Quantcast Short-Circuit Current Behavior

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steady-state value some time after the initiation of the fault. Since a generator continues to be
driven by its prime mover, and has its field energized from its separate exciter, the steady-state
value of fault current will persist unless interrupted by some circuit interrupter. Most properly
applied fault protective devices, such as circuit breakers or fuses, operate before steady-state
conditions are reached.
(c) Synchronous motors supply current to a fault in much the same manner as do
synchronous.generators. The drop in system voltage due to a fault causes the synchronous motor
to receive less power from the system for driving its load. The inertia of the motor and its load
acts as a prime mover, and with field excitation maintained, the motor acts as a generator to
supply fault current. This fault current diminishes because the motor will slow down as the
kinetic energy is dissipated, reducing the voltage generated, and because of the decay of motor
field excitation.
(d) The fault-current contribution of an induction motor results from generator action
produced by inertia driving the motor after the fault occurs. In contrast to the synchronous
motor, the field flux of the induction motor is produced by induction from the stator rather than
from a direct current field winding. Since this flux decays on removal of source voltage resulting
from a fault, the contribution of an induction motor drops off rapidly, ultimately disappearing
completely upon loss of voltage. As field excitation is not maintained, there is no steady-state
value of fault current as with synchronous machines.
(e) Capacitor discharge current, because of its very short time constant of less than one
cycle, can be neglected in most cases. However, there are applications in industrial and
commercial power systems in which very high transitory short-circuit currents can be developed
when a short circuit occurs close to a bank of energized capacitors. These transitory currents,
generally of much higher frequency, may exceed in magnitude, the power-frequency short-circuit
currents and persist long enough to impose severe duty on the circuit parts carrying this current.
4.2.2.3 Short-Circuit Current Behavior. When a short circuit occurs, a new circuit is
established with lower impedance and the current consequently increases. In the case of a bolted
short circuit, the impedance is drastically reduced and the current increases to a very high value
in a fraction of a cycle. Figure 4-1 represents a symmetrical short-circuit current wave; that is, a
short-circuit current that has the same axis as the normal current which flowed before the fault
occurred. To produce a symmetrical short-circuit current (under the usual condition that the
short-circuit power factor be essentially zero) the fault must occur exactly when the normal
voltage is maximum. In Figure 4-1 the system voltage is assumed to remain constant, although
the current changes.
(a) The total short-circuit current consists of components from any source connected to
the circuit (Figure 4-2). The contributions from rotating machinery decrease at various rates.
This causes the symmetrical current to decrease until a steady-state value is reached. This
4-4





 


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