What is Transformer-Type, Parallel Operation Of Transformers

The three-phase power transformer is the main device of the power system. Considering the insulation part of the generator, the generation of electrical power is limited 11KV to 22KV, but to transmit power at a higher voltage level i.e 132KV or more is economical because of losses in lower voltage transmission is more and larger conductor size is required due to higher current.

Every ac power transmission line starts with a three-phase electrical transformer and ends with it by stepping down 3 phase high voltage to usable level using a step-down type transformer i.e 415V for three-phase and 215V for single phase user.

What is the transformer?

The electrical transformer is a static piece device means it does not have any moving part in it. that's why the efficiency of it is high compared to other electrical machines.

The transformer basically transfers energy from one electrical circuit to other circuits through a magnetic field. Hence also called an electromagnetic energy conversation device. The circuit which transfers energy called primary winding and which received energy refers as the secondary winding of a transformer which further connected to the load.

Here mainly to Note that primary and secondary are not connected electrically to each other but connected magnetically.

Magnetic coupling allows transferring energy from a higher voltage level to a lower level or from low voltage to higher i.e energy transfer in either direction. The transformer converts voltage level from one voltage to other voltage levels without a change in frequency or power.

Working principle of transformer


The working principle of the transformer is base on Faraday's law of electromagnetic induction.According to Faraday's law, an emf induced in a coil if it links to changing flux. When primary winding is fed with ac supply voltage " V1 " therefore an ac current " IФ " starts flowing through winding N1 turns.

Alternating mmf N1I creates alternating flux ϕ. Which starts flowing through the magnetic core and link to the secondary winding and induced voltage E2 in the secondary winding due to transformer action.

If a load is connected across secondary of X'mer, current start flowing in secondary through the load.

Types of transformer

There are in general two types of transformer

Shell type and core type transformer These two types differ from each other in the manner in which winding wound on the magnetic core.

Core type


In core type, the magnetic core is built of laminations to form a rectangular frame and the windings are arranged concentrically with each other around the legs or limbs. The top and bottom horizontal portions of the core are called a yoke. The yokes connect the two limbs and have a cross-sectional area equal to or greater than that of limbs.

Each limb carries one-half of primary and secondary windings. The two windings are closely coupled together to reduce the leakage reactance. The low voltage winding is wound near the core and high voltage winding is wound over low voltage winding away from the core in order to reduce the amount of insulating materials required.

Shell Type


In shell-type, the windings are wound around the central limb and the flux travels through two side limbs and completes its circuit. The central limb carries total mutual flux while the side limbs forming a part of a magnetic circuit carry half the total flux. The cross-sectional area of the central limb is twice that of each side limbs

Step-up Transformer:

A transformer that changes voltage from a lower voltage to a higher voltage called a step-up transformer. Step-up X'mer has more number of turn in secondary winding than primary hence voltage per turn increase, it increases voltage level, mostly use at generating station to transmit electrical power in transmission line at a higher voltage.

Step Down Transformer:

A transformer that lowers down voltage to utilization voltage called a step-down transformer, it reduced the voltage from higher to lower voltage level. It has less number of a turn on secondary winding compare to the primary winding.

It is used in a distribution network, where the transmission line end. mostly delta - star-connected windings, the primary is connected in delta and secondary is in star for deriving neutral point.

Note that step-down x'mer can be used as step up transformer, in which secondary of a step down x'mer becomes primary and primary of the step down becomes secondary winding.

Step up or step down working operation of the transformer are decided after installation on site.

Parts of the transformer :

Magnetic core:

The magnetic core is made up of magnetic material like CRGO (cold rolled grain oriented steel). this material has high permeability. The core is a stack of thin silicon-steel lamination to reduced eddy current losses and separated from each other by using thin layers of varnish.

Winding :

Winding is made up of copper conductor and wound on a magnetic core.
  • In a core-type transformer, most of the part of the core is surrounded by the winding.
  • On the other hand in shell type transformer core surround a major part of the winding.
  • The advantage of the core type is it reduced core material but required more copper for winding
  • shell type required less conductor material but more core material.
  • The vertical portion of the X'mer called a limb and the horizontal portion called a yoke.
  • 1 phase shell type x'mer has 3 limbs and winding wound on the central limb and core type has two limbs and winding wound on both limbs.

Conservator :


Transformer oil should not be allowed to come in contact with atmospheric air as it may take up moisture resulting reduction in the dielectric strength of the oil. also, air may cause acidity and sludging of oil. To prevent this transformer provided with a conservator.

The function of a conservator is to take up construction and expansion of transformer oil during working without allowing it to come in contact with outside air. The conservator consists of an airtight metal drum-like structure fitted above the transformer top and connected with it by a pipe.

The main tank completely filled with the oil when cold. The conservator tank partially filled with oil. So space for expansion of oil is provided and sludge formed remains in the conservator itself and does not go to the main tank.

Breather :

When temperature changes, the oil expands or contracts and there is a movement of air takes place. When the transformer cools down, the oil level goes down and the air is drawn inside. this process is known as breathing. The air, drawn in is passed through a breather for removing moisture from the air. The breather consists of a small vessel containing a silica gel crystal impregnated with cobalt crystal.

Parallel Operation

Transformers are said to be connected in parallel operation when their primary connected to a common supply and secondary connected to a common load. 

The function of the transformer is to deliver power to the locality, city. But in some cases, a single transformer unit is unable to fulfill the load requirement. in this case, Parallel Operation Of Transformer is preferred instead of a single transformer, multiple parallel transformers are installed instead of one large unit. 

The below diagram shows the two single-phase transformers are connected in parallel, connected to the same voltage source on the primary side. 

To check polarities of terminals voltmeter"v" connected in series with two secondaries. Zero voltmeter readings show proper polarities. 

Using two or more transformers in parallel instead of a large single unit is uneconomical but has certain advantages.

Reasons For Parallel Operation

The reason for the parallel operation of transformers are given as follows:

1) With two or more transformers in parallel, the power becomes more reliable if one of the transformers developed a fault, it can be removed from operation and load can be supplied from the remaining transformer thought at a reduced level. 

2) Parallel transformers can be switch on and off as per load demand as loads are uncertain during different times of the day.

3) The cost of the stand-by unit is much less when two or more transformers are installed. 

4) As time passage the power demand increases and become more than that of the rated KVA of the existing transformer. Under such circumstances, the need for an extra transformer arises. Hence to supply such excessive demand parallel transformers are installed. 

Conditions of Parallel Operation Of Transformers 

Necessary Conditions for parallel operation of the transformer
  1. Transformers must be connected with regard to Polarity. This condition must be fulfilled strictly. If secondary terminals connected with the wrong polarity large circulating current will flow and the transformers may get damaged.
  2. The turn ratios of the transformer should be equal.
Desirable Conditions for parallel operation of the transformer

1) Transformers must have the same voltage ratio, this condition should satisfy as accurately as possible since different secondary voltage would give rise to undesired circulating current.  

2) The equivalent impedances should be inversely proportional to the respective KVA ratings. 

3) The ratio of equivalent resistance to the equivalent reactance of all transformers should be the same. If the ratio of reactance/resistance of transformer to operate in parallel are not equal, the power factor of load supplied by transformers will not be equal.
4) Per unit impedances of the transformer should be equal. If this condition is not met, the parallel operated transformers will not share the load according to their respective KVA rating. Sometimes this condition is not fulfilled by the design of the transformer in this situation, external resistances or reactances of the proper amount added in series with either the primary or secondary circuit of the transformer. where the impedance is below the value required to satisfy the condition.

Parallel operations of Three-Phase Transformers

The conditions for parallel operation of a three-phase transformer are the same as that of the single-phase transformer except for some conditions as follows. 

  • The phase displacement between primary and secondary voltage must be the same for all transformers to be connected in parallel. 
  • The phase sequence must be the same. 

Why transformers rated output is in KVA instead of KW?

The rated output voltage is express in KVA rather than kilowatt (KW) because the rated output is limited by heating and hence by losses in the transformer. These losses depend on voltage (core loss) and current (I2R) loss and remain unaffected by the load power factor.
For this reason, we mention output in KVA instead of KW.

Advantages of transformer :

  • Used for stepping up or down supply voltage.
  • High efficiency compared to other electrical machines.
  • with the use of a x'mer, long-distance ac power transmission is possible.
  • Distribute power at high voltage.
  • Insulate circuits/establish separately derived circuits.
  • Provide 3-wire secondary circuits..
  • Provide electrostatic shielding transient noise protection.

Auto transformer Starter
Buchholz Relay Working
Open Circuit and Short Circuit Test
Polarity Test

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