1. Internship Report
SECTION: ED (Electrical Distribution)
(13-07-2015 to 14-07-2015)
Submitted By:
Taimoor Muzaffar Gondal
University:
Comsats Wah
Submitted To:
Assistant Manager
ED (A-34)

2. AKNOWLEDGMENTS
First of all, I want to express all my humble thanks to ALLAH who is very
sensitive about each and every activity Of all his man and without whose help, I
am unable to accomplish any objective in my life. Secondly, I am great full to my
worthy and devoted teacher for providing me the Opportunity of doing internship
in POF. I am also thankful to all other Teachers as the knowledge imparted by
them Enable me to study the organization in a best way. I am also thankful to my
friend Zafran Khan Jadoon, Ali Hassan who helps me in any problem in the
preparation of internship report. I am also thankful to all staff of Electrical
Distribution Section.

3. Table of Contents
ED 1 ………………………………………………………………………………….………………………………………………….
Over headed lines …………………………………………………………………………………………………………………
Conductors ……………………………………………………………. ……………………………………………………………
Protection devices…………………………………………………………………………………………………………………
Relays……………………………………………………………………………………………………………………………………..
Substation ………………………………………………………………………………….…………………………………………
Step Down Transformers………………………………………………….………………........…………………………….
Grid station …………………………………………………………………………………..……………………………………….
Single Line Diagram……………………………………………………………………..………………………………………..
Billing Section…………………………………………………………………………………………………………………………
Meter Maintenance………………………………………………………………………………………………………..……..
Symbols used in power System…………………………………………………………………………………………….
Pump maintenance……………………………………………………………………………………………………………….

4.  Experience At ED (A-34):
A review of Two day training at ED (A-34) section is given below.
o Single Line Diagram of power system.
o Identification of the symbols used in Single Line Diagram.
o Types of cables, breakers, relays, insulators etc.
o Billing system.
o Meter maintenance.
o Visit to substation 7A.
o Visit to the grid station.
o Better understanding of over headed Distribution and transmission system.

5. ED section (A-34)
there are two departments in ED (A-34)-:
i. ED 1
ii. ED 2
First of All ED 1…
ED 1 section:
This department deals with the
i. Over headed lines
ii. Substations
 Over headed lines:
An overhead power line is a structure used in electric power transmission and
distribution to transmit electrical energy along large distances.
Some of the main parts of an over headed lines are given below:
o Conductors
o Insulators
o Capacitor banks
o Fuses and other protection devices
o Distribution Transformers
 Conductors:
Conductors in over headed system are power cables and wire.
The following types of the power cable are mainly used for distribution purpose in the
medium-voltage (MV) range
.
o Paper-insulated lead covered (PILC) cables.
o Cross-linked polyethylene (XLPE) cable.
o PVC
PVC cables are used mainly in the low voltage (LV) range.
For LV distribution purposes, the choice is basically between XLPE and PVC-
insulated cable.
The XLPE cables are higher current ratings then PVC cables for the same
Conductor size. Normally cables tend to be slightly more expensive than PVC.

6.  Paper-insulated lead covered (PILC) cables.
o The PILC cables are manufactured by using lays of paper impregnated with
compound mineral oil as insulating medium, both as individual core and overall
Insulation.
o A lead sheath in constructed as an outer core layer to mainly provide a seal for
the compound in the paper layers.
o A steel tape layer is used main mechanical protection.
o The outer sheath may be a PVC layer or other type of insulating and waterproof
material.
 Cross-linked polyethylene (XLPE) cables:
o XLPE, PEX and PVC are used as conductor insulating material in these cables.
o XLPE is a semiconductor and provide partial insulation as well as electrical
stree relieving.
o The conductors with their XLPE layers are embedded in PVC to provide total
insulation.
o Steel wires are used for mechanical strength. And also could be used to provide
a path for earth fault current.

7. o The outer sheath in normally a PVC sheath.
 Insulator:
There are three main types of insulators, their names are given below:
o Pin type
o Shackle type
o Suspension type
 Pin Type:
o Pin Insulator is earliest developed overhead insulator, but still popularly used in
power network up to 33KV system.
o Pin type insulator can be one part, two parts or three parts type, depending upon
application voltage.
o In 11KV system we generally use one part type insulator where whole pin
insulator is one piece of properly shaped porcelain or glass.
o In order to obtain lengthy leakage path, one, two or more rain sheds or petticoats
are provided on the insulator body.
o These rain sheds or petticoats are so designed, that during raining the outer
surface of the rain shed becomes wet but the inner surface remains dry and non-
conductive. So there will be discontinuations of conducting path through the wet
pin insulator surface.
o For 33KV tow parts and for 66KV three parts pin insulator are generally used.

8.  Shackle Type Insulator:
o The shackle insulator or spool insulator is usually used in low voltage
distribution network.
o It can be used both in horizontal and vertical position.
o The use of such insulator has decreased recently after increasing the using
of underground cable for distribution purpose
o The tapered hole of the spool insulator distributes the load more evenly and
minimizes the possibility of breakage when heavily loaded. The conductor
in the groove of shackle insulator is fixed with the help of soft binding wire.
 Suspension Type Of insulator:
o In higher voltage, beyond 33KV, it becomes uneconomical to use pin insulator
because size, weight of the insulator become more. Handling and replacing
bigger size single unit insulator are quite difficult task. For overcoming these
difficulties, suspension insulator was developed.
o In suspension insulator numbers of insulators are connected in series to form a
string and the line conductor is carried by the bottom most insulator. Each
insulator of a suspension string is called disc insulator because of their disc like
shape.
o 1. Each suspension disc is designed for normal voltage rating 11KV so by using
different numbers of discs, a suspension string can be made suitable for any
voltage level.
o If any one of the disc insulators in a suspension string is damaged, it can be
replaced much easily

9.  Fuses And Protection Devices:
Protection devices consist of relays, circuit breaker and fuses etc.
 Circuit Breakers:
Electrical circuit breaker is a switching device which can be operated manually as well as
automatically for controlling and protection of electrical power system respectively.
Types of circuit breaker:
According to their arc quenching media the circuit breaker can be divided as-
1. Oil circuit breaker.
2. Air circuit breaker.
3. SF6 circuit breaker.
4. Vacuum circuit breaker.
 Oil circuit breaker.
o Mineral oil has better insulating property than air. In oil circuit breaker the fixed
contact and moving contact are immerged inside the insulating oil
o Whenever there is a separation of current carrying contacts in the oil, the arc in circuit
breaker is initialized at the moment of separation of contacts, and due to this arc the oil
is vaporized and decomposed in mostly hydrogen gas and ultimately creates a hydrogen
bubble around the arc.
o This highly compressed gas bubble around the arc prevents re-striking of the arc after
current reaches zero crossing of the cycle.
o The oil circuit breaker is the one of the oldest type of circuit breakers.
 Air Circuit Breaker Air:
o This type of circuit breakers is that kind of circuit breaker which operates in air at
atmospheric pressure.
o It creates an arc voltage in excess of the supply voltage.
o It may increase the arc voltage by cooling the arc plasma. As the temperature of arc
plasma is decreased, the mobility of the particle in arc plasma is reduced; hence more
voltage gradient is required to maintain the arc.
o Splitting up the arc into a number of series arcs also increases the arc voltage.

10.  SF6 Circuit Breaker:
A circuit breaker in which the current carrying contacts operate in sculpture
hexafluoride or SF6 gas is known as an SF6 circuit breaker.
o The attachment of electron with SF6 gas molecules may occur in two different ways,
o SF6 has excellent insulating property. SF6 has high electro-negativity. That means it has
high affinity of absorbing free electron.
o Whenever a free electron collides with the SF6 gas molecule, it is absorbed by that gas
molecule and forms a negative ion.
o That’s why this is used voltage and high voltage electrical power system.
o These circuit breakers are available for the voltage ranges from 33KV to 800KV and
even more.
 Vacuum Circuit Breaker:
A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes
place in vacuum. The technology is suitable for mainly medium voltage application.
o The operation of opening and closing of current carrying contacts and associated arc
interruption take place in a vacuum chamber in the breaker which is called vacuum
interrupter. The vacuum interrupter consists of a steel arc chamber in the center
symmetrically arranged ceramic insulators.

11. o The vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6
bar.
o The material used for current carrying contacts plays an important role in the
performance.
o Service life of vacuum circuit breaker is much longer than other types of circuit
breakers.
o There is no chance of fire hazard as oil circuit breaker.
o It is much environment friendly than SF6 Circuit breaker.
 Relays:
Here is a detailed list of the different types of relays.
1. Latching Relay:
o Latching relays are also called impulse relays.
o They work in the bitable mode, and thus have two relaxing states.
o They are also called keep relays or stay relays because as soon as the current
towards this relay is switched off, the relay continues the process that it was
doing in the last state.
o This can be achieved only with a solenoid which is operating in a ratchet and
cam mechanism
o In the ratchet and cam method, power consumption occurs only for a particular
time. Hence it is more advantageous than the others.
2. Reed Relay:
o These types of relays have been given more importance in the contacts.
o In order to protect them from atmospheric protection they are safely kept inside
a vacuum or inert gas.
o Though these types of relays have a very low switching current and voltage
ratings, they are famous for their switching speeds.
3. Buchholz Relay:
o This relay is actually used as a safety device.
o They are used for knowing the amount of gas present in large oil-filled
transformers.
o They are designed in such a way that they produce a warning if it senses either
the slow production of gas or fast production of gas in the transformer oil.
4. Overload protection Relay:
o As the name implies, these relays are used to prevent the electric motors from
damage by over current and short circuits.

12. o For this the heating element is kept in series with the motor. Thus when over
heat occurs the bi-metallic strip connected to the motor heats up and in turn
releases a spring to operate the contacts of the relay.
5. Mercury Wetted Relay:
o This relay is almost similar to the reed relay explained earlier
o . The only difference is that instead of inert gases, the contacts are wetted with
mercury.
o This makes them more position sensitive and also expensive. They have to be
vertically mounted for any operation.
o They have very low contact resistance and so can be used for timing
applications. Due to these factors, this relay is not used frequently.
6. Machine Tool Relay:
o This is one of the most famous industrial relay.
o They are mainly used for the controlling of all kinds of machines. They have a
number of contacts with easily replaceable coils.
o This enables them to be easily converted from NO contact to NC contact.
o Many types of these relays can easily be setup in a control panel. Though they
are very useful in industrial applications.
o The invention of PLC has made them farther away from industries.

13. Substation and Grid station
POF has its own generating station, but due to higher demand of electricity organization
also receives power supply from WAPDA.
 Sub-station:
o A substation is a part of an electrical generation, transmission, and
distribution system. Substations transform voltage from high to low, or the
reverse between the generating station and consumer.
o Generally substations are unattended, relying on SCADA for remote
supervision and control.
o A distribution substation transfers power from the transmission system to the
distribution system of an area. It is uneconomical to directly connect
electricity consumers to the main transmission network, unless they use
large amounts of power, so the distribution station reduces voltage to a level
suitable for local distribution.
o Substations generally have switching, protection and control equipment, and
transformers. In a large substation, circuit breakers are used to interrupt any
short circuits or overload currents that may occur on the network. Smaller
distribution stations may use recloser circuit breakers or fuses for protection
of distribution circuits. Substations themselves do not usually have
generators, although a power plant may have a substation nearby. Other
devices such as capacitors and voltage regulators may also be located at a
substation.
o A distribution substation transfers power from the transmission system to
the distribution system of an area. It is uneconomical to directly connect
electricity consumers to the main transmission network.

14.  SCADA:
SCADA (supervisory control and data acquisition) is a system operating with
coded signals over communication channels so as to provide control of remote
equipment (using typically one communication channel per remote station).
The control system may be combined with a data acquisition system by adding
the use of coded signals over communication channels to acquire information
about the status of the remote equipment for display or for recording functions.
These processes include industrial, infrastructure, and facility-based processes,
as described be
o Infrastructure processes may be electrical power transmission and
distribution, wind farms,
o A SCADA system usually consists of the following subsystems
o Remote terminal units (RTUs)
o Programmable logic controller (PLCs)
o telemetry
o human–machine interface
o A supervisory (computer) system, gathering (acquiring) data on the process
and sending commands (control) to the SCADA system
 Grid Station:
WAPDA Grid System:
Transmission System of WAPDA links lower generating stations and load
centers of the entire country creating one of the world's largest contiguous grid
system. The national grid, connecting hydel generation in the north and thermal
generation in mid country and the south, consists of a large network of
transmission lines and grid stations of voltage levels from 500kV to 33kV to
transmit Power to load centers throughout the country to serve over ten (10)
million consumers of various categories.

15.  Single Line Diagram of distribution system:
Stepwise Parts Of a Grid Station:
132KV -> isolators -> lightning arrestors -> sub isolators -> CT -> Circuit breakers ->
132KV/11KV -> Transformer -> panel -> substation -> consumer

16.  Symbols Used in the Distribution Sector:

17.  Billing and Metering Maintenance Section:
Official Consumers (employees) Approximately 11000
Private Consumers Approximately 5000
Commercial Users Approximately 1400
Industrial Users 7
Recovery of bill from official users is made by pay.
POF has lower rates per unit then WAPDA.
In maintenance section streets lights have been removed, and replace by the LEDs.
PUMP maintenance:
There are 31 tube wells under ED. This department also deals with the
maintenance of these pumps.