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Page 31

| Figure 2-12 Method of gas-oil seal application to power transformers. |

| produces a constant breathing or interchange of air in the top of the tank and the surrounding air. This breathing may result in the lowering of the temperature of the enclosed air to a dew point, resulting in condensation of water vapor within the tank. The gradual accumulation of quantities of moisture will decrease the insulating quality (dielectric strength) of the oil. Large drops of water may collect, and, as water is heavier than oil, the globules of water will fall through the oil to the windings, which may result directly in a breakdown. |

| Isolation of the oil from the air by using an inert gas (Fig. 2-12), such as nitrogen, above the surface of the oil in a sealed transformer tank often eliminates this source of possible trouble. |

| Some outdoor transformers are provided with oil conservators. These are auxiliary tanks used for oil expansion to eliminate the air space above the transformer tank and to isolate the hot oil from the surrounding air. This is done by completely filling the transformer tank with suitable piping. The conservator is open to the surrounding air through a breathing device and is provided with a sump from which water from condensation may be drawn off (see Fig. 2-13 on page 32). |

| The use of oil conservators eliminates breathing and moisture condensation in the transformer tank containing the core and coils, thus preserving the original quality of the oil there, protecting it from sludging. |

| Large substation transformers are equipped with a diaphragm covered vent, usually at the top of the transformer. These transformers are usually sealed against the weather so that the air (or inert gas) and the oil vapor existing above the oil level may generate excessive pressures within the tank. The purpose of this diaphragm, known as a relief diaphragm, is to rupture when pressures exceed a predetermined amount, thus relieving the pressure in |

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