Types of Three Phase Transformer

In this topic, you study Types of Three Phase Transformer.

As in the case of single phase transformers, three phase transformers also have core type and shell type constructions as shown in Fig. 1. Core type transformers generally have three limbed core construction (Fig. 1 (a)) with each limb having the primary and secondary windings arranged concentrically. With very large three phase core type transformers, it becomes necessary to reduce the overall height so as to facilitate their transportation along the road or railway routes. In such cases, five-limbed cores with reduced yoke sizes are employed (Fig. 1 (b)). In shell type (Fig. 1 (c)), each phase of the transformer has a magnetic circuit independent of the others. Hence, the three phases are more independent of one another than they are in the core type.

(a)

 

(b)

(c)

Fig. 1: Common types of three phase transformers : (a) Three phase core type transformer having three limbs, (b) Three phase core type transformer having five limbs, (c) Three phase shell type transformer

In practice, for three phase circuits, use of a three phase transformer is generally preferred over a bank of three single phase transformers. The relative advantages and disadvantages of three phase transformers can be outlined as follows.

Advantages of Three Phase Transformer

1. For a given output, three phase transformers have considerably less weight and occupy much less floor space.
2. Lower cost. For equal rating, the cost of a three phase transformer is about 15
3. Higher efficiency and better voltage regulation.
4. Being compact and light, they are easy to handle. This decreases the cost of transportation.

Disadvantages of Three Phase Transformer

1. Anything which injures the winding of one phase is likely to injure the other windings.
2. If any one phase becomes faulty, the whole transformer is to be removed from the service. This is not the ease with a bank of three single phase transformers. If the fault is developed on any one transformer, the system may be maintained with reduced capacity using V or open-delta connection.
3. Greater cost of a spare unit.
4. Greater cost of repairs.