Conditions for parallel operation of transformers

Two transformers are in parallel when their primaries are connected to the same supply bus and their secondaries to the same load bus. Parallel operation is used to meet load beyond one unit's rating, for reliability (one can be taken out for maintenance) and to add capacity as load grows.

  Supply bus ===+=============+====
                |             |
              [ T1 ]        [ T2 ]
                |             |
  Load bus  ====+=============+==== -> load

Conditions and reasons

1. Same voltage ratio (same rated primary and secondary voltages)

• Reason: if the secondary EMFs differ, their difference drives a circulating current $I_c = \frac{E_A - E_B}{Z_A + Z_B}$ through the closed loop of the two secondaries even at no load. Because $Z_A + Z_B$ is small, even a small voltage difference gives a large current, causing extra copper loss, heating and reduced capacity.

2. Correct polarity

• Reason: if connected with opposite polarity, the two EMFs add round the loop instead of opposing. The loop then has $2E$ acting on a very small impedance, which is a dead short circuit and will damage the transformers.

3. Equal per-unit (percentage) impedance

• Reason: load divides in inverse ratio of ohmic impedance. Only when per-unit impedances on their own ratings are equal does each transformer carry load in proportion to its kVA rating. Otherwise the one with lower pu impedance gets overloaded before the bank reaches its full rating.

4. Same ratio of reactance to resistance (X/R)

• Reason: if X/R differ, the currents of the two transformers are not in phase with each other. Each then operates at a different power factor from the load, and the arithmetic sum of their currents exceeds the load current, causing extra loss and reduced useful capacity.

5. Same phase sequence (three-phase)

• Reason: if the sequence differs, in each cycle pairs of phases are short-circuited, giving very large currents.

6. Same phase displacement / vector group (three-phase)

• Reason: e.g. a Y-y (0°) and a Y-d (30°) cannot be paralleled; the 30° difference in secondary voltages gives a large circulating current. Transformers must be from the same group (e.g. Dyn11 with Dyn11).

Summary