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Page 81

| The impedance of the transformers need not be exactly the same, but it is desirable so as to hold down current in the neutral resulting from unbalances due to unequal voltage drops. |

| Single-Bushing Transformer... |

| In the wye-wye connection, transformers having only one high-voltage or primary bushing are often used in a three-phase bank. With this type of transformer, the single, high-voltage bushing is connected to the power line. The special connection on the outside of the tank takes the place of the second high-voltage bushing and must be connected among the three transformers and to the neutral wire or ground. The distribution transformers have a factory-built connection installed between the neutral low-voltage bushing and the tank. While the elimination of one high-voltage bushing restricts this type transformer to use only in the wye primary connection, it does make for a less expensive transformer, and simpler installation. |

| Harmonics: Third Winding |

| When a voltage is transformed from the primary to the secondary, certain side reactions take place. The sine wave of the primary voltage becomes distorted because of the uneven molecular movements within the iron core, producing distorted sine-wave magnetic fields which influence the shape of the sine waves in both the primary and secondary. This distorted wave can be considered to be made up of the original sine wave and a series of harmonics, or waves, whose frequency may be 3 times the 60 cycles per second, 5 times the 60 cycles per second, etc., which are referred to as the third harmonic, fifth harmonic, etc., of the original sine wave. These harmonics or fluctuations in the sine wave can either flow to ground when the transformer is provided with a ground, or when transformers are connected in delta, these fluctuations circulate around the delta producing a little heat, but resulting in a sine wave being formed in the secondary. This is a simplified explanation of the phenomenon. |

| In large, high-voltage station-type power transformers connected to a wye transmission circuit without a neutral back to the source, these harmonics are particularly bothersome. To overcome this, each of the transformers is provided with an extra small-capacity auxiliary winding. The three auxiliary windings are connected in delta as shown in Fig. 5-14 on page 82 (even though the main primary and secondary windings are connected in wye), and this provides a circuit in which the triple-frequency (and other harmonics) components of the exciting current can flow. A sine wave applied voltage on the high side will then produce a more pure sine wave voltage on the low side. |

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