Three-Phase Transformer

In practice, electrical power is generated, transmitted and distributed by using three-phase system. Therefore, we require a three-phase transformer to step-up or step-down the voltage at various stages of a power system network.

We can construct a three-phase transformer in one of the following two ways −

We can connect three separate single-phase transformers for 3-phase operation. This arrangement is known as a three-phase bank of transformers.
We can construct a single three-phase transformer which consists of a magnetic core and having windings for all the three phases. This whole assembly is combined in a single structure.

The windings of a three-phase transformer may be connected in the following ways −

Star-Star Connection − In this case, both primary and secondary windings are connected in star connection.
Delta-Delta Connection − In this case, both primary and secondary windings are connected in delta connection.
Delta-Star Connection − In this case, the primary winding is connected in delta, while the secondary winding is connected in star.
Star-Delta Connection − In this case, the primary winding is connected in star while the secondary winding is connected in delta.

Construction of Three Phase Transformer

A three phase transformer can be constructed in two ways namely core-type construction and shell-type construction.

Core Type Construction

In the core type construction of 3-phase transformer, the magnetic core has three vertical limbs and two horizontal sections as shown in Figure-1. Here, one pair of primary and secondary windings is placed on each limb. The low voltage (lv) winding is placed next to the core while the high-voltage (hv) winding is wound around the lv winding.

Shell Type Construction

A shell type three-phase transformer can be constructed by stacking three single-phase shell-type transformers together as shown in Figure-2. In this case, both primary and secondary windings are placed on the central limb and the two outer limbs serve the path for flux. The behavior of a shell-type three-phase transformer is almost similar to that of a bank of three single-phase transformers.

Advantages of a Bank of Three Single Phase Transformers

The following are the major advantages that a bank of three single-phase transformers have over a three-phase unit transformer −

When one 1-phase transformer of a bank of transformers is damaged and isolated from the service, the remaining two transformers may be used to supply power in open-delta connection.
In the bank of transformers, we can provide a single-phase transformer with higher kVA rating than the others to supply an imbalance load.
For a bank of three single-phase transformers, the standby requirement is lesser.
It is more convenient to transport a 1-phase transformer than a 3-phase transformer.

Advantages of a Three Phase Unit Transformer

For the same kVA rating, a three-phase unit transformer has the following advantages over a bank of three single-phase transformers −

A three-phase unit transformer is smaller in size, light in weight and cheaper.
It is more efficient than bank of transformers.
Its installation is simple.

Depending upon the requirements, we use both bank of transformers and a three-phase unit transformer. However, it is a common practice to use a three-phase unit transformer.