Parallel Operation Of Transformers

The following conditions must exist for transformers to operate satisfactorily in parallel:

1. Connection diagrams must be identical. Paralleling transformers with different connection diagrams is similar to short circuiting their secondary windings.
2. Voltage ratios must be the same. If voltage ratios are not the same, circulating currents will flow in the secondaries with no or little load, and the division of load will be improper.
3. Percent impedance, including primary and secondary leads to each transformer, should be nearly equal. If the impedance's are equal and the turns ratios are identical, the paralleled transformers will divide the load currents (properly) in proportion to their kVA ratings. If the percent impedance's are different, the transformer with the lower percent impedance will take more than its proper share of the load.

Example: Operation with unequal turns ratios:

Two transformers with similar characteristics (both rated 7,200-240 Volts, 3% Z, equal X/R ratios but with different kVA ratings) are to be operated in parallel to serve a 75 kVA load. One unit is rated 25 kVA and the other is rated 50 kVA.

By mistake, the 25 kVA unit has its primary tap set 5% low (6840 V tap) which would give an open-circuit voltage of 252.6 V instead of the desired 240 V.

When paralleled and energized from a 7,200 V primary, the circulating current is 117 percent of rated current for the 25 kVA unit before any load current is drawn. The open-circuit voltage from the combination is 244.2 V.

Solution:

Difference between O-C voltages = 12.6 V (= 5.25%)
Loop impedance (25 kVA base) = 4.50%
Circulating current (use per-unit values) = 1.17 per unit
(by Ohm's Law) = 117%
Impedance voltage divider ratio = 1.5%/4.5% = 1/3
Terminal voltage rise above 240 V = (1/3) x (12.6 V) = 4.2 V
"No load" terminal voltage = 240 V + 4.2 V = 244.2 Volts