Internship Report
SUBMITTED TO: RESIDENT ENGINEER AMJAD
HUSSAIN
SUBMITED BY
MUHAMMAD YASIR KHALIL
REGISTRATION # 15PWELE4807
DEPARTMENT OF ELECTRICAL ENGINEERING
(POWER)
STUDENT OF UET PESHAWAR
INTERNSHIP AT PESHAWAR UNIVERSITY GRID STATION, DANISH
ABAD PESHAWAR |
CONTENTS
1) Introduction
a) Power system
b) Grid station
c) Single line diagram
2) Switch Yard
3) Tower
4) Green Tree
5) Lightning arrester
6) Wave Trap
7) Line Isolator along with Earth Switch
8) Circuit Breaker
a) Quenching Media
b) Operating Media
9) Current Transformer:
10) Potential Transform:
11) Grounding:
12) Bus bar
13) Transformer Bay
14) Power transformer
a) Core
b) Winding
c) Main Tank
d) Conservator Tank
e) Breather
f) Buchholz’s Relay
g) Cooling System
h) Tape Changer
i) Oil Drain Valve
j) Oil Filter Valve
15) Relays
16) Capacitor Bank
17) Battery
1) Introduction:
University Grid 132kV is located at Danish Abad Peshawar. It is powered by the
following Four Transmission lines :
PUN-1: Peshawar Cantt Grid
JMR-2: Jamrud Grid Peshawar
PIN-3: Small industrial Kohat Road Grid
PUN-4: Sheikh Muhammad Grid
a) Power system:
The electrical system in which different systems are connected to transfer the
power from the source to consumer. It is divided into
 Generation
 Transmission
 Distribution
The electrical power is achieved by using different source of energy. Hydro Energy
is the cheapest source of energy, so the turbine produces 11.5 kV or 6.6 kV at
generation station. This voltage is step up to 500 kV to reduce the power losses.
500 kV is used as primary transmission then it step down to 220 kV which is
known as secondary transmission then again step down to 132 kV which is
known as primary distribution, 66 kV is also included in primary distribution but
gradually it is obsoleting. This voltage is again step down to 11kV which is known
as secondary distribution.
b) Grid Station:
Grid station is a place where many circuits are interconnect for the purpose of
controlling and voltage regulations.
c) Single Line Key Diagram:
The grid system is consisting of 3 phase 3 wire system. We draw the circuit
diagram of grid station for the purpose of studying the power flow and fault
detection. If we draw the
circuit diagram for many
incoming and outgoing
circuit so it will be very
complicated for us to
analyze.
To
remove
complexity, we draw the
circuit diagram as a single
line diagram in which 3
phase 3 wire system is
represented as a single
line. We use symbols for
different components, which is international accepted. The single line key diagram
of University Grid station is given below.
2) Switch Yard:
The region in which different components are physically connected for the sake of
protection, controlling and distribution of Electrical power. The components which
are used in switch yard is listed below.
3) Tower:
Tower is a long stand like a Minar which carry the transmission line from one
station to another. Towers are usually at free spaces like fields and not in towns to
avoid hazards from magnetic field
of high voltage transmission.
Towers are triangular in shape
having Shielding wire at top and
transmission lines at its arms. Shield
wire protect the tower from thunders
and acts as a neutral wire. Towers
have Insulation Discs which is at the
end of one transmission line and
beginning of adjacent transmission
line. Discs provide insulation to
tower as a result being a conductor,
tower do not conduct high voltage to
the ground. Each disc represents
11kv. Counting number of discs
connected to a line will show the
voltage across the respective
transmission line. Transmission
lines have vibrations due to high
voltage across it or due to wind, to
avoid such vibrations Damper weight / Balancing weight are use which result in
regulation of vibrations.
4) Lightning / Surge Arrester:
It provide voltage regulation to the system and keep constant voltage by removing
the excess voltages which may occur due to thunder lightening.
5) Line Isolator along with Earth Switch:
Line isolator along with earth switch helps in
protection of the grid system. It isolates the
grid electrical machinery from the input
supply of 132kV during fault or tripping
condition. Isolator work on no load.
On full load it should not operate because
high arc will produce which is dangers for
human body !
6) Circuit Breaker:
Circuit breaker provide isolation to the grid station in
case of tripping. The breaker operates on DC and DC
is provided by DC System . It can operate in faulty
condition means abnormal current or at full load or at
no load because it has quenching media. At full load it
produces arcs and this quenching media helps to
absorb the arc. In 11kv circuit breakers we mostly use
vacuum tube in which there is no flam produce in
vacuum because of absences of oxygen.
There is different type of circuit breakers like:
 Vacuum circuit breaker
 Air blast circuit breaker
 Oil filed circuit breaker
Vacuum Cricuit Breaker
 SF6 circuit breaker.
Due to high voltage there is a spark inside
the circuit breaker. To absorb or eliminate
the spark we use different quenching media
such as SF6(Sulfur Hexa Flouride) Gas in
modern circuit breakers while oil in old age
breakers.
7) Current Transformer:
Current transformer is used to step down the
current. At University Grid station the CT
ratio is 400/5 that means it convert 400 A to
5 A. Protection is very important for any high rated device. CT step down the
current to 5A which is then use by different Relays as they operate on 5A only. CT
helps in protection of the system through Relays. Through CT we measure the
input and output current and we can detect the fault if incase occur through its
readings or directly from Auxiliary panel of respected CT in control room. CT lies
on each line. Use of current transformer is due to following reasons:
a) Protection
b) Measurement
c) Metering
8) Potential Transformer:
Potential Transformer converts 132 kV to 110 V which is then suitable for Bus Bar
and other power cables to work on. Secondly for single common Bus Bar there will
be only one PT while there could be many CTs depending upon circuits connected
to bus bars. Use of potential transformer is due to following reasons:
a) Protection
b) Measurement
c) Metering
9) Grounding:
Earth is a protection element. Grounding provides a very low resistive path to the
flow of high current in an abnormal condition. Each equipment is grounded in
University grid station for safety and protection purpose and there is an earth mesh
under the yard and the bodies of all devices are grounded on different points in the
yard. And also each pole is grounded in the yard. The yard has floor made from
crushed stone and sand. The following are the reason why we use ground like that :
a)
b)
c)
d)
10)
In a Rainy day we can easily walk in the yard
It has property of rigidity
It is highly insulated material
It has less chances of formation of standing water in rainy weather.
Bus Bar:
Bus Bar is a set of conductors in the grid station
which energizes the transformers. Here in Uni
Grid station the bus bar energizes four
transformers but one of them is damaged and will
repair by the company soon. The voltage on the
bus bar is constant, here we have 132kV bus bar at
the yard. Bus bar is perpendicular to the line bay.
Bus Bar may be of 132Kv or 11Kv.
11)
Transformer:
Transformer comprises of following units.
a)
b)
c)
d)
CT
132 kV circuit Breaker
Lightening Arrester
Power transformer
We are familiar with the work of CT as it can step down the current from 400A to
5A so that the Relay woks with their rated current. Likewise, Circuit Breakers
provide protection and act as a switch to disconnect the grid system when needed.
Now sometime thunderstorm take place which are hazardous to whole system, to
avoid the effects of Surge Voltages and Thunder voltage we use Lightening
Arrester. Lightening Arrestor act as protection element for the Power transformer.
12)
Power transformer:
Power transformers at Uni Grid Station has an apparent power of 40 MVA, 31.5/40
MVA. At normal condition it can give us power approximately about 31.5MVA
and if we turn ON the fans it can give us upto 40MVA. Power transformer works
on the principle of Mutual Induction. There are various important parts of power
transformer:
a) Core:
Core of transformer is made up of Ferromagnetic Material which has the property
to get quickly magnetize and help in Quick Mutual Induction phenomena.
b) Winding:
There are two main important windings in Transformer, Primary and Secondary
but sometime Tertiary winding is also present in auto transformer
.
c) Main Tank:
Main tank contain core with windings and is filled of
special type of Oil (Difloro Dichloro ethyl benzene)
which act as a cooling reagent. The oil has latent heat
of vaporization higher than any other liquids though
less than water. We cannot use water as a cooling
reagent because it has the ability to corrode the Core
which made up of Pure Iron.
d) Conservator Tank:
This tank also contains Oil of similar nature and is at the top
of Power Transformer. This tank provides additional oil to
the Main tank when it is needed. Conservator tank is not
fully filled, it has 30% free space in upper portion, which is
filled with air by Breather when the oil from conservator
tank moves to main tank.
The above is the cross section of conservator tank.
e) Breather:
Breather is a tube which contain oil and Silica
gel(SiO2). Silica gel is hygroscope agent which helpe
in absorbing the moisture in air. Breather breathe the
air when conservator tank provide oil to the main tank.
This is due to low pressure at the top of conservator
tank and air moves from outside to inside of Power Transformer.
f) Buchholz’s Relay:
Buchholz’s Relay is installed between main tank and conservator tank. It detect
interior fault of the transformer. It operates when a bubble is formed inside the oils
then it tries to evaporate and then the Buchholz relay perform its function and the
bubble is captured by relay and if the size of the bubble is small then alarm
operates and P&I team visits the grid and if the bubble is large then it trip the
transformer and also alert the staff by alarm.
g) Cooling System:
There are two types of nature of Power
transformer for cooling purpose.
i.
ii.
Oil Nature
Air Nature
Oil is use as a cooling reagent whereas
air is also use as cooling reagent. Oil is
pass through pipes horizontally which is
Cooling Fan
known as radiators. Tubes are exposed
to high power fans that create strong cool blow towards the pipes and cool the oil
inside it. Cooling fans are on either side of the transformer in order to confirm the
Radiator
cooling nature of power transformer.
h) Tape Changer:
Tap Changer is a device which is used to change the turn ratio. usually we install
the tap changer at primary side of the step down transformer (11kV) because
primary side contain high voltage (132kV) but low current and 11kV at the
secondary side of the power transformer. And it is controlled remotely from the
control room .If it is doesn’t operate from the Control Room then we operate it
from the tap changer panel at the transformer side if it fails then we operate
manually by using a specific tool which is inside the tap changer panel.
i) Oil Drain Valve(DN-80):
It is also called bottom valve. It is used to extract
water vapours from the main tank by connecting a
dehydration plant and the oil of the main tank
passes through the dehydration plant and it filters
the oil by extracting the water vapours this
phenomena is just like dialysis and it happens for
3 days.
j) Oil Filter Valve(DN-40):
It is used for testing purposes. When P&I team visits
the grid stations they want to test the Quality of the
Oil and Quantity of vapours in it. Using DN-40 they
collect a sample of the transformer oil. Then they
analyze the sample and perform the required
operation.
Relays:
a) Buchholz’ Relay: Buchholz relay are of two types;
Main Tank Buchholz Relay:
Buchholz’s Relay is installed between main tank and conservator tank. It detect
interior fault of the transformer. It operates when a bubble is formed inside the oils
then it tries to evaporate and then the Buchholz relay perform its function and the
bubble is captured by relay and if the size of the bubble is small then alarm
operates and P&I team visits the grid and if the bubble is large then it trip the
transformer and also alert the staff by alarm.
Tap Changer Buchholz Relay:
It detects the fault inside the housing (shell like) of the tap changer winding of the
transformer.
b) Impedence /Distance Relay: The relay whose working depends on the distance
between the impedence of the faulty section and the position on which relay
installed is known as the impedence relay or distance relay.
It divides the distance in zones ie
Zone 1: it operate on 85% area. It works in 20msec
Zone 2: it operate on 130% area. It works in 0.75msec
Zone 3: it operate on 150% area. It works in 0.35msec
d) Differential Relay: It is installed between 132kV and 11Kv current
transformer. When it detect the difference in current between 11kV and
132kV CT it operates.
d)Over Current Relay: An overcurrent relay is a type of protective relay
which operates when the load current exceeds a pickup value. It is of two
types:
Instantaneous over current (IOC) relay
Definite time overcurrent (DTOC) relay
e) Earth Fault relay: Earth Fault Relay (EFR) It is a
safety device used in electrical installations with high
earth impedance. It detects small stray voltages on the
metal enclosures of electrical equipment. The result is to
interrupt the circuit if a dangerous voltage is detected.
Capacitor Bank:
Capacitor Bank is for improving
power Factor. Improving Power
factor means reducing losses and
reduce reactive power. It also
decreases the load and provide
Relief. Each Transformer has its
capacitor Bank. Number of cells
in capacitor bank depends upon
the Power Rating of Transformer
13)
Battery:
200KVAR Small
Size cell
Batteries are used to operate the DC system of the grid station because all the
relay, panels and circuit breaker works on DC. If the DC fails then we have to shut
down the grid station because we will not be in such a state to operate the grid
station. then we will not tackle the fault as a result it may damage the grid station
in faulty condition because the relays sense faults and
400KVAR
then the circuit breaker operates and circuit breaker and
Capacitor cell
relays need DC supply.
These batteries are rechargeable. They are charged by
Battery charger which take three phase AC and Rectify it
to 110 V DC. Batteries are connected in series to provide
110 V DC.
The electrolyte use in these batteries is combination of Distillated water and
sulphuric acid in ratio of 1 and 4 respectively. We have to check the specific
Gravity every morning with Hydrometer.
1 cell produce 2 – 2.5 volts
110 volts we use 55 cells connected in series.
Overall battery capacity is 150AH which means if we have 10A load we can
supply power to it for 15 hours, non-stop or 150Amps for an hour continuously.