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Arabic (عربي) Running two or more transformers in parallel is the standard way to add capacity to an existing installation, to provide redundancy, or to improve efficiency by switching units in and out with the load. It works reliably — but only when a specific set of conditions is met. Ignoring any one of them produces circulating currents, uneven loading, or in the worst case, damage to both machines.
The Four Conditions
Vector Group Compatibility
The vector group determines the phase relationship between HV and LV windings. Two transformers with different clock numbers produce LV voltages that are out of phase with each other, and connecting them in parallel places that phase difference across the two windings.
A 30° phase difference between two 400 V systems creates a voltage difference of roughly 207 V across the connection — driven only by the transformer impedances, which are deliberately low. The resulting current is limited only by the sum of the two impedances and can exceed the short-circuit rating.
The full notation and clock convention are covered in Transformer Vector Groups Explained.
Load Sharing Between Transformers
When two transformers of different rating operate in parallel, they share load in inverse proportion to their impedances — not in proportion to their kVA ratings. The unit with lower impedance takes a larger share.
This is why matching impedances matters. Two transformers of the same rating but different impedance will not share load equally, and the total usable capacity is less than the sum of the two ratings.
Worked Example — Matched Impedances
Two units in parallel: 1000 kVA at 6 % and 630 kVA at 6 %, total connected load 1400 kVA.
Ratio of shares: (1000 / 6) : (630 / 6) = 166.7 : 105 = 1.587 : 1
Unit 1 carries: 1400 × 166.7 / 271.7 = 859 kVA (86 % of rating)
Unit 2 carries: 1400 × 105 / 271.7 = 541 kVA (86 % of rating)
Both units loaded equally in percentage terms — the ideal case.
Worked Example — Mismatched Impedances
Now the same two units with their standard impedance values — 1000 kVA at 6 % and 630 kVA at 4 %, as given in the table above.
Ratio of shares: (1000 / 6) : (630 / 4) = 166.7 : 157.5 = 1.058 : 1
Unit 1 carries: 1400 × 166.7 / 324.2 = 720 kVA (72 % of rating)
Unit 2 carries: 1400 × 157.5 / 324.2 = 680 kVA (108 % of rating)
The smaller unit is overloaded by 8 % while the larger runs at 72 %.
To keep the 630 kVA unit within its rating, total load would have to be limited to roughly 1300 kVA — meaning 330 kVA of installed capacity cannot be used. This is why transformers intended for parallel operation are often specified with matched impedance rather than the standard value for their rating.
Circulating Current from Voltage Ratio Mismatch
If the two transformers produce slightly different secondary voltages — because of different turns ratios, or because they are on different tap positions — a current circulates between them even with no external load.
Because transformer impedances are low by design, even a small voltage difference produces a significant current. Two 1000 kVA units at 6 % impedance with a 1 % voltage difference circulate roughly 8 % of rated current continuously — current that produces losses and heating without delivering any useful power.
Fault Level with Units in Parallel
Parallel operation increases the available short-circuit current at the common busbar, because both transformers contribute to a downstream fault. For two identical units the fault level approximately doubles.