Department of Electrical and Computer Engineering
Electrical Power Engineering
ELCE 3001
Lecturer: Mr. Khairy A. Sado
Room: C2
Email: Kerry.sado@uod.ac
Introduction
The main components of an electrical power system are generation, transmission,
distribution, and load.
Power system network
Generation Subsystem
Generators – An essential component of power systems is the three
phase ac generator known as synchronous generator or alternator.
In a power station, several generators are operated in parallel in the
power grid to provide the total power needed. They are connected at a
common point called a bus.
Insulation requirements and other practical design problems limit the
generated voltage to low values, usually in advanced countries is 30 kV
while in others is 11 kV or 16 kV.
Transformers – The transformer transfers power with very high
efficiency from one level of voltage to another level. Using a step-up
transformer will reduce losses in the line, which makes the
transmission of power over long distances possible.
Transmission subsystem
An overhead transmission network transfers electric power from
generating units to the distribution system which ultimately supplies the
load. Transmission voltage lines are standardized at 69 kV, 115 kV, 138
kV, 161 kV, 230 kV, 345 kV, 400 kV, 500 kV, and 765 kV line to line.
Transmission voltages above 230 kV are usually referred to as extrahigh voltage (EHV).
High voltage transmission lines are terminated in substations, which are
called high-voltage substations, receiving substations, or primary
substations.
Very large industrial customers may be served from the transmission
system.
The portion of the transmission system that connects the high-voltage
substations through step-down transformers to the distribution
substations is called the sub transmission network.
Typically, the sub transmission voltage level ranges from 69 to 138 kV.
Some large industrial customers may be served from the sub
transmission system.
Distribution subsystem
The distribution system connects the distribution substations to the
consumers’ service-entrance equipment.
The primary distribution lines from 4 to 34.5 kV and supply the load
in a well-defined geographical area. Some small industrial customers are
served directly by the primary feeders.
The secondary distribution network reduces the voltage for utilization
by commercial and residential consumers. Distribution subsystems are
both overhead and underground.
Distribution substations change the medium voltage to low voltage by
using step down transformer. Low voltage ranges from 220 V single
phase to 400 V, 3 phase 50 Hz in Kurdistan. It serves to power private
residences, commercial and institutional and small industry.
Load
Power systems loads are divided into:
1. Industrial load: Industrial loads are composite loads, and induction
motors form a high proportion of these loads. These composite loads
are functions of voltage and frequency and form a major part of the
system load.
2. Commercial load and residential load: Commercial and residential
loads consist largely of lighting, heating, and cooking. These loads
are independent of frequency and consume negligibly small reactive
power.
Power Stations
Power plants (generation stations):
A power plant or generating station employs a prime mover coupled to a
generator for production of electrical 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. Depending on the form of energy converted into
electrical energy, the generating stations are classified as
Steam power stations
Hydroelectric power stations
Diesel power stations
Nuclear power stations
Gas power stations
Generation of electrical energy
Steam turbine power plant
A steam power plant
Overview of steam power
The most universal method of electric power generation is
accomplished using thermal generation, and the most common machine
for this production is the steam turbine.
Fossil generation uses primarily coal, natural gas, and oil as fuels.
The Concept: A steam power station basically works on physical
principle of the vapor power cycle or Rankine cycle. Steam is produced
in the boiler by utilizing the heat of fuel 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:
1. fuel and water are available in abundance.
2. a large amount of electric power is to be generated.
The advantages of the steam turbine over reciprocating engines are:
(1) The balanced construction.
(2) Relatively high efficiency.
(3) Few moving parts.
(4) Ease of maintenance.
(5) Availability in large sizes.
Advantages
(1) The fuel (
coal) used is quite cheap.
(2) Less initial cost as compared to hydroelectric and nuclear stations.
(3) It can be installed at any place irrespective of the existence of fuel.
(4) It requires less space as compared to the hydroelectric power station.
(5) The cost of generation is lesser than that of the diesel power station.
Disadvantages
(1) It pollutes the atmosphere due to the production of smoke and fumes.
(2) It is most costly in running cost as compared to hydroelectric plant.
Hydroelectric power station
A hydro power plant
Overview of hydro power
The generation of hydroelectric power is accomplished by means of
hydraulic turbines that are directly connected to synchronous generators.
Worldwide, hydropower plants produce ~24% of the world's electricity
and supply >1 billion people with power.
The world's hydropower plants output a total of 675,000 MW, the
energy equivalent of 3.6 billion barrels of oil (according to the National
Renewable Energy Laboratory).
The concept - The water in the reservoir is stored energy. When the
gates open, the water flowing through the penstock becomes kinetic
energy due to in motion. 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. Used water is carried through pipelines, called
tailraces, and re-enters the river downstream.
The most common turbines are the impulse or Pelton turbine, the
reaction or Francis turbine, the propeller or Kaplan turbine, and Deriaz
turbine which combines some of the best features of the Kaplan and
Francis designs.
Advantages
(1) It requires no fuel as water is used for the generation of electrical
energy.
(2) It is quite neat and clean as no smoke or ash is produced.
(3) It requires very small running charges because water is the source of
energy which is available free of cost.
(4) It is comparatively simple in construction and requires less
maintenance.
(5) It does not require a long starting time like a steam power station. In
fact, such plants can be put into service instantly.
(6) It is robust and has a longer life.
(7) Such plants help in irrigation and controlling floods.
(8) 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
(1) It involves high capital cost due to construction of dam.
(2) There is uncertainty about the availability of huge amount of water
due to dependence on weather conditions.
(3) Skilled and experienced hands are required to build the plant.
(4) It requires high cost of transmission lines as the plant is located in
hilly areas which are quite away from the consumers.
Nuclear Power Station
A nuclear power station
Overview of nuclear power
A generating station in which nuclear energy is converted into
electrical energy is known as a nuclear power station.
The Concept: Nuclear generation uses fission reactors that operate by
breakup of high-mass atoms from heavy elements such as Uranium
(U235) or Thorium (Th232) to yield a high energy release. The heat
released is utilized in raising steam at high temperature and pressure. The
nuclear fission is done by bombarding the fuel rods with slow moving
neutrons.
Nuclear reactor
The nuclear reactor controls the initiation and maintenance of a
controlled rate of fission by the absorption of a neutron in a chain
reaction. In the so-called "thermal" reactors a moderator, principally
water, heavy water, or graphite, is required to slow down the neutrons
and thereby enhance the probability of fission.
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.
Advantages
(1) The amount of fuel required is quite small. Therefore, there is a
considerable saving in the cost of fuel transportation.
(2) A nuclear power plant requires less space as compared to any other
type of the same size.
(3) It has low running charges as a small amount of fuel is used for
producing bulk electrical energy.
(4) This type of plant is very economical for producing bulk electric
power.
(5) 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.
(6) 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.
(7) It ensures reliability of operation.
Disadvantages
(1) The fuel used is expensive and is difficult to recover.
(2) The capital cost on a nuclear plant is very high as compared to
other types of plants.
(3) The erection and commissioning of the plant requires greater
technical know-how.
(4) The fission by-products are generally radioactive and may cause a
dangerous amount of radioactive pollution.
(5) Maintenance charges are high due to lack of standardization.
(6) Nuclear power plants are not well suited for varying loads as the
reactor does not respond to the load fluctuations efficiently.
(7) The problem of radioactive waste is still a big problem.
Combustion turbine (gas turbine) and combined-cycle power
plants
Combustion turbine power plant
A single-shaft combustion turbine generator system.
Overview of gas 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.
This system utilizes a gas turbine using Brayton cycle. The axialflow compressor and the generator are driven by the turbine. Here, air
is used as the working fluid.
The Concept: 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 hydroelectric stations, as a starting plant for driving auxiliaries in power
plants etc.
Advantages
(1) It is simple in design as compared to steam power station since no
boilers and their auxiliaries are required.
(2) It is much smaller in size as compared to steam power station of the
same capacity.
(3) The initial and operating costs are much lower than that of
equivalent steam power station.
(4) It requires comparatively less water as no condenser is used.
(5) The maintenance charges are quite small.
(6) Gas turbines are much simpler in construction and operation than
steam turbines.
(7) It can be started quickly form cold conditions.
(8) 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
(1) 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.
(2) Since a greater part of power developed by the turbine is used in
driving the compressor, the net output is low.
Combined-cycle power plant
In the combined-cycle power plant, the prime mover duty is divided
between a gas (combustion turbine) and a heat recovery steam
turbine, with each turbine powering its own generator.
This system utilizes a combination of a gas turbine using Brayton
cycle and a steam turbine using Rankine cycle.
Power plants use the combined-cycle prime mover has higher
efficiencies than those use the combustion turbine prime mover.
A combined-cycle power plant
Diesel Power Station
Overview of diesel power plant
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
(1) The design and layout of the plant are quite simple.
(2) It occupies less space as the number and size of the auxiliaries is
small.
(3) It can be located at any place.
(4) It can be started quickly and can pick up load in a short time.
(5) There are no standby losses.
(6) It requires less quantity of water for cooling.
(7) The overall cost is much less than that of steam power station of the
same capacity.
(8) The thermal efficiency of the plant is higher than that of a steam
power station.
(9) It requires less operating staff.
Disadvantages
(1) The plant has high running charges as the fuel (i.e., diesel) used is
costly.
(2) The plant does not work satisfactorily under overload conditions for
a longer period.
(3) The plant can only generate small power.
(4) The cost of lubrication is generally high.
(5) The maintenance charges are generally high.