Parallel Operation of Transformers – Reasons and Conditions

The transformers are said to be connected in parallel when their primary windings are connected to a common supply and the secondary windings are connected to a common load.

Reasons for parallel operation

The reasons for operating the transformers in parallel are as follows:

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, the capacity of the sub-station can be expanded by connecting a transformer in parallel with the existing transformer.
3. The total load requirement can be supplied by number of transformers of standard size. This 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 for parallel operation of transformers:

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 same turns or transformation ratio. If transformers have slightly 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 X/R 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.

Conditions for parallel operation of Single-Phase transformers:

Necessary conditions

1. The transformers must have the same polarities.
2. The transformers should have equal turn ratios.

Desirable conditions

1. The voltages at full load across the transformer’s internal impedance should be equal.
2. The ratios of their winding resistances to reactances should be equal for both transformers. This condition ensures that both transformers operate at the same power factor, thus sharing active power and reactive voltamperes according to their ratings.

Three-phase transformers in parallel

The conditions for proper parallel operation of single-phase transformers are as follows:

1. The polarities of the transformers should be the same.
2. Identical primary and secondary voltage ratings.
3. Impedances inversely proportional to the kVA ratings.
4. Identical X/R ratios in the transformer impedances.

The condition for the parallel operation of single-phase and three-phase transformer is the same but with the following additions:

1. The phase sequence of the transformers must be identical.
2. The primary and secondary voltages of all the transformers connected in parallel must have the same phase shift.

Conclusion:

If the two transformers have equal impedances (i.e., equal resistance and equal reactance) –
They will share a load equally.

• If their per unit impedance is same, they will share a load in proportion to their ratings.
• Both the transformers will operate at the same power factor.
• Currents IA and IB will be in phase and the total current I will be the arithmetic sum of two currents.

If the two transformers have unequal impedances –

• Transformers will not share the load in proportion to their ratings.
• Transformers will not operate at the same power factor.
• Total current I will be the vector or phasor sum of two currents IA and IB.

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