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What we need to know about dry-type transformers

Oil immersion was introduced almost as soon as transformers were developed in the 1880s for power system use, and is standard today for the largest ratings on electric utility circuits as well as for most smaller distribution transformers such as in residential areas (where open, self-ventilated construction is unsuited to outdoor service).

Indoors, where oil presents a fire hazard, and fire-resistant alternative fluids involve high cost or the risk of leakage, dry type (air cooled) transformers are increasingly used for commercial building loads up to 34.5 kV and at least 5000 kVA.

Unlike the liquid filled design, in which winding insulation is usually cellulose-based (such as various forms of paper and pressboard), the dry type insulation system resembles that of a rotating machine (polyester, mica, silicone, etc.). Thermal classifications are similar to those of a motor or generator, as is the winding impregnation (VPI treatment is common). However, in contrast to the rotating machine, transformer ratings include a specific "hottest spot" allowance, which can be calculated or measured much more readily than in a motor winding (there's even an ANSI/IEEE standard,øC57.134, on how to determine the value). Construction differences also allow a BIL (Basic Impulse Level) transient overvoltage rating.

Dry-type transformers differ in several ways from their liquid filled counterparts. Higher operating temperatures are allowed (because of the need for only one transfer of heat, directly to air, rather than winding-to liquid-to air). Their overload capability with auxiliary fans ranges from 33% to 50%, typically twice that for the liquid filled type. Although failure rates appear comparable, surveys indicate that repair time for a dry type unit is much shorter. Whereas lengthy and sometimes complex tests of the liquid itself are necessary for a fluid-filled unit, cleaning and inspection for the dry type constitute simpler maintenance.

Variations introduced during the 1960s are the "cast coil" transformer, in which the windings are fully encapsulated, and the sealed un it built into a inert gas-filled enclosure. The chief advantage of both is protection from moisture or contamination, useful in especially hostile atmospheres.

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