Parallel operation (connections) of transformers|Conditions for parallel operation of transformers

Definition:

Parallel operation of transformer is nothing but basically it is connections in which the primaries of two or more transformers are energized from the same source and the secondaries are connected to supply the same load.
In other words the transformers are said to be connected in parallel when their windings are connected to HV and LV bus-bars common on both.

Need of parallel operation:

1. If the amount of power to be transformed is greater than that which can be handled by one transformer,it becomes necessary to employ two or more units in parallel.
2. With increase in demand on power system it becomes necessary either to replace the existing unit by unit of greater capacity or to install additional unit to carry portion of the load. If we replace the existing unit by greater capacity unit then the cost of new unit is very high, hence it is better to add an unit in parallel unit with existing unit.
3. The cost of standby unit is reduced since spare transformer units are invariably required to ensure continuity of service in case of damage or some fault, it is sometimes found desirable to supply the load through two or more units and thereby to reduce the size of the spare unit.

Conditions for parallel operation of transformer:

For satisfactory parallel operation of transformers, the following conditions to be fulfilled are:

1. polarities of the transformers are the same.
2. The voltage ratings of both the transformers are the same.
3. Same frequency ratings.
4. Percentage impedances of the transformers are the same.(same in magnitude and have the same phase angle)
5. Phase sequence should be same.

          The main drawback of operating two or more transformers in parallel is that available short-circuit current is increased, sometimes to a level beyond the interrupting capacity of the existing over-current devices.