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Qassim University
College of Engineering
Electrical Engineering Department
Course: EE-340: Fundamentals of Power Systems
Prerequisite(s): EE-202 & EE-203
• Electronic Materials & Web Sites :Course materials (PP Slides
are edited by the instructor as teaching aided tool to be used on the
smart board & HW problem sheets) will be provided to the students
during the teaching of this course.
• The Text and Reference Books as identified in the First Day
Material uploaded on College Web-Site are used to prepare the
course material alongwith the material collected from other sources.
Introduction
• Electricity is the most preferred used form of energy used in
industry, homes, businesses and transportation.
• It can be easily and efficiently transported from the
production centers to the point of use. It is highly flexible in
use as it can be converted to any desired form like
mechanical, thermal, light, chemical etc.
• An electrical power system is made up of many components
connected together to form a large, complex system that is
capable of generating, transmitting and distributing electrical
energy over large areas.
The structure of electrical power system
• The first commercial electric power generating station
was installed in New York, USA on September 4, 1882
A.D. by Thomas Edition with the total power output of
33kW. Since then its growth has been continuously
increasing all over the world. And still this rate is 6%-7%
in most of the developing countries.
• The function of an electric power system is to generate,
transmit and distribute electric energy in an economical,
safe, reliable, environmentally compatible and socially
acceptable manner.
• A power system is usually divided into three parts:
generation, transmission and distribution system.
• Generation
 Electricity is produced by converting the mechanical energy
into electrical energy. In majority of cases, the mechanical
energy is either obtained from thermal energy or provided by
the flowing water.
 The main sources of thermal energy sources are coal, natural
gas, nuclear fuel and oil. The use of non-fossil fuels such as
wind, solar, tidal, and geothermal and biogas in electricity
generation is also increasing.
• Hydro-power is the main non-thermal source of mechanical
energy used in electricity generation. The conversion of
mechanical to electrical energy is done using synchronous
generators in majority of power plants. Few wind generation
systems use induction generators.
• The power is usually generated at low voltage, between 11 and 35
KV, and then fed into the transmission system using a step up
transformer.
• Normally the generating stations have;
Fuel supply system, prime movers associated with control devices,
AC generators, step up transformers, Station monitoring and
measurement devices, grounding and protection devices, station
auxiliaries and power supply systems.
 Transmission system
• The electricity is generated in bulk in the generating stations and
then transmitted over long distances to the load points.
• The transmission system interconnects all the generating
stations and major load centers in the system. It forms the back
bone of the power system.
• Since the power loss in a transmission line is proportional to the
square of line current, the transmission lines operate at the
highest voltage levels, usually 220 KV and upwards.
• Usually the transmission network has a meshed structure in
order to provide many alternate routes for the power to flow
from the generators to the load points. This improves the
reliability of the system.
• High voltage transmission lines are terminated at substations.
• Very large industrial customers may be provided power directly
from these substations. At these substations, the voltage is
stepped down to a lower level and fed into the sub-transmission
system.
• This part of the transmission system connects the high voltage
substation through step down transformers to distribution
substation.
• Typically the sub-transmission voltage levels are from 66 KV to
132 KV. Some large industrial consumers may be served directly
from the sub-transmission system.
• The transmission lines connect the neighboring power systems
at transmission levels, thus forming a grid.
• The grid is the network of multiple generating resources and
several layers of transmission network. The operation of power
systems in interconnected (grid) fashion has many advantages.
Distribution system
• The distribution represents the final stage of power transfer
to the individual consumer.
• The distribution network is generally connected in a radial
structure.
• The primary distribution voltage is typically between 11 KV
and 33 KV.
• Small industrial customers are supplied by primary feeders
at this voltage level.
• The secondary distribution feeders supply residential and
commercial at 415/240 V.
• Small generating plants located near the load centers are
usually connected to sub-transmission or distribution system
directly.
Power System Symbols
The following set of standard symbols are used for electrical diagrams of a
power system.
Single line diagram
Power systems are usually three phase balanced systems. They are
represented by simple diagrams called a single-line or one-line
diagram. It indicates by a single line and standard symbols how the
transmission lines and associated apparatus of an electric system are
interconnected. It provides the information about the systems in a
very concise way.
Power System Layout of a Typical Power Supply
Network
Key Features of Typical Power System
• Generation is usually in the 11-36 kV range and generators
feed power to the transmission grid, sub-transmission or
primary distribution network.
• The transmission grid usually operates at voltages in the range
of 110-765 kV.
• Connections to the other grids are done at the transmission
voltage level.
• Sub-transmission system operates normally in the 66-132 kV
voltage range.
• Primary distribution is usually in the range of 11-34.5 kV while
secondary distribution has the voltage range of 110-400 V.
Isolated and Interconnected Systems
• In the era of the development of power systems, these were
operated as isolated systems. The each city or region of a
country had its own electrical power system without any
connection to the power systems of other neighbouring
region.
• Due to many technical and economical reasons, they have
been put to operate in interconnected fashion. Few of the
advantages are;
Quality:
• The voltage profile of the transmission network improves as
more generators contribute to the system, resulting in an
increased total system capability. This also improves the
frequency behavior of the system following any load
perturbation due to increased inertia of the system.
Economy:
• In interconnected systems, it is possible to reduce the total
set of generating plants required to maintain the desired level
of generation reserve. This results in reduction of operational
and investment costs. Also, operational (including plant startups and shut down) and generation scheduling of units can be
more economically coordinated.
Security:
• In case of emergency, power can be made available from the
neighboring systems and each system can benefit even when
individual spinning reserves may not be sufficient for isolated
operation.
Synchronized operation:
• The electricity supply over the entire system is synchronized
and a common frequency exists in the interconnected system.
• Presently, Saudi Arabia’s network is operating at 60 Hz as its
common frequency, while in past, when operating as isolated
power systems, some systems operated at 50Hz and others at
60Hz frequency.
Challenges of Interconnected Systems
• Power systems having different frequencies may not be
interconnected. However, with the use of DC link as
interconnection enables the interconnection of power
systems operating at different frequencies but it involves
additional cost and new technology requirement.
• As system becomes more complex due to increased size.
• The total current during faults (short circuits) becomes very
large.
• Control of system frequency and voltage at various buses
and power flow calculations becomes very challenging.
• Sometimes a fault in one part of the network can spread to
the neighbouring systems in a cascading manner causing
black outs.
Choice of Transmission Voltage
• The choice of the transmission voltage is very important as
it effects; the weight of the conductors, power losses, the
transmission efficiency, line voltage drop, the maximum
loading capability of the line, right of way and other system
equipment.
• For a given current density, (i) the power loss in a line is
inversely proportional to the square of the system voltage
and (ii) the transmission efficiency increases with the
increase of voltage.
• The percentage voltage drop due to line impedance and the
weight of the conductor used is inversely proportional to
the system voltage.
• The maximum power that can be transferred is
proportional to the square of the voltage, therefore, system
stability is improved at higher voltage.
•
•
•
•
There are few disadvantages of using high voltage
transmission systems.
The cost of insulation for higher voltage lines increases.
The cost of mechanical supporting structures and right-of-way
for the line increases
The cost of transformers, switchgear and other terminal
equipment increases significantly.
Corona and electric field effects are encountered.
Keeping these merits and demerits, the major power lines are
designed to operate at optimum voltage level which can be
calculated for specified values of transmitted power and the
line length.
• When regional or national grids are interconnected, voltage
levels which are close are standardized at one level to achieve
many benefits.
• In Saudi Arabia, the following nominal values of voltage levels
are standardized ;
• Primary Distribution
: 13.8 kV and 33/34.5 kV
• Secondary Distribution
: 127/220 V and 220/380 V
• Sub-transmission
: 69 kV, 110kV and 132kV
• Transmission
: 220 kV and 380 kV
Electric Power Generation
• Law of conservation of energy states that “The energy can
neither be created nor destroyed”. It can change forms.
• Therefore, at an electric power generation station, energy is
converted from other forms to the electric form.
• Majority of the fuels are converted to mechanical energy by a
prime mover.
• The prime mover provides this mechanical energy input to an
AC synchronous generator or drives the generator, the output
of which consists of sinusoidal voltages and currents.
• The economically available fuel used so far for power
generation is very important.
Types of Fuels
• Coal
• It is most abundant and economical fossil fuel.
• However, burning of coal introduces smoke and pollution as well
as flue gases; sulfur and carbon dioxides into the atmosphere.
• Carbon dioxide is considered as a green house gas since it traps
the sun’s energy in the earth’s atmosphere thereby leading to a
gradual global warming.
• Sulfur dioxide results in acid rain.
• Oil
• It is second most popular fuel for electric power generation.
• However, it has similar environmental effects as the coal.
• The rapid rate of its consumption may lead to its fast depletion in
future.
• Natural gas
• It is the cleanest fuel and has become most popular for electric
power generation.
• However, it has finite available sources and the rising demand in the
long run is not expected to be fulfilled.
• Water
•
•
•
•
Energy of flowing water is used for hydroelectric generation.
It is the free source of energy available at many places.
However, it has limited resources and mostly depends on rain falls.
Moreover, the amount of local solar days available are limited at
many locations.
• These some factors and high cost of required technology has limited
the use of this source for electric power generation.
• However, the on going developments in this area are encouraging .
• Nuclear fuel
• Nuclear fission fuels are available in plenty.
• However, technology for processing such fuels and generating
electricity with such fuels is expensive and restricted to technically
advanced countries.
• There are serious health hazards associated with radio active
emissions.
• The concerns about long term waste disposal methods, risks of
leakage emissions leakage safety for aging plants are high.
• Solar radiation
• It is main source of energy for life on earth and is really abundant.
• However, energy density in sun light is comparatively low and its
availability is limited to day time only.
• Most suitable and feasible sites for hydroelectric generation are
generally located very far away from load centres so the costs of site
development and the transmission and distribution network
required are high.
• Wind
• It is a free source of energy, but is unpredictable and is mostly
fluctuating .
• However, interest in this source is growing and consequently its
development is now receiving more attention, a trend which is
expected to continue.
• Bio-mass
• These fuels are relatively new and are economically promising.
• However, their production technology needs more developments
before bio-mass is put for its large scale use.
• The Current and Projected Share of Fuel Sources
World Wide for Electric Power Generation
• It is shown in Table below;
Fuel
1995
19999
2010
2020
Oil
9.6%
9.9%
9.1%
10.0%
Natural
gas
17.7%
18.8%
22.2%
26.5%
Coal
36.6%
34.1%
33.4%
30.9%
Nuclear
16.5%
17.0%
14.6%
12.0%
Renewable 19.7%
20.2%
20.7%
20.6%
Types of Power Plants
• The most important types of generating stations presentlt in use
are;
•
•
•
•
Thermal generating stations
Hydro generating stations
Nuclear generation stations
More than 95% of the electric energy generated worldwide
is generated by these three types of stations.
• In Saudi Arabia and other Arab Gulf Countries , about 100%
of electric energy is produced by thermal generating plants.
• In USA, the ratio of thermal, hydro and nuclear energy
produced in 2000 AD was about 65%, 13% and 22%
respectively.
Thermal Power Plants
• The burning of fuel produces heat energy. This heat energy is
converted into mechanical energy by using one of the following
alternate methods.
A. In a conventional power plant, energy from coal, oil and natural
gas fuel is used to produce steam by heating water in a boiler.
The steam is then used to produce mechanical power with the
help of steam turbine which acts as a prime mover for the AC
generator.
Fig. 3.1 shows the main components of a steam turbine power
plant. Whereas, Fig. 3.2 describes the schematic diagram of a coal
fired thermal generating plant.
A thermal plant based on oil or gas and using steam turbine
technology will be similar in principle except for the fuel handling
and combustion parts. To meet the huge amount of water
required , these plants are located near lakes, seas or rivers.
B. Second type of thermal power plants use oil or gas as fuel.
When the fuel is burnt in the combustion chamber, the high
temperature gases are produced which are used to drive the gas
turbine.
such gas turbine then works as prime mover for the AC generator.
These thermal power plants are termed as Gas Turbine Thermal
Plant and can be installed at any place since they do not need large
quantity of water.
The components of such plants are shown in Fig. 3.3.
C
In a third type of thermal power plant, both gas and steam turbine
methods are combined to achieve a better overall thermal
efficiency.
In the first stage the fuel is burnt to produce hot gases to drive the
gas turbine.
The heat from the hot exhaust gases of the gas turbine is extracted
using a heat exchanger and is then used for production of steam,
which is used to drive a steam turbine.
Such stations are known as Combined Cycle Thermal Power Plants
and have better efficiency than the conventional steam or gas
turbine thermal plants alone.
Fig. 3.4 shows the schematic diagram of such plants.
D Another type of power generating station using diesel oil as a
fuel is sometimes used.
In such plants, an internal combustion diesel engine is used as
a prime mover to drive the AC generator.
Such plants exist usually only in industrial, health related and
other governmental facilities for use as emergency or standby-units.
E Sometimes co-generation thermal plants are also used.
In such plants, the waste energy from steam turbine type
thermal plant is used in factories for process heat or other
purposes.
Thus the plant produces electricity as well as hot water
and/or steam for use in industrial applications.
Hydro power plants
• Hydro power plants use the energy of running water to drive hydro
turbines.
• Dams are built to create water reservoir on rivers and lakes where
hydraulic turbines are installed. Such turbine works as prime mover
for the AC generator.
• In some cases, small hydro plants are installed on small rivers and
canals.
• Other forms of hydro plants can convert portion of energy in the sea
waves and tides into electrical energy.
Nuclear power plants
• Such power plants have controlled nuclear fission reaction to
generate heat in the nuclear reactors. The heat is used to produce
steam for steam turbine.
• Therefore, nuclear plants are essentially of steam turbine type.
• The fuel used in the plants is usually uranium or plutonium.
• In a conventional nuclear power plant, heavy isotopes are
broken into two or more lighter isotopes in a controlled
manner.
• If the mass of heavy isotopes is m1 and the combined mass of
resulting lighter isotopes is m2, then the heat energy
produced by such a reaction is equal to (m1-m2) C*C where
C= speed of light.
• Therefore, by using a small quantity of nuclear fuel, very large
amount of energy can be produced.
Renewable Energy Sources Based Power Plants
Hydro electric power plants, Wind energy plants, Solar energy
power plants/photo-electric plants, Bio-mass power plants,
Geothermal power plants.
Principle of AC Generation
• Generally 3-phase AC synchronous generators are used in power
systems.
• These generators have a rotating part (rotor) which has the DC field
winding and a stationary part (stator) which has the AC armature
winding.
• Therefore, the DC field winding rotates and the AC generating
coils are stationary.
• A schematic of the cross-section of cylindrical rotor type
synchronous generator is shown in Fig. 3.5.
• Here the slots in the rotor carry the DC field winding which is
distributed along the rotor periphery so as to produce a nearly
sinusoidal distribution of flux density in the air gap.
• The stator shows 3-coils for 3-phases.
• The placements of upper coil sides of the phases are marked by
a,b,c and lower sides by a’, b’, c’, respectively.
• These generators are normally used in thermal power plants and
are called turbo-generators.
• Another type of generator uses salient pole type rotor construction.
• A 4-pole, salient rotor type, synchronous generator is shown below.
• These machines normally have many poles whereas cylindrical rotor
machines have mostly 2-poles.
• Referring the cylindrical rotor type synchronous generator;
 When the DC field current If is passed, flux density B is set-up
in the air gap.
 This has a sinusoidal spatial distribution around the rotor
periphery with a maximum value of Bmax.
 When the rotor is rotated with a constant speed (N rpm)in
clock wise direction, 3-phase AC voltages of positive phase
sequence, sinusoidal waveforms and frequency f are induced
in different stator coils.
The rms magnitude E of the induced emf per phase under no load
condition can be given as;
• Thus, for a 2-pole (cylindrical rotor type) and 12-pole (salient pole
rotor type) machines, the required values of N at 6o Hz are 3600
rpm and 600 rpm respectively.
• Due to this simple reason, thermal power plants employ
cylindrical rotor type, high speed generators whereas hydro and
wind power plants employ salient pole rotor type, low speed
machines.
• Again, due to the mechanical stresses at high and low speeds,
cylindrical machines have longer rotors of smaller diameters
whereas salient pole type machines employ shorter rotors of
larger diameters.
Per phase circuit of round rotor synchronous
generator
• The open circuit terminal voltage of a synchronous is equal to the
induced emf, E, given by equation (3.1).
• However, when the generator is loaded and the stator currents
flow, they will modify the flux linkages and consequently the
terminal voltage is modified for each phase as;
Field Control of AC Generator
 Isolated Generator



It is clear from eqn(3.1) that open circuit voltage of Ac generator
depends on design parameters (k).
Also, E depends on prime mover speed N and maximum flux
density Bmax.
Usually, N or f (frequency) is kept constant and emf is controlled
by DC excitation or field current If. Which determines Bmax
 If we operate the machine in linear region of hysteresis loop, E α If.
 Besides, the terminal voltage of an isolated generator also depends
on R, X, and Ia as well as its phase angle.
Field control of Interconnected Generators
• In power systems, generators are usually operate in interconnected
or synchronized fashion with other generators.
• Therefore, different generators feed their share of power to the
system to which various loads are also connected and the entire
system operates at the same frequency.
• It is useful to study the behavior of one generator when
synchronized to an infinite bus.
• An infinite bus is an ideal voltage source.
• When any generator is connected to a very large system, such that
the total available generation in the power system is much larger
than the power rating of the one generator, the system can be
assumed as an infinite bus.
• In this case, any single generator cannot affect the frequency or the
voltage of entire system appreciably.
• To connect or synchronize a 3-phase AC generator with an infinite
bus, the following conditions must be satisfied (Refer Fig.3.8);
• In this condition, if we change the field current (If ), of the incoming
generator, it will change the induced emf(E) of this generator.
• However, it will not affect the terminal voltage of infinite bus (V).
• Therefore If controls the magnitude and phase angle of the
generator’s current in reference to its terminal voltage.
Power Control of AC Generator
• For an isolated system, field current is used to adjust the internal
induced emf.
• For interconnected system, this control is used to adjust the
reactive power output and the operating p.f. of the generator.
• For active power control, assume the AC generator is synchronized
and is connected to infinite bus. The per phase equivalent circuit is
shown in fig. 3.9.
• Assume that R =0, induced emf Ea =E (angledelta) and Va =
V(angle(o).