Dry Type Transformers

Dry-Type Transformers are for indoor and outdoor applications in schools, hospitals, industrial plants, commercial buildings and anywhere that safe and dependable power are important considerations.

Dry-type transformers can have their windings insulated various ways. A basic method is to preheat the conductor coils and then, when heated, dip them in varnish at a high temperature. The coils are then baked to cure the varnish. This process is an open-wound method and helps ensure penetration of the varnish. Cooling ducts in the windings provide an efficient and economical way to remove the heat produced by the electrical losses of the transformer by allowing air to flow through the duct openings. This dry-type insulation system operates satisfactorily in most ambient conditions are also sealed with an epoxy resin mixture.

Another version of the dry-type transformer is a cast coil insulation system. It is used when addition coil strength and protection are advisable. Theses type of transformer are used in located  where environments are harsh,  such as cement and chemical plants and outdoor installations where moisture, salt spray, corrosive fumes, dust, and metal particles can destroy other types of dry-type transformers. These cast coil units are better able to withstand heavy power surges, such as frequent but brief overloads experienced by transformers serving transit systems and various industrial machinery. Cast coil units  are now being used where previously only liquid-filled units were available for harsh environments. They can have the same high levels of BIL while still providing ample protection of the coils and the leads going to the terminals.

Information on dry-type transformer loading from ANSI/EEE C57.96-1989 indicates that you can have a 20-yr life expectancy for the insulation system in a transformer. For dry-type transformers having a 220 (degrees) C, insulating system and a winding hot-spot temperature of 220 (degrees) C, and with no unusual operating conditions present, the 20 yr life expectancy is a reasonable time fame. However, due to degradation of the insulation, a transformer might fail before 20 years. Most 150 (degrees) C rise dry-type transformers are build with 220 (degrees) C insulation systems. Operating such a transformer at rated kVA on a continuous basis with a 30 (degrees) C average ambient should equate to a "normal" useful life.

The life of a transformer increases appreciably if the operating temperature is lower than the maximum temperature rating of the insulation. However, you should recognize that the life expectancy of transformers operating at varying temperatures is not accurately known. Fluctuating load conditions and changes in ambient temperature make it difficult, if not impossible, to arrive at such definitive information.

Dry-type transformers are available in three general classes of insulation. The main features of insulation are to provide dielectric strength and to be be able to withstand certain thermal limits. Insulation classes are 220(degrees) C (Class H). 185 (degrees) C (Class F), and 150 (degrees) C (Class B). Temperature rise ratings are based on full-load rise over ambient (usually 40 (degrees) C above ambient and are 150 (degrees) C (available only with Class H insulation), 115 (degrees) C (available with Class H and Class F insulation) and 80 (degrees) C (available with Class H, F, and B insulation). A 30 (degrees) C winding hot spot allowance is provided for each class.

The lower temperature rise transformers are more efficient, particularly at loadings of 50% and higher. Full load losses for 115 (degrees) C transformers are about 30% less that those of 150 (degrees) C transformers. And 80 (degrees) C transformers have losses that are about 15% less than 115 (degrees) C transformers and 40% less than 150 (degrees) C transformers. Full load losses for 150 (degrees) C transformers range from about 4% to 5% to 30 kVA and smaller to 2% for 500 kVA and larger.

When operated continuously at 65% or more of full load, the 115 (degrees) C transformer will pay for itself over  the 150 (degrees) C transformer in 2 yrs or less( 1 yr. if operated at 90% of full load). the 80 (degrees) C transformer requires operation at 75% or more of full load for a 2-yr payback, and at 100% load to payback in 1 yr over the 150 (degrees) C transformer. If operated continuously at 80% or more of full load, the 80 (degrees) C transformer will have a payback over the 115 (degrees) C transformer in 2 yrs or less.

You should note that at loadings below 50% of full load, there is essentially no payback for either the 115 (degrees) C or the 80 (degrees) C transformer over the 150 (degrees) C transformer, Also at loadings below 40% the lower temperature rise transformers become less efficient that the 150 (degrees) C transformers. Thus, not only is there no payback, but also the annual operating cost is higher.

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