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What are the necessary conditions for parallel operation of MV/LV transformers?

The following conditions for the connection of power transformers in parallel are required:
It is preferred to connect in parallel transformers having the same characteristics stated below:
  • Voltage ratio
  • Rated power
  • Short circuit impedance.
  • Coupling symbol of windings as for example: Dyn 11
  • Impedances of the LV links between the transformers and the main LV switchboard where the paralleling is realized.
For transformers having unequal rated power their internal impedances are in the ratio of the rated power of the transformers.
Connection in parallel of transformers having a power ratio equal or higher than two is not recommended.
When the transformers do not comply with the above requirements, recommendations for their paralleling shall be asked to the manufacturer
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