What are the main parts of a transformer? and its protective accessories

What is a Transformer

A transformer is static equipment, with two or more windings linked to a common magnetic field. By using electromagnetic induction this device will transfer electrical energy from one circuit to another without any frequency change.

What are the functions of a Transformer

• It changes the voltage level

• It changes the current level

• It changes the impedance

• It can provide isolation

• Filters DC from a waveform that is a mixture of AC and DC

Working principle of a transformer

The working principle of a transformer is depended upon the Faraday’s law of electromagnetic induction, in an electrical transformer the mutual induction between two or more windings is responsible for the transformation action

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Alternating current is supplied to one of the winding and when the current will reach the electric coil there will be an alternating flux surrounded to that coil so if we bring another coil near to the first one then there will be an alternating flux linkage with that second coil and so there will be a rate of change in flux with respect to time in the second coil and thus EMF will be induced in it.

Type of service

Power transformer

• These transformers are used in generating stations and these are above 500 KVA

• They have maximum efficiency near full load

• Iron losses are 5-6 times when compared to distribution transformer

Distribution transformer

• It is used in the distribution section and they are up to 500 KVA

• They are operated for 24 hours

• It has maximum efficiency near 50% of full load

Parts of a transformer

• Core

• Windings

• Transformer tank

• Conservator

• Breather

• Cooling arrangements

• Tap Changer

• Bushings

• Accessories

Core

It guides the magnetic flux created by the current that flow in the winding, it acts as a magnetic link between the primary and the secondary windings. The core is laminated to reduce the eddy current loss

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Parts of a core

The part which carries transformer windings is called as legs and the part which connect the legs, which also closes the magnetic circuit is called as yokes.

Windings

The winding is the electrical circuit of the transformer, it is made up of highly conducting copper with high dielectric strength

These winding needs to be strong mechanically and electrically to withstand both over-voltage and mechanical stress

Dielectric circuit

It consists of insulation used at different places in a transformer all the magnetic and conducting part of the transformer must be insulated well

Major insulation

This insulation is between the core and low voltage winding and the similar insulation is used for low and high voltage winding

Minor insulation

It is done between the elements of a given winding, insulating turns, layers, coil, conductor

Transformer tank

It provides a protective cover to the core, it provides an external surface for heat dissipation, it is filled with insulating oil for placing core and coil assembly. It does the protection of the windings and other internal parts

Conservator

It is a cylindrical metal drum, the main function of the conservator is to keep the main tank of the transformer completely filled with oil in any circumstances It takes up the contraction and the expansion of transformer oil and keeps the main tank full of oil and it reduces the rate of oxidation of the oil because less oil surface is exposed to the air and thus the sludge formation can be reduced mostly the capacity of the conservator is nearly 10 to 12 percent of the volume of oil of the main tank

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Breather

During the expansion of the oil, the air is expelled out and during the contraction, the air is drawn in during this process the entered air is passed through a breather so that it could absorb the moisture from the air and the material used for dehydrating will be silica. the Material will be blue if it is dry and whitish pink when damp

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Bushings

It does the connection of the transformer to the network system in which it is placed a Bushing is needed when the conductor is taken through a metallic tank one end of it will go inside the transformer tank while the other end is in the air It has a conducting rod and a porcelain cylinder to isolate the conducting part

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Tap Changers

It is used to get a proper balance in system voltage variation so that we could get a required output voltage on the secondary load side

There are two types of tap changers

• Off-load tap changers

• On load tap changers

Tap changers are selected according to the requirement of current, voltage and number of tapping required It is mostly connected to high voltage winding to get a wide range of voltage adjustment

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  **Accessories** 

Temperature indicators

These are specially designed for the protection of the transformer. There two separate instruments to indicate oil and winding temperatures.

Functions of a temperature indicator

• It will indicate maximum oil temperature and hottest spot temperature of windings

• Alarm or a trip circuit will be operated at a predetermined temperature

• If the winding attains a preset high-temperature value, cooling equipment will switch on and it turns off when the temperature decreases

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Oil level indicator

• It monitors the level of oil in a transformer

• If the oil level drops near to an empty position in the conservator it will actuate an alarm

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Buchholz relay

It is used as a protection device to determine certain faults like inter-turn faults, core faults due to the insulation failure of the core bolt The insulation failure will create a short circuit and subsequent arcing

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Pressure relief device

It prevents the rapid pressure build-up in the transformer tank, pressure build-up can cause an explosion. These devices are designed to operate and discharge any pressure more than their set value to the atmosphere.

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