Maximum and minimum fault currents 1

[wonderchi]

Hi,

I am performing fault analysis to rate protective equipment in one of our stations. Is it necessary to find the minimum fault current?

My point is, isnt true that you only want to know the maximum fault current so your protective equipment can be rated above this level. So how does minimum fault currents come into the picture?

Thanks

 

[davidbeach]

You only need maximum fault currents for equipment rating. For protection you need both minimum and maximum; minimum currents lets you determine whether or not your settings will allow all faults to be cleared.

 

[wonderchi]

Can you expand more on the purpose of minimum currents for protection settings.

thanks

 

[mpparent]

Minimums are also good for AF Calc's.

Mike

 

[jraef]

You want your protection coordination scheme to isolate the fault at the lowest level possible in order to keep the resultant outages confined to the minimum area. For instance, you don't want a ground fault on a 50kVA distribution transformer to cause an entire city to go black. So on your primary system, the minimum fault level is coordinated with the maximum level of the system directly below it, allowing the lower system to clear the fault if possible.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[davidbeach]

You can also have a condition where the minimum fault at the far end of a distribution circuit is less than the load current at the source end of the circuit. Find that, you know that you have to break up the circuit with a recloser or fuses.

 

[Borti]

Max. and min. fault current is very important in feeder protection. Whether you use reclosers or just overcurrent relays, you should have one element covering the entire feeder and one for close faults. The minimum fault current you would consider is the one calculated for the fault at the end of your feeder. If you have to set a recloser, the low instantaneous set would be set according to this fault current with some overreach. The timed overcurrent should be set to coordinate with the protective device at the end of the feeder. Again, you have to consider the minimum fault.
The high instantaneous set has to be low enough to operate on close faults and high enough not to see the end of the feeder faults. You have to consider both minimum and maximum fault currents when setting this element.