page_65

< previous page | page_65 | next page >

Page 65

| tion do not reach dangerous temperatures. The oil, when used for drying, may be so hot as to be flammable and easily ignited. The transformer may be considered dry when a series of Megger readings remain constant for a 24-hour period. The time periods required may be long, in the nature of 3 to 4 weeks. Method 4 may take longer than the others. |

| Radiators |

| Radiators may be removed for cleaning and repairs. The oil in the transformers may have to be removed before these can be unbolted from the tank. |

| In some designs, there are flanges and valves so arranged that with the closing of the valves, the radiators may be peened over and caulked; larger holes may be welded. Dents deep enough to affect oil circulation may have to be cut out and a patch welded in place. |

| Electrical Grounds. |

| Electrical grounds associated with transformers are of the greatest importance. By providing low resistance paths to unbalanced or neutral currents, they help maintain output voltages reasonably balanced and within safe limits. Under fault or lightning conditions, they are essential to drain off surge voltages, holding voltages within the transformer to safe values. This is especially true of substation installations, where such grounds not only protect the transformers, but also help insure the positive operation of relays, further assuring the correct operation and safety of electrical systems. Such grounds should be checked periodically to ascertain not only that their circuit connections are unbroken, but also that their resistance be reasonably low. The Megger is often used to check the resistance of these grounds (Fig. 4-8). |

| Cautions |

| Though some items of inspection may be accomplished without deenergizing the transformer, thorough inspection and maintenance, particularly of larger units, require that they be deenergized when work is done on them. |

| To provide maximum safety to the workers, both the high voltage and the low-voltage leads should be disconnected from the transformer. Preferably, there should be a visual break between the transformer terminals and the high and low-voltage lines. If this is not possible, grounds should be applied to both the high and low-voltage terminals of the transformer. It should be kept in mind that the transformer may be energized not only from the high voltage (usually the primary) side, but also the high-voltage terminals or bushings even though disconnected, will continue to receive full voltage |

< previous page | page_65 | next page >