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Conditions for Paralleling two Transformers

Transformers may need to be paralleled with other transformers to share loads greater than the capacity of the individual transformer.  When transformers are to be connected in parallel, it is necessary for them to satisfy certain basic conditions. 

They are

The Same Voltage Ratio.

Two transformers in Parallel should have the same primary and secondary voltage ratings.  Any error in the voltage ratio would cause heavy circulating currents to flow between the transformers.   This circulating current will result in a corresponding imbalance in the primary currents, and result in overloading of one transformer.  This circulating current will result in increased copper losses. 

The Same Percentage impedance

For two transformers of different capacity to share the load proportionally, their impedances should be in the inverse ratio of their  ratingsThat is, the percentage resistance and the percentage reactance of the two transformers should be equal to ensure equal sharing of the active and reactive loads between the transformers. 

The Same Polarity

Two Transformers in parallel should have the same polarity.  Connecting transformers with wrong polarity can result in circulating currents or short circuits.  Some transformers may have inherent polarity errors which needs to be detected before connecting them.

The Same Phase Sequence

Only Transformers having the same Phase sequence can be connected in parallel.  The phase sequence can be detected using a phase sequence indicato.

The Same Phase Angle

The difference in the Phase angle between the secondary voltages should be zero.  This is ensured by checking the compatibility of the vector groups of the transformers to be paralleled.