It is given that at any area where a utility operates; there will always be an increase in load as time goes by. System planning is very crucial and that it is always necessary to forecast well the growth of loads in the area. Having that increase in loads, utilities should upgrade or revamp existing lines, transformers and other electrical equipment.

While it is known mostly by electrical engineers that paralleling of single phase transformers are operational, many still don’t know that it is possible to parallel three phase transformers. Two or more three phase transformers, or two or more banks made up of three single phase units, can be connected in parallel for additional capacity.

While paralleling of three phase transformers is operational, there are certain conditions that should be met.

  1. Not all possible connections of paralleling three phase transformers are possible. Refer to tables below showing the operative and non-operative connections in paralleling three phase transformers.
  2. Phase angular displacements between high and low voltages must be the same for both three-phase transformers. DELTA-DELTA can be connected in parallel with WYE-WYE but cannot be connected with either DELTA-WYE or WYE-DELTA and vice-versa.
  3. Tap setting must be identical.
  4. It is recommended that kVA ratings of three phase transformers must be the same to avoid very high circulating currents and a possibility of overloading for the smaller transformer.
  5. Voltage ratings should be identical.
  6. Frequencies of both transformers must be identical.
  7. The percent impedance of one transformer must be between 92.5% and 107.5% of the other. Otherwise, circulating currents between the two transformers would be excessive.
Operative Parallel Connections of 3-Phase Transformers.
1 Delta Delta Delta Delta
2 Y Y Y Y
3 Delta Y Delta Y
4 Y Delta Y Delta
5 Delta Delta Y Y
6 Delta Y Y Delta
7 Y Y Delta Delta
8 Y Delta Delta Y




In-operative Parallel Connections of 3-Phase Transformers.
1 Delta Delta Delta Y
2 Delta Delta Y Delta
3 Y Y Delta Y
4 Y Y Y Delta