Electrical Power Conversion Systems 299
the transformer windings, and the power lines. To counteract the induc-
tive effects, utilities use power-factor-corrective capacitors.Capacitors for Power Factor Correction
Static capacitors are used for power factor correction in the system.
They are constructed similarly to the smaller capacitors used in electrical
equipment, which have metal-foil plates separated by paper insulation.
Ordinarily, static capacitors are housed in metal tanks, so that the plates
can be immersed in an insulating oil to improve high-voltage operation.
The usual operating voltages of static capacitors are from 230 volts to 13.8
kilovolts. These units are connected in parallel with power lines, usually
at the industrial plants, to increase the system power factor. Their primary
disadvantage is that their capacitance cannot be adjusted to compensate
for changing power factors.
Power factor correction can also be accomplished by using synchro-
nous capacitors connected across the power lines. (Three-phase synchro-
nous motors are also called synchronous capacitors; see Chapter 14 for
a discussion of three-phase synchronous motors.) The advantage of syn-
chronous capacitors over static capacitors is that their capacitive effect can
be adjusted as the system power factor increases or decreases. The capaci-
tive effect of a synchronous capacitor is easily changed by varying the DC
excitation voltage applied to the rotor of the machine. Industries consider-
ing the installation of either static or synchronous capacitors should first
compare the initial equipment cost and the operating cost against the sav-
ings brought about by an increased system power factor.THREE-PHASE LOAD CHARACTERISTICS
In Chapter 2 single-phase circuit fundamentals were discussed.
However, the major type of load in electrical power systems is the three-
phase load. Therefore, you should become familiar with the characteris-
tics of three-phase loads.Balanced Three-phase Loads
A balanced three-phase load means that the resistance or impedanc-
es connected across each phase of the system are equal. Three-phase mo-
tors are one type of balanced load. The following relationships exist in a
balanced, three-phase delta system. The line voltages (VL) are equal to the