University of Technology

Electromechanical Engineering Department

unmanned aerial vehicle

2023/2024
Electrical transformers

BY (stage[1] group B uav)students:

‫فاروق عمر‬ ‫اواب محمود ابراهيم‬
‫محمد نزار‬
‫مريم صالح دغيم‬ ‫نور الهدى ماجد حاتم‬

Supervised by

D.Zainab Bashir

P a g e 0 | 17
introduction
An electrical transformer is one of the electrical devices
through which variable electrical energy (AC) is transferred
from one circuit to another, through the mutual
electromagnetic effect between two coils, with the
possibility of raising or lowering the voltage or current in
the second circuit. Since the transformer does not provide
any increase in capacity, it is possible to work to raise the
voltage in the second circuit at the expense of reducing the
current in the circuit. Compatible with it, and vice versa of
course. The operation of the transformer is based on the
principle of mutual influence of adjacent coils. Transformers
differ in the amount of electrical energy that can be
transferred from one circuit to another. They range from
large transformers used in electrical power transmission
and distribution networks that transmit power measured in
megawatts (MW), to very small transformers used. In
communications devices that transmit small power
measured in milliwatts, and through this article you will
learn about the transformers used in electrical devices,

Some types of transformers

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whose rated power ranges from one watt to some.
Hundreds of watts.
In this research, we will discuss: of transformers, their
components, working mechanisms, types, applications, and
causes of Transformer Failure, and how to reduce the
likelihood of failure

 COMPONENTS
1. Core
2. Winding
3. Insulation
4. Tank
5. Terminals and bushings
6. Transformer oil
7. Oil Conservator
8. Breather
9. Radiators and fans
10. Explosion vent
11. Tap Changers
12. Buchholz relay

1. Core
The core provides a low reluctance path for electromagnetic
flux and supports the primary and secondary windings. It is
made by stacking thin sheets of high-grade grain-oriented
steel which are separated by thin insulating material. In
order to minimize the hysteresis and eddy currents, the

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carbon content of the core steel is maintained below 0.1%.
When it is alloyed with silicon, eddy currents can be
reduced.
A typical three-phase transformer core is shown in the
picture above. Each limb carries the primary and secondary
winding of each phase. The limbs are magnetically coupled
by the yokes. There are two types of core constructions:
core type and shell type. In shell-type construction, the

windings are surrounded by the core as shown below:

2. Winding
The transformer carries two sets of winding per phase –
Primary winding and secondary winding. These winding
consists of several turns of copper or aluminum conductors,
insulated from each other and the transformer core. The
type and arrangement of winding used for transformers
depend upon the current rating, short circuit strength,
temperature rise, impedance, and surge voltages.
Out of the primary winding and secondary winding, the one
which is rated for higher voltage is known as High voltage

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(HV) winding and the other is known as Low voltage (LV)
winding.

The high-voltage winding conductors are thinner than the

low-voltage conductors and surround the LV winding from
the outside. The LV winding is placed close to the core.
In shell-type transformers, the winding is split into several
coils (few turns of a conductor). The HV coils are
sandwiched between the LV coils. Whereas in core-type
transformers, windings are classified into four types: Multi-
layer windings, Helical windings, Disc winding, and foil
winding. The choice of winding type is determined by the
number of turns and its current carrying capacity.

3. Insulation
Insulation is the most important part of transformers.
Insulation failures can cause the most severe damage to
transformers. Insulation is required between the windings
and the core, between windings, between each turn of the
winding, and between all current-carrying parts and the
tank. The insulators should have high dielectric strength,
good mechanical properties, and high-temperature
withstand ability. Synthetic materials, paper, cotton, etc are
used as insulation in transformers.
The core, winding and insulation are the most basic parts of
a transformer and are present in all types.

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4. Tank
The main tank is a part of a transformer that serves two
purposes:
1. Protects the core and the windings from the external
environment.
2. Serves as a container for oil and support for all other
transformer accessories.
Tank bodies are made by fabricating rolled steel plates into
containers. They are provided with lifting hooks and cooling
tubes. In order to reduce weight and stray losses, aluminum
sheets are also used instead of steel plates. However,
aluminum tanks are costlier than steel ones.

5. Terminal and bushings

For connecting incoming and outgoing cables, terminals are
present in transformers. They are mounted upon the
bushings and connected to the windings’ ends.
Bushings are insulators that form a barrier between the
terminals and the tank. They are mounted over the
transformer tanks. They are a safe passage for the
conductors connecting terminals to the windings. They are
made from porcelain or epoxy resins.

6. Transformer oil
In all oil-immersed transformers, transformer oil provides
added insulation between the conducting parts, better heat
dissipation, and fault detection features. Hydro-carbon

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mineral oil is used as transformer oil. It is composed of
aromatics, paraffin, naphthenes, and olefins. Transformer
oil has a flashpoint of 310 degrees Celsius, a relative
permeability of 2.7, and a density of 0.96 kg/cm3

7. Oil Conservators

The oil conservator is moved on top of the transformers and

is located well above the tank and bushings. Normally a
rubber bladder is present in some oil conservators. The
transformer oil expands and contracts with an increase and
decrease in temperature. The oil conservator provides
adequate space for oil expansion. It is connected to the
main tank through a pipe. A level indicator is fitted to the

conservator to indicate the oil level inside.

8. Breather
Breather is present in all oil-immersed transformers that
have a conservator tank. It is necessary to keep the oil-free
from moisture. As the temperature variations cause the
transformer oil to expand and contact, air flows in and out
of the conservator tank. This air should be free from
moisture. Breather serves this purpose.

A breather is attached to the end of the air pipe such that

the air enters and exits the conservator through it. The
silica gel present in the breathers removes moisture from
the air and delivers moisture-free air to the conservator.

9. Radiators and fans

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The power lost in the transformer is dissipated in the form
of heat. Dry transformers are mostly natural air-cooled. But
when it comes to oil-immersed transformers, a variety of
cooling methods are followed. Depending on the kVA rating,
power losses, and level of cooling requirements, radiators
and cooling fans are mounted on the transformer tank.
Parts of a transformer: Radiators and cooling fans
The heat generated in the core and winding is passed to the
surrounding transformer oil. This heat is dissipated at the
radiator. In larger transformers forced cooling is achieved
with the help of cooling fans fitted to the radiators.
10. Explosion vent
An explosion vent acts as an emergency exit for oil and air
gases inside a transformer. It is a metallic pipe with a
diaphragm at one end, held slightly above the conservator
tank. Faults occurring under oil elevate the pressure inside
the tank to dangerous levels. Under such circumstances, the
diaphragm ruptures at a relatively low pressure to release
the forces from within the transformer to the atmosphere.

11. Tap changers

Tap changers are used to adjust the secondary voltage of
transformers. They are designed to change the turn ratio of
the transformer as required. There are two types of tap
changers: On-load tap changers and Off-load tap changers.
On-Load tap changers
Off-load tap changers are designed to operate only when the
transformer is not supplying any loads whereas on-load tap
changers are capable of operating without interrupting the
current flow to the load. Automatic tap changers are also
available.

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12. Buchholz relay
Buchholz relay is one of the most important parts of oil-
immersed transformers rated over 500kVA. It is an oil and
gas actuated relay that is used to sense faults occurring in
the parts immersed in the oil.

Short circuits occurring under the transformer oil generate

enough heat to decompose the oil into hydrogen, carbon
monoxide, methane, etc. These gases gradually move
toward the conservator tank through the connecting pipe.
Buchholz relay, which is mounted on the pipe connecting
the conservator tank and the main tank, senses these gases
and activates the trip and alarm circuits. The trip circuit
opens the circuit breaker supplying current to the primary
winding and interrupts the current flow.

 WORKING MECHANISMS

The transformer works on the principle of Faraday’s law of

electromagnetic induction and mutual induction.

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There are usually two coils – primary coil and secondary coil – on the
transformer core. The core laminations are joined in the form of
strips. The two coils have high mutual inductance. When an
alternating current passes through the primary coil, it creates a
varying magnetic flux. As per Faraday’s law of electromagnetic
induction, this change in magnetic flux induces an EMF
(electromotive force) in the secondary coil, which is linked to the core
having a primary coil. This is mutual induction.

Overall, a transformer carries out the following operations

1. Transfer of electrical energy from one circuit to another

2. Transfer of electrical power through electromagnetic induction
3. Electric power transfer without any change in frequency
4. Two circuits are linked with mutual induction

The figure shows the formation of magnetic flux lines around a

current-carrying wire. The normal of the plane containing the flux
lines is parallel to the normal of a cross-section of a wire.

`PAGE 9
The figure shows the formation of varying magnetic flux lines around
a wire wound. The interesting part is that the reverse is also true;
when a magnetic flux line fluctuates around a piece of wire, a current
will be induced in it. This was what Michael Faraday found in 1831,
which is the fundamental working principle of electric generators, as
well as transformers.

 TYPES
Power transformers can be categorized based on different
criteria, such as their construction, function, application,
etc. Some of the common types of power transformers
are:

 Step-up and step-down transformers: These

transformers are used to increase or decrease the
voltage level of an AC supply. A step-up transformer
has more turns in the secondary winding than in the
primary winding, while a step-down transformer has
fewer turns in the secondary winding than in the
primary winding.
 Single-phase and three-phase transformers: These
transformers are used to handle single-phase or three-

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 phase AC supplies. A single-phase transformer has one
primary winding and one secondary winding, while a
three-phase transformer has three primary windings
and three secondary windings that are connected in
star or delta configuration.
 Two-winding and autotransformers: These transformers
have either two separate windings or one common
winding for both primary and secondary circuits. A two-
winding transformer is used when the voltage ratio is
greater than 2, while an autotransformer is used when
the voltage ratio is less than 2.
 Distribution and power transformers: These
transformers are used for different purposes in the
power system network. A distribution transformer is
used to step down the voltage for distribution to
domestic or commercial users. It has good voltage
regulation and operates at full load or near full load
most of the time. A power transformer is used to step
up or step down the voltage for transmission between
generating stations and substations. It has poor voltage
regulation and operates at variable loads depending on
the demand.
 Instrument transformers: These transformers are used
to measure high voltages and currents in a circuit by
stepping them down to lower values that can be
measured by conventional instruments. They include
current transformers (CT) and potential transformers
(PT).
 Oil-cooled and dry-type transformers: These
transformers differ in their cooling methods. Oil-cooled
transformers use mineral oil as a cooling medium that
circulates through radiators or heat exchangers. Dry-
type transformers use air as a cooling medium that
flows through vents or fans.

`PAGE 11
 Core type and shell type transformers: These
transformers differ in their core shapes and winding
arrangements. A core-type transformer has a
rectangular core with two vertical limbs and a
horizontal yoke. The windings are cylindrical and
concentric and are placed on both limbs. A shell-type
transformer has a central limb and two outer limbs that
form a shell around the windings. The windings are
sandwiched between the limbs and have multiple
layers.
 Outdoor and indoor transformers: These transformers
differ in their installation locations and protection
levels. Outdoor transformers are designed to withstand
harsh weather conditions and are usually oil-cooled and
enclosed in metal tanks. Indoor transformers are
designed to operate in controlled environments and are
usually dry-type and enclosed in metal cabinets.

 APPLICATIONS
 The power transformer is used to increase otherwise
decrease voltage within a power distribution network.
 Distribution transformer is mainly used to decrease a
voltage for distribution to commercial & residential
users.
 Instrument transformer is used to decrease high
voltage as well as current and after that, it can be

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 measured & carefully used through conventional
devices.
 Single-phase transformer is frequently used to provide
power for receptacle, residential lighting, AC &
heating requirements.
 A three-phase transformer is used to achieve cost-
effective power distribution.
 Autotransformers & Two-winding are normally used to
increase or decrease voltage from the grids like
transmission to distribution.
 Oil cooled transformers are used in electrical
substations or power distribution
 Air-cooled transformers provide an inexpensive
technique of correcting a lower otherwise higher
rating of voltage to operate electrical equipment
efficiently.
 causes of Transformer Failure

1. Age – The age of the transformer can lead to its

failure. However, it’s not necessarily the calendar time,
but the cumulative operating hours at a high load. A
transformer with a continuous 24/7 cycle of high load
will age about four times as fast as one with a 5-day 8-
hour load cycle.
2. Major Leakages – Moisture and oxygen can enter a
transformer through leaking gaskets causing
accelerated ageing of insulation and insulation failure.
Leaks can be caused by cracks, tank damage, sealant
damage, deformation, weld cracking and many other
issues and have the potential to cause environmental
harm if not adequately contained.

`PAGE 13
3. Inadequate Maintenance – Transformer
maintenance is primarily concerned with ensuring the
level and condition of the oil and ensuring moisture
does not enter the tank. Annual maintenance is the
easiest way to be proactive in reducing the likelihood
of transformer failures.
4. Overloading / Overheating – Maintenance allows you
to make sure the electrical load settings are
appropriate for the specific type of transformer being
used. Overloading causes overheating, and eventually,
thermal degradation which will reduce the
effectiveness of the winding insulation.
5. Moisture – Moisture in a liquid-filled transformer can
cause issues that result in irreversible damage to the
insulation. If the transformer tank is not properly
sealed, moisture will eventually work its way into the
insulating fluid. In the case of free-breathing
transformers, it is also possible for moisture to enter a
transformer during the natural breathing process if the
silica gel is not well maintained.
6. Left to Disrepair – Transformers that are left to
disrepair are often a tell-tale sign that transformer
failure could happen. Substations that are overgrown
with shrubbery, have the potential to result in damage
by trees, obstruction of radiators and other external
factors like animals, which could all influence the
failure of the transformer.
7. Lightning Surges – Lightning surges are very capable
of destroying the function of a transformer however
due to a combination of transformer design and the
low lightning density within the UK, this is less
common a phenomenon compared to the above factors.

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  How to reduce Transformer Failure

It is key to ensure transformers are designed and manufactured for their intended
application. A good construction transformer fit for purpose should not fail
prematurely provided an adequate maintenance regime is adopted. Transformers
have a long-life expectancy unless something outside the design specification
happens. It is vital to perform regular checks on the loading of your transformers.
Electrical transformers that operate below the maximum load capacity have a
longer lifespan.

Planned maintenance, inspection and testing will significantly reduce the number
of transformer failures. Transformer preventive maintenance is very important,
immediate preventative action can be

 conclusion
the significance of electrical transformers as a vital
component in the power system is evident. This
technology plays a crucial role in enhancing the
efficiency and effectiveness of electricity transmission.
Careful consideration of technical and environmental
factors is imperative to ensure the sustainability of
these devices and guarantee their continuous
performance. Amidst the future challenges in electrical
energy, the development and utilization of
transformers remain a critical aspect of achieving
more efficient and sustainable energy systems.

Sources:

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1. https://www.electrical4u.com/electrical-power-

transformer-definition-and-types-of-transformer/

#:~:text=Some%20of%20the%20common

%20types%20of%20power%20transformers,both

%20primary%20and%20secondary%20circuits.

%20...%20More%20items

2. https://www.theengineerspost.com/types-of-

transformers/

3. Types of Transformers : Working and Their Applications

(elprocus.com)

4. Seven of the most common causes of Transformer Failure, and

how to reduce the likelihood of failure | Bowers Electrical Ltd

5. 12 different parts of a transformer | Transformer

parts (electricalclassroom.com)

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