T&D System
• The demand for energy increases over time as populations
grow and create a need for more homes, factories, office
buildings, consumer products, and public infrastructure. To
meet this demand, electricity must not only be produced, but
also transmitted to areas where people live.
Like any other industry, the electric power system may be
thought of as consisting of three main divisions:
• 1. Manufacture, production or generation, cogeneration,
• 2. Delivery or transmission and distribution,
• 3. Consumption.

DISTRIBUTION SYSTEM

The grid of today
• Transmission network
• To transport the electric power
from the point of generation to
the load centers
• All above a certain voltage
• (Sub transmission)
• Distribution network
• To distribute the electric power
among the consumers
• Below a certain voltage

Planning and Design
• New transmission and distribution systems are planned and
designed by a variety of professionals. Transmission planning
engineers use information collected by utility forecasters to
identify when and where more power lines are needed for the
system to operate reliably. Siting, land rights, and permitting
agents work with government agencies and landowners to
acquire land and the rights to use land. CAD designers design
the layouts for new transmission and distribution lines and
facilities.

Construction and Maintenance
• Crews of line workers and utility workers build and fix
transmission and distribution lines. Substation mechanics
build and maintain the substations that step power down from
power lines to be distributed for residential and commercial
use. Millwrights help install and secure large
equipment. Machinists fabricate special tools that are needed
to construct and operate facilities.

Operation and Control
• The transmission of electricity across the grid from generation
plants to distribution facilities is done in control rooms
by system control operators. Reliability coordinators ensure
that enough energy is available from generating facilities and
elsewhere in the grid to meet demand. They also respond to
and help resolve emergency conditions. When power outages
occur, outage coordination dispatchers coordinate the
response and send workers out to fix the problems.

Safety and Regulations
• Electricity is an essential component to modern life, but is also
very dangerous. There are many regulations that govern the
safety and reliability of power. Safety and occupational health
coordinators coordinate safety programs. Training and
development specialists lead classes that teach employees
how to do their jobs safely and responsibly.

The distribution system is
particularly important to an
electrical utility for two reasons
• it’s close proximity to the customers
• it’s high cost of investment

Mainly distribution systems
are two types
• Primary Distribution (33KV/11KV)
• Secondary Distribution (11KV/440V)
Household electricity is alternating current (AC)
Household voltages are typically 120V or 240V

Distribution System Considerations
design of distribution systems, three
broad classifications of choices need
to be considered:
• The type of electric system: dc or ac, and if ac, single-
phase or poly phase.
• The type of delivery system: radial, loop, or network.
Radial systems include duplicate and throw over systems.
• The type of construction: overhead or underground.

• The type of electric system: dc or ac, and if ac, single-
phase or poly phase.
• The type of delivery system: radial, loop, or network. Radial systems
include duplicate and throw over systems.
• The type of construction: overhead or underground.

Types of Delivery Systems:
• Primary distribution
• Secondary distribution

Primary distribution
• which carries the load at higher than utilization voltages from
the substation (or other source)to the point where the voltage
is stepped down to the value at which the energy is utilized by
the consumer.

Primary distribution systems
include three basic types:
• Radial systems, including duplicate and throwover systems
• Loop systems, including both open and closed loops
• Primary network systems

Secondary distribution
• which includes that part of the system operating at utilization
voltages, up to the meter at the consumer’s premises.

• The maximum generation voltage in advanced countries is 33 kV
while that in India is 11 kV. The amount of power that has to be
transmitted through transmission lines is very large and if this
power is transmitted at 11kV the line current and power loss will be
large. There fore the voltage is stepped to a higher level by using
step-up transformers located in sub-stations.

Distribution system is consist of:
• Substation
• Utility or Distribution Pole
• Primary wires
• Cross arm
• Insulators
• Lighting Arrestor
• Cut out
• Transformer
• Neutral wire
• Secondary wire
• Grounding
• Guy wire

Substation

Substation
• A substation is a part of an electrical generation, transmission,
and distribution system.
• Substations may be owned and operated by an electrical
utility, or may be owned by a large industrial or commercial
customer.

Utility pole

Utility pole
• A utility pole is a column or post used to support overhead
power lines and various other public utilities, such
as cable, fibre optic cable, and related equipment such
as transformers and street lights.

• Most utility poles are made of wood, pressure-treated with
some type of preservative for protection against rot, fungi and
insects. Southern yellow pine is the most widely used species
in the United States; however, many species of long straight
trees are used to make utility poles, including Douglas-fir, Jack
pine, lodge pole pine, western red cedar, and Pacific silver fir.

Cross arm
• The woods most commonly used for crossarms are Douglas Fir
or Longleaf Southern Pine because of their straight grain and
durability. The top surface of the arm is rounded so that rain
or melting snow and ice will run off easily. The usual cross-
sectional dimensions for distribution crossarms are 3-1/2
inches by 4-1/2 inches; their length depending on the number
and spacing of the pins.

Cross arm

Insulator

Insulator
• An electrical insulator is a material whose internal electric
charges do not flow freely, and therefore make it very hard to
conduct an electric current under the influence of an electric
field. A perfect insulator does not exist, but some materials
such as glass, paper and Teflon, which have high resistivity, are
very good electrical insulators.

Types of Insulators
• Pin type insulator
• Suspension insulator
• Strain insulator
• Shackle insulator
• Line post insulator
• Station post insulator
• Cut-out

Pin type insulator
• As the name suggests, the pin type insulator is mounted on a
pin on the cross-arm on the pole. There is a groove on the
upper end of the insulator. The conductor passes through this
groove and is tied to the insulator with annealed wire of the
same material as the conductor. Pin type insulators are used
for transmission and distribution of electric power at voltages
up to 33 kV. Beyond operating voltage of 33 kV, the pin type
insulators become too bulky and hence uneconomical.

Pin type insulator

Suspension insulator
• For voltages greater than 33 kV, it is a usual practice to use
suspension type insulators shown in Figure. Consist of a
number of porcelain discs connected in series by metal links in
the form of a string. The conductor is suspended at the
bottom end of this string while the other end of the string is
secured to the cross-arm of the tower. The number of disc
units used depends on the voltage.

Suspension insulator

Strain insulator
• A dead end or anchor pole or tower is used where a straight
section of line ends, or angles off in another direction. These
poles must withstand the lateral (horizontal) tension of the
long straight section of wire. In order to support this lateral
load, strain insulators are used. For low voltage lines (less than
11 kV), shackle insulators are used as strain insulators.
However, for high voltage transmission lines, strings of cap-
and-pin (disc) insulators are used, attached to the crossarm in
a horizontal direction.

Strain Insulator

Shackle insulator
• In early days, the shackle insulators were used as strain
insulators. But now a day, they are frequently used for low
voltage distribution lines. Such insulators can be used either in
a horizontal position or in a vertical position. They can be
directly fixed to the pole with a bolt or to the cross arm.

Shackle insulator

Conductor
• a conductor is an object or type of material that permits the
flow of electrical current in one or more directions. For
example, a wire is an electrical conductor that can carry
electricity along its length.

Conductor

Lightning Arresters
• A lightning arrester is a device used on electrical power
systems and telecommunications systems to protect the
insulation and conductors of the system from the damaging
effects of lightning. The typical lightning arrester has a high-
voltage terminal and a ground terminal. When a lightning
surge (or switching surge, which is very similar) travels along
the power line to the arrester, the current from the surge is
diverted through the arrestor, in most cases to earth.

Lightning Arresters

Wire Sizes
• In the United States, it is common practice to indicate wire
sizes by gage numbers. The source of these numbers for
electrical wire is the American Wire Gage (AWG) (otherwise
known as the Brown & Sharpe Gage). A small wire is
designated by a large number and a large wire by a small
number

American Standard Wire Gage
(AWG)

American Standard Wire Gage
(AWG)

Cut out
• a fuse cutout or cut-out fuse is a combination of a fuse and a
switch, used in primary overhead feeder lines and taps to
protect distribution transformers from current surges and
overloads. An overcurrent caused by a fault in the transformer
or customer circuit will cause the fuse to melt, disconnecting
the transformer from the line. It can also be opened manually
by utility linemen standing on the ground and using a long
insulating stick called a "hot stick".

Cut out

Transformers
• transfers energy between two or more circuits
through electromagnetic induction.

transformers

Phone/Cables Wires
• These wires were typically copper, although aluminium has
also been used, and were carried in balanced pairs separated
by about 25 cm (10") on poles above the ground, and later
as twisted pair cables. Modern lines may run underground,
and may carry analog or digital signals to the exchange, or
may have a device that converts the analog signal to digital
for transmission on a carrier system.

Grounding
• Electrical circuits may be connected to ground (earth) for
several reasons. In mains powered equipment, exposed metal
parts are connected to ground to prevent user contact with
dangerous voltage if electrical insulation fails. Connections to
ground limit the build-up of static electricity when handling
flammable products or electrostatic-sensitive devices.

Guy wire
• A guy-wire or guy-rope, also known as simply a guy, is a
tensioned cable designed to add stability to a free-standing
structure. They are used commonly in ship masts, radio
masts, wind turbines, utility poles, fire service extension
ladders used in church raises and tents.

Guy wire

Guy wire

Overhead Power Line
• An overhead power line is a structure used in electric power
transmission and distribution to transmit electrical energy
along large distances. It consists of one or
more conductors (commonly multiples of three) suspended
by towers or utility poles. Since most of the insulation is
provided by air, overhead power lines are generally the
lowest-cost method of power transmission for large quantities
of electric energy.

Overhead Power Line

Overhead power transmission lines are
classified in the electrical power industry by
the range of voltages:
• Low voltage (LV) – less than 1000 volts, used for connection
between a residential or small commercial customer and the utility.
• Medium voltage (MV; distribution) – between 1000 volts (1 kV) and
to about 33 kV, used for distribution in urban and rural areas.

• High voltage (HV; subtransmission less than 100 kV;
subtransmission or transmission at voltage such as 115 kV and
138 kV), used for sub-transmission and transmission of bulk
quantities of electric power and connection to very large
consumers.
• Extra high voltage (EHV; transmission) – over 230 kV, up to about
800 kV, used for long distance, very high power transmission.
• Ultra high voltage (UHV) – higher than 800 kV.

Underground wires
• Undergrounding refers to the replacement of overhead
cables providing electrical power or telecommunications, with
underground cables.
• Underground cables take up less right-of-way than overhead
lines, have lower visibility, and are less affected by bad
weather. However, costs of insulated cable and excavation are
much higher than overhead construction. Faults in buried
transmission lines take longer to locate and repair.

Underground wires

ARELIABILITYTESTSYSTEMFOREDUCATIONAL
PURPOSES
-BASICDISTRIBUTIONSYSTEMDATAANDRESULTS
• It describes an electrical distribution system for use in
teaching power system reliability evaluation. It includes all the
main elements found in practical systems. However, it is
sufficiently small that students can analyze it using hand
calculations and hence fully understand reliability models and
evaluation techniques. The paper contains all the data needed
to perform basic reliability analyses. It also contains the basic
results for a range of case studies and alternative
design/operating configurations.

IEEE Application of
Probability Methods (APM)
• Subcommittee published a Reliability Test System (RTS) in
1979 .This has proved to be a valuable and consistent
reference source for copying alternative techniques and
computer programs. It has been used extensively in recent
years in reliability assessment of generation systems and in
composite systems by utilities, consultants and universities. Its
major advantage is that it provides a consistent set of data,
since extended in Refs [4,5], enabling a wide range of
techniques and applications to be much more easily compared
than previously possible. It is sufficiently large that practical
factors can be realistically modeled and assessed but also
sufficiently small that the effect of sensitivity studies can be
easily identified. The major weakness of the RTS is that it
requires the web of computer programs to perform the vast
majority of the reliability analyses.

DESCRIPTIONOF THE DISTRIBUTION
NETWORKS
• The RBTS has 5 load bus bars (BUS2-BUS6). Two of these bus
bars (BUS2 and BUS4) were selected and distribution
networks designed for each. BUS2 has generation associated
with it and BUS4 does not. This permits the effects and
differences caused by the generation and transmission system
on the overall load point indices to be seen. The peak loads
defined in the RBTS for different customer types are shown in
Table 1.

SYSTEM STUDIES
• The studies performed include:
11kV feeders. These studies consider the 11kV feeders
only and ignore any failures in the 33kV system, the 33/11kV
substation and the 11kV breakers. They assume the 11kV source
breaker operates successfully when required, disconnects are
opened whenever possible to isolate a fault, and the supply
restored to as many load points as possible using appropriate
disconnects and the alternative supply if available.

33kV system. These studies evaluate the reliability indices at the
11k V supply point busbars. They ignore any failures on the
incoming 33kV supply circuits. They include the effect of passive
and active failures [8] on all components from the 33kV busbars
down to the 11kV supply point busbars together with active
failures on the outgoing 11kV feeder breakers.

This paper has presented an extension to the EUBTS by
providing all the basic data for teaching reliability assessment of
distribution systems. All the networks, 33kV and 11kV, can be
analyzed using hand calculations, permitting full understanding
and use of the basic models and evaluation techniques.
Students can then either use existing computer programs or
develop their own in order to analyze more practical systems
and to perform an increasing number of sensitivity studies. A
selected number of results are included in this paper in order to
give confidence to students in their endeavors. These should
first be repeated at the initial stage of the teaching program.
They can then be followed by a greater number and range of
studies.