Working Principle and Parts of Transformer

What is Transformer?

The transformer is a static device( i.e the one which does not contain any rotating parts or moving parts) that is used to transfer electrical energy from one ac circuit to another ac circuit with an increase or decrease in voltage and current without any change in frequency.
What is transformer ?

The transformer works on the principle of faraday's law of electromagnetic induction which states that the magnitude of produced voltage is proportional to the rate of change of flux.

It is important to remember that input to a transformer and output from a transformer both are alternating(AC) Quantities.  The electrical power is generated and transmitted at very high voltages. The voltage is to be reduced to a lower value for its domestic and industrial use. 

This is done by using a transformer. Thus it is possible to reduce the voltage level using a transformer. A transformer that has an input voltage that is very high and an output voltage that is very low is called a step-down transformer. A transformer that has an input voltage that is low and the output voltage is very high is called a step-up transformer.  

Principle of Operation of a transformer

The construction of a single-phase transformer is shown in below fig.
It consists of two highly inductive coils or windings wound on air iron or steel core. The winding connected to the ac supply is called primary winding whereas the other one is called the secondary winding. The ac supply is connected to the primary winding whereas the load is connected to the secondary winding. 

The primary windings and secondary windings are separated from each other as well as from the laminated iron core. So there is not any physical contact between the primary windings and secondary windings. 

The operating principle of the transformer has been explained step by step.
1. As soon as the primary winding of the transformer is connected to the single-phase ac supply and an ac current starts flowing through primary windings.
2. The ac primary current produces an alternating magnetic and changing magnetic flux surrounding the winding.
3. Most of this changing flux gets linked with the secondary winding through the iron core.
4. The varying flux will induce voltage into the secondary winding according to faraday's laws of electromagnetic induction.

Thus due to the primary current, there is an induced voltage in the secondary winding due to mutual induction. Hence the emf induced in the winding is called the mutually induced emf. 

If more number turns on the primary side and fewer numbers turn on the secondary side, this transformer decreases the voltage.

If less number turns on the primary side and more number turns on the secondary side, this transformer increases the voltage. 

The transformer consists of primary windings, secondary windings, Iron core, radiators, oil tank, conservator tank, pressure release valve, Buchholz relay, magnetic oil gauge, Tap changer, HV and LV bushings. 

The transformer core is used to provide a low reluctance path between the primary and secondary windings which means the maximum amount of flux produced by the primary winding and liked with the secondary winding.  

When the transformer is switched on and the current is passed through the primary and secondary winding is called the inrush current of the transformer. 

Transformer parts and construction

The Parts of the transformer are
1. Electrical circuit
2. Magnetic circuit
3. Step-up transformer 
4. Step down transformer 
5. Shell type transformer 
6. Core type transformer 
7. Dielectric circuit 
8. Conservator tank 
9. Breather 
10. Magnetic oil gauge
11. Explosion vent 
12. Bushings
13. Radiator 
14. Oil temperature indicator 
15. Winding temperature indicator 
16. Tap changer
17. Buchholz relay

1. Electrical Circuit 

Electrical circuit of transformer

The electrical circuit consists of the primary and secondary winding. The conductor of winding are made from copper and has a rectangular or circular cross-section as per requirement. 

2. Magnetic circuit 

Magnetic circuit of transformer

The magnetic circuit consists of a laminated core and yoke. The magnetic circuit provides the low reluctance path to the changing magnetic flux.

3. Step-up transformer 

Step Up transformer

A transformer that has a high output(secondary side) voltage and low input voltage (primary side) is called a step-up transformer.

The step-up transformer reduced the current level on the secondary side and increase the voltage level on the secondary side. 

4. Step down transformer 

Step Down transformer

A transformer that has a low output(secondary side) voltage and high input voltage(primary side) is called a step-down transformer.

The step-down transformer increases the current level on the secondary side and decreases the voltage level on the secondary side. 

5. Shell type transformer 

Shell type transformer

This transformer has a rectangular shape and consists of three important parts one core and two winding namely primary and secondary winding. The shell type transformer's core surrounds the windings and these transformers have disc or sandwich windings. 

In a shell-type transformer, laminations are cut in E and L shape form. In shell-type transformers, primary and secondary windings are wound on a central limb of the core.

Shell type transformer is used for low ratings and low voltage applications.  In a shell-type transformer, cooling is not effective. 

6. Core type transformer 

This transformer has a square type shape and consists of three parts one core and two winding namely primary and secondary winding. In core-type transformer winding surrounds the core and these transformers have concentric or cylindrical windings.
Core type transformer

In a core-type transformer, laminations are cut in form of L shape form. The primary and secondary winding are wound on the side limb of the core. 

The core type transformer is used for high voltage applications like a distribution transformer, power transformer etc. 

7. Dielectric circuit 

Dielectric circuit of transformer

The dielectric circuit consists of various insulations which are used for insulating the live parts from the earth. The core of the transformer is laminated by using varnish or paper. In transformer laminated core is used to reduce eddy current losses. 

8. Conservator Tank 

Conservator tank in transformer

The conservator tank is used to provide enough space for transformer oil to spread during heating. 
The conservator tank is cylindrical in shape and is provided at the roof of the transformer. When the transformer is loaded, the temperature increases and the oil of the transformer is heated and it starts spreading. The conservator tank works as a pond for transformer oil. 

The Conservator tank consists of rubber bellow, magnetic oil gauge, breather. 

9. Breather 

Breather in transformer

The breather acts as the heart for the transformer. When the load in the transformer is increased, the temperature of the transformer increases and the oil of the transformer get heated and get expands. 

The transformer oil get expands and contracts during its working condition. When load increases, transformer oil expands, transformer breath out and when load decrease, transformer oil get contracts, transformer breath in. The process of air tanking in and out is called the breathing of a transformer. 

The level of oil increase and decrease when the transformer breathes out and breath in for oil cooling purpose. During the breath in and breath out process, air that is taken by the transformer is contained moisture. So the quality of oil deteriorated due to the bad quality of air. 

The air which is taken by the breather is contained the chemical material silica gel. The silica gel breather absorbs the moisture from the atmospheric air which is taken by the transformer during the breathing out process.  

10. Magnetic oil gauge

MOG in transformer

The magnetic oil gauge is connected at the bottom of the conservator tank. The magnetic oil gauge supervises the level of transformer oil in the conservator tank in the power transformer. When transformer oil goes low it gives the alarm to operating personnel. The magnetic oil gauge has a mercury switch to give indication during low oil levels.

11. Explosion vent

Explosion Vent in transfomer

The explosion vent is used to protect the transformer from damage. When an internal fault occurs in the transformer, the temperature increases and also high pressure is created inside the transformer tank. The exposition vent is an aluminium pipe that is provided at top of the transformer main tank. 

High pressure inside the transformer tank is released through the explosion vent.

12. Bushings 

Bushing in transformer

The transformer consists of low voltage and high voltage bushings. The bushing of the transformer is made from porcelain material. The bushings insulate live conductors from the earth. In low voltage transformers, solid bushings are used and in high voltage transformers, oil-filled bushings are used. 

13. Radiators

Radiator in transformer

The radiator is used to cool down the oil during the working condition. The radiators are consists of upper and lower valves which are attached to the main tank at the top and bottom.

When the load on the transformer is increased, transformer oil gets heated. The heated oil gets upward in the main tank and enters into the radiators through the top valve. The oil inside the radiator is cooled down by various cooling process and again enter into the main tank through the bottom valve. 

14. Oil Temperature Indicator 

OTI in transformer

The oil temperature indicator is used to indicate and monitor the temperature of transformer oil during the working condition. 

The oil temperature indicator consists of two mercury switches that are connected to the indicating unit. When the temperature of transformer oil increased from a certain value, OTI gives the alarm and after a certain limit, it gives the tripping.

15. Winding Temperature Indicator

WTI in transformer

The winding temperature indicator is used to indicate and monitor the temperature of winding during the working conditions. 

The Oil temperature indicator also gives the alarm and tripping for a certain value of winding temperature.

16. Tap changer 

Tap changer of transformer

The transformer consists of two types of tap changers one is an on-load tap changer and the second is off load tap changer. In a high voltage transformer, on-load tap changer is used which regulate the voltage under load condition. 

The offload tap changer is used to regulate the voltage under no-load conditions and the transformer is in off condition. 

The tap changer is connected to the HV side of the transformer. The voltage on the low voltage side can be controlled by changing the tap of the tap changer. 

17. Buchholz relay 

Buchholz relay in transformer

The Buchholz relay is a gas actuated relay. This relay is used for the protection of the transformer during an internal winding fault. When a fault occurs in the winding, the temperature of winding increases and also the temperature of oil also increased. Due to an increase in oil temperature, gases are produced in the main tank.

The Buchholz relay is connected between the conservator tank and the main tank. The Buchholz relay consists of two floats with mercury switches. When the fault occurs in the transformer, oil goes into the conservator tank through the Buchholz relay and the Buchholz relay gives the alarm and tripping and protects the transformer from major damage. 

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