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| and 2-16). Combinations of these methods are often used; in special cases, circulating water around radiators or in pipes within the transformer is sometimes employed. |

| 6. Bushings take the terminals of the coils through the tank, insulating them from the tank. Usually, these consist of a conductor through an insulating collar, usually porcelain (Fig. 2-17). The distance from the line terminal to the tank may be increased by petticoats, reducing the possibility of flashover from rain or dirt. For higher voltages, the porcelain cylinders may also be filled with oil (Fig. 2-18) or contain layers of insulation with metal foil inserted between them to equalize electric stresses among the layers. |

| 7. The primary coil sometimes has taps which permit changes in the turn ratio or ratio of transformation (Fig. 2-19). Taps on the winding are brought to a terminal board and must conform to the same specifications as the bushing leads. Changes in taps are usually made with the transformer deenergized; they may also be changed while the unit is energized, as in voltage regulators. |

| 8. For economy and to restrict damage when a voltage surge, such as lightning, is applied to the transformer, the insulation of the internal parts is usually stronger than that of the bushings, which in turn is stronger than that provided by the protective device, usually a lightning arrester. Flashover will then be apt to occur outside the transformer tank, restricting damage to more easily replaceable parts (Fig. 2-20). |

| 9. Transformers may be constructed to include more than two windings, may incorporate several units in a poly-phase transformer for large or substation application (Fig. 2-21), and may be made mobile (Fig. 2-22) for special application. |

| 10. So-called polymer insulators are replacing porcelain; lighter in weight, better rain sheding properties, do not fragmentize, less attractive as targets to vandals. |

| STUDY QUESTIONS |

| 1. What are the three essential parts of a transformer? What are some other important parts? |

| 2. Why are transformer cores laminated? |

| 3. What are the principal types of core construction and what are their differences? What determines what and where they are used? |

| 4. What factors affect the design of coils? |

| 5. Why and how are transformers cooled? |

| 6. What affects the quality of insulating oil in a transformer, and how may this be remedied? |

| 7. How may the effectiveness of cooling be improved? |

| 8. What is a bushing? What types are there and where used? |

| 9. What are taps, and how may they be changed? |

| 10. What is meant by insulation coordination? |

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