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 CHAPTER 4. POWER SYSTEM PROTECTION AND COORDINATION.
4.1 SYSTEM PROTECTION METHODS. This chapter briefly presents system protection
methods. These methods include application and coordination of components required to protect
power systems against abnormalities which are reasonably expected to occur during normal
system operation. This chapter deals almost exclusively with the quick isolation of the affected
portion of the system.
4.1.1 System Protection and Coordination. Electrical system protection should not only be
safe under all service conditions, but should also be selective to insure continuity of service. A
selective system is a system that isolates only the faulted circuit without disturbing any other part
of the system. Overcurrent devices should provide short-circuit, as well as low overcurrent,
protection for system components (bus, wire, motor controllers, etc.).
4.1.1.1 Definition. System protection should guard electrical equipment against thermal
damage and electromechanical stress, while providing the highest possible degree of coordination
among protective devices. Coordination (or selectivity) of electrical protective devices is
achieved when the devices react, under fault conditions, to isolate faulty equipment, while
maintaining service to the remainder of the system.
4.1.1.2 Objectives. Protection for power systems allows for the following objectives:
(a) Personnel injury prevention.
(b) Prevention of damage to equipment.
(c) Interruption of power minimization.
(d) Minimization of the effect of faults on the system, both in extent and duration.
(e) Minimization of the effect of disturbances on the utility system.
4.1.1.3 Methods. There are several methods to minimize the effects of faults on the
system and load. The basic features that are incorporated in the design of power systems will
perform the following:
(a) Quickly isolate the affected portion of the system, while maintaining normal service
to as much of the system as possible, and minimizing damage to the affected portion.
(b) Minimize the magnitude of the available short circuit current to minimize potential
system damage to the system, components, and load.
4-1
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