is low.

(iii) The overall efficiency of such plants is low (about 20%) because the exhaust gases from the turbine contain sufficient heat.

(iv) The temperature of combustion chamber is quite high (3000oF) so that its life is comparatively reduced.

Selection of Site for Nuclear Power Station

Selection of  Site for Nuclear Power Station:

The following points should be kept in view while selecting the site for a nuclear power station :

(i) Availability of water.  As sufficient water is required for cooling purposes, therefore, the plant site should be located where ample quantity of water is available,  e.g., across a river or by sea-side.

(ii) Disposal of waste. The waste produced by fission in a nuclear power station is generally radioactive which must be disposed off properly to avoid health hazards.The waste should either be buried in a deep trench or disposed off in sea quite away from the sea shore.Therefore, the site selected for such a plant should have adequate arrangement for the disposal of radioactive waste.

(iii) Distance from populated areas.  The site selected for a nuclear power station should be quite away from the populated areas as there is a danger of presence of radioactivity in the atmosphere near the plant.  However, as a precautionary measure,  a dome  is used in the plant which does not allow the radioactivity to spread by wind or underground waterways.

(iv) Transportation facilities.  The site selected for a nuclear power station should have adequate facilities in order to transport the heavy equipment during erection and to facilitate the movement of the workers employed in the plant.

From the above mentioned factors it becomes apparent that ideal choice for a nuclear power station would be near sea or river and away from thickly populated areas.

Schematic Arrangement of Nuclear Power Station

Schematic Arrangement of Nuclear Power Station:

The schematic arrangement of a nuclear power station is shown in Figure.The whole arrangement can be divided into the following main stages :

(i) Nuclear Reactor

(ii) Heat Exchanger

(iii) Steam Turbine

(iv) Alternator

Schematic Arrangement Of Nuclear Power Station

(i) Nuclear Reactor.  It is an apparatus in which nuclear fuel (U235) is subjected to nuclear fission. It controls the  chain reaction that starts once the fission is done.If the chain reaction is not controlled, the result will be an explosion due to the fast increase in the energy released. A nuclear reactor is a cylindrical stout pressure vessel and houses fuel rods of Uranium, moderator and control rods (See Figure).The fuel rods constitute the fission material and release huge amount of energy when bombarded with slow moving neutrons.The moderator consists of graphite rods which enclose the fuel rods. The moderator slows down the neutrons before they bombard the fuel rods. The control rods are of cadmium and are inserted into the reactor. Cadmium is strong neutron absorber and thus regulates the supply of neutrons for fission.  When the control rods are pushed in deep enough, they absorb most of fission neutrons and hence few are available for chain reaction which, therefore, stops.However, as they are being withdrawn, more and more of these fission neutrons cause fission and hence the  intensity  of chain reaction (or heat produced) is increased. Therefore, by pulling out the control rods, power of the nuclear reactor is increased, whereas by pushing them in, it is reduced. In actual practice, the lowering or raising of control rods is accomplished automatically according to the requirement of load.The heat produced in the reactor is removed by the coolant, generally a sodium metal.  The coolant carries the heat to the heat exchanger.

Nuclear Reactor

(ii) Heat Exchanger. The coolant gives up heat to the heat exchanger which is utilized in raising the steam. After giving up heat, the coolant is again fed to the reactor.

(iii) Steam Turbine. The steam produced in the heat exchanger is led to the steam turbine through a valve.  After doing a useful work in the turbine, the steam is exhausted to condenser. The condenser condenses the steam which is fed to the heat exchanger through feed water pump.

(iv) Alternator. The steam turbine drives the alternator which converts mechanical energy into electrical energy.The output from the alternator is delivered to the bus-bars through transformer, circuit breakers and isolators.

Schematic Arrangement of Diesel Power Station

Schematic Arrangement of Diesel Power Station:

The given figure shows the schematic arrangement of a typical diesel power station.  Apart from the diesel-generator set, the plant has the following auxiliaries :

(i) Fuel Supply System.  It consists of storage tank, strainers, fuel transfer pump and all day fuel tank. The fuel oil is supplied at the plant site by rail or road.This oil is stored in the storage tank. From the storage tank, oil is pumped to smaller all day tank at daily or short intervals.From this tank, fuel oil is passed through strainers to remove suspended impurities.The clean oil is injected into the engine by fuel injection pump.

(ii) Air Intake System. This system supplies necessary air to the engine for fuel combustion.It consists of pipes for the supply of fresh air to the engine manifold.Filters are provided to remove dust particles from air which may act as abrasive in the engine cylinder.

(iii) Exhaust System.  This system leads the engine exhaust gas outside the building and discharges it into atmosphere. A silencer is usually incorporated in the system to reduce the noise level.

Schematic Arrangement of Diesel Power Station

(iv) Cooling System. The heat released by the burning of fuel in the engine cylinder is partially converted into work.The remainder part of the heat passes through the cylinder walls,piston, rings etc. and may cause damage to the system.In order to keep the temperature of the engine parts within the safe operating limits, cooling is provided.The cooling system consists of a water source, pump and cooling towers.The pump circulates water through cylinder and head jacket.The water takes away heat form the engine and itself becomes hot.The hot water is cooled by cooling towers and is recirculated for cooling.

(v) Lubricating System. This system minimizes the wear of rubbing surfaces of the engine.It comprises of lubricating oil tank, pump, filter and oil cooler.The lubricating oil is drawn from the lubricating oil tank by the pump and is passed through filters to remove impurities.The clean lubricating oil is delivered to the points which require lubrication. The oil coolers incorporated in the system keep the temperature of the oil low.

(vi) Engine Starting System. This is an arrangement to rotate the engine initially, while starting, until firing starts and the unit runs with its own power.Small sets are started manually by handles but for larger units, compressed air is used for starting.In the latter case, air at high pressure is admitted to a few of the cylinders, making them to act as reciprocating air motors to turn over the engine shaft.The fuel is admitted to the remaining cylinders which makes the engine to start under its own power.

Equipment of Hydro-electric Plant

Equipment of Hydro-electric Plant:

The constituents of a hydro-electric plant are  

(1) Hydraulic structures(dam, spillways, headworks, surge tank, penstock and accessory works.)

(2) Water turbines and

(3) Electrical equipment.  

We shall discuss these items in turn.

1. Hydraulic Structures. 

Hydraulic structures in a hydro-electric power station include dam, spillways, headworks, surge tank, penstock and accessory works.

(i) Dam. A dam is a barrier which stores water and creates water head.  Dams are built of concrete or stone masonary, earth or rock fill.The type and arrangement depends upon the topography of the site. A masonary dam may be built in a narrow canyon.An earth dam may be best suited for a wide valley. The type of dam also depends upon the foundation conditions, local materials and transportation available, occurrence of earthquakes and other hazards.At most of sites, more than one type of dam may be suitable and the one which is most economical is chosen.

(ii) Spillways. There are times when the river flow exceeds the storage capacity of the reservoir. Such a situation arises during heavy rainfall in the catchment area. In order to discharge the surplus water from the storage reservoir into the river on the down-stream side of the dam, spillways are used.Spillways are constructed of concrete piers on the top of the dam.Gates are provided between these piers and surplus water is discharged over the crest of the dam by opening these gates.

(iii) Headworks. The headworks consists of the diversion structures at the head of an intake. They generally include booms and racks for diverting floating debris, sluices for  by-passing debris and cavitation.  For this purpose, it is necessary to avoid sharp corners and abrupt contractions or enlargements.

(iv) Surge tank.  Open conduits leading water to the turbine require no protection.However, when closed conduits are used, protection becomes necessary to limit the abnormal pressure in the conduit. For this reason,closed conduits are always provided with a surge tank.A surge tank is a small reservoir or tank (open at the top) in which water level rises or falls to reduce the pressure swings in the conduit. A surge tank is located near the beginning of the conduit.When the turbine is running at a steady load, there are no surges in the flow of water through the conduit i.e.the quantity of water flowing in the conduit is just sufficient to meet the turbine requirements.However, when the load on the turbine decreases, the governor closes the gates of turbine, reducing water supply to the turbine. The excess water at the lower end of the conduit rushes back to the surge tank and increases its water level.Thus the conduit is prevented from bursting.On the other hand, when load on the turbine increases, additional water is drawn from the surge tank to meet the increased load requirement. Hence, a surge tank overcomes the abnormal pressure in the conduit when load on the turbine falls and acts as a reservoir during increase of load on the turbine.

(v) Penstocks. Penstocks are open or closed conduits which carry water to the turbines.They are generally made of reinforced concrete or steel.Concrete penstocks are suitable for low heads (< 30m) as greater pressure causes rapid deterioration of concrete.The steel penstocks can be designed for any head; the thickness of the penstock increases with the head or working pressure.

Various devices such as automatic butterfly valve, air valve and surge tank  are provided for the protection of penstocks. Automatic butterfly valve shuts off water flow through the penstock promptly if it ruptures.Air valve maintains the air pressure inside the penstock equal to outside atmospheric pressure.When water runs out of a penstock faster than it enters, a vacuum is created which may cause the penstock to collapse.Under such situations, air valve opens and admits air in the penstock to maintain inside air pressure equal to the outside air pressure.

Penstock Protective Devices

2.Water Turbines.

Water turbines are used to convert the energy of falling water into mechanical energy.The principal types of water turbines are :

         (i) Impulse turbines                                 (ii) Reaction turbines

(i) Impulse turbines . Such turbines are used for high heads. In an impulse turbine, the entire pressure of water is converted into kinetic energy in a nozzle and the velocity of the jet drives the wheel.  The example of this type of turbine is the Pelton wheel(shown in figure).

Pelton Wheel

It consists of a wheel fitted with elliptical buckets along its periphery.The force of water jet striking the buckets on the wheel drives the turbine.The quantity of water jet falling on the turbine is controlled by means of a  needle  or spear (not shown in the figure) placed in the tip of the nozzle.The movement of the needle is controlled by the governor.If the load on the turbine decreases, the governor pushes the needle into the nozzle, thereby reducing the quantity of water striking the buckets.  Reverse action takes place if the load on the turbine increases.

(ii) Reaction turbines. Reaction turbines are used for low and medium heads.In a reaction turbine, water enters the runner partly with pressure energy and partly with velocity head.The important types of reaction turbines are :

      (a) Francis turbines                                        (b) Kaplan turbines

A Francis turbine is used for low to medium heads.It consists of an outer ring of stationary guide blades fixed to the turbine casing and an inner ring of rotating blades forming the runner.The guide blades control the flow of water to the turbine.Water flows radially inwards and changes to a downward direction while passing through the runner. As the water passes over the “rotating blades” of the runner,both pressure and velocity of water are reduced.This causes a reaction force which drives the turbine.

A Kaplan turbine is used for low heads and large quantities of water. It is similar to Francis turbine except that the runner of Kaplan turbine receives water axially.Water flows radially inwards through regulating gates all around the sides, changing direction in the runner to axial flow.This causes a reaction force which drives the turbine.

3.Electrical Equipment. The electrical equipment of a hydro-electric power station includes alternators, transformers, circuit breakers and other switching and protective devices.

Schematic Arrangement of Hydro-electric Power Station

Schematic Arrangement of Hydro-electric Power Station:

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 schematic arrangement of a modern hydro-electric plant is shown in given Fig.

Schematic Arrangement of Hydro-electric Power Station

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 gates close, there is a sudden stopping of water at the lower end of the penstock and consequently the penstock can burst like a paper log.The surge tank absorbs this pressure swing by increase in its level of water.

Choice of Site for Hydro-electric Power Stations:

The following points should be taken into account while selecting the site for a hydro-electric power station :

(i) Availability of water. Since the primary requirement of a hydro-electric power station is the availability of huge quantity of water, such plants should be built at a place ( e.g ., river, canal) where adequate water is available at a good head.

(ii) Storage of water. There are wide variations in water supply from a river or canal during the year. This makes it necessary to store water by constructing a dam in order to ensure the generation of power throughout the year.The storage helps in equalizing the flow of water so that any excess quantity of water at a certain period of the year can be made available during times of very low flow in the river.This leads to the conclusion that site selected for a hydro-electric plant should provide adequate facilities for erecting a dam and storage of water.

(iii) Cost and type of land.The land for the construction of the plant should be available at a reasonable price.Further, the bearing capacity of the ground should be adequate to withstand the weight of heavy equipment to be installed.

(iv) Transportation facilities. The site selected for a hydro-electric plant should be accessible by rail and road so that necessary equipment and machinery could be easily transported.It is clear from the above mentioned factors that ideal choice of site for such a plant is near a river in hilly areas where dam can be conveniently built and large reservoirs can be obtained.

Equipment Of Steam Power Station

Equipment Of Steam Power Station:

A modern steam power station is highly complex and has numerous equipment and auxiliaries. However, the most important constituents of a steam power station are :

1. Steam generating equipment 

2. Condenser 

3. Prime mover

4. Water treatment plant 

5. Electrical equipment.

1. Steam Generating Equipment. This is an important part of steam power station.  It is concerned with the generation of super-heated steam and includes such items as boiler, boiler furnace, super-heater, economizer, air pre-heater and other heat reclaiming devices.

(i) Boiler. A boiler is closed vessel in which water is converted into steam by utilizing the heat of coal combustion.  Steam boilers are broadly classified into the following two types :

         (a) Water tube boilers                                     (b) Fire tube boilers

In a water tube boiler, water flows through the tubes and the hot gases of combustion flow over these tubes. On the other hand, in a fire tube boiler, the hot products of combustion pass through the tubes surrounded by water.Water tube boilers have a number of advantages over fire tube boilers viz., require less space, smaller size of tubes and drum, high working pressure due to small drum, less liable to explosion etc.Therefore, the use of water tube boilers has become universal in large capacity steam power stations.

(ii) Boiler furnace. A boiler furnace is a chamber in which fuel is burnt to liberate the heat energy. In addition, it provides support and enclosure for the combustion equipment i.e.,burners.The boiler furnace walls are made of refractory materials such as fire clay, silica, kaolin etc.These materials have the property to resist change of shape, weight or physical properties at high temperatures. There are following three types of construction of furnace walls :

(a) Plain refractory walls

(b) Hollow refractory walls with an arrangement for air cooling

(c) Water walls.

The plain refractory walls are suitable for small plants where the furnace temperature may not be high. However, in large plants, the furnace temperature is quite high and consequently, the refractory material may get damaged. In such cases, refractory walls are made hollow and air is circulated through hollow space to keep the temperature of the furnace walls low.The recent development is to use water walls.These consist of plain tubes arranged side by side and on the inner face of the refractory walls.  The tubes are connected to the upper and lower headers of the boiler.  The boiler water is made to circulate through these tubes.  The water walls absorb the radiant heat in the furnace which would otherwise heat up the furnace walls.

(iii) Super-heater. A super-heater is a device which superheats the steam  i.e.,it raises the temperature of steam above boiling point of water.This increases the overall efficiency of the plant. A super-heater consists of a group of tubes made of special alloy steels such as chromium-molybdenum.

These tubes are heated by the heat of flue gases during their journey from the furnace to the chimney

The steam produced in the boiler is led through the super-heater where it is super-heated 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 super-heater                          (b) Convection super-heater

The radiant super-heater 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 super-heater falls with increase in steam output.Due to these limitations, radiant super-heater is not finding favour these days.  On the other hand, a convection super-heater 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 super-heater increases with the increase in steam output. For this reason, this type of super-heater is commonly used these days.

(iv) Economizer. 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 economizer 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.  Super-heaters and economizers 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 temperature 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 required 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, disadvantages of this type of condenser are : high initial cost, requires large floor area and high maintenance 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 efficiency. 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.

Site Selection and Efficiency for Steam Power Stations

Choice 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 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.

It is clear that all the above factors cannot be favorable 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.

Efficiency 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, radiation, ash etc.

(ii) Overall efficiency. The ratio of heat equivalent of electrical output to the heat of combustion of coal is known as overall efficiency of steam power station i.e

    Overall  efficiency, η(overall)=Heat equivalent of electrical ouput/Heat of combustion of coal

The overall efficiency of a steam power station is about 29%.  It may be seen that overall efficiency is less than the thermal efficiency.  This is expected since some losses (about 1%) occur in the alternator.  The following relation exists among the various efficiencies.

                        Overall  efficiency = Thermal  efficiency ×  Electrical  efficiency

Schematic Arrangement of Steam Power Station

 Schematic Arrangement of Steam Power Station:

Although steam power station simply involves the conversion of heat of coal combustion into electrical energy, yet it embraces many arrangements for proper working and efficiency.The schematic arrangement of a modern steam power station is shown in given Fig.  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

 Schematic Arrangement of Steam Power Station

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 pulverized (i.e ., crushed into small pieces) in order to increase its surface exposure, thus promoting rapid combustion without using large quantity of excess air.  The pulverized coal is fed to the boiler by belt conveyors.  The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal.  The removal of the ash from the boiler furnace is necessary for proper burning of coal.It is worthwhile to give a passing reference to the amount of coal burnt and ash produced in a modern thermal power station. A 100 MW station operating at 50% load factor may burn about 20,000 tons of coal per month and ash produced may be to the tune of 10% to 15% of coal fired i.e .,2,000 to 3,000 tons.  In fact, in a thermal station, about 50% to 60% of the total operating cost consists of fuel purchasing and its handling.

2. Steam generating plant.   

The steam generating plant consists of a boiler for the production of steam and other auxiliary equipment for the utilization of flue gases.

(i) Boiler. The heat of combustion of coal in the boiler is utilized to convert water into steam at high temperature and pressure.  The flue gases from the boiler make their journey through super heater, economizer, air pre-heater and are finally  exhausted to atmosphere through the chimney.

(ii) Super-heater.  The steam produced in the boiler is wet and is passed through a super-heater where it is dried and super-heated (i.e ., steam temperature increased above that of boiling point of water) by the flue gases on their way to chimney.  Super-heating 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 super-heated steam from the super-heater is fed to steam turbine through the main valve.

(iii) Economizer.  An economizer is essentially a feed water heater and derives heat from the flue gases for this purpose. The feed water is fed to the economizer before supplying to the boiler. The economizer extracts a part of heat of flue gases to increase the feed water temperature.

(iv) Air pre-heater.  An air pre-heater increases the temperature of the air supplied for coal burning by deriving heat from flue gases.  Air is drawn from the atmosphere by a forced draught fan and is passed through air pre-heater 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 economizer.  This helps in raising the overall efficiency 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.

Generating Stations

Generating Stations:

Bulk electric power is produced by special plants known as generating stations  or power plants.

A generating station essentially employs a 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 energy produced by the generating station is transmitted and distributed with the help of conductors to various consumers.  It may be emphasized 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) Hydro-electric power stations

(iii) Diesel power stations 

(iv) Nuclear power stations

In this modern world, the dependence on electricity is so much that it has become a part and parcel of our life.The ever increasing use of electric power for domestic, commercial and industrial purposes necessitates to provide bulk electric power economically.This is achieved with the help of suitable power producing 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 minimum 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.

1-Steam Power Station(Thermal 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 utilizing 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.

Advantages:

(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 transported 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.

Disadvantages:

(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-Hydro-Electric Power Station:

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

Hydro-electric power stations are generally located in hilly areas where dams can be built conveniently 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 drinking purposes.

Advantages:

(i) It requires no fuel as water is used for the generation of electrical energy.

(ii) It is quite neat and clean as no smoke or ash is produced.

(iii) It requires very small running charges because water is the source of energy which is 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 construction, yet for operation, a few experienced persons may do the job well.

Disadvantages:

(i) It involves high capital cost due to construction of dam.

(ii) There is uncertainty about the availability of huge amount of water due to dependence on weather conditions.

(iii) Skilled and experienced hands are required to build the plant.

(iv) It requires high cost of transmission lines as the plant is located in hilly areas which are quite away from the consumers.

3-Diesel Power Station:

A generating station in which diesel engine is used as the prime mover for the generation of electrical energy is  known as   diesel power station.In a diesel power station, diesel engine is used as the prime mover. The diesel burns inside the engine and the products of this combustion act as the “working fluid” to produce mechanical energy. The diesel engine drives the alternator which converts mechanical energy into electrical energy. As the generation cost is considerable due to high price of diesel, therefore, such power stations are only used to produce small power.Although steam power stations and hydro-electric plants are invariably used to generate bulk power at cheaper cost, yet diesel power stations are finding favour at places where demand of power is less, sufficient quantity of coal and water is not available and the transportation facilities are inadequate.  These plants are also used as standby sets for continuity of supply to important points such as hospitals, radio stations, cinema houses and telephone exchanges.

Advantages:

(i) The design and layout of the plant are quite simple.

(ii) It occupies less space as the number and size of the auxiliaries is small.

(iii) It can be located at any place.

(iv) It can be started quickly and can pick up load in a short time.

(v) There are no standby losses.

(vi) It requires less quantity of water for cooling.

(vii) The overall cost is much less than that of steam power station of the same capacity.

(viii) The thermal efficiency of the plant is higher than that of a steam power station.

(ix) It requires less operating staff.

Disadvantages:

(i) The plant has high running charges as the fuel ( i.e ., diesel) used is costly.

(ii) The plant does not work satisfactorily under overload conditions for a longer period.

(iii) The plant can only generate small power.

(iv) The cost of lubrication is generally high.

(v) The maintenance charges are generally high.

4-Nuclear Power Station:

A generating station in which nuclear energy is converted into electrical energy is known as a nuclear

power station.In nuclear power station, heavy elements such as Uranium (U235) or Thorium (Th232) are subjected to nuclear fission in a special apparatus known as a reactor.The heat energy thus released is utilized in raising steam at high temperature and pressure.The steam runs the steam turbine which converts steam energy into mechanical energy.  The turbine drives the alternator which converts mechanical energy into electrical energy.The most important feature of a nuclear power station is that huge amount of electrical energy can be produced from a relatively small amount of nuclear fuel as compared to other conventional types of power stations.  It has been found that complete fission of 1 kg of Uranium (U235) can produce as much energy as can be produced by the burning of 4,500 tons of high grade coal. Al-though the recovery of principal nuclear fuels(i.e ., Uranium and Thorium) is difficult and expensive,yet the total energy content of the estimated world reserves of these fuels are considerably higher than those of conventional fuels,  viz., coal, oil and gas. At present, energy crisis is gripping us and, therefore, nuclear energy can be successfully employed for producing low cost electrical energy on a large scale to meet the growing commercial and industrial demands.

Advantages:

(i) The amount of fuel required is quite small.  Therefore, there is a considerable saving in the cost of fuel transportation.

(ii) A nuclear power plant requires less space as compared to any other type of the same size.

(iii) It has low running charges as a small amount of fuel is used for producing bulk electrical energy.

(iv) This type of plant is very economical for producing bulk electric power.

(v) It can be located near the load centers because it does not require large quantities of water and need not be near coal mines.  Therefore, the cost of primary distribution is reduced.

(vi) There are large deposits of nuclear fuels available all over the world.  Therefore, such plants can ensure continued supply of electrical energy for thousands of years.

(vii) It ensures reliability of operation.

Disadvantages:

(i) The fuel used is expensive and is difficult to recover.

(ii) The capital cost on a nuclear plant is very high as compared to other types of plants.

(iii) The erection and commissioning of the plant requires greater technical know-how.

(iv) The fission by-products are generally radioactive and may cause a dangerous amount of radioactive pollution.

(v) Maintenance charges are high due to lack of standardization.  Moreover, high salaries of specially trained personnel employed to handle the plant further raise the cost.

(vi) Nuclear power plants are not well suited for varying loads as the reactor does not respond to the load fluctuations efficiently.

(vii) The disposal of the by-products, which are radioactive, is a big problem.  They have either to be disposed off in a deep trench or in a sea away from sea-shore.