Transformers
For the parallel operation of single phase transformers it is necessary that they should have

same polarity
same efficiency
same number of turns on the secondary side.
same kVA rating

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Transformers
An ideal transformer is one which has

no losses and magnetic leakage
core of stainless steel and winding of pure copper metal
a common core for its primary and secondary windings
interleaved primary and secondary windings

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