Wye-Wye Connection

Three single-phase transformers with their primaries and secondaries both connected in wye are shown in Figs. 6-23(a) and 6-23(b). The primary neutral is shown connected to the neutral of the source and the secondary neutral connected to that coming from the load. In many applications the neutral connection consists of ground. Connecting the primary neutral to the neutral of source assures balanced line-to-neutral voltage even if the load is unbalanced or if the transformers have unequal exciting admittances. The equivalent circuits of Fig. 6-11, 6-13, and 6-14 apply to each of the three transformers connected wye-wye with or without the ideal transformer just as they do in the delta-delta arrangement. Here also, if the transformers are identical and supply balanced 3-phase load, each transformer carries one-third of the 3-phase load.

It can be seen from Fig. 6-23 that the current in the transformer winding is the line current in the wye connection. The secondary currents I_a, I_b, and I_c are therefore practically in phase with the primary currents I_A, I_B, and I_C and if the exciting current is neglected, the current ratios are the reciprocals of the turns ratio, i.e.

[6-89]

Also, if the leakage impedance is neglected, the voltage ratios equal the turns ratios, thus

[6-90]

Phasor diagrams are shown for the wye-wye arrangement in Fig. 6-24. The wye connection is generally used in high-voltage applications because

Figure 6-23. Wye-wye connection, (a) Common physical arrangement of three single-phase transformers; (b) schematic diagram showing primary neutral connected to the source and secondary going to the neutral of the load. The load is not shown.

Figure 6-24. Phasor diagrams for wye-wye bank of ideal transformers supplying balanced noninductive load, (a) Primary; (b) secondary.

the voltage across the transformer winding is only 1/ or 0.58 of the line-to-line voltage, which follows from Fig. 6-23 and the phasor diagrams in Fig. 6-24 as shown below

and

from which

[6-91]

Similarly

[6-92]

and

[6-93]

The wye-wye arrangement requires a neutral connection between the source and the primary of the transformers not only to assure balanced line-to-neutral voltage, but also to provide a path for the third-harmonic component in the exciting current of the transformers. Without the primary neutral connection, serious unbalances in the line-neutral voltage may result from (a) unequal exciting admittances among the three transformers and (b) unbalanced line-to-neutral loads in the secondary. Furthermore, if the third harmonics are suppressed in the exciting current, large third harmonic components may appear in the line-to-neutral voltages. The wye-wye arrangement is, therefore, a four-wire system if balanced voltages are to be assured. The delta-delta arrangement is, on the other hand, a 3-wire system.