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| Figure 5-14 Connection of third winding in each of three single-phase transformers connected Y-Y. |

| Polarity of Three-Phase Transformers |

| The order of bringing out and marking leads in a three-phase transformer is shown in Fig. 5-15. |

| The determination of the polarity of three-phase transformers is not as simple as for single-phase transformers; considerations of phase rotation, marking of leads, and types of internal connection are also involved. |

| Consider first delta-delta or Y-Y connections. For these two connections, the corresponding line voltages may have either a 0° or 180° displacement. With 0° displacement, if two corresponding leads are connected together (say H1 and X1), then H1 H2 is in phase with X1 Y2, and H2 H3 with X2 X3, etc. With 180° displacement, on the other hand. H1 H2 will be in phase opposition to X1 X2, etc. The vector diagrams for 0° and 180° displacement phase are shown in Fig. 5-16. |

| In the case of Y-delta connection, when two corresponding leads of high and low side are connected together, the other corresponding voltages (H1 H2 and X1 X2, H2 H3 and X2 X3, H3 H1, and X3 X1, may be 30° leading or lagging from each other. The vector diagrams for Y-delta or delta-Y connections, for this condition, are shown in Fig. 5-17. Figure 5-18 shows the vector diagram for a 30° lag of wye side to a delta side. It will be noted that in Fig. 5-17, if X1 is placed on H1 the voltages X1 X2, X2 X3, and X3 X1 will lead H1 H2, H2 H3, and H3 H1 by 30° for a delta-Y connection and lag by 30° for the Y-delta connection. The reverse is true for Fig. 5-18. |

| Figure 5-15 Arrangement of leads on a three-phase transformer. |

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