Large power transformers are
used in electric power systems. Industrial
and residential large power tranformers that
operate at the line frequency (60 Hz in the
U.S.), may be single phase or three-phase,
and are designed to handle high voltages and
currents. Efficient power transmission requires
a step-up large power transformer at the power-generating
station to raise voltages, with a corresponding
decrease in current. Line power losses are
proportional to the square of the current
times the resistance of the power line, so
that very high voltages and low currents are
used for long-distance transmission lines
to reduce losses. At the receiving end, step-down
large power transformers reduce the voltage,
and increase the current, to the residential
or industrial voltage levels, usually 115
to 600 V.
Large power transformers must be efficient
and should dissipate as little power as possible
in the form of heat during the transformation
process. Efficiencies are normally above 99
percent and are obtained by using special
steel alloys to couple the induced magnetic
fields between the primary and secondary windings.
The dissipation of even 0.5 percent of the
power transmitted in a large power transformer
generates large amounts of heat, which requires
special cooling provisions. Typical large
power transformers are installed in sealed
containers that have oil or another substance
circulating through the coils to transfer
the heat to external radiatorlike surfaces,
where it can be discharged to the surrounding
atmosphere.
