Module 4

Generating stations, transmission and distribution of electric power

Generating stations• Electric energy is considered superior to all other forms of

energy• Cheaper, economical for all purpose of uses• Very convenient form of energy- can be converted into other

forms• A generating station generates electricity• A generating station essentialy employs a prime mover

coupled to an alternator.• Prime mover converts some form of energy into mechanical

energy.• Alernator converts mechanical energy to electrical energy

• Generating stations are classified as mainly, Hydro-electric power stations Thermal stations Nuclear power stations Diesel power stations

Hydro-electric power stations

• Is a power plant utilizing the potential energy of water at a high level for the electricity generation.

• The main requirement is a dam with a high head water content.

Schematic arrangement of a hydro –electric plant

Advantage of Hydro-electric power stations

• No fuel is required by such plants as water is the source of energy. Hence operating costs are low.

• The plant is highly reliable and it is cheapest in operations and maintenance.

• It is very neat and clean plant because no smoke or ash is produced

• Such plant are robust and have got longer life.• Highly skilled engineers are required only at the time of

construction but later on only a few experienced person will be required.

• Such plants in addition to generation of electric power also serve other purpose such as irrigation and flood control.

Disadvantages

• It requires large area• Its construction cost is very high and takes long

time for erection.• Long transmission lines are required as the

plants are located in hilly areas which are quite away from the load centre.

• There is uncertainty about the availability of huge amount of water due to dependence of weather conditions.

Thermal stations( steam power plants)

• The heat of combustion of fuels (coal, oil, or gas ) is utilized by the boilers to raise steam at high pressure and temperature.

• Steam produced used in driving the steam turbines coupled to generators.

Schematic arrangementthe plant can be divided into four main circuits.1. Fuel and ash circuits2. Air and fuel gas circuits3. Feed water and steam circuit4. Cooling water circuit

• Fuel and ash circuito Fuel from the storage fed to boiler through fuel feeding

deviceo Ash produced by combustion collects and removed to

ash storage through ash handling equipment.• Air and fuel gas circuito Forced draught fan draws atmospheric air through air-

pre heatero It heated by flue gsses passing to chimney. Then pass to

furnaces.o Flue gases after passing boiler and super heater drawn

by induced draught fan through economizer and air pre heater finally to chimney.

• feed water and steam circuito Condensed water is extracted from the condenser by

condensate pump then forced to HP feed water heater where its temperature raised by bled steam.

o It is then pumped to boiler through economizer in which it is further heated by flue gasses.

o In boiler water is converted into high pressure steam, which is wet

o Wet steam is passed through super heater, where it dried then supplied to steam turbine

o In condensor heat extracted from steam and converted to feed water

• Cooling water circuito circulate through the condensor to condense the steam and

discharges at the source it self.

Advantages of steam power plant

• Fuel used is cheaper• Less space is required in comparison with that of

hydroelectric plants• Cheaper in initial cost in comparison with other types

of power plants of same capacity• Cheaper in production cost in comparison with that

of diesel power plant• Such plants can be installed at any place irrespective

of the existence of fuels, while hydro-electric plants can be developed only at the source of water power

disadvantages

• High maintenance and operating costs• Pollution of atmosphere due to fumes and residues

from pulverized fuels• Requirement of water in large quantity• Handling of coal and disposal of ash is quite difficult• Requires long time for erection and put into action

Nuclear power plants

• Nuclear energy is converted to electrical energy.

• Main fuels are uranium and thorium.• Fuel subjected to nuclear fission releases large

amount of energy.

Advantages of nuclear power plants

• The amount of fuel required is very small, therefore , there is no problem of transportation, storage etc.

• These plants require less area as compared to any other type

• These can be located near the load centers, therefore primary distribution cost is reduced

• These plants are most economical in large capacity• 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

disadvantages

• The initial capital cost is very high as compared to other types of power plants

• The erection and commissioning of the plant requires greater technical knowledge

• The fission by products are generally radio-active and may cause a dangerous amount of radio active pollution

• Fuel is expensive• The disposal of products, which are radio-active, is a big

problem. They have either to be disposed off in a deep trench or in a sea away from sea-shore

Non conventional energy sources

• The non conventional energy sources areSunWindTidalgeothermal

Solar power generation

• Sun is the prime source of energy• The solar electrical energy is of special

importance in India thatI. There is a need for conservation of fossil fuel

resources, such as gas, oil, coal etcII. There is a need for reducing atmospheric and

thermal pollution which are having serious detrimental effects on our environment.

III. There is a need for supply of electrical energy in remote areas.

Solar thermal system

• Advantages1. Renewable source: no fuel required2. There is no pollution• Disadvantage1. Main disadvantage is sun energy available for only

part of the day, and cloudy and hazy atmospheric conditions effects most

2. Cost is large3. Solar tracking system required for sun angle tracking

Solar photo voltaic systems (SPV)

• Advantage1. renewable source2. No pollution3. No moving parts, so maintenance cost low4. Long life• Disadvantages1. High cost2. Generation possible for 6 hours during day time only3. Output is in DC. So DC-AC converter is requires

Tidal energy

• Tides will rise and fall due to change in gravitational force.

• The rised tide stored in some area, it may increase upto meters

• And this head used to rotate a turbine.

Wave energy

Wind energy

• Advantages1. Renewable energy source2. No pollution3. Installation can made fast4. Round the clock availability• Disadvantages1. Weather dependent2. Most of the time plant may operate at a lower

output compared to capacity3. May create noise pollution4. May damage in extreme wind speed conditions5. Large area required

Geothermal energy

• Using heat from interior of the earth energy is converted to electrical energy.

• Two main techniqueso To use hydrothermal sourceso To extract heat from hot dry rocks.

Transmission & distribution of electric power

Various systems of power transmission• DC systemsi. DC two wireii. DC two wire with mid point earthediii. DC three wire system• Single phase AC systemsi. Single phase two wireii. Single phase two wire with mid point earthediii. Single phase three wire• Two phase AC systemsI. Two phase four wireII. Two phase three wire• Three phase AC systemsi. Three phase three wireii. Three phase four wire

Comparison of dc and ac transmission

• DC• Advantages of dc transmissioni. Requires two conductorii. no inductance, capacitance and phase differenceiii. No skin effectiv. Free from most of the losses• Disadvantages of dc transmissioni. Cannot step up or step downii. Generation is difficult

• AC• Advantages of AC transmissioni. Possible to generate high voltage ii. Can step up or step downiii. Maintenance of ac substation is easy and cheap• Dis advantages of AC transmissioni. Require more conductor than dcii. L, C affects regulationiii. Due to skin effect effective resistance increasesiv. Construction of ac transmission line is more

complex

Advantage of high transmission voltage

• Volume of conductor is reduced and hence the cost is less

• Voltage drop in the lines is reduced• Efficiency of transmission line is increased

Power transmission scheme

• Primary transmissionTransmission line network connects generating stations

with main substations. Main voltage levels are 66, 110, 132, 220, 400kV.

• Secondary transmissionlink between main receiving end substation to secondary

substations. voltage levels 66, 33kV• Primary distributionLine supply power from substation to distribution

transformer. mainly 11kV. Consumers with load more than 50kW supplies 11 kV

• Secondary distribution Three phase 4 wire n/w. low tension low power

consumers. Normalyv415/240 V

Primary distribution

Secondary distribution

• Feeders• Distributors• Service mains

substation• Classification of substations1. According to service requirementa) Transformer substationsb) Switching sub stationsc) Frequency changer sub stationsd) Converting substations2. According to constructional featuresa) Indoor substationsb) Outdoor substationsc) Underground substations

Pole mounted substation

Substation equipments

1. Bus bars2. Insulators3. Isolators4. Circuit breaker5. Power transformer6. Current transformer7. Potential transformer8. Protective relays9. Lightning arresters10.Carrier current equipment

Advantage of underground system over overhead system of power distribution

1. More safe2. Low maintenance cost3. Very few chances of fault4. Few chances of accident5. Good appearance6. Free from lightning effects7. No interference with communication systems

Disadvantage of underground system over overhead system of power distribution

1. More expensive2. Difficult to locate the fault3. Cannot be worked above 66kV due to

insulation difficulties