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Summary:
"What
we need to know about dry-type transformers"
They
differ from liquid filled units in many ways and separate industry
standards apply to their design, construction, and maintenance
By
Richard L. Nailen, EA Engineering Editor
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.
Copyright 2002, Barks Publications, Inc., Chicago.
Reproduction by any means prohibited.
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