Need and conditions for parallel operation of transformers

Need for parallel operation

Two or more transformers are connected in parallel (primaries to a common supply bus, secondaries to a common load bus) because:

• Load growth: a second unit can be added when the load exceeds the rating of the existing one.
• Reliability: if one transformer fails or is taken out for maintenance, the others still supply (part of) the load.
• Efficiency: at light load some units can be switched off so the rest run near maximum efficiency.
• Transport and spares: one very large unit is hard to transport, and a small spare unit is cheaper to keep.

  HV bus  ==========+==============+=====
                    |              |
                  [T1]           [T2]
                    |              |
  LV bus  ==========+==============+===== --> load

Conditions for parallel operation

1. Same voltage ratio (same rated primary and secondary voltages). If the ratios differ, the no-load secondary emfs are unequal and a circulating current $I_c = (E_A - E_B)/(Z_A + Z_B)$ flows round the loop even with no load. Since the impedances are small, even 1% difference gives a large current, which causes extra copper loss and heating and reduces the useful capacity.

2. Same polarity (essential). The terminals must be connected with correct polarity. With wrong polarity the two secondary emfs add in the closed loop, which acts like a dead short circuit, and very large currents flow.

3. Same per-unit (percentage) impedance. Load is shared in the inverse ratio of impedances: $S_A/S_B = Z_B/Z_A$. If the per-unit impedances are equal, each transformer takes load in proportion to its kVA rating, so none is overloaded while the others are under-loaded.

4. Same X/R ratio. If X/R differ, the currents in the two units are not in phase; the total current is less than the arithmetic sum, and the units work at different power factors, so the combined capacity is not fully used.

5. For three-phase transformers, additionally:

   • Same phase sequence (essential) – otherwise pairs of phases are short-circuited.
   • Same vector group / phase displacement (essential) – e.g. a Dyn11 unit cannot be paralleled with a Dyn1 unit, as the 60° phase difference between secondary voltages would cause huge circulating currents. Units of the same group (0°, 180°, −30°, +30°) can be paralleled.

Conditions 2 and 5 (polarity, phase sequence, phase displacement) must be met exactly; conditions 1, 3 and 4 should be met as closely as possible.