216 Electrical Power Systems Technology
distances at high voltages since less current is required at high voltages,
and therefore line loss (I^2 R) is reduced significantly. A typical high-volt-
age transmission line may extend a distance of 50 to 100 miles from the
generating station to the first substation. These high-voltage power trans-
mission lines typically operate at 100,000 to 500,000 volts by using step-up
transformers to increase the voltage produced by the AC generators at the
power station. Various substations are encountered along the power distri-
bution system, where transformers are used to reduce the high transmis-
sion voltages to a voltage level such as 480 volts, which is suitable for in-
dustrial motor loads, or to 120/240 volts for residential use.
Transformers provide a means of converting an AC voltage from one
value to another. The basic construction of a transformer is illustrated in
Figure 8-8. Notice that the transformer shown consists of two sets of wind-
ings that are not physically connected. The only connection between the
primary and secondary windings is the magnetic coupling effect known
as mutual induction, which takes place when the circuit is energized by an
AC voltage. The laminated iron core plays an important role in transfer-
ring magnetic flux from the primary winding to the secondary winding.
Transformer Operation
If an AC current, which is constantly changing in value, flows in the
Figure 8-8. Basic transformer construction: (A) Pictorial design, (B) Schematic
diagram