Circulating Current

[fgonz007]

I've noticed in distribution stations, when paralleling feeder breakers on different buses, the difference in the load of the two transformers divided by 2 becomes added to the breaker as circulating current. Once the load is transferred fully to one breaker, the circulating current "disappears". What causes this circulating current?

Thanks!!

 

[pwrtran]

1) Are the transformers from same primary source or different sources?
2) Are two transformers similar or dissimilar?

If both 1) and 2) is true in terms of turn ratio, % impedance and MVA, ULTC and position of the fixed tap if applicable. Then the circulating current may cause by the transformers sitting on different tap position.

If only 1) is true, then again the circulating current is caused by delta V of the secondary voltage.

If 1) is NOT true then 2) is not a factor, you may have active and reactive power flow in different directions.