page_62

< previous page | page_62 | next page >

Page 62

| Askarels |

| The same general procedures for testing and maintaining oils are also used with askarels. The main exception is that samples are taken from the top rather than the bottom of the transformer tank. Askarel may have its moisture removed and filtered by means of a filter press using activated clay (Fuller's earth) rather than filter paper, which is inadequate for decontaminating askarels. |

| Oil and askarels mix readily, and it is practically impossible to separate them once they are mixed. It is extremely important that they be kept apart, because oil changes the characteristics of askarels, making them flammable and explosive. The equipment used in decontaminating askarels should be used solely for askarels. |

| Gaskets |

| New gaskets properly cemented to cleaned surfaces should be used whenever any opening in the transformer case is made or the cover removed. Care should be used in preparing and placing the gaskets, making sure that the nuts and bolts are tightened uniformly. Gaskets for oil-filled transformers may be of cork or neoprene and other plastics. Those of cork should be compressed to one-half their thickness while those of neoprene should be compressed to two-thirds. Only cork should be used in askarel-filled transformers as askarels tend to dissolve plastic materials. |

| Winding Insulation |

| Winding insulation may be checked with a Megger (Figs. 4-4 and 4-5), a small dc generator used for measuring resistance. Refer to Chap. 7 for a description of a Megger. Readings are taken between windings at the terminals (Fig. 4-6 on page 64), and the ground. Minimum safe insulation resistance values (in ohms or megohms) at different temperatures are usually specified for different voltage ratings of the transformers by the manufacturers. Each winding is tested separately to the ground. |

| If one winding under test indicates a very low resistance, that winding may be faulted to ground. The terminals should be checked to ascertain whether the fault is in the terminal board or in the winding itself. The board or one coil (if several constitute the winding) may need replacement. |

| If all the windings indicate lower than acceptable resistance values, but approximately proportionally equal, it may be that moisture has entered the insulation of the windings. The windings may require drying out. |

< previous page | page_62 | next page >