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When a 1-phase transformer is connected in parallel, there are four conditions for proper parallel operation, but when two or 3-phase transformer paralleling is done, there are two additional points along with those four conditions applied for 1-phase. Statement 1 - The phase sequence must be the same. Statement 2 - The phase shift between primary and secondary voltages must be the same for all transformers connected in parallel. From the above two Statements (s), which is/are the correct conditions in this regard?

When a 1-phase transformer is connected in parallel, there are four conditions for proper parallel operation, but when two or 3-phase transformer paralleling is done, there are two additional points along with those four conditions applied for 1-phase. Statement 1 - The phase sequence must be the same. Statement 2 - The phase shift between primary and secondary voltages must be the same for all transformers connected in parallel. From the above two Statements (s), which is/are the correct conditions in this regard? Correct Answer Statement 1 is true, Statement 2 is true

Condition for Parallel Operation of Three Phase Transformers:

Following are the necessary conditions for parallel operation of 3 phase transformers:

  • The line voltage ratio of the transformers must be the same.
  • The transformers should have equal per unit leakage impedance.
  • The ratio of equivalent leakage reactance to equivalent resistance should be the same for all the transformers.
  • The transformers should have the same polarity.
     

The above four conditions are also applicable for the parallel operation of single-phase transformers.

Apart from the above four conditions, there exist two more conditions that should be fulfilled for parallel operation of three-phase transformers:

  • The relative phase shift between the secondary line voltages of all transformers should be zero.
  • The phase sequence of secondary line voltages of all the transformers should be the same.

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