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Page 77

| Open-Delta Connection.. |

| Three-phase transformation may also be accomplished by using two similar transformers in open delta (Fig. 5-9). Because of the manner in which the current of the three phases divides among the two transformers as compared to that when three transformers are used, each transformer can only be loaded to 86.6 percent of its rating. |

| Figure 5-9 Two, single-phase transformers additive polarity, connected open D. |

| In the open delta connection, the impedances of the two transformers need not be the same, although it is usually preferable that they be so. Should it become necessary to close the delta with a third transformer, then the three must have practically identical impedance. |

| The open delta connection is often used as a temporary expedient, either in an emergency should one transformer fail, or pending a contemplated increase in load. |

| The regulation of an open delta bank is not as good as a closed delta bank. The voltage drop across the open delta is greater than across each of the separate transformers. |

| Similarly, as in a closed delta bank, the midpoint of the secondary of one of the transformers can be used to provide a supply to small lighting or other single-phase requirement. |

| Delta-Wye Connection |

| Frequent use is made of this type connection in supplying combined three-phase and single-phase loads, where the single-phase loads may be comparatively large. The two halves of the secondary winding are reconnected in |

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