SHORT CIRCUIT STUDIES PURPOSE ON POWER SYSTEM TUTORIALS

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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