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Page 79

| Figure 5-11 Three, single-phase transformers additive polarity, connected Y-D. |

| The impedance of the three transformers need not be the same in this type connection. |

| By reconnecting the primary windings from a delta connection to this wye connection, the incoming primary or transmission voltage may be raised (increasing the capacity of the circuit) without the need of replacing the transformers. It is often used in this fashion. |

| The common connection, on the wye high-voltage side, may be or may not be grounded and carried as a fourth primary conductor to the source of power. |

| Open Wye-Open Delta Connection |

| This is similar to the open delta connection, with the primary of each transformer connected between the common or neutral point and one of the three primary wires (Fig. 5-12 on page 80). The secondaries of the two transformers are connected to the secondary mains in the same fashion as for the delta connection, except that the third transformer is not used. The secondaries are connected in open delta and only 86.6 percent of the rated capacity of the two transformers can be utilized. |

| Like the open delta connection, this connection may be used in emergency or for expediency awaiting increases in load to install the third unit. It is also often used when relatively small three-phase power loads require supply in which case the power transformer, the one without the midpoint of the secondary brought out, may be of very small capacity as compared to the other transformer. |

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