IEC 60909 vs ANSI/IEEE 141 Short-Circuit Current Calculations

Short-circuit calculation is one of the most important work to be undertaken in plant power distribution. A comprehensive, detailed and accurate short-circuit currents will be used in switchgear selection, protection settings and coordination.

Traditional short-circuit calculation primarily calculate the steady-state currents as they are significantly easier to calculate than transient or electromagnetic simulations. Steady-state currents however often fail to correctly calculate the AC and DC decay resulting from rotating machines which have notable contribution to the overall fault current at the fault. Steady-state calculations use only fixed impedances for rotating machines.

ANSI/IEEE 141 gives detailed guidelines on the short circuit calculations methodology and data requirements. The method presented in this standard is one of the simplest steady-state SCC in terms of the computational requirements. The machine impedances used for the calculation are mainly sub-transient reactances. The standard assumes unloaded network conditions, which means that the pre-fault load flow calculation for the network is not required.

The peak short-circuit current can be calculated multiplying the initial momentary SC current by a crest factor of 2.7. AC and DC decaying are encountered through use of the specified decrement factors. These factors depend on the X/R ratio of the fault impedance and the circuit breaker contact parting time. These will be different for near-to and far-from generators faults.

IEC 60909 short-circuit calculation is similar to the short-circuit current calculation ANSI/IEEE 141. The most important differences are the use of a voltage correction factors (c=1.1 for high-voltage networks) and impedance correction factors. The use of a voltage correction factor c increases the pre-fault magnitude of the voltage source applied to the network by 10%. This makes the fault current at least 10% higher and sometimes leads to more conservative results. The impedances of rotating machines are constant in this calculation. These constant impedances for synchronous machines are calculated using sub-transient impedances and the corresponding impedance correction factors. The impedances used for asynchronous (induction) motors are calculated using the locked rotor impedance values.

References:
1. IEEE Std 141-1993 IEEE Recommended Practice for Electric Power Distribution for Industrial Plants
2. CIRED - 19th International Conference on Electricity Distribution Vienna, 21-24 May 2007 Paper 0562

About the Author

Ver Pangonilo
A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia and Professional Electrical Engineer (PEE) - Philippines with extensive experience in concept select, front-end engineering, HV & LV detail design, construction and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and waste water pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).
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