Fault current Analysis 2

[28021984]

Dear All

i am designing a Distribution system having one 2MVA 22kV/415V Transformer and one MAIN DISTRIBUTION BOARD (MDB) and 5 Distribution Board (DB). i obtain 40kA Fault in Transformer secondary and in every DB the Fault level is more than 30kA.

I want to reduce the Fault current level less than 15kA in every DB.

Please help me is any solutions are there to reduce the fault level at DB.

Regards,

LSK

 

[davidbeach]

Increase the length of the feeders between the MDB and the DB until the fault current is low enough for you.

 

[dpc]

Try a 500 kVA transformer.

 

[28021984]

Thanks for your reply.

The system is already designed. so it is impossible to increase the length between MDB and DB aswellas the size of the transformer.

So please let me know is the any other solution is available.

Regards,

LSK

 

[waross]

I have seen specs where there was a lower limit on the length of the feeders. The extra cable was looped in the cable tray. (Big loops for the close in panels.)
Probably a lot cheaper than reactors and no worries about the magnetic field from a large air core reactor heating up nearby structural metal.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

The only way to reduce fault current is to add impedance. If there are no practical means of adding impedance, there is no practical means of reducing fault current. Basic Ohm's Law.

 

[dpc]

And you will need to add a lot of impedance to get the fault current down to 15 kA. You may wish to looking to series rating of current-limiting fuses and circuit breakers to obtain a higher interrupting rating - at the expense of selectivity.

No offense, but the nature of your question indicates you need to find someone with more experience to assist you.

Also, the fact that a system is designed does not preclude changes. The fact that a system is built and operating does not preclude changes.

 

[Skogsgurra]

I think that you are saying "i am designing a Distribution system". How, then, can it be that it can't be changed?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

In support of David's post:

davidbeach (Electrical)
10 Jan 12 12:52
Increase the length of the feeders between the MDB and the DB until the fault current is low enough for you.

I suggest that you at least calculate the length of feeder cable required to reduce the fault current to an acceptable level.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

The only way to reduce fault current is to add impedance.

And if you increase impedance, you increase voltage drop under load and during motor starting.

You might want to consider current limiting fuses. In the USA, this would only be helpful if the equipment were tested with the selected protective equipment and given a "series rating". I don't know if there is a similar procedure for IEC equipment.

The best solution would be to purchase higher rated equipment. It doesn't seem reasonable to have an MDB rated for 3 kA load with only a 15 kA fault rating.

 

[FreddyNurk]

IEC has similar provisions for 'series rating' of either fuses and circuit breakers, or specific manufacturer tested combinations of circuit breakers (typically certain types of moulded case circuit breakers).

That said, to rate the DBs at 15kA would still require further work depending on what is installed in them. Assuming that we're discussing the style of DB I'm thinking of, say, light & power distribution within buildings, most of the miniature circuit breakers are only rated to 6kA, sometimes lower. This would mean either putting more expensive circuit breakers in for every circuit, or revisiting the whole design.

If it were DBs for some sort of industrial application, I'd be rating all the sub boards higher.

 

[7anoter4]

If you cannot use a longer cable- as waross proposed-you can nevertheless to increase the cable reactance by increasing the distance between single-core cables.
For instance if-let's say the DB is supplied by 3*1/c 1000 MCM copper triplexed cables- approx.20 m [65ft]-and the reactance will be 0.002 ohm if the cables will run at 2 ft distance apart then the reactance will be 0.0049 and the short circuit current will be 22 KA. Of course you have to check the voltage drops [mainly in the case of induction motor starting] as jghrist indicated.