By American National Standards Institute
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Additional resources for IEEE Recommended Practice for Calculating Short-Circuit Currents in Industrial and Commercial Power Systems
For circuits with resistance, the absolute maximum fault current peak occurs before the symmetrical current 24 Copyright © 2006 IEEE. All rights reserved. DESCRIPTION OF A SHORT-CIRCUIT CURRENT IEEE Std 551-2006 peak and before one-half cycle as shown on Figure 2-9. Figure 2-9 is drawn for fault in a circuit with a relatively low X/R ratio of 2 to emphasize these important characteristics. This analysis assumes fundamental 60 Hz voltage, linear impedances, no ac decaying sources, and no pre-fault load currents.
The current now contains harmonic terms. Because they result from a violation of analytical restraints, they will not appear in the calculated results. Their character and magnitude must be determined by other means and the result artificially introduced into the solution for fault current. A similar type of non-linearity may be encountered in electromagnetic elements in which iron plays a part in setting the value of L. If the ferric parts are subject to large excursions of magnetic density, the value of L may be found to drop substantially when the flux density is driven into the saturation region.
The initial magnitude 2 E/Z × sin (α – φ) decays in accordance with the exponential expression. This dc component eventually disappears. The second term represents the steady-state ac component of the solution. The second term is a sinusoidal function of time whose crest value is simply the maximum peak value of the supply voltage divided by the magnitude of the Thevenin equivalent system impedance ( 2 E/Z) as viewed from the fault. The difference between the initial fault current magnitude and the final steady-state fault current magnitude depends only on the X/R ratio of the circuit impedance and the phase angle α of the supply voltage when the fault occurs.