Can current-limiting fuses increase MVA rating on existing SWGR? 5

[AJ2002]

Can an existing 13.8 KV GE magna-blast 500MVA rated switchgear be retrofitted with current limiting fuses on the supply side to protect the switchgear due too changes on the utility side and with the addition of on site generation?

The SC calculations are just above the 500MVA. Fuses would avoid the negatives of installing reactors and limit expenses.

Does any one have advice, references and lessons learned on this topic? Can the fuses be set high enough to limit the current to 500 MVA and maintain coordination?

Thanks for your help.

 

[RonShap]

Unless the fuses are UL listed as a series combination with the switchgear breakers, you cannot use them to satisfy the need for higher SWGR ratings due to higher fault current availability.

Although if in the USA, and you are a PE, you could make some assumptions and guesses and sign off on the installation in conjunction with NEC.

http://www.eng-tips.com/viewthread.cfm?qid=118092&page=1

 

[nightfox1925]

In one of the projects I have been involved with long time ago. We had a requirement for an 80kA S.C. rated 480VAC switchgear and one manufacturer has offered a switchgear with fused power circuit breakers.

I am not aware if it is also employed in 13.8kV switchgears. You have to consult the parent manufacturer if this can be possibly done physically. If yes, then you have to consider some points:

1. The retrofit should be tested as a complete assembly
and certified by the manufacturer.(series rating test)

2. Ensure that the fuse rating and curve will allow
coordination with the overcurrent devices attached to
the associated breaker.

The primary disadvantage of fused breakers is that you tend to have fuse stocks since this fuses blows for every fault on the associated feeders. Did you checked out whether IS limiters will be fine with your application or series reactors (as long as you dont get voltage problems).

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[rbulsara]

I am more surprised at 500MVA fault level at 13.8kV!! Sounds very very unusual. Although it is not your question.

I have not heard of or applied fuses to get "series rating" of an assembly at 13.8kV. It is acceptable at low voltage levels if UL listed combination is used.

Technically you need to review the time current curves of all the devices invloved and see at if the fuse will open if breaker fails to open, within breakers withstand limit.

 

[AJ2002]

The site has two paralleled 25MVA 115kV to 13.8kV Delta-Wye Z=9.4% tranformers (OA Rated) which operate with a tie breaker closed. If the utility is considered infinite it alone is beyond the MVA availability of the switchgear (530MVA).

If the tie breaker is openned the problem goes away. This additional S.C. is a result of paralleling the transformers.

 

[dpc]

http://www.gwelec.com/product_current/clip.cfm

I generally agree with other comments that from a Code compliance viewpoint, any upstream fuse would have to be a tested combination with the downstream switchgear and I'm not aware of any tested combinations.

However, the CLip device by G&W has been used by some utilities to deal with this problem. It is supposed to be very fast, and may operate quickly enough to keep the switchgear within its momentary rating (Close and Latch). I have no direct experience and certainly don't want to be standing close to one when it "operates". Basically, it uses a pyrotronic device to force the fuse into the circuit when the current level exceeds the breaker rating.

With a standard CLF, you will be sacrificing coordination for any close in fault.

Also, you're probably aware of this, but the medium voltage circuit breakers are actually rated for two short circuit currents (not MVA), at least in the ANSI world.

1. Momentary
2. Interrupting

The momentary current is the peak current that occurs in the first half-cycle. The breaker must be able to withstand the mechanical forces associated with this current and also be able to close into a fault at this level and latch (so it can then trip).

Interrupting current is less than momentary and is the current expected when the breaker contacts actually get around to parting open.

ANSI and IEC have specific methods of calculating these currents. Since you are close to the breaker ratings, you really should be doing a fault calculation that is consistent with the breaker rating and testing method, and not relying on MVA calculations.

 

[rbulsara]

AJ2002:

I am not sure someone red flagged my last post or i did not hit submit button.

Details always help so thanks for posting your site details. dpc makes good points and he deserves the star more than I do.

I must say, IMHO, using CLF is a cheeky option at best to justify use of marginally rated equipment. I would prefer reactors to CLFs to mitigate short circuit rating. Based on your description, I would think money should not be a object to the right thing at this facility. Physical constraints may be different issue.

 

[nightfox1925]

Please be aware that, as dpc had pointed out, that you are going to loose some degree of selectivity if these fuses are employed on these circuit breakers.

The manufacturer will select a fuse that would be capable of limiting the short circuit level at a value within the rated maximum S.C. rating of the circuit breakers. The fuse rating(with associated let-through value) will be selected to allow the circuit breaker to trip if the S.C. value is in such amount that is within its S.C. ratings. Hence, for a fault value that is within the power circuit breaker ratings, the breaker and their associated relay settings will actuate without the fuse blowing. However, if the short circuit value is in such amount that is going to exceed the breaker S.C. ratings, then the fuse blows and overrides any coordination downstream. This is what the manufacturer would test and certify. You loose selectivity at this extent. If the S.C. is going to be consistently high all the time, then you end up with the fuses blowing for every fault and the down time will be longer due to fuse replacement and of course keeping fuses at your stock all the time.

You may find some other ways to limit the S.C. like the use of 1)reactors on the main incomers or at the bus tie with a by pass circuit breaker, 2) replacing the switchgear with a new one with sufficient S.C. rating, 3) re-distributing the existing loads to minimize the contribution, 4)higher transformer impedance (which is costly so forget it) or 5) if you are using a normally closed bus tie, try to consider a normally open tie to decrease the level fault level by 50% and just do control modification on the bus tie control.

If the exisitng switchgear is old and obsolete, it might be high time do do such upgrade to replace it with a new one with sufficient rating and new protection scheme.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[dpc]

Keep in mind that the only way a current-limiting fuse "limits" short circuit current is by melting. If it doesn't melt, it is providing NO current-limiting. And to be considered "current-limiting", it must clear the fault within the first half-cycle. So if the fuse is going to protect the breaker, it has to melt, it has to do it in the first-cycle and it will clear before the downstream breakers even think about tripping.

 

[nightfox1925]

Thanks for the supplement dpc. As an example, if you require an 80kA switchgear/circuit breaker and the manufacturer 'A' could only offer a 65kA circuit breaker. They will offer a fused-circuit breaker and ensure you that is suitable for 80kA. But in reality is that if the fault is within 65kA, the breaker trips as per your device coordination settings. However, if the fault is beyond 65kA then the fuse blows. The vendor will select a fuse with melting and let-through characteristics that would eventually limit the current at 65kA and beyond. If anybody offers you fused circuit breakers, ensure that the operating characteristics of the fuse and breakers as a combination are well inderstood and cleared out by the b vendor and from there, determine whether such application will be acceptable to you as the "end-user".

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata