Electrical Machines
Maximum allowable tan - delta (tanδ) of dielectric dispassion factor of power transformer insulating oil is generally taken as

 0.5 at 90°C.
 2 at 90°C.
  1 at 90°C.
 3 at 90°C.

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Electrical Machines
For satisfactory parallel operation of two transformers which of the following conditions is most essential,

 MVA rating of all transformer inversely proportional to impedance.
 percentage impedance of all transformers must be same.
 voltage ratio of all transformers must be same.
  polarities of all transformers must be same.

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