Electrical power systems are, in general, fairly complex
systems composed of a wide range of equipment devoted to generating,
transmitting, and distributing electrical power to various consumption centers.
The very complexity of these systems suggests that failures are unavoidable, no
matter how carefully these systems have been designed.
The feasibility of designing
and operating a system with zero failure rate is, if not unrealistic, economically
unjustifiable. Within the context of short-circuit analysis, system failures
manifest themselves as insulation breakdowns that may lead to one of the
following phenomena:
— Undesirable current flow patterns
— Appearance of currents of excessive magnitudes that could
lead to equipment damage and downtime
— Excessive overvoltages, of the transient and/or sustained
nature, that compromises the integrity and reliability of various insulated
parts
— Voltage depressions in the vicinity of the fault that
could adversely affect the operation of rotating equipment
— Creation of system conditions that could prove hazardous
to personnel
Because short circuits cannot always be prevented, we can only
attempt to mitigate and to a certain extent contain their potentially damaging
effects. One should, at first, aim to design the system so that the likelihood
of the occurrence of the short circuit becomes small.
If a short circuit occurs, however, mitigating its effects
consists of a) managing the magnitude of the undesirable fault currents, and b)
isolating the smallest possible portion of the system around the area of the
mishap in order to retain service to the rest of the system. A significant part
of system protection is devoted to detecting short-circuit conditions in a
reliable fashion.
Considerable capital investment is required in interrupting
equipment at all voltage levels that is capable of withstanding the fault
currents and isolating the faulted area. It follows, therefore, that the main
reasons for performing short-circuit studies are the following:
— Verification of the adequacy of existing interrupting
equipment. The same type of studies will form the basis for the selection of
the interrupting equipment for system planning purposes.
— Determination of the system protective device settings,
which is done primarily by quantities characterizing the system under fault
conditions. These quantities also referred to as “protection handles,”
typically include phase and sequence currents or voltages and rates of changes
of system currents or voltages.
— Determination of the effects of the fault currents on
various system components such as cables, lines, busways, transformers, and
reactors during the time the fault persists. Thermal and mechanical stresses
from the resulting fault currents should always be compared with the corresponding
short-term, usually first-cycle, withstand capabilities of the system
equipment.
— Assessment of the effect that different kinds of short circuits
of varying severity may have on the overall system voltage profile. These studies
will identify areas in the system for which faults can result in unacceptably
widespread voltage depressions.
— Conceptualization, design and refinement of system layout,
neutral grounding, and substation grounding. Compliance with codes and
regulations governing system design and operation, such as the National
Electrical Code® (NEC®) (NFPA 70-1996) [B6],1 article 110-9.
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