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.