by Ed Zientek, P.E.

Sometimes existing transformers are paralleled in industrial and commercial facilities when facility engineers, consultants or maintenance staffs are looking for ways of making power systems more reliable, provide better power quality, prevent voltage sags, or for additional load requirements. This article addresses the limiting conditions of connecting transformers in parallel and loading considerations when turn ratios, impedances and kVA ratings are different. Most engineers know that these parameters are important when paralleling transformers but there are some misconceptions of when circulating currents actually exist.