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This paper presents a comprehensive review of dry-type transformer design and provides some application guidelines including insulation system characteristics; BIL; losses, overloading, and loss-of life; and price. A large number of distribution transformers, currently utilized in low voltage (LV) power systems are of dry-type design. This includes, the IEEE Class II ventilated dry-type design with 3-phase kVA ratings in the approximate range of 500 kV A -2.5 MV A and primary voltage ratings ranging from 4.16 kV to 34.5 kV and the secondary voltage ratings of 600 V and below.

In the 1930's, the dry-type distribution transformer for indoor applications that met the fire-resistant requirement were built with Class B (130°C) insulation. Many dry-type transformers during the !950's, instead, used Class H (!80°C) insulation for operation at higher temperatures. Since, air has a poor dielectric strength and thermal conductivity, these transformers were sealed in nitrogen gas to increase operating voltage and cooling efficiency. The ventilated dry-type transformers with 150°C winding temperature rise (or 220°C absolute temperature and Class C insulation) were also introduced in the same period.

In the 1990's and at present, dry-type transformers are safe, reliable, cost effective, and have low maintenance cost and fire risk. In many commercial and industrial (C&I) applications, they are replacing liquid-filled transformers (primacy voltages up to 34.5 kV and ratings up to 2.5 MVA are very common). These applications include power plants, utilities, hospitals, schools, multi-story buildings, paper and steel mills, mining, oil and gas refineries, chemical plants and subway system.

It is to be recognized here, that electric utilities own about 90% (~ 40 million) of all liquid-filled distribution transformers. Dry-type transformers, on the other hand, are primarily utilized by the commercial and industrial (C&[) customers (~12 million). The common secondary voltages are 277/480 V (3-phase, 4-wire, grounded Y) or 120/208 (3- phase, 4-wire, grounded Y) or 120-240 V (I-phase, 3-wire).

Utilities, for the most part, have already established their own loss evaluation criteria11H2l (popularly called the Total Owning Cost or "TOC" method and identified by "A " and "B" factors) before buying their transformers. With the introduction of improved materials and manufacturing techniques, as a result, utility-owned distribution transformer efficiencies have steadily improved from the 1950's to the I990's. Manufacturers responded by tailoring their products to the energy evaluation factors specified by customers, a practice that continues to this day.

Assessment of Transformer Overloading Capacity: An Application Guideline