PARALLELING TRANSFORMER’S LOADING CONSIDERATIONS

PARALLELING TRANSFORMER’S LOADING CONSIDERATIONS

Paralleling transformers has been practiced mostly in commercial and industrial facilities along with electric utilities where reliability and the quality of power is the main objective. For many years, it has been a practice that transformers installed for paralleling have the same kVA, turns ratio and impedances which served also as a reason why most of the engineers today is having a hard time understanding load sharing and circulating currents. Often times during transformer replacement or upgrades, they tend to not know the impact of paralleling transformers with different parameters and could later result to transformer failures.

Simple transformer paralleling with the same kVA, turns ratio and impedance is not enough in practicing this kind of activity. An engineer must consider all possible scenarios and simulate the effects of his decisions.

The most appropriate type of transformers that should be use in paralleling must have the same turns ratio, per cent impedance, and X/R ratios. This is necessary to ideally divide the current that passes through the transformer and will result an even loading between the two. So when transformers are connected in parallel even though they have at least one of the parameters mentioned to be different will create circulating currents or unwanted current division in the transformers. This set-up tends to lower the efficiency and reduces the maximum capacity that the two combined transformers can carry.

According to Schneider Electric’s Application Guide, transformers should not be operated in parallel when:

• The division of load is such that, with the total load current equal to the combined kVA rating of the transformers, one of the transformers is overloaded.
• The no-load circulating currents in any transformer exceed 10% of the full load rating.
• The combination of the circulating currents and full load current exceed the full load rating of either transformer.

From the list above, the circulating currents represent the current flowing at no load in the high and low voltage windings, excluding exciting currents. Full load current is the current flowing in the transformer with a load connected, absent of exciting and circulating currents.

Recommended Connections for Transformer Paralleling

Same Impedances— Same Turns Ratios—Same kVA

Same Impedances— Same Turns Ratios—Different kVA

Different Impedances— Same Turns Ratios— Same kVA

Not Recommended Connections for Transformer Paralleling

Different Impedances— Same Turns Ratios—Different kVA

Different Impedances— Different Ratios—Same kVA

Different impedances— Different ratios—Different kVA

Transformers with different winding connections like a Delta-Wye transformers should never be attempted to be paralleled with Delta-Delta transformers regardless whether they have all equal parameters. The reason of which is in Delta-Wye transformers the secondary line to neutral voltages possesses a phase shift of 30 degrees which can result to extremely high circulating currents in the transformers that would later result to transformer overheating.