page_59

< previous page | page_59 | next page >

Page 59

| overload or by voltage surges (lightning or switching), and may be an indication of potential insulation failure inside the transformer. Discoloration of the bushings, and of the associated lightning arrester, may also provide evidence of such flashovers. |

| Discoloration of terminals and leads may be evidence of heating, which may be evidence of loose connections. Connections to the ground should receive special attention for, as a safety measure, they prevent possible energization of the case. |

| Excessive rust or corrosion of the tank may also indicate excessive transformer heating, although it may be caused be corrosive elements in the ambient. Signs of oil overflow on the tank may indicate excessive heating or the occurrence of a severe internal stress during a temporary fault which may have cleared itself; of course, it may also be due to having initially overfilled the transformer tank with oil, or accidental spill during installation. |

| Where they exist, in large, station-type transformers, pressure relief diaphragms should be inspected and replaced if any signs of imminent rupture appear. |

| Unusual noises or unusual heat during periods of normal loads may indicate that the unit may have been subjected to severe operating conditions, such as overloads or frequent short circuits. |

| The level of oil in the transformer may be observed externally in some instances where such oil gauges are installed, usually on the larger, station-type transformers. In addition, the color and the clarity of the oil may also indicate the existence of moisture or carbon or sludge; moisture may indicate leaks from improperly tightened bushings, cover or ports in the transformer; carbon or sludge may indicate the existence of excessive internal temperatures. |

| Internal Inspection |

| Where oil gauges do not exist, the oil level may be visually inspected by opening the handhole (if any) or by removing the tank cover. |

| The condition of the gaskets should be observed for signs of wear. If worn, they should be replaced with new ones so that the cover may be securely fastened when replaced. If they are even slightly loose, rain or snow may be driven into the transformer ultimately causing transformer failure; loose covers also may cause noise problems. |

| The condition of the oil in the transformer should also be part of the routine inspection. A very small quantity of moisture will spoil the insulating qualities of the oil; a fraction of a teaspoonful of water in the tank will often necessitate changing the oil. A sample may be removed by a simple glass or metal suction tube (rubber should never be used since it contains sulphur which will dissolve in the oil and attack the copper windings and terminals) of |

< previous page | page_59 | next page >