The momentary or first cycle current is used to evaluate the closing or latching of the high voltage circuit breaker. Other selection criteria are the interrupting and symmetrical short circuit currents.

The sub-transient reactance of the utility sources or generators, the impedance of transmission lines and transformers are used for the momentary current calculations. The reactance of the synchronous motors and induction motors are modified according to Table 4.1 from IEEE Standard 141-1993, IEEE Recommended Practice for Electric Power Distribution for Industrial Plants.

Table 4-1 Rotating-machine reactance (or impedance) multipliers
Type of rotating machine First-cycle network Interrupting network
All turbine generators; all hydrogenerators with amortisseur windings; all condensers
Hydrogenerators without amortisseur windings 0.75{X_d}prime 0.75{X_d}prime
All synchronous motors
Induction motors
 Above 1000 hp at 1800 r/min or less
 Above 250 hp at 3600 r/min
 All others, 50 hp and above
 All smaller than 50 hp neglect neglect
Source: Based on IEEE Std C37.010-1979 and IEEE Std C37.5-1979.


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