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 excitation. This magnetizing current loss is very small in well designed and well constructed
transformers, since this current does not usually exceed 5 percent of the full-load current, and in
larger transformers may even be as low as 1 to 2 percent.
(b) Load losses (commonly called copper losses or short-circuit losses).
o I 2 R loss due to load currents.
o Eddy-current loss in conductors due to leakage fields.
The I 2 R losses due to load and exciting currents are inherent in the transformer design. The
losses are determined by the length of the windings and the frame dimensions. The loss due to
the additional current required to maintain the rated output is usually negligible. The
eddy-current loss is set up by stray magnetic fields. This loss is also inherent in the transformer
design and is usually calculated as a percentage of the I 2 R loss. The I 2 R losses are functions
of the transformer core and winding design, and depend upon voltage transformation ratios,
winding conductor sizes, and magnetic core dimensions and loss characteristics. The core
hysteresis and eddy-current losses are a function of core dimensions, magnetic materials, and
lamination thicknesses.
3.2.3 Insulation. Insulation mediums used in power transformers are either liquid or gas. In
both cases some solid insulation for major separations is used. Solid insulation separates the
high-voltage winding from the low-voltage winding. Liquid systems include oil, askarel, and
high-fire-point liquids such as silicone. The gas systems include air, nitrogen, and fluorogases
(such as sulfur hexafluoride). The selection of the insulation medium is dictated mainly by the
installation site and cost.
3.2.3.1 Oil. Low cost, high dielectric strength, and ability to recover after dielectric
overstress make mineral oil the most widely used transformer insulating material. Due to its
flammable property, oil-insulated transformers are normally used for outdoor installations.
Indoor installations require vaults and venting or fire suppression systems.
3.2.3.2 Askarel. Askarel is a generic term for a group of nonflammable synthetic
chlorinated hydrocarbons used as electrical insulating media. Each manufacturer of askarel
transformers applied a special name for this material. For example, Inerteen(R) is the trade name
for the nonexplosive insulating and cooling liquid produced by Westinghouse. Inerteen(R),
nearly water white in color, is produced by chlorination of biphenyl (a common chemical). The
resulting polychlorinated biphenyls (PCBs) are relatively insoluble in water but soluble in fat,
and are extremely persistent in the environment. While Inerteen(R) is generally regarded as
being nontoxic but carcinogenic to humans, very high standards of control against pollution must
be exercised. The combustion of Inerteen(R) and other askarels has been known to create
dioxins as a solid residue or dust. In the public sector, there have been several well documented
fires in which askarel filled transformers have become involved in large building fires. In each
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