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| lators to maintain desired outgoing supply voltages. The arrangements employ auto-transformers (these will be discussed in the section relating to the autotransformers in Chap. 6). |

| Insulation Coordination |

| The insulations associated with the several parts of the transformers must not only withstand the normal operating voltages imposed on them, but must also be able to withstand higher voltage impulses or surges that result from lightning or switch operations and that may find their way into the transformer. The insulation from coils to ground (steel cores, tanks, etc.), from coil to coil, and from turn to turn, must be strong enough to withstand the voltages produced by the impulses at these points. |

| To provide adequate insulation without the use of excessive and expensive insulating material, the insulation requirements of the several parts are coordinated. In general, this means that the internal parts are insulated as nearly equal as practical, but generally stronger than that of the bushings, thus insuring that any breakdowns that may occur would be outside the tank, where damage would be comparatively light. Further, the bushing is usually protected by an air gap or lightning arrester (which will be discussed later), whose insulating value under the surge conditions is even lower than that of the bushing, so that the failure will be by flashover across the bushing and the tank, rather than by puncture of the bushing insulation (Fig. 2-20). Finally, the insulation of the weakest point in the transformer is stronger, by great enough margin, than the protective device. Such a coordinated arrangement of insulation whereby the transformer is stronger against voltage impulses or surges than the bushings, and the bushings are stronger than the protective devices, tends to restrict damage. |

| Figure 2-20 Insulation coordination. |

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