Electrical Transformer - Electrical C & V

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06 April 2020

Electrical Transformer

A Transformer is a passive electrical device that transfers electrical power one circuit to another circuits.
Generally "Transformer is a simple static or static Electro-magnetic electrical device that works on Faraday's law of induction."

Electrical Transformer
Transformer

 Transformers are mostly used in circuits to step up (increase) or step down (decrease) the voltage levels. That is, its main use is to decrease and increase the voltage.


Working principle of transformer

The transformer operates on Faraday's law of electromagnetic induction. According to Faraday's rule "When current (current) changes in the primary coil, then the flux attached to the secondary coil also changes, therefore the secondary carrier induces (induced) electric force."


Parts of Transformer

The transformer has three main parts -

1. Primary winding
2. Core
3. Secondary winding

1) Primary winding - The winding that receives electrical power from the source is called primary winding.

Electrical Transformer
Parts of Transformer
                             
2) Core - It is used to provide a path to the magnetic flux generated in the transformer. The core of the transformer is not a solid bar of steel, it is a structure that consists of thin laminated sheets or layers of steel. This structure is used because heat reduces considerably from this structure. This structure is used to reduce and eliminate heat.

Two types of Cores are used in transformers.

i) Core type - In this type of core winding are wrapped outside the laminated core.

ii) Shell type - In this type of core winding is inside the laminated core.

Electrical Transformer
Core type & Shell Type Transformer
                                   
3) - Secondary winding - The winding that gives us the desired output voltage is called secondary winding.


Working of Transformer

When an input voltage is applied to the primary winding, the alternating current (alternating current) starts to flow in the primary winding. This generates a magnetic field in the core of the transformer, when the magnetic field cuts the secondary winding, then the alternating voltage is generated in the secondary winding.
Electrical Transformer
Working of Transformer 
By the ratio of the turns of both winding wires we can determine what type of transformer is the transformer and what will be the output voltage.

The ratio of output voltage and input voltage is equal to the ratio of wire turns of both windings.

The output voltage of a transformer is greater than the input voltage if the number of turns of the secondary winding wire is greater than the turns of the wire of the primary winding. This type of transformer is called step up transformer.

If the secondary winding turns lower then the output voltage will be less than the input voltage, hence it is called step down transformer.

The winding in the transformer are not electrically connected to each other but are magnetically connected to each other.

Electrical Transformer
Working of Transformer

Transformer emf equation

If flux in core is sinusoidal then relationship between rms (root mean square) voltage (Erms) and frequency (f), no of turns (n), core cross sectional area (a), peak magnetic flux density (Bp) The universal EMF is given by the equation.



Types of Transformer

There are many types of transformers, which are used for different purposes in electrical power systems. For example, in power generation, transformer is used in power distribution and in transmission.

Transformers can be classified on the basis of voltage level, core medium , winding arrangements and their usage.

Types of transformers based on voltage levels

1. Step-up transformer- A transformer that increases the voltage between primary winding to secondary winding is called step-up transformer. It has more than secondary windings, primary windings.
Electrical Transformer
Step up Transformer
2.Step-down transformer - A transformer that reduces the voltage between the primary winding to the secondary winding is called a step-down transformer. It has fewer than secondary windings, primary windings.
Electrical Transformer
Step Down Transformer
Depending on the type of transformer core -

These types of transformers are classified based on the core between the primary and secondary winding.

1. Air core transformer - In such transformers the primary and secondary windings are wrapped in an asymmetric strip. Here the flux linkage between the primary and secondary windings is by wind.

The mutual inductance of this type of transformer is less than that of iron core.

In this type of transformer, hysteresis and eddy current loss are completely eliminated.

2. Iron core transformer - In iron core transformers the primary and secondary windings are wrapped in an iron core. Which provides the best linkage path to the generated flux. The efficiency of these transformers is higher than air core type transformers.

Depending on the type of transformer winding -

Auto-Transformer

The auto transformer has only one winding in which the primary and secondary windings share the same winding. The same winding is used as both primary and secondary.
Electrical Transformer
Auto-Transformer

Benefits of Auto-transformer

1) - Because the auto-transformer has only one winding, its size is reduced compared to the other transformers, which also reduces its cost.

2) - Its efficiency is more than two winding transformer.

3) - The voltage regulation of auto-transformer is good.

Disadvantage of Auto-transformer

1) - Leakage flux between primary and secondary windings is less, which also reduces impedance. It may also lead to short circuit under incorrect conditions.

2) - Due to common neutral in star-star connected autotransformer, only one side cannot be neutral earth, both sides have to be neutral earth.

Applications of Auto-transformer

1) - Auto-transformer is used for induction and starting synchronous motor.

2) - It is used to regulate the voltage of transmission lines.

Types of transformers depending on use

1. Power Transformer - they are large in size. These high voltage (more than 33kv) are used. And they are used in power generation stations and transmission substation.

2. Distribution transformer - A distribution transformer is used to distribute power produced in power generation plants to remote areas. It is used to distribute electrical energy of low voltage level (less than 33kv).

3. Measuring transformer - It is used to measure electrical quantity like voltage, current, power. can be classified into protection transformer and current transformer.

4. Protection transformer - This type of transformer is used for protection of the component. Protection transformers are more accurate than measuring transformers.


Losses of transformer

Most of the losses are winding and core losses.

Losses in transformers vary with changing load. Losses also occur at no load, even at full load, and at half load.

hysteresis and eddy current loss are constant at every load level and when there is no load then these losses increase. The same winding loss increases with increasing load.

To reduce losses, we need big core, good quality silicon steel or amorphous steel and thick wire, which increases our cost.

1. Winding Joule loss- The rising current in the conductor of windings produces joule heating due to the resistance of the wire. As the frequency increases, the resistance of winding makes a difference due to the skin effect and the proximity effect. Therefore losses increase.

2. Core loss- core losses are of two types, a hysteresis loss and eddy current loss.

3. Hysteresis loss- As often as magnetic field reverse happens, a small part of energy is wasted due to hysteresis of the core.

4. Eddy current loss – changing magnetic field induce eddy currents in the conductive metal transformer core. And when this current passes through the resistance of iron, heat is generated in the core. In order to reduce eddy current losses, the core is made into a stack consisting of thin laminated sheets that are insulated from each other.

5. Humming loss - The magnetic flux increases and shrinks a ferromagnetic region such as the core with every cycle of the magnetic field, producing a sound called transformer hum.

6. Stray losses - The leakage inductance in the transformer is very low because whatever energy is supplied to the magnetic field is returned to the supply in the next half cycle. But still some leakage flux stops the close conductive nature, which increases the eddy currents and converts to heat. This is called stray losses.


Configuration of Transformers

There are different types of configurations for single phase and three phase systems of transformers.

1. Single phase transformer - Like any other electrical device, single phase transformers can also be connected in serial or parallel. For example distribution transformers can usually be connected in serial or parallel with low voltage windings.

2. Three phase transformer- 3-phase is used in generation, transmission and distribution of electrical energy as well as industrial work.

A 3-phase supply has more electrical benefits than a single phase supply. When we talk about a 3-phase transformer, then we have to take care of three alternating voltage and current which are different from each other in 120 degree phase-time.

The transformer cannot function like a phase changing device. To make a transformer compatible with 3-phase supplies, we have to connect the connectors in a certain way so that it becomes a 3-phase transformer configuration.

To make a 3-phase transformer we can either connect three single phase transformers or we can use a pre-assembled and balanced 3-phase transformer, wherein three pairs of single phase winding are mounted in a single laminated core. Can.

3-phase transformer winding can be connected in three forms-

1) Star connection (Y)

2) Delta connection ()

3) Interconnected star ()


Three phase transformer can be connected in different configurations-

i) star - star

ii) delta - delta

iii) star - delta

iv) delta - star

i) Star - Star configuration– The star - star connection is used for small high voltage transformers. Because the number of turns / phase decreases due to the star connection (because the phase voltage in the star connection is 1 / √3 times the line voltage). So therefore the amount of insulation also decreases. The ratio of the line voltage of the primary side to the secondary side is equal to the transformation ratio. The line voltages on both sides are in phase of each other. This connection can only be used with balanced load.

ii) Delta - Delta configuration– This connection is used for large low voltage transformer. In this, the number of required phase / turns is more than star-star connection. The ratio of primary and secondary side line voltages is equal to the transformation ratio. This connection can also be used for unbalanced load.

iii) Star - Delta configuration - In this connection, the primary winding is connected to the star, in which the neutral is grounded and the secondary winding is connected to the delta. This connection is used in step down transformer. The ratio of secondary to primary line voltage is 1 / √3 times the transformation ratio. There is a phase shift of 30 ° between the primary and secondary line voltage.

iv) Delta - Star configuration - In this, the primary winding is connected to delta and the secondary winding is connected to the star, which remains neutral grounded. Thus this configuration can be used to provide three phase 4 wire service. This type of connection is mainly used in the step-up transformer at the beginning of the transmission line. The ratio of secondary to primary line voltage is √3 times the transformation ratio. There is a 30 ° phase shift between primary and secondary line voltage.


Cooling of transformer 

There is a lot of losses in the transformer, so heat is also generated in it, so to reduce that heat, the transformer needs cooling.

Transformer can be divided into two types -

1) Dry type transformer 2) Oil immersed transformer

1) Dry type transformer -

i) Air natural- This method is used in small transformer. In this, the transformer is allowed to cool by natural air on its own.

ii) Air blast- Transformer rated higher than 3MVA cannot be cooled by natural air. So in this method, air is thrown with the help of wings in the core and windings. This method is used to cool transformer rating up to 15MVA.

2) Oil immersed transformers -

i) Oil natural air natural- This method is used for oil immersed transformer. In this method, when heat is generated in the core and winding, this oil is transferred. According to the principle of convection, the hot oil flows upwards and then goes into the radiator. Now, the cold oil goes in the empty space which is made at the bottom. The heat in oil goes into the atmosphere by natural air flow. In this way, this cycle goes on. This method is used for transformers up to 30MVA.

ii) Oil natural air forced-heat in a better way, we can use forced air at the place where the heat is destroyed. Heat dissipates quickly when forced air is used. In this method, fans are mounted near the radiator. This cooling method is used to cool transformers up to 60MVA rating.

iii) Oil forced air forced- in this method oil is rotated with the help of pump. In this, oil is forced into the heat exchanger and compressed air flow is done with the help of fans in the heat exchanger. This type of cooling method is used for cooling transformers with higher ratings, which are at power stations.

iv) Oil forced water forced- This method is similar to oil forced air forced method but water is used in it to destroy the heat from heat exchanger. The oil is forced to flow through the heat exchanger with the help of a pump where the heat is dissolved in water and then the heated water is taken away and put in coolers.This method is used in transformers with very high ratings, which have a rating of one MVA.
 


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