Internship Report

ON
Protection & Instrumentation
OF
132KV Grid Station Qasim Pur Colony
Multan
Submitted By:
M.Dewan iqbal (2k19-ELE-109) NFC IET

Nadeem Hayat (2k19-ELE-125) MULTAN

M.Asif (2k19-ELE-124)
Submitted To:
Engr. Sohaib (P&I Division Multan)
TRANSFORMERS:
Power Transformer Definition
A power transformer is a static machine used for transforming power from one circuit
to another without changing the frequency. As there is no rotating or moving parts, a
transformer is classified as a static device. Transformer operates on an AC supply.
Transformers operate based on the principle of mutual induction.

Use of Power Transformers

Generation of electrical power in low voltage level is very much cost effective.
Theoretically, this low voltage level power can be transmitted to the receiving end.
This low voltage power if transmitted results in greater line current which indeed
causes more line losses.

Types of Transformers
Step Up Transformer and Step Down Transformer
Step up transformers converts the low voltage (LV) and high current from the
primary side of the transformer to the high voltage (HV) and low current
value on the secondary side of the transformer.

Step down transformers converts the high voltage (HV) and low current from
the primary side of the transformer to the low voltage (LV) and high current
value on the secondary side of the transformer.

Three Phase Transformer and Single Phase Transformer

A three phase transformer is generally used in three phase power system as it
is more cost effective than single phase transformers. But when size matters, it
is preferable to use a bank of three single phase transformer vs a three phase
transformer, as it is easier to transport than one single three phase transformer
unit.
Electrical Power Transformer, Distribution Transformer and
Instrument Transformer
Power transformers are generally used in transmission network for stepping
up or down the voltage level. It operates mainly during high or peak loads
and has maximum efficiency at or near full load.
Distribution transformer steps down the voltage for distribution purpose to
domestic or commercial users. It has good voltage regulation and operates 24
hrs a day with maximum efficiency at 50% of full load.
Instrument transformers include C.T and P.T which are used to reduce high
voltages and current to lesser values which can be measured by conventional
instruments.

Two Winding Transformer and Autotransformer

A two winding transformer is generally used where the ratio between the
high voltage and low voltage side is greater than 2.

It is more cost effective to an autotransformer in situations where the

ratio between the high voltage and low voltage sides is less than 2.

Outdoor Transformer and Indoor Transformers

As the name suggets: outdoor transformers are designed for installing
outdoors.
Whereas indoor transformers are designed for installing at indoors (who
would have guessed!).
Oil Cooled and Dry Type Transformer
This classification pertains to the transformer cooling system used
within the transformer.
In oil cooled transformers, the cooling medium is transformer oil.
Whereas in the dry type transformer, air cooling is used instead.

Core type Transformer

There are two main types of transformer windings – core type, and shell-
type. There are also berry type transformers.

A core type transformer has two vertical legs or limbs with two
horizontal sections named yoke. The core is rectangular in shape with a
common magnetic circuit. Cylindrical coils (HV and LV) are placed on
both the limbs.
Shell Type Transformer
A shell type transformer has a central limb and two outer limbs. Both
HV, LV coils are placed on the central limb. The double magnetic
circuit is present.
 Parts of a Power Transformer

Basic Parts of a Transformer

These are the basic components of a transformer:

1. Laminated core
2. Windings
3. Insulating materials
4. Transformer oil
5. Tap changer
6. Oil Conservator
7. Breather
8. Cooling tubes
9. Buchholz Relay
10. Explosion vent
 Core

The core acts as a support to the winding in the transformer. It also

provides a low reluctance path to the flow of magnetic flux. It is made of
laminated soft iron core in order to reduce eddy current loss and
Hysteresis loss.

Why Are Windings Made of Copper?

 Copper has high conductivity. This minimizes losses as well as the

amount of copper needed for the winding (volume & weight of
winding).
 Copper has high ductility. This means it is easy to bend conductors into
tight windings around the transformer's core, thus minimizing the
amount of copper needed as well as the overall volume of the winding.

Winding

Two sets of winding are made over the transformer core and are
insulated from each other. Winding consists of several turns of copper
conductors bundled together and connected in series.

Winding can be classified in two different ways:

1. Based on the input and output supply
2. Based on the voltage range

Insulating Materials

Insulating paper and cardboard are used in transformers to isolate

primary and secondary winding from each other and from the
transformer core.

Transformer oil is another insulating material. Transformer oil performs

two important functions: in addition to insulating function, it can also
cool the core and coil assembly. The transformer's core and winding
must be completely immersed in the oil. Normally, hydrocarbon mineral
oils are used as transformer oil. Oil contamination is a serious problem
because contamination robs the oil of its dielectric properties and
renders it useless as an insulating medium.

Conservator

The conservator conserves the transformer oil. It is an airtight, metallic,

cylindrical drum that is fitted above the transformer. The conservator
tank is vented to the atmosphere at the top, and the normal oil level is
approximately in the middle of the conservator to allow the oil to expand
and contract as the temperature varies. The conservator is connected to
the main tank inside the transformer, which is completely filled with
transformer oil through a pipeline.

Breather

The breather controls the moisture level in the transformer. Moisture can
arise when temperature variations cause expansion and contraction of
the insulating oil, which then causes the pressure to change inside the
conservator. Pressure changes are balanced by a flow of atmospheric air
in and out of the conservator, which is how moisture can enter the
system.

Tap Changer

The output voltage of transformers varies according to their input

voltage and the load. During loaded conditions, the voltage on the output
terminal decreases, whereas during off-load conditions, the output
voltage increases. In order to balance the voltage variations, tap
changers are used. Tap changers can be either on-load tap changers or
off-load tap changers. In an on-load tap changer, the tapping can be
changed without isolating the transformer from the supply. In an off-
load tap changer, it is done after disconnecting the transformer.
Automatic tap changers are also available.

Cooling Tubes

Cooling tubes are used to cool the transformer oil. The transformer oil is
circulated through the cooling tubes. The circulation of the oil may
either be natural or forced. In natural circulation, when the temperature
of the oil rises, the hot oil naturally rises to the top, and the cold oil sinks
downward. Thus the oil naturally circulates through the tubes. In forced
circulation, an external pump is used to circulate the oil.

Buchholz Relay

The Buchholz Relay is a protective device container housed over the

connecting pipe from the main tank to the conservator tank. It is used to
sense the faults occurring inside the transformer. It is a simple relay that
is operated by the gases emitted during the decomposition of transformer
oil during internal faults. It helps in sensing and protecting the
transformer from internal faults.

Explosion Vent

The explosion vent is used to expel boiling oil in the transformer during
heavy internal faults in order to avoid the explosion of the transformer.
During heavy faults, the oil rushes out of the vent. The level of the
explosion vent is normally maintained above the level of the
conservatory tank.

Transformer Protection

Transformer Protection for Different Types of Transformers

The protection system used for a power transformer depends on the
transformer's categories. A table below shows that,

Transformer Rating - KVA

Category
1 Phase 3 Phase

I 5 - 500 15 - 500

II 501 - 1667 501 - 5000

III 1668 - 10,000 5001 - 30,000

IV > 10,000 >30,000

  Transformers within the range of 500 KVA fall under (Category I
& II), so those are protected using fuses, but to protect
transformers up to 1000 kVA (distribution transformers for 11kV
and 33kV) Medium Voltage circuit breakers are usually used.
 For transformers 10 MVA and above, which falls under (Category
III & IV), differential relays had to be used to protect them.

Additionally, mechanical relays such as Buchholtz relays, and sudden

pressure relays are widely applied for transformer protection. In
addition to these relays, thermal overload protection is often
implemented to extend a transformer’s lifetime rather than for detecting
faults.

Common Types of Transformer Protection

1. Overheating protection
2. Overcurrent protection
3. Differential Protection of Transformer
4. Earth Fault Protection (Restricted)
5. Buchholz (Gas Detection) Relay
6. Over-fluxing protection

Overheating Protection in Transformers

Transformers overheat due to the overloads and short circuit conditions.
The allowable overload and the corresponding duration are dependent on
the type of transformer and class of insulation used for the transformer.

Higher loads can be maintained for a very short amount of time if it is

for a very long, it can damage the insulation due to temperature rise
above an assumed maximum temperature. The temperature in the oil-
cooled transformer is considered maximum when its 95*C, beyond
which the life expectancy of the transformer decreases and it has
detrimental effects in the insulation of the wire. That is why overheating
protection becomes essential.

Overcurrent Protection in Transformer

The overcurrent protection system is one of the earliest developed
protection systems out there, the graded overcurrent system was
developed to guard against overcurrent conditions. power distributors
utilize this method to detect faults with the help of the IDMT relays. that
is, the relays having:

1. Inverse characteristic, and

2. Minimum time of operation.

The capabilities of the IDMT relay is restricted. These sorts of relays

have to be set 150% to 200% of the max rated current, otherwise, the
relays will operate for emergency overload conditions. Therefore, these
relays provide minor protection for faults inside the transformer tank.

Differential Protection of Transformer

The Percentage Biased Current Differential Protection is used to protect
power transformers and it is one of the most common transformer
protection schemes that provide the best overall protection. These types
of protection are used for transformers of rating exceeding 2 MVA.

The transformer is star connected on one side and delta connected the
other side. The CTs on the star side are delta-connected and those on the
delta-connected side are star-connected. The neutral of both the
transformers are grounded.
Buchholz (Gas Detection) Relay

The above picture shows a Buchholz relay. The Buchholtz relay is

fitted in between the main transformer unit and the conservator tank
when a fault occurs within the transformer, it detects the resolved gas
with the help of a float switch.

If you look closely, you can see an arrow, gas flows out from the main
tank to the conservator tank, normally there should not be any gas in the
transformer itself. Most of the gas is referred to as dissolved gas and
nine different types of gasses can be produced depending on the fault
condition. There are two valves at the top of this relay, these valves are
used to reduce the gas build-up, and it's also used to take out a gas
sample.
 Over-fluxing Protection
A transformer is designed to operate at a fixed flux level exceed that flux
level and the core gets saturated, the saturation of the core causes
heating in the core that quickly follows through the other parts of the
transformer that leads to overheating of components, thus over flux
protection becomes necessary, as it protects the transformer core. Over-
flux situations can occur because of overvoltage or a reduction in system
frequency.

To protect the transformer from over-fluxing, the over-fluxing relay is

used. The over-fluxing relay measures the ratio of Voltage / Frequency
to calculate the flux density in the core. A rapid increase in the voltage
due to transients in the power system can cause over fluxing but
transients die down fast, therefore, the instantaneous tripping of the
transformer is undesirable.

What is Switchgear?
Definition of Switchgear: The apparatus used for switching, controlling
and protecting the electrical circuits and equipment is known as
switchgear.

Essential Features of Switchgear

The essential features of switchgear are :
1. Complete Reliability
2. Absolutely certain discrimination
3. Quick operation
4. Provision for manual control

Classification of Switchgear
Switchgear can be classified on the basis of the voltage level into the
following
1. Low voltage (LV) Switchgear
2. Medium voltage (MV) Switchgear
3. High voltage (HV) Switchgear

Switchgear Equipment
Switchgear covers a wide range of equipment concerned with switching
and interrupting currents under both normal and abnormal conditions. It
includes switches, fuses, circuit breakers, relays, current transformer,
and other equipment.
Read switchgear in the substation for more details.
A brief account of these devices is given below.
1. Switches
A switch is a device which is used to open or close an electrical
circuit in a convenient way. It can be used under full-load or no-load
conditions but it cannot interrupt the fault currents.
 When the contacts of a switch are opened, an arc is produced in the air
between the contacts. This is particularly true for circuits of high voltage
and large current capacity.
The switches may be classified into
1. air switches
2. oil switches
The contacts of the former are opened in the air and that of the latter is
opened in oil.
i. Air-break switch – It is an air switch and is designed to open a circuit
under load. In order to quench the arc that occurs on opening such a
switch, special arcing horns are provided. Read different types of air
break switch.
ii. Isolator or disconnecting switch – It is essentially a knife switch and
is designed to open a circuit under no load.
iii. Oil switches – As the name implies, the contacts of such switches are
opened under oil, usually transformer oil.

Air Break Switch

2. Fuses
A fuse is a short piece of wire or thin strip which melts when excessive
current flows through it for sufficient time. It is inserted in series with
the circuit to be protected.
When a short circuit or overload occurs, the current through the fuse
element increases beyond its rated capacity. This raises the temperature
and the fuse element melts (or blows out), disconnecting the circuit
protected by it.

Electrical Fuses Switchgear

Read: What is a FUSE and How it Works?

3. Circuit Breakers
A circuit breaker is an equipment which can open or close a circuit
under all conditions viz. no-load, full load and fault conditions. It is so
designed that it can be operated manually (or by remote control) under
normal conditions and automatically under fault conditions. For the
latter operation, a relay circuit is used with a circuit breaker.
Low Voltage Circuit Breakers: Miniature Circuit Breaker, Moulded
Case Circuit Breaker, Residual Current Circuit Breaker, Ground Fault
Circuit Interrupter
High Voltage Circuit Breakers: Vacuum Circuit Breaker, SF6 Circuit
Breaker, Oil Circuit Breaker, Air Blast Circuit Breaker
Read Circuit Breaker – Operating Principle and Arcing Phenomenon.
4. Protective Relays
Protective relays are vital parts of the switchgear equipment.
A relay is a device which detects the fault and supplies information to
the breaker for circuit interruption.
The function of a protective relay is to initiate a signal to circuit breakers
for disconnecting the elements of the power system when it develops
a fault.
When a fault occurs the relay contacts are closed and the trip coil of the
circuit breaker is energized to open the contacts of the circuit breaker.
There have been rapid developments in relaying technology during the
last two decades. The most important advancement has been due to the
advent of computer technology which has helped in the development of
numerical relays.
5. Instrument Transformers
Instrument transformers (current transformer and voltage transformer)
are used in switchgear installations for the measurement of electrical
parameters for protection and metering purposes.
An instrument transformer in which the secondary current is
substantially proportional to the primary current and differs in phase
from it by approximately zero degrees is called a current transformer
(CT).
A voltage transformer (VT) is an instrument transformer in which the
secondary voltage is substantially proportional to the primary voltage
and differs in phase from it by approximately zero degrees.
6. Surge Arresters
Surge Suppressors or Surge Arrestors are very important components
of switchgear and substation installations.
These are used to protect the substation equipment from temporary over-
voltages, switching impulses, and lightning impulses, and to a certain
extent, very fast transient over-voltages.
Historically, spark gaps with air insulation were used as surge
suppressors. Lightning arrestors, surge capacitors, surge suppressing
reactors, and thyrite resistors with series gaps were used in the past for
this purpose.
Innovation in this field has resulted in the advent of two commonly used
types, viz. the metal oxide-based (ZnO) type and C-R type of surge
arrestors/ suppressors.
7. Auto Reclosures and sectionalisers
Auto reclosures and sectionalisers are used in the distribution
networks of medium voltage switchgear up to 33 kV class.
These equipment are useful for the fast automatic restoration of supply
following transient faults in the system. The faults may be due to
frequent lightning surges and in areas where power lines run through
forests and trees.
These types of equipment are extensively used in the continents of
America and Australia due to their distinct advantages over conventional
switchgear.
8. Disconnect Switch / Isolator
Disconnectors (Isolators) are devices which are generally operated off-
load to provide isolation of main plant items for maintenance, on to
isolate faulted equipment from other live equipment.
Air Insulated or open terminal disconnectors are available in several
forms for different applications.