244 Electrical Power Systems Technology
POWER SYSTEM PROTECTIVE EQUIPMENT
There are many devices that are used to protect electrical power sys-
tems from damage due to abnormal conditions. For instance, switches, fus-
es, circuit breakers, lightning arresters, and otective relays pr are all used for this
purpose. Some of these devices automatically disconnect the equipment
from power lines before any dam age can occur. Other devices sense varia-
tions from the normal operation of the system and make the changes nec-
essary to compensate for abnormal circuit conditions. The most common
electrical problem that requires protection is short circuits. Other problems
include overvoltage, undervoltage, and changes ill frequency. Generally, more
than one method of protection is used to protect electrical circuits from
faulty conditions. The purpose of any type of protective device is to cause
a current-carrying conductor to become inoperative when an excessive
amount of current flows through it.
Types of Fuses
The simplest type of protective device is a fuse. Fuses are low-cost
items and have a fast operating speed. However, in three-phase systems,
since each hot line must be fused, two lines are still operative if only one
fuse burns out. Three-phase motors will continue to run with one phase
removed. This condition is undesirable, in most instances, since motor
torque is greatly reduced, and overheating may result. Another obvious
disadvantage of fuses is that replacements are required. All protective de-
vices, including fuses, have an operating-characteristic time curve, such as
the one shown in Figure 9-2, prevent any possible damage to equipment,
Figure 9-1. Transformer tap-changer voltage regulator