Conditions For Transformer Parallel Operation

Parallel operation transformer:

Operating two are more transformer in parallel with each other such operation is called transformer parallel operation. Condition for Parallel operation,

Transformer Parallel Operation
Transformer Parallel Operation

Important condition for Transformer parallel operation:

The secondary voltage of the all transformer should be equal in magnitude and phase.

The voltage ratio of the all transformer must be equal. i.e if the single transformer has the ratio of 11000/440 Volts means, you should connect the another transformer with the voltage ratio of 11kV/440 Volts. In many websites such as circuit globe, electrical4u, electrical easy etc. have written like this the voltage ratio should be same. Do you accept this?. I never accept this condition because of the voltage ratio means the ratio between the primary to secondary voltage is called voltage ratio. Actually, the real facts to operate transformers in parallel operation, the secondary voltage of each transformer parallel transformer should be same. For example, if you have the two different transformer 11 kV/110 volts and 110kV/1100 Volts. Can we operate these two transformer in parallel? No it is highly impossible, but their voltage ratio is same. That why I say the two transformers secondary voltage should be same. That is a correct answer.


Example: If can connected two transformer 11000V/440 Volts and 22000V/440 Volts surely we can perform parallel operation.

Phase sequence should be same:

The transformer should have same phase sequence. If we connect the transformer in wrong phase sequence, it creates high short circuit current in the transformers primary and secondary windings. Typically, we can say it as short circuit.

Vector group of the transformer should be same:

The transformer vector group should be same otherwise, the high circulating current will flow on the transformer’s windings.

Frequency of the transformer:

The parallel transformer should have same frequency.

Polarity should be same of the all transformer..

Considerable condition for transformer parallel operation:

Considerable condition means we can run the transformer without these conditions.

The percentage impedance of the parallel transformer can be equal. Because of the percentage impedance is only responsible for sharing the KVA output of the transformer. Again, current flow of these transformers are inversely proportional to their internal impedance. From these two statements it can be said that, impedance of transformers running in parallel are inversely proportional to their MVA ratings. In other words, percentage impedance or per unit values of impedance should be identical for all the transformers that run in parallel.

The ratio of their winding resistances to reactance’s should be equal for both the transformers. This condition ensures that both transformers operate at the same power factor, thus sharing their active power and reactive volt-amperes according to their ratings.


Advantage of parallel Operation:

  • Load sharing to multiple loads
  • Reduce installation cost. Instead of installing New MCC panel, PCC panel, new accessories etc. we can simply extend the existing PCC.
  • Maximize the power transfer to various loads
  • To maximize electrical power system availability

Disadvantage of Parallel operation:

  • During running condition, if one transformer tripped means then the parallel transformer also trips. There unnecessary power system shut down.
  • If the load demand is high, then we cannot create load schedule.
  • To maintain a single transformer, we should shut down the entire power system.
  • Poor Fault analysis.

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