Example: Fault Currents At Medium Voltages

Assume that a plant is served by a 7,500 kVA, 13,200-4,160Y/2,400-V transformer with an impedance of 5.5% (0.055). On the secondary side there is one main breaker and four feeder breakers, one of them serves:

1. a synchronous motor, rated 3000 hp, 0.8PF, 900 RPM, 421 amperes, with sub-transient reactance of 20%, and
2. an induction motor rated 1,000 hp, 1,200 RPM, 142 amperes, sub-transient reactance of 17%.

These 4,000-Volt motors are connected to the switchgear through negligible cable lengths. The symmetrical ampere contribution of either type motor to the momentary or interrupting duty level of fault current is determined from the equation:

For the synchronous motor the ANSI Standard lists 1.0 as the reactance multiplier to be used for momentary duty contribution and 1.5 for interrupting duty contribution. Therefore, this motor will contribute

or 2105 symmetrical amperes to the momentary duty, and

or 1403 amperes to the interrupting duty.

For the induction motor of this rating the specified reactance multipliers are 1.2 for momentary duty contribution and 3.0 for interrupting duty contribution. Using the same equation, the amperes are calculated to be 696 for momentary duty and 278 amperes for interrupting duty.

And furthermore . . .

Main Breaker Calculations:
Feeder Breaker Calculations: