Parallel Operation of Transformers

For supplying a load in excess of the rating of an existing transformer, a second transformer may be connected in parallel with it.

When the primaries and secondaries of the two or more transformers are connected separately to the same incoming and outgoing lines to share the load, the transformers said to be connected in parallel.

parallel operation of transformers

It is seen that primary windings are connected to the supply bus bars and secondary windings are connected to the load bus-bars. In connecting two or more than two transformers in parallel, it is essential that their terminals of similar polarities are joined to the same bus-bars.

Necessity 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. When the load on the transmission lines increases beyond the capacity of the installed transformer. To overcome this problem one way is to replace the existing transformer with the new one having larger capacity and the other way is to place one more transformer is parallel with the existing one to share the load. The cost of replacing the transformer is much more than placing another one in parallel with the existing one.
  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 in the event of 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.