Three Phase Transformer and its Construction


A three way transformer is considered to be the backbone of the electrical power distribution system. It is used to transfer a huge amount of power. Its main function is to step up and step down the voltages at different stages of a power system network. however, it is also possible to use three interconnected single phase transformer as a replacement for three phase transformers but there are certain advantages of using three way transformers which include-

  • It occupies less space than the three single phase transformers
  • Cost effective
  • The overall bus-bar structure, switchgear and installation of ‘three phase transformer’ is less complicated

Three phase transformer construction

A three phase transformer is constructed in the following ways

  • Three separate single phase transformers are joined to perform the three phase operation which is known as core type three phase transformer
  • A single three phase transformer is used in which the cores and windings for all the three phases are merged into a single structure which is known as the shell type three phase transformer

Core type construction

The core is composed of three legs or limbs. Each limb contains primary and secondary windings in cylindrical form. The core is generally made up of three limbs in the same plane which is built with stack lamination. Each of these limbs carries high and low voltage winding. The low voltage windings are insulated from the core than the windings of high voltage. The low windings are located next to the core with proper insulation between the core and the low voltage windings. The high voltage windings are positioned over the low voltage windings with appropriate insulation between them. The construction is shown in the figure below.

Shell type three phase

When it comes to three phase transformer, three phases are more independent than they are in core type. Each phase comes with its individual magnetic circuit. Both the construction of three phase core type transformers and three phase shell type transformers are similar to each other.

The functioning principle of a three phase transformer is similar to the working principle of a single phase transformer. Power is shifted from primary to secondary windings by the phenomenon of mutual induction. The biggest disadvantage of a three phase transformer is that even in case of a fault in one phase, the whole transformer has to be removed from service for repairing purpose.