Medium Voltage Power fuses

[unclebob]

Hi all!

I'm having a discussion with a consultant about the caliber to use for the primary side of a 5000 kva transformer.

I'm using the nominal current to size my fuse. Primary is 25 kV so the nominal current is about 116 A. I'm going with a 150E SM-5 S&C fuse.

The consultant wants to use a 100E to fully coordinate with the upstream relays. By using another type of fuse, he claims he can use a 80E !
I'm trying to convince him with this paper (see attachment), but he's using the fuse TCC to prove to me that a 100E is good enough up to 200A!

The 5000 A, 600 V switchgear fed by the transformer doesn't have a main breaker. I'm suspecting that he wants to lower the incident energy at the 600V side by using a smaller caliber. Also, I think he obsessed with coordination...

I'm running out of ideas to convince him he's not right.

Do you have any comments?

BOB

 

[dpc]

I think your consultant probably knows what he is doing. S&C publishes a pamphlet with recommended fuse sizes for various transformer primaries at various voltages. I'd consult that, or ask your local S&C rep.

A 150E expulsion fuse is not going to think about melting until about 300 amps.

 

[dpc]

http://www.sandc.com/edocs_pdfs/edoc_025854.pdf

 

[WoodrowJWeen]

Unclebob, I have to agree with you.
100E will probably limit your transformer loading capacity.
Here's a link to another S&C guide I use.
It's a large file and you may have to extrapolate for a 5000kVA.

http://www.sandc.com/edocs_pdfs/EDOC_006526.pdf

 

[LionelHutz]

I don't deal with those fuses, but I do deal with other lower voltage fuses such as E or R rated fuses. Still, I have always understood that most if not all of these high voltage fuses have a rated current and a minimum interrupt rating. The rated current is the most current they can support without thermal damage. The minimum interrupt rating is typically 2 or more times the rated current and it's the minimum current required for the fuse to interrupt without over-heating. So, you should not continously operate between these 2 values and you need a protection relay to provide the protection between these 2 values.

So, I don't see the theory behind selecting a fuse that is has a rating less than the transformer rated current. I do suppose the fuse could work if the enviroment was cooler than the specified ambient temperature used to specify the rated current.

I'll have to read the S&C guides if I have some time. Maybe they say something different.

 

[HornTootinEE]

You may limit your transformer overload ability with a 100E, but what does a 150E do compared to the transformer damage curve? And coordination is a big deal. If you ever want to be able to pin point protection operation causes or limit outages to the smallest possible area, coordination is everything.

This size of transformer at that voltage could easily be a coordination concern when trying to blow a fuse to limit transformer damage on a through fault, but also allow maximum possible loading up to and possibly excedding nameplate value.

Sounds to me like you guys are having a philosophy argument vs. fuse melt characteristics.

 

[dpc]

There could be a lot of valid reasons for using a smaller fuse size, including better protection against transformer through-fault damage. For delta-wye transformers it can be difficult (impossible) to provide full range protection per ANSI frequent fault transformer damage curve for ground faults on the wye side using fuses on the primary.

Also, the arc-flash incident energy at the low side bus (since no main breaker) will be a function of the primary fuse time-current curve. One size smaller fuse can make a significant difference in the arc-flash level.

 

[LionelHutz]

I guess I should have expanded on my answer. I believe selecting a 100E fuse means you can operate up to 80% of transformer capacity and selecting a 80E fuse means operating at 64% of transformer capacity. Sure, the co-ordination may be better, but it doesn't make sense to install 5MVA if you limit it to only using 3.2MVA.

I actually read the question again and it appears you have no other protection besides these fuses. In that case, I'd have to say that's a rather poor installation. As dpc posted, E rated fuses by themselves are lousy at providing full range protection. I'd also doubt any practical sized upstream fuse could get the 600V side to a safe workable arc flash rating.