Chapter 6. Dry-Type Transformers

Chapter 6. Dry-Type Transformers - 6 Dry-Type Transformers...

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6 Dry-Type Transformers Paulette Payne Powell Potomac Electric Power Company 6.1 History . .............................................................................. 6 -1 6.2 Transformer Taps. .............................................................. 6 -2 6.3 Cooling Classes for Dry-Type Transformers. .................. 6 -2 6.4 Winding Insulation System. ............................................. 6 -3 6.5 Application. ........................................................................ 6 -3 6.6 Enclosures . ......................................................................... 6 -4 6.7 Operating Conditions. ...................................................... 6 -5 6.8 Limits of Temperature Rise. ............................................. 6 -5 6.9 Transformer Loading . ....................................................... 6 -6 6.10 Accessories. ......................................................................... 6 -7 6.11 Fault Protection. ................................................................ 6 -7 6.12 Surge Protection. ............................................................... 6 -8 6.13 Harmonics. ......................................................................... 6 -8 6.14 Dry-Type Transformer Maintenance . .............................. 6 -8 A dry-type transformer is one in which the insulating medium surrounding the winding assembly is a gas or dry compound. Basically, any transformer can be constructed as ‘‘dry’’ as long as the ratings, most especially the voltage and kVA, can be economically accommodated without the use of insulating oil or other liquid media. This section covers single- and three-phase, ventilated, nonventilated, and sealed dry-type transformers with voltage in excess of 600 V in the highest-voltage winding. Many perceptions of dry-type transformers are associated with the class of design by virtue of the range of ratings or end-use applications commonly associated with that form of construction. Of course, the fundamental principles are no different from those encountered in liquid-immersed designs, as discussed in other chapters. Considerations involving harmonics are especially notable in this regard. Consequently, this chapter is brief, expounding only on those topics that are particularly relevant for a transformer because it is dry. Dry-type transformers when compared with oil-immersed transformers are lighter and nonflam- mable. Increased experience with thermal behavior of materials, continued development of materials, and transformer design has improved transformer thermal capability. Upper limits of voltage and kVA have increased. Winding insulation materials have advanced from protection against moisture to protection under more adverse conditions (e.g., abrasive dust and corrosive environments). 6.1 History The history of dry-type transformers can be traced to demonstration of the principle of electromagnetic induction and development of ac lighting systems. In 1831, Michael Faraday demonstrated induction of ß 2006 by Taylor & Francis Group, LLC.
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current in a secondary coil by the electromagnetic effect of current in the primary coil. Sir William Grove was the first to connect a transformer to an ac source needing high-voltage power for his laboratory work, but its significance as a commercial application was not realized. In 1882, Lucien Gaulard and John Dixon Gibbs were granted a patent for a secondary generator placed in series to supply an ac arc lamp lighting system. The system, demonstrated in England in 1883 and Turin, Italy, in 1884 was not practical, but stimulated transformer development in several countries [1].
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Chapter 6. Dry-Type Transformers - 6 Dry-Type Transformers...

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