Paralleling 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. In addition to requirements listed before for single-phase transformers, phase angular displacements (phase rotation) between high and low voltages must be the same for both. The requirement for identical angular displacement must be met for paralleling any combination of three-phase units and/or any combination of banks made up of three single-phase units.


CAUTION: This means that some possible connections will not work and will produce dangerous short circuits. See Table 2 on Picture 2 below.


For delta-delta and wye-wye connections, corresponding voltages on the high-voltage and low-voltage sides are in phase. This is known as zero phase (angular) displacement. Since the displacement is the same, these may be paralleled. For delta-wye and wye-delta connections, each low-voltage phase lags its corresponding highvoltage phase by 30 degrees. Since the lag is the same with both transformers, these may be paralleled. A delta-delta, wyewye transformer, or bank (both with zero degrees displacement) cannot be paralleled with a delta-wye or a wye-delta that has 30 degrees of displacement. This will result in a dangerous short circuit. Table 1 on Picture 2 shows the combinations that will operate in parallel, and Table 2 on Picture 2 shows the combinations that will not operate in parallel.

Picture 1: Delta-Wye and Wye-Delta Connections Using Single-Phase Transformers for Three-Phase Operation

Picture 2: Parallel Connections of Three-Phase Transformers


Wye-wye connected transformers are seldom, if ever, used to supply plant loads or as GSU units, due to the inherent third harmonic problems with this connection. Delta-delta, delta-wye, and wye-delta are used extensively at Reclamation facilities. Some rural electric associations use wye-wye connections that may be supplying structures in remote areas. There are three methods to negate the third harmonic problems found with wye-wye connections:

1. Primary and secondary neutrals can be connected together and grounded by one common grounding conductor.
2. Primary and secondary neutrals can be grounded individually using two grounding conductors.
3. The neutral of the primary can be connected back to the neutral of the sending transformer by using the transmission line neutral.

In making parallel connections of transformers, polarity markings must be followed. Regardless of whether transformers are additive or subtractive, connections of the terminals must be made according to the markings and according to the method of the connection (i.e., delta or wye).

CAUTION: As mentioned above regarding paralleling single-phase units, when connecting additive polarity transformers to subtractive ones, connections will be in different locations from one transformer to the next.