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## Principles of transformer paralleling

Electrical systems have been using paralleled transformers for many years. Electrical utilities are ideal examplesÂ of these applications. Their main objectives are reliability and power quality along with keeping consumersÂ on-line.

Utility power engineers understand very well the operation of paralleling transformers when it comesÂ to load sharing between transformers and when circulating currents exist.

Many times auto-tap transformersÂ are used to adjust voltage levels due to loading conditions. Often times these tap changes produce circulatingÂ current in parallel-operated transformers.

Power engineers in industrial and commercial facilities are less likely to understand circulating currents andÂ load sharing because most transformers installed in parallel have the same kVA, turn ratios and impedances.

However, as systems change over time, and transformers are replaced or added, then users need to knowÂ the impact of paralleling transformers with different parameters.

1. Principles of Paralleling
2. Limiting Conditions
3. Equal Impedances â€“ Equal ratios â€“ Same kVA
4. Equal Impedances â€“ Equal ratios â€“ Different kVA
5. Unequal Impedances â€“ Equal ratios â€“ Same kVA
6. Unequal Impedances â€“ Equal ratios â€“ Different kVA
7. Unequal Impedances â€“ Unequal ratios â€“ Same kVA
8. Unequal Impedances â€“ Unequal ratios â€“ Different kVA
9. Delta-Delta to Delta-Wye

Transformers connected in parallel have the same voltage on the primary and the secondary windingÂ terminals. The difference in the voltage between the primary and secondary windings is the turns ratio.

For these terminal voltages to be the same for the paralleled transformers their impedance drop mustÂ be identical. Therefore, under any condition of load, the current will be divided such that the product ofÂ impedance and current in one transformer is equal to the product of impedance and current in the other.

Also, if the turn ratios of the transformers are different, but the primary and secondary terminal voltagesÂ are the same in both transformers, then circulating currents must be flowing between the transformersÂ even at no load.