Prospective Short Circuit Current - Utility and Industrial

Utility and Industrial

In power transmission systems and industrial power systems, often the short-circuit current is calculated from nameplate impedances of connected equipment and the impedance of interconnecting wiring. For simple radial distribution systems with only a few elements, hand calculation is feasible, but computer software is generally used for more complex systems. Where rotating machines (generators and motors) are present in the system, the time-varying effect of their contribution to a short circuit may be evaluated. Stored energy in a generator may contribute much more current to a short circuit in the first few cycles than later on; this affects the interrupting rating selected for circuit breakers and fuses. An isolated generator may be specially designed to ensure it can source enough current on a short-circuit to allow subordinate overcurrent protection devices to operate properly.

Where an industrial system is fed from an electrical utility, the short circuit level at the point of connection may be specified, often with minimum and maximum values or values to be expected after system growth. This allows calculation by an industrial customer of its internal fault levels within its plant. If the prospective short circuit current from the utility source is very large compared to the customer's system size, an "infinite bus" is assumed, with zero effective internal impedance; the only limit to the prospective short circuit current is then the impedances after the defined "infinite bus".

In polyphase electrical systems, generally phase-to-phase, phase to ground (earth), and phase to neutral faults are examined, as well as a case where all three phases are short-circuited. Because impedances of cables or devcies varies between phases, the prospective short cicuit current varies depending on the type of fault. Protection devices in the system must respond to all three cases. The technique of symmetrical components is used to simplify analysis of unsymmetrical faults in three-phase systems.

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