Minimum Fault Level??


In the "Electrical Design basis" for a project, I read that the minimum impedance (max. MVA) will be used to ensure an adequate fault interrupting rating of the equipment, the maximum impedance (min. MVA) will be used to ensure that the motors can start under the weakest system conditions.

I know of only one Fault level - maximum fault MVA. What is the minimum fault MVA? How is it calculated under what conditions?
Fault current or short circuit currents are often given in MVA. This value is calculated simply by multiplying the fault current by the driving voltage and the square root of three if it’s three phase. The maximum fault MVA occurs at the minimum system impedance thus giving the highest current magnitude.

Assume me for a moment that your system has multiple systems in parallel or a system that may be supplied from two separate sources. In. The parallel case, assume you just lost one supply. With the loss of that supply, the new total system impedance increases and the fault MVA would decrease. In the two separate sources condition where you may choose to use source one or source two, you could have a different system impedance from source one than you do for source two and thus two different fault MVAs.

Last consider that that you have a large load that may be say a 4000 HP motor. Starting this motor may not be a problem for the parallel sources condition but for loss of one source you may have a problem with voltage levels need to successfully start the motor. This would be due to the higher system impedance which was identified with the lower fault MVA considered for the single source condition.

For the two source condition, the fault MVA for source one is higher than source two. In that case, source one would likely be the better source for starting your 4000 HP motor.

What constitutes the minimum MVA condition? When a Utility considers this they look at various system configurations that may be used or may become necessary due to storm or other emergency conditions and certain customers may not be able to operate their equipment if they are transferred to an alternate source. In that case, the Utility would likely supply both the highest expected fault MVA and the minimum expected fault MVA.

Hope this helps.