Explain need of parallel operation of transformer and write necessary condition for parallel operation
1 Answer


The transformers are said to be connected in parallel when their primaries are connected to a common supply and the secondaries are connected to a common load.

Reasons for operating transformers in parallel:

1) In case of large loads, it is not practical and economical if all the load is supplied by a single large transformer.

2) If the load becomes grater than the capacity of already installed transformer, capacity of the sub- station can be expanded by connecting a transformer in parallel with the existing transformer.

3) The total load requirement number of transformers of standard size. This can be supplied by reduces the spare capacity requirement.

4) There will be no interruption of supply in case of breakdown in a transformer or when the transformer is disconnected for maintenance.


Conditions which must be satisfied for satisfactory parallel operation:

1) Primary windings of transformers must be suitable for the supply system voltage and frequency.

2) The transformers should be properly connected with regard to polarity. Terminals of similar polarity must be joined together. A wrong polarity results in dead short circuit.

3) The transformers should have the sameturns or transformation ratio. different transformation ratio, then parallel operation is possible. But due to unequal secondary voltages, circulating current will flow in transformers even at no load.

4) The percentage impedance of transformers should be equal in magnitude and their XIR ratio must be same. The parallel operation is still possible. But the power factor at which transformers operate will be different from the power factor of load. Therefore, transformers will not share the load in proportion to their kVA ratings.

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