Republic of the Philippines
NUEVA VIZCAYA STATE UNIVERSITY
Bambang, Nueva Vizcaya
College of Engineering
INSTRUCTIONAL MODULE
IM NO.: EE 18-2S-2021-2022
College: Engineering
Campus: Bambang
DEGREE
PROGRAM
SPECIALIZATION
Bachelor of Science in
Electrical Engineering
COURSE NO.
EE 18
COURSE TITLE
YEAR LEVEL
5
TIME FRAME
Electrical Transmission and Distribution
System
12hrs WK NO.
1-2 IM NO.
1
I.
UNIT TITLE/CHAPTER TITLE
Generation, Transmission & Distribution of Electrical Energy
II.
LESSON TITLE
Overview of distribution system
III.
LESSON OVERVIEW
Typical Electric Power Supply Systems Scheme; Objective of an electric power system;
Combined Process of Power System; Components of Distribution System
IV.
DESIRED LEARNING OUTCOMES
1. Explain the Typical Electric Power Supply Systems Scheme.
2. Discuss the objective of an electric power system.
3. Explain the combined Process of Power System
4. Discuss the components of Distribution System
V.
LESSON CONTENT
Typical Electric Power Supply Systems Scheme
(Generation, Transmission & Distribution of Electrical Energy)
& Elements of Distribution System
An electric power system or electric grid is known as a large network of power generating plants
which connected to the consumer loads.
As, it is well known that “Energy cannot be created nor be destroyed but can only be
converted from one form of energy to another form of energy”. Electrical energy is a form of
energy where we transfer this energy in the form of flow of electron. So, electrical energy is obtained by
converting various other forms of energy. Historically, we have done it from chemical energy using cells
or batteries. However, as the invention of generator had occurred, it became the technique to first
convert some form of energy into mechanical form of energy and then converting it into electrical form
of energy using generator. Generators produce two type of power AC and DC. Nevertheless, 99% of
the present power systems use AC generators.
Electrical energy has grown immensely over two centuries because the flexibility it provides for
its use. The variety of use has led its demand to increase monotonously. However, as the load or
demand has increased practically one requirement is consistent. That is, we must generate the amount
required by the load at that very instant because this large amount cannot be stored for delivering this
high amount of demand.
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Therefore, the generation of electrical energy is happening simultaneously as we use it. In
addition, our demand is always varying. Therefore, the generation is also varying with it. Apart from
varying demand, the type of current we consume also varies. These variations put many constraints
and conditions. This is the reason of the complex and big control rooms across the whole power system.
The lines network between Generating Station (Power Station) and consumer of electric power
can be divided into two parts.
1.
Transmission System
2.
Distribution System
We can explore these systems in more categories such as primary transmission and secondary
transmission as well as primary distribution and secondary distribution. This is shown in the fig 1 below
(one line or single line diagram of typical AC power systems scheme).
It is not necessary that the entire steps which are shown
in the figure below, fig 1 must be included in the other power
schemes. There may be difference. For example, there is no
secondary transmission in many schemes, in other (small)
schemes of power system, there is no power transmission, but
only distribution.
The main objective of an electric power system is to
obtain electrical energy and make it reachable safely to the load
point where it is being used in usable form. This is done in five
stages namely
1.
Generating Station
2.
Primary Transmission
3.
Secondary Transmission
4.
Primary Distribution
5.
Secondary Distribution
Fig. 1
After these five levels, the energy must be available as the stated form in terms of voltage
magnitudes, frequency and consistency. Generation means the conversion of a form of energy into
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electrical energy. Transmission implies the transport of this energy to very long distance with very high
amount of voltage magnitude. Moreover, distribution is fulfilling the demand of the consumers at
certified voltage level and it is done in terms of feeders. Feeders are the small-small chunks of load
distributed at different places, physically.
Generation or Generating Station
The place where electric power produced by the parallel connected three phase
alternators/generators is called Generating Station (i.e. power plant).The ordinary power plant capacity
and generating voltage may be 11kV, 11.5 kV 12kV or 13kV. But economically, it is good to step up the
produced voltage from (11kV, 11.5kV Or 12 kV) to 132kV, 220kV or 500kV or more (in some countries,
up to 1500kV) by Step up transformer (power Transformer).
Generation is the part of power system where we convert some form of energy into electrical
energy. This is the source of energy in the power system. It keeps running all the time. It generates
power at different voltage and power levels depending upon the type of station and the generators used.
The maximum number of generators generate the power at voltage level around 11kV-20kV. The
increased voltage level leads to greater size of generator required and hence the cost involved.
Presently the generating stations we employ mainly over the world are following:i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
Thermal power plant
Hydel power plant (Hydro-electric)
Nuclear power plant
Diesel power plant
Gas power plant
Solar power plant
Tidal power plant
Wind power plant. Etc
We generate electric energy through these power plants at different voltage levels and at
different locations depending upon the type of the plant. They are used for different purposes viz.
Base load plant:- When the plant is used to handle the base load demand on the system
Peak load plant:- When the plant is made to handle the peak load demand on the system
Accordingly, the plant is made to handle the load. This categorization is important for the quality
of power is being developed. It is also important for the fact that the power must be generated at the
same instant when the load is taking up the power. So, as we know the type of load and approximate
amount of load at the station, different type of generating station is chosen.
For example; Thermal plant, Hydel plant, Nuclear plant, Solar plant, Wind plant and Tidal plant
are chosen to handle the base load on the system whereas Gas plants, Diesel plants are used to
handle peak load demand. This is mainly governed by the nature of the time they take in the process of
starting the delivery of power. Base load plants take more time in delivering the power whereas peak
load plants must start very fast to supply the demand.
Primary Transmission
The electric supply (in 132kV, 220 kV, 500kV or greater) is transmitted to load center by three
phase three wire (3 Phase – 3 Wires also known as Delta connection) overhead transmission system.
As the voltage level which is generated is around (11-20) kV and the demand is at various
levels of voltage and at very far away places from the generating station. For example, the generating
station can be generating voltage at 11kv, but the load center is 1000km apart and at the level of 440V.
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Therefore, for the delivery of electrical energy at such a long distance, an arrangement must be
there to make it possible. Hence, the transmission system is essential for the delivery of electrical
energy. This is made possible by using the transmission lines of different length. These are overhead
transmission lines in almost every cases. Some exceptions occur when it is needed to cross an ocean.
Then there is a compulsion to use underground cables.
But, as the system grew and load demand increased, the challenge in this process has become
very complex. At low voltage level, the amount of current flowing through the line for high load demand
is more and hence the voltage drop due to the resistance and reactance of the transmission line is very
significant. This leads to more losses in the transmission lines and the decrease in the voltage at the
load end.
This affects the cost of the system and the working of the equipment the consumers use. So,
transformer is used to increase the voltage level at certain values ranging from 220kV to 765kV. This
makes the current value lesser for the same load that would be having higher values of current at
certain load. The current value can be calculated using formula:-
Where,
= RMS value of line to line voltage
= RMS value of line current
* denotes the conjugate of a phasor.
The increased demand and the constraint of location of generating station has made possible
the need of a very complex system called ‘Grid’. This system connects multiple generating stations
generating voltage at different levels being connected together as a combined system.
This makes the system to reach out to various load centers and this provides a great system of
having higher reliability. Presently, this system has grown to size of a country. One more system is
being used now a days is the use of HVDC. HVDC is used for greater distances and sometimes used
to connect two grids of different voltage or frequency levels. HVDC also provides lower corona losses,
lower communication interferences, elimination of inductive effect and elimination of frequency of
operation.
Transmission lines vary in sizes. This size determines its characteristics and its behavior in the
system. For example, in long transmission lines the voltage at the consumer end becomes higher than
its rated value during light load condition due to the dominating capacitive nature of the transmission
lines.
Secondary Transmission
Area far from the city (outskirts) which have connected with receiving stations by lines is called
secondary transmission. At receiving station, the level of voltage reduced by step-down transformers up
to 132kV, 66 or 33 kV, and electric power is transferred by three phase three wire (3 Phase – 3 Wires)
overhead system to different sub-stations.
Primary Distribution
At a sub station, the level of secondary transmission voltage (132kV, 66 or 33 kV) reduced to
11kV by step down transforms.
Generally, electric supply is provided to those heavy load consumer (commercial power supply
for industries) where the demands is 11 kV, from the lines which caries 11kV ( in three phase three wire
overhead system) and they make a separate sub station to control and utilize the heavy power in
industries and factories.
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In other cases for heavier load consumers (at large scale), the demand is upto132 kV or 33 kV.
So electric supply provided them directly by secondary transmission or primary distribution (in 132 kV,
66kV or 33kV) and then step down the level of voltage by step-down transformers in their own sub
station for utilization ( i.e. for electric traction etc).
When the transmission lines get closer to the demand centers, the voltage level is reduced to
make it practical to distribute at different places of load. Therefore, power is taken from the grid and
stepped down to 30-33kV, depending upon the places where it is being delivered. This is then
transmitted to substations. For example, the system voltage at substation level in India is 33KV.
Many control mechanisms are provided in the substations to make the power delivery a
controlled and continuous process without much disturbance. These substations deliver power to
smaller units called ‘Feeders’. This is done by either ‘Overhead lines’ or ‘Underground cables’. These
feeders are in towns, cities, or villages or it may be some group of industries, which takes the power
from the substation, and convert its voltage level according to its own use.
For domestic use, the voltage is further reduced at 110V-230V (phase to ground) to be used by
the individuals at different power factor. The combined amount of demand is the load on the entire
system and that must be generated at that instant.
Depending upon the scheme of the distribution system, it is categorized as radial or ring mains.
It gives different degree of reliability and stability to the system. All these systems are protected using
various protection schemes comprising of circuit breakers, relays, lightening arresters, ground wires etc.
Many measuring and sensing elements are also associated like ‘Current transformer’ and
‘Potential transformer’ and metering at all the places from the substations to feeders to the consumers’
places.
Secondary Distribution
Electric power is transferred by (from primary distribution line i.e.11kV) to distribution sub station
is known as secondary distribution. This sub station is located near by domestic & consumers areas
where the level of voltage reduced to 440V by step down transformers.
These transformers called Distribution transformers, three phase four wire system (3 Phase – 4
Wires also known as Star connection). So there is 400 Volts (Three Phase Supply System) between
any two phases and 230 Volts (Single Phase Supply) between a neutral and phase (live) wires.
Residential load (i.e. Fans, Lights, and TV etc) may be connected between any one phase and
neutral wires, while three phase load may be connected directly to the three phase lines.
In short, secondary power distribution may be divided in three sections such as , feeders,
distributors and service lines (details below).
Combined Process of Power System
The entire structure of the power system is consisting of the source (Generating station),
transfer (Transmission and Distribution) and the load (Consumer). The objectives are:1.
Rated voltage and frequency to the load centres.
2.
Reliability of the system so that power delivery is continuous.
3.
Flexibility of the system so that the power is available at different voltage levels
4.
Faster clearance of faults so that the runs well for longer time and it life elongates
5.
The cost of power must be as low as possible
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6.
The losses in the system must be as low as possible
These all objectives are met by using different sets of generating stations, transmission systems,
distribution systems and the enhanced quality of safety equipment.
At any instant, our load varies in different magnitudes. Therefore, for following the demand, the
generation must change and catch up the demand. For this purpose, there are many control
mechanisms like governing valve in thermal plants, control rods in nuclear plants, which changes the
amount of power, which is being generated. And, for this purpose, there is a set of arrangements made
to communicate the demand to the generating station. They are PLC, SCADA, Fiber optical
communication, GSM communication etc.
In addition, some state estimation techniques are being used in a power system to predict the
load demand at different instant of times. It helps in determining the amount of power to be generated
at the right time. Now, with the advent of new techniques, a very promising technique is using ‘Soft
Computing techniques’ for the control of the operation of power system. In addition, it is accompanied
by various software and numerical techniques. Therefore, it can be stated that the steps in which a
power system operates are following:1.
2.
3.
4.
Load demand variation
Communication between substation and generating station
Control operations at the generating stations
Continuous evaluation at the substation for changes in demand
Modern power system operates and literally handles such a great amount of power supply by
these four basic steps. The more controlled the power delivered, the more will be the quality of power
because the quality of power is simply the maintenance of the rated value of voltage and frequency at
every place. This objective is obtained, only when the entire system operates in continuous
coordination and effectiveness.
As our load varies from lightly loaded condition to heavily loaded condition, the substation
communicates to generating station to increase the power generation and it keeps checking the
requirement so that the continuous delivery of power happens.
The communication is done according to the load magnitude and the cost involved in the
process. Moreover, this increase in demand is then acknowledged by the generating station by varying
its power input to the generator. In addition, from the generating station to load centers, various levels
(viz. Transmission and Distribution) are there.
Therefore, for the power quality and reliability, there are many devices are employed to
efficiently perform the various control mechanisms that consists the fault management systems, the
power factor improvement systems, the measurement systems etc.
All these operations are being done continuously in any power system around the world to make
the power delivery possible and efficient. With the increase demand, there happened the increase in
the inventions of various devices.
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In addition, the revenue collected from the power distribution made it possible to have further
invention and the use of new technologies. This enables us to use energy in such simple form whereas,
in reality, many complex operations are being done constantly.
Below is a complete typical AC electric power supply system scheme, in other words, the above
whole story in figure below.
OVERVIEW OF DISTRIBUTION SYSTEM
What is a Distribution System?
The part of the power system that distributes electric power for local use is called as distribution
system. Generally, a distribution system is the electrical system between the substation fed by
transmission system and the consumer’s meters. A typical distribution system is shown in the figure.
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Components of Distribution System
Distribution Sub-Station – A distribution sub-station is the electrical system which transfers
power from transmission system to the distribution system of an area.
Feeders – A feeder is a conductor which connects the distribution sub-station to the area where
power is to be distributed. The current in a feeder remains the same throughout its length
because no tapings are taken from it. The main consideration in the design of a feeder being its
current carrying capacity.
Distribution Transformers – The distribution transformer is a step-down transformer in which
primary and secondary are delta and star connected respectively. It is also termed as service
transformer. The output voltage of distribution transformer is 440 V in 3-phase system whereas
230 V in 1-phase system in India.
Distributor – A distributor is a conductor from which tapings are taken for supply to the
consumers. Due to the taping is done at various places in a distributor, the current being not
same throughout its length. The main design consideration of a distributor is the voltage drop
across its length because the statutory limit of voltage variations is ± 6 % of rated voltage at the
consumer’s terminals
Service Mains – Service Mains is a small cable which connects the distributor to the
consumer’s meter
Elements of a Distribution System
Secondary distribution may be divided into three parts as follow.
1.
Feeders
2.
Distributors
3.
Service Lines or Service Mains
Classification of Distribution System
Classification based on the nature of current −
i.
AC Distribution System
In most of the conditions, the power consumer or load requires AC power.
Therefore, the electric power is generated, transmitted, and distributed in the form of AC
power. Because an AC voltage can be easily step up and step down with the help of a
transformer.
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According to the voltage level, the AC distribution system is further classified into
two types;
1.
Primary distribution system
2.
Secondary distribution system
Primary Distribution System
The voltage level of a primary distribution system is higher than the utilization voltage
level. In most cases, the primary distribution system uses a three-phase three-wire system and
the voltage level is in the range of 3.3 kV, 6.6 kV, and 11 kV.
The primary distribution system supplies power to big consumers like industries or large
commercial complexes, etc. The voltage level is stepped down at the utilization level by a stepdown transformer. This transformer is placed near to the consumer premises.
The typical arrangement of a primary distribution system is as shown in the figure below.
Secondary Distribution System
In a secondary distribution system, the power is distributed at the utilization level. The
primary distribution system ends with a transformer that is used to convert 11 kV to 415 V. And
this power is directly distributed to the small consumers.
In most cases, the primary winding of this transformer is connected in delta connection
and secondary winding is connected in star connection to provide a ground terminal. Therefore,
the secondary distribution system uses a three-phase four-wire system.
Single-phase supply is taken by using any one phase with a neutral terminal that
provides a voltage magnitude of 230 V or 120V (according to country standards). For small
shops and residential purposes, a single-phase supply is used.
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Some consumers require three-phase supply like small scale industries, flour mills, etc.
This type of consumer uses a three-phase supply by using R, Y, B, and N terminals. The
arrangement for the secondary distribution network is shown in the figure below.
ii.
DC Distribution System
Most of the load connected to the power system is AC load. But there is a certain
application where we required DC power. To fulfill these applications, we use DC power
in the distribution system and this system is known as the DC distribution system.
In this condition, generated AC power is converted into DC power with the help of
a rectifier or rotary converter. The applications where we need DC power are; traction
purpose, DC motors, Battery charging, and electroplating.
According to the connection of DC system, it is classified into two types;
1.
Two-wire DC distribution system
2.
Three-wire DC distribution system
Two-wire DC Distribution System
This type of distribution system requires only two wires; one wire is at a positive potential
level and the second wire is at a negative or zero potential levels. The wire connected with a
positive potential level is known as a healthy wire.
The load is connected in parallel between two lines. Generally, the load connected in
this system is lamp-load or motors. The load has only two terminals can be connected in this
type of system. The connection diagram of this system is shown in the figure below.
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Three-wire DC Distribution System
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This type of distribution system requires three wires; two wires are healthy wires and one
wire is the neutral wire. The main advantage of this system is that we have two voltage levels in
this system.
Let say, two healthy wires are at a voltage level of +V and -V. The neutral wire is at zero
potential networks. If a load is connected between one healthy wire and a neutral wire, the
available voltage is V volt. And if a load is connected between both healthy wires, the available
voltage is 2V volt.
Hence, the load requires a higher voltage that is connected between healthy wires, and
the load requires a lower voltage that is connected between one healthy wire and a neutral wire.
The connection diagram of a three-wire distribution system is as shown in the figure below.
Classification According to Method of Connection
The distribution system is classified into three types according to the method of
connection;
1.
Radial system
2.
Ring main system
3.
Interconnected distribution system
Radial System
In a radial system, a separate feeder is used to feed power from the substation to each
area. And this feeder feeds power to a distributor in one direction only. The design of the radial
system is simple and easy to implement in the system.
The initial cost of this system is less compared to other systems. But the reliability of this
system is very low. If one feeder is out of step condition, the entire system will stop. This type of
system is only used for a short-distance distribution system.
The consumer far from the feeder may suffer poor voltage regulation and voltage
fluctuation with a variation of load. Due to this advantage, this type of system is only used to
supply the load which is near to the feeder. The single line diagram of the radial system is
shown in the figure below.
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Ring Main System
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In the ring main system, the distribution transformer is connected in a loop and supplied
by a substation from one end. It means each distribution transformer has two different ways to
connect with the substation. A single-line diagram of the ring main system is shown in the figure
below.
We can compare this arrangement with two feeders connected in parallel. For example,
if a fault occurs between B and C points. In this condition, the part between B and C will isolate
from the system. And substation supply power in two different ways.
It makes the system more reliable. And there is less voltage fluctuation at the
consumer’s end. Each part of the ring carries less current. So, less conductor material is
required compared to radial system.
Interconnected Distribution System
In an interconnected distribution system, a loop is supplied by more than one substation
at different points. This system is also known as a grid distribution system. The single-line
diagram of the interconnected system is shown in the figure below.
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Classification of Distribution System According to Type of Construction
According to the construction of distribution system is classified into two types;
1.
Underground distribution system
2.
Overhead distribution system
Underground Distribution System
As the name suggests, in this type of system, the conductors are placed under the
surface of streets or sidewalks. The underground distribution system is safer than the overhead
distribution system. But the initial cost of the underground system is very high. Because it
requires trenching, conduits, manholes, and special cables.
In this system, the cables lie under the streets. Hence, there are fewer chances of the
fault and it gives a good appearance as cables are invisible. When a fault occurs in the
underground system, it is difficult to locate and also it is difficult to repair. The life of an
underground system is very long. The useful life of this system is more than 50 years.
Overhead Distribution System
In the overhead distribution system, the conductors are mounted on wooden, concrete,
or steel poles. This system is also known as a conventional system for the distribution network.
The conductor is placed on the poles, there are more chances of fault and hazards compared to
the underground distribution system.
But the initial cost of the overhead system is less. The overhead transmission system is
more flexible compared to the underground system. Because this system is permanently placed
once installed. Hence, it is easy for load expansion and laying a new line. The conductors are
placed over the surface. So, the appearance of this system is not good as an underground
distribution system.
In the overhead system, the conductors are placed with taking proper space. Hence, the
air is used as an insulation medium. So, it does not require special insulated cables. The current
carrying capacity of the overhead system is higher compared to the underground system.
Requirements of a Distribution System
Some of the requirements of a good distribution system are −
Proper Voltage – The voltage variations at consumer’s terminals should be as low as possible.
The statutory limit of voltage variations is ± 6 % (India) of the rated voltage at
consumer’s
terminals.
Availability of Power on Demand – The electric power must be available to the consumers in
any amount that they may require from time to time.
Reliability – The modern industry is almost dependent on electric power for its operation. This
calls for reliable service as much possible.
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VI.
LEARNING ACTIVITIES
VII.
EVALUATION (Note: Not to be included in the student’s copy of the IM)
VIII.
ASSIGNMENT
IX. REFERENCES
Turan Gonen. Electrical Power Distribution. Third Edition
Westinghouse Electric Corporation: Electric Utility Engineering Reference Book-Distribution
Systems, Vol. 3, Westinghouse Electric Corporation, East Pittsburgh, PA, 1965.
NNVSU-FR-ICD-05-00
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