Is there a standard definition of "minimum available fault current"?

[constantlylearning]

I’m embarrassed to ask this, but can anyone tell me if there is a definitive guide to what is meant by “minimum available fault current” on transmission systems? Traditionally, when I’ve been asked for this and I do not get more information from the requesting party, I give the fault current/system impedance based on the following conditions:

[ul]
[li]All generation local to the bus in question is turned off.[/li]
[li]The strongest line source OR strongest transformer source is out of service.[/li]
[li]I’ve seen some suggestions that a fault impedance should be included. However, I feel that this is not correct. The requesting party can take the system impedance data and model and add fault impence if they choose.[/li]
[/ul]

For “maximum available fault current”, I always turn all generation on and keep all lines and transformers in service (that would normally be in service).

Finally, I think it is always a good idea (though I’ve not always followed the practice) to give the requesting party system impedances rather than fault currents. Of course, with one, you can derive the other. But, if they can’t make the conversion, they probably shouldn’t be applying the data.

 

[livewire9]

I am a civil-structural engineer who has worked in this industry for some time now. What keeps me interested is learning more and more about the electrical side of the business; though it is not really my responsibility. I do feel that the more I understand the more I can contribute to a project team. That said, could anyone explain how to derive fault current by knowing the impedance of a circuit? What are the boundaries for determining system impedance/fault current? Generator to final point of load? Or can boundaries be considered at transformers or breakers?

Thanks in advance for helping out a structural guy!

 

[FreddyNurk]

An aside on the 'minimum available fault current' which isn't really relevant for transmission systems, is the consideration on whether or not there is actually enough fault current to trip protection. I've been involved in a number of remote power station sites where multiple small diesel engines are used in a station, with 11kV or 22kV reticulation. The fault current performance for a bolted fault at the station isn't too bad, but put a couple of transformers in between the station and consumers and the available current to trip the magnetic stages of small breakers gets rather ordinary.

It doesn't come up that much as normally there's a much stiffer source in place on the network, but if its a particularly weak source it can have unexpected consequences, thus minimum available fault current can be relevant for such applications.

 

[cranky108]

LiveWire, If it were easy we would be happy to tell you. But the fact is a transmission grid is modeled as a matrix of complex numbers, where most of the elements are zero because there are no lines there. Each row is a network node, and the numbers are the line impedance. Therefor there are three matrix, one for each sequence network. From there on the math gets deeper.

Boundaries? transformers are typically higher impedance devices, so to assume a small system equivlance on the high side, would allow one to calculate a not to exceed fault value on the low side, but that only works where there is a single feed. It becomes more difficult if there are two feeds, or generators involved.

Breakers don't change, or limit fault current. They interrupt it. Only current limiting fuses can limit fault current.

 

[livewire9]

Thanks for the reply, cranky. That sets a buoy for deep water I dare not tread. No one can know everything and it is good for one to know the limits of their need to pursue further understanding.

 

[HamburgerHelper]

Sometimes in industrial systems, you will see breakers in parallel with an inductive choke to limit the fault current below the rating of the switchgear. The times I have seen this was when the system was allowed as an abnormal condition to be fed by two independent sources, which would increase the fault current. When that happens, the breakers open to allow the chokes to limit the fault current.

 

[cranky108]

We typically order the transformer with a higher impedance to limit the fault current on the low side. That seems to cause fewer problems then replacing distribution equipment that might be under rated.

Don't get me wrong, a lower impedance transformer does save on losses, but if you spend those savings on replacing equipment, that transfers future losses to the present as a cost of replacing equipment.

Reactors are nothing new, but could have been included in the transformer (so much for being too concerned about energy losses).

 

[tinfoil]

We spec a minimum xmfr impedance to limit fault currents, and then spec a 'loss equation' to try to minimize operating costs.

Seems to work.