Power transformers are static devices used to transmit electrical power between circuits without changing frequency. They operate using electromagnetic induction and are used to step up or down transmission voltages. Power transformers have ratings between 33-400 kV and above 200 MVA. They are essential for minimizing energy losses during long distance power transmission by increasing voltage for transmission then decreasing it for distribution. Power transformers work by inducing an emf in the secondary winding through a fluctuating magnetic field produced in the primary winding according to Faraday's law of induction. The number of turns in each winding determines whether the transformer steps up or down the voltage. Key components include the core, windings, insulating materials, tap changers, bushings, tank, conservator, breather

1. Power transformer
Power transformers are electrical instruments used in transmitting electrical power from
one circuit to another without changing the frequency. They operate by the principle of
electromagnetic induction. They are used in transmitting electrical power between
generators and distribution primary circuits. Power transformers are used to step up or
step down the voltage in distribution networks. Since they have no rotating or moving
parts, these instruments are considered static devices. These instruments work based on
an alternating current (AC) electrical system.
A power transformer is a mere classification of transformers with a voltage range varying
between 33 kV-400 kV and a rating above 200 MVA. The voltage ratings of power

2. transformers available in the market include 400 kV, 200 kV, 110 kV, 66 kV, and 33 kV. The
other types of transformers include distribution (230 V-11kV) and instrument transformers.
Power transformers are essential in minimizing substantial energy losses, due to Joule’s
effect, in the transmission of large amounts of electrical power over long distances by
converting it into high-voltage current then stepping it down to a safer low-voltage current.
They are commonly found in power plants, industrial plants, and electric utility companies.
Operating Principle of Power Transformers
Faraday’s Law of Electromagnetic Induction
Power transformers operate based on Faraday’s law of electromagnetic induction. This law
is the working principle of all transformers, inductors, motors, generators, and solenoids.
Faraday’s law states that when a closed-loop is brought near a fluctuating magnetic field,
an electromotive force (emf) will be induced across it.
When alternating current is allowed to flow through a coil, an alternating or fluctuating
magnetic flux surrounds the coil (primary winding). The magnetic flux produced by the
primary winding passes through a ferromagnetic core to be transmitted effectively to a
secondary winding. The magnetic flux will then induce an emf in the secondary winding

3. due to electromagnetic induction. The induced emf will stimulate the flow of current in the
secondary winding.
Stepping Voltages Up or Down
The total voltage in a winding is equal to the voltage per turn of the coil multiplied by the
number of turns. Since the voltage per turn of the primary and secondary windings are the
same, the induced voltage in the secondary winding can be related to the input voltage on
the primary winding. This relationship is expressed by the equation:
Vs = Vp/Np x Ns
Where V represents the total voltage in the winding, N represents the number of turns of a
winding, and the subscripts p and s refer to the primary and secondary windings,
respectively. The ratio of the number of turns in the secondary winding to that of the
primary winding (Ns/Np) is called the turns ratio.
If the number of turns in the secondary winding is fewer than the number of turns in the
primary winding, the voltage output is lower than the input voltage (step-down
transformer). On the other hand, if the number of turns in the secondary winding is more
than the number of turns in the primary winding, the voltage output is higher than the
input voltage (step-up transformer).

4. Since energy is conserved, the relationship between the alternating current in the primary
and secondary windings is represented by the below equation:
Vp Ip = Vs Is
Where I represents the current.
Components of Power Transformers
The basic parts of transformers are the core and the primary and secondary windings,
which are discussed in more detail in this chapter.
Core Components
The core supports the windings and provides a low reluctance path for the magnetic flux. It
is made by stacking and laminating thin steel sheets. The sheets are insulated from each
other by a coating. To reduce eddy current losses and hysteresis losses, the iron or steel
sheets are less than one millimeter thick, and their carbon content is maintained below
0.1%. Eddy current is further reduced by alloying the steel with silicon. The vertical sections
of the core in which the windings are carried are r

5. eferred to as the limbs, while the
horizontal sections of the core that couples the limbs are referred to as the yokes.
Windings in Power Transformers
The windings are made up of copper or aluminum conductor coil with a specific number of
turns. Copper is the preferred material since it offers high electrical conductivity and high
ductility; these properties reduce the amount of winding and make the material easier to
wrap around the core.
A transformer consists of at least two windings- the primary and the secondary
windings. The primary winding is the winding in which the input voltage is applied, while
the secondary winding is the winding that receives the output voltage. The primary and the
secondary windings in a phase of a transformer can play as the high voltage (HV) winding
or the low voltage (LV) winding:
 HV Winding The HV winding has a greater number of turns and consists of a thinner
conductor than the LV winding.

6.  LV Winding The LVwinding has a fewer number of turns. It consists of a thicker
conductor than the HV winding since a higher current is carried in the LV winding.
Other parts of power transformers include the following:
Insulating Materials
Insulating materials are used to isolate the windings from the core, the primary and the
secondary windings, and each turn of the windings. These materials protect the
transformer from damage. Transformer insulators should have high dielectric strength,
good mechanical properties, and can withstand high temperatures.
Paper and pressboard can be used as an insulator (i.e., dry-type transformers); however,
they have limited service lives and require frequent replacement as these materials can
degrade. Hence, transformer oils are more common compared to solid insulating
materials. They provide enhanced insulation between conducting parts, act as a coolant for
the coil and windings assembly, and have fault detection features. Hydrocarbon mineral
oils consisting of aromatics, paraffin, naphthene, and olefins are used as transformer oils.
Oil contamination must be prevented to preserve the oil’s dielectric properties and
insulating features.
Tap Changer
Tap changers are devices that regulate the transformer’s output voltage as it responds
accordingly to the varying input voltage and load by adjusting the number of turns in one
winding. This adjustment, therefore, changes the turn ratio. During offloading conditions,
the output voltage increases, whereas during loaded conditions, the output voltage
decreases. Tap changers are typically connected in the HV winding to make fine voltage
regulations and minimize core losses of the transformer. The current is also lower in the
HV winding, which minimizes the risk of sparking and igniting the transformer oil.
There are two types of tap changers. Onload tap changers are designed to tap the voltage
without disrupting the current flow to the load. Whereas offload tap changers require
disconnecting the load of the transformer before operating.

7. Bushings in Transformers
Bushings are insulated barriers that contain the terminal that connects the current-carrying
conductor from an electrical network to the ends of the transformer windings. The bushing
insulation is typically made from porcelain or epoxy resin. The bushings are mounted over
the main tank.

8. Transformer Tank
The transformer tank (or the main tank) houses and protects the core, windings, and other
components from the external environment. It serves as the container for the transformer
oil. It is constructed from rolled steel plates or aluminum sheets.
The following are present in large transformers insulated with hydrocarbon mineral oil:

9. Conservator Component
The conservator is a tank that serves as the reservoir of the transformer oil and is located
above the main tank and bushings. Transformer oil from the conservator is supplied to the
main oil tank inside the transformer through a pipeline. The conservator has a flexible
bladder that allows the expansion and contraction of the oil. It has an adequate space to
allow the expansion of the oil during high ambient temperatures. The conservator is
vented to the atmosphere to balance the pressure changes during the expansion and
contraction of the oil by intaking or releasing air.
Breather Component
The breather delivers moisture-free air to the conservator by passing air through a small
bed of silica gel inside a cylindrical container. The silica gel acts as an air filter that strips
and controls the moisture level inside the conservator and the main tank. The breather is
connected by a pipeline to the conservator.
Moisture can degrade the insulating properties of the transformer oil or may even lead to
internal faults. Therefore, it is necessary to remove the moisture.

10. Cooling System
The cooling system is a critical component of transformers regardless of the insulating
material utilized. Power losses occurring in the transformers are in the form of heat
increasing the temperature of the windings and the core. Consequently, the temperature
of the insulating material will also increase. Without a cooling system, these components
may be damaged or decomposed if heated continually. The cooling system of transformers
consists of fans, radiators, and cooling tubes. Heat transfer mechanism occurs by natural
and/or forced convection and radiation.
For dry-type transformers, cooling may be accomplished by the following methods:
 Air Natural
 Air Forced
For oil-immersed type transformers, cooling may be accomplished by the following
methods:
 Oil Natural Air Natural
 Oil Natural Air Forced
 Oil Forced Air Forced
 Oil Natural Water Forced
 Oil Forced Water Forced
Explosion Vent
The explosion vent is a metallic pipe with a diaphragm at its free end located slightly above
the conservator tank. It releases gases, transformer oil, and energy during internal faults to
relieve the excessive pressure inside the transformer, thus preventing the explosion of the
transformer. Faults elevate the internal pressure of the transformer to dangerous levels.
When such circumstances occur, energy will be released into the atmosphere, destroying
the diaphragm at relatively low pressure.
Buchholz Relay
The Buchholz relay is a device installed along the pipeline connecting the conservator and
the main tank. It detects faults in the transformer by sensing the emitted gases to activate
the trip and alarm circuits. Once the trip circuit is activated, the circuit breaker will then
disrupt the current flow to the primary winding. Emitted gases are generated by the heat
released induced by faults.