Demonstrative Video
A 3-\(\Phi\) transformer can be constructed in two different ways:
Using one pre-assembled and balanced 3-\(\Phi\) transformer consisting of three pairs of single phase windings mounted onto one single laminated core
connecting together three 1-\(\Phi\) transformer known as transformer bank
Merits of 3-\(\phi\) over Bank of Three 1-\(\phi\) Transformers
The advantage of a single 3-\(\Phi\) transformer is that for the same KVA rating it will be smaller, cheaper, and lighter than three individual 1-\(\Phi\) transformers connected together because the copper and iron core are used more effectively
requires smaller quantity of iron and copper. Hence, for an equal rating its cost is nearly 15% lesser
It has smaller size and can be accommodated in smaller tank and hence needs smaller quantity of oil for cooling.
Because of smaller size, it occupies less space, has less weight.
It needs less number of bushings.
It operates at slightly better efficiency and regulation.
Disadvantages of a single 3-\(\phi\) unit
difficult and costly to repair three-phase transformers
difficult to transport single large unit of three-phase transformer than to transport three single phase transformers individually.
Three 1-\(\Phi\) cores are positioned at \(120^\circ\) to each other
For balanced 3-\(\Phi\)e sinusoidal voltages applied the fluxes \(\Phi_a\), \(\Phi_b\), and \(\Phi_c\) will also be sinusoidal and balanced
The total combined flux in the merged leg becomes zero
Therefore, the central leg can be removed as it carries no flux
This structure is not convenient for the core
Other way is core made up of three limbs in the same plane built using stack lamination
Each leg carries the low and high voltage winding
The magnetising paths of the leg a and c are greater than that of leg b (central), thus construction is not symmetrical, and result in imbalance in magnetising current
Shell type is constructed by stacking three single phase shell transformer
Winding direction of the central unit b is made opposite to that of units a and c
If the system is balanced with phase sequence a-b-c, the flux will also be balanced
Magnitude of the combined flux is equal to the magnitude of each of its components
3-Phase Transformers-Advantages and Disadvantages
Advantages of 3-\(\Phi\) transformer:
For the same KVA rating, the core material required is very less as compared to bank of three 1-\(\Phi\) transformer
It is lighter and smaller
It requires less space to install
Higher efficiency
Low cost compared with three units of 1-\(\Phi\) transformer
Bus bar structure and switchgear installation for single 3-phase unit is simpler
Only three terminals are required as compared to six terminals of 1-phase
Disadvantages of 3-\(\Phi\) transformer:
In case of fault or loss of one phase results in complete unit shut down because a common core is shared for all the three units.
The core of the defective unit would immediately saturate because of absence of an opposing magnetic field
This causes escape of magnetic flux to the metal enclosure from the core and raises the heating of the metallic parts
In some cases heating results in fire
Cost of repair is more for 3-phase transformer
To restore the service, spare unit cost is more as compared to one single transformer
Under self cooling the capacity is reduced