UFC 3 -520-01
June 10, 2002
2-4.2.1.2 Generators. Short circuit current flow from a generator is limited by the
generator impedance and the circuit impedance between the generato r and the fault.
The magnitude of the generator fault current depends on the armature and field
characteristics, the time duration of the fault, and the load on the generator.
2-4.2.1.3 Motors. Synchronous motors appear as a source of generation during a
current, but the current supplied decays faster than with a synchronous motor .
2-4.2.1.4 Transformers. Supply transformers are not sources of short circuit current,
but they have a significant impact on the magnitude of short circuit current supplied to a
fault location. The transformer impedance will function to limit the short circuit current,
but the short circuit current available at the transformer will be magnified or reduced in
accordance with the turns ratio as it passes through the transformer.
2-4.2.2 Short circuit current from the above sources varies with time; not a ll sources
can sustain the peak short circuit current. Instead, the short circuit current available in
the system reaches some peak value and decays over the next few cycles to a smaller
steady-state value. With time, the short circuit current provided b y rotating machines
falls to zero as they brake to a complete stop. Short circuit current is modeled as a
function of time by three distinct impedances:
Subtransient reactance (Xd'')-- the effective reactance defining the short circuit
current during the first few cycles after a fault occurs. Use this value in all short
circuit studies.
Transient reactance (Xd')-- the effective reactance during the period after the first
few cycles up to about 30 cycles after a fault occurs. Use this value in voltage
regulation and stability studies.
Synchronous reactance (Xd)--the effective reactance after a steady-state condition
has been reached. In terms of short circuits, steady-state conditions occur several
seconds after the fault occurs.
2-4.3
Symmetrical Versus Asymmetrical Current.
2-4.3.1 Include the effect of asymmetric current in all short circuit studies. Normal
current is symmetric about the zero axis. S hort circuit current tends to have symmetric
and asymmetric behavior. The degree to which the current waveform is asymmetrical
depends on when the fault occurs in relation to the voltage waveform peak or zero, and
the proportions of resistance and reactance in the circuit. If a short circuit occurs in an
inductive reactive circuit at the peak of the volta ge waveform, the resulting short circuit
current is completely symmetrical as shown in Figure 2 -2. If a short circuit occurs in the
same circuit but at the zero of the waveform, the resulting short circuit current will be
completely asymmetrical.
2-6
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