kAIC xfrm rating/utility impedance discrepancy 1

[HowdyEE]

An installed transformer vault has 4 1000kVA transformers 13.8kV - 480V. The utility rates this installation at 200kAIC and (145, j1385) uOhms. There are a couple of issues I am having with this.

1) Why don't the Utility ratings and the xfrm ratings match? Assuming 5% impedance on the xfrms (is this a reasonable assumption?), the short-circuit current of 4 1000kVA is 96kAIC, correct?

2) The Utility will not accept current-limiting fuses as a means of reducing the kAIC ... Why?

 

[dpc]

Too little information. The transformer impedance for a standard ANSI transformer will be nominal 5.75%. You should not make assumptions about transformer impedances for transformers of this size.

The utility may be giving you 200 kA fault duty as a design criteria. This will certainly require the use of current-limting fuses in your equipment to interrupt faults, so I'm not clear on your Question #2.

If the utility is saying that they will not install CLF to reduce your available fault current, then I agree with that.



David Castor

http://www.cvoes.com

 

[rbulsara]

Don't want to speak on behalf of your utility company, but based on experience...

1) Utility co. reserves the right to make changes (read upgrades) to the vault -different transformer size, different impedance etc, in the future so they are giving you a conservative but not outrageous SCCR requirement for your equipment. Also utility co. may use bank of single phase oil-filled transformers, which may or may not comply with ANSI standards, meaning they can have much lower Z than you would expect.


2) This has been discussed in many times here, and dpc said use of CLF is not permitted by any standards to reduce available SCC so that you can use a equipment rated less than 200 kAIC. On the other hand if the equipment is rated (UL listed) for 200kAIC (stamped on the nameplate), you should be able to them. In fact there will be CLF either in the breakers or the switch-fuse units to have 200 kAIC rating, but those have to be factory assembled and labeled equipment.

Rafiq Bulsara

http://www.srengineersct.com

 

[HowdyEE]

Thanks for your responses, and sorry for any vagueness.

DPC, I think you are right about the transformer; I did not realize distribution-sized transformers can have %Z as low as 2.3. That is right in line with the impedance and AIC the utility gave.

Let me make sure I understand CLF's in SCC applications: They are not accepted ways of limiting SCC UNLESS they are a U/L listed Fuse/Breaker pair. Correct?

So if we have 200kAIC supply and 65kAIC equipment, there needs to be a UL listed current limiting device, which under no circumstances (even if the Current-Time chart shows no interferences) can be a fuse.

 

[dpc]

So if we have 200kAIC supply and 65kAIC equipment, there needs to be a UL listed current limiting device, which under no circumstances (even if the Current-Time chart shows no interferences) can be a fuse.

No, you could use a fuse, as long as it is a tested combination with everything downstream you are trying to protect. But this will likely not be the case when you get down to the smaller breakers. It depends on the size of the fuse. You may be able to add CLFs strategically throughout the system, going to smaller fuses as you work your way down, but again, these have to be a tested combination with the breakers. If your equipment is already purchased and on site, you have some problems.

A current-limit fuse doesn't really limit current unless it clears the fault in the first 1/2 cycle. To limit current, it has to melt. So for a high magnitude fault, it is never going to coordinate with anything downstream - it has to melt to protect the breaker.

David Castor

http://www.cvoes.com

 

[HowdyEE]

I see where I'm missing something: current-time curves are only applicable to low-level and over-current faults. For high-level faults (a.k.a. short-circuit) the energy let-through and total clearing time needs to be considered.

 

[dpc]

On a TCC, if you are looking at a time greater than .008 sec, nothing is considered current-limiting. Any device that is in current-limiting mode will already have cleared the fault by then. The TCC may start at 0.01 sec, but things are happening before that time is reached.

So even for a current-limting fuse, the fuse curve normally shown on a TCC has nothing to do with current-limiting.





David Castor

http://www.cvoes.com

 

[HowdyEE]

Thanks for your help David. Looks like a current-limiting reactor is going to be the best/simplest/easiest solution.

There are hundreds of CB's that are either already existing or being installed by too many subcontractors to try to coordinate some kind of current-limiting fuse at the meter (at this point). Besides, having such a specific string of fuses and breakers will most likely not be followed by maintenance at a later date, which would nullify any current-limiting fuses.

 

[dpc]

It will take a fairly high reactor impedance (relatively speaking) to reduce fault current from 200 kA to 65 kA. That may create some voltage regulation issues that you will want to look into. as well as heat rejection from the CLR. And finding some place to put it. But it may be the best option given the situation.

David Castor

http://www.cvoes.com

 

[HowdyEE]

Luckily we have some long conductor lengths that will take care of about half the needed impedance. I roughly calculate the reactor to be 4.7mOhms.

Voltage drop at rated current is 1.5% with the above sized reactor and conductor impedance. Not too bad.

The CLR is basically a big inductor, correct? At that point, can a series-listed CLF and CB pair be used to limit AIC upstream of the reactor?

 

[dpc]

Yes, if I understand the question correctly. To protect the reactor, upstream of the reactor, you would need a tested combination CLF and CB that is rated to interrupt an available fault current of 200 kA.

The CLR is a big inductor but in the real world inductors have some resistance as well as reactance. Reactor manufacturer can give you an estimated X/R ratio.

David Castor

http://www.cvoes.com

 

[HowdyEE]

Interestingly I just found the rated voltage stamped on the transformer: 6.92%. That makes an actual SCC of 69kAIC.

I can understand a small overstatement, but this is ridiculous. Do you think providing actual values of the transformer and reduced SCC to an Advisory Committee would be a worthwhile adventure?

 

[rbulsara]

Does not hurt to ask. Perhaps that will provide them the opportunity to state their reasoning as well.

Rafiq Bulsara

http://www.srengineersct.com