Three-Phase Transformer Connections

Three-phase power is attainable with one three-phase transformer, which is constructed with three single-phase units enclosed in the same tank or three separate single-phase transformers. The methods of connecting windings are the same, whether using the one three-phase transformer or three separate single-phase transformers.

Wye and Delta Connections

The two common methods of connecting three-phase generators, motors, and transformers are shown in Picture 1. The method shown in at Picture 1 (a) is known as a delta connection, because the diagram bears a close resemblance to the Greek letter ∆, called delta. The other method, Picture 1 (b), is known as the star or wye connection. The wye differs from the delta connection in that it has two phases in series. The common point “O” of the three windings is called the neutral because equal voltages exist between this point and any of the three phases. When windings are connected wye, the voltage between any two lines will be 1.732 times the phase voltage, and the line current will be the same as the phase current. When transformers are connected delta, the line current will be 1.732 times the phase current, and the voltage between any two will be the same as that of the phase voltage.

Picture 1: Three-Phase Connections: (a) Delta; (b) Wye

Three-Phase Connections Using Single-Phase Transformers

As mentioned above, single-phase transformers may be connected to obtain three-phase power. These are found at many facilities, at shops, offices, and warehouses. The same requirements must be observed as in section 2.18, “Parallel Operation of Single-phase Transformers for Additional Capacity,” with one additional requirement—in the manner connections are made between individual single-phase units. ANSI standard connections are illustrated below in the following figures. There are other angular displacements that will work but are seldom used. Do not attempt to connect single-phase units together in any combination that does keep the exact angular displacement on both primary and secondary; a dangerous short circuit could be the result. Additive and subtractive polarities can be mixed (see the following pictures). These banks also may be paralleled for additional capacity if the rules are followed for three-phase paralleling discussed below. When paralleling individual three-phase units or single-phase banks to operate three phase, angular displacements must be the same.

Picture 2: Delta-Delta Connections, Single-Phase Transformers for Three-Phase Operation

Picture 3: Wye-Wye Connections, Using Single-Phase Transformers for Three-Phase Operation


Note: These connections are seldom used because of 3rd harmonic problems.

Picture 2 shows delta-delta connections. Picture 3 shows wye-wye connections, which are seldom used at Reclamation facilities, due to inherent third harmonic problems. Methods of dealing with the third harmonic problem by grounding are listed below. However, it is easier just to use another connection scheme (i.e., deltadelta, wye-delta, or delta-wye), to avoid this problem altogether.