Three-Phase Transformers

Three-phase voltages may be transformed by means of three-phase transformers. The core of a three-phase transformer is made with three legs, a primary and a secondary winding of one phase being placed on each leg. It is possible to construct the core with only three legs since the fluxes established by the three windings are 120° apart in time phase. Two core legs act as the return for the flux in the third leg. For example, if the flux is at a maximum value in one leg at some instant, the flux is half that value and in the opposite direction through the other two legs at the same instant.

The three-phase transformer takes less space than do three single-phase transformers having the same total capacity rating since the three windings can be placed together on one core. Furthermore, three-phase transformers are usually more efficient and less expensive than the equivalent single-phase transformer banks. This is especially noticeable at the larger ratings. On the other hand, if one phase winding becomes damaged, the entire three-phase transformer has to be removed from the service. Three-phase transformers can be connected in any of the aforementioned connection types. The difference is that all connections are made inside the tank.

Figures 3.53 through 3.57 show various connection diagrams for three-phase transformers. Figure 3.53 shows a Δ–Δ connection for 120/208/240 V three-phase four-wire secondary service at 0° angular displacement. It is used to supply 240 V three-phase loads with small amounts of 120 V single-phase load. Usually, transformers with a capacity of 150 kVA or less are built in such a design

Figure 3.53 Three-phase transformer connected in delta-delta

Figure 3.54 Three-phase transformer connected in open-delta.

Figure 3.55 Three-phase transformer connected in Y–Δ.

that when 5% of the rated kilovolt-amperes of the transformer is taken from the 120 V tap on the 240 V connection, the three-phase capacity is decreased by 25%.
Figure 3.54 shows a three-phase open-Δ connection for 120/240 V service. It is used to supply large 120 and 240 V single-phase loads simultaneously with small amounts of three-phase load. The two sets of windings in the transformer are of different capacity sizes in terms of kilovolt-amperes. The transformer efficiency is low especially for three-phase loads. The transformer is rated only 86.6% of the rating of two sets of windings when they are equal in size, and less than this when they are unequal.
Figure 3.55 shows a three-phase Y–Δ connection for 120/240 V service at 30° angular displacement. It is used to supply three-phase 240 V loads and small amounts of 120 V single-phase loads.
Figure 3.56 shows a three-phase open-Y open-Δ connection for 120/240 V service at 30° angular displacement. The statements on efficiency and capacity for three-phase open-Δ connection are also applicable for this connection.
Figure 3.57 shows a three-phase transformer connected in Y–Y for 120/208Y-V service. The connection
allows single-phase loads to balance among the three phases.

Application of Distribution Transformers

Figure 3.56 Three-phase transformer connected in open-wye open-delta.

Figure 3.57 Three-phase transformer connected in Y–Y.