750kva on 200A primary fuses

[Richard3009]

11kv/440-750kva 3 phase tx on 200A primary fuses, spotted this at work, whats the max current per phase you can draw on the secondary without rupturing the fuses.

 

[Lucien Nunes]

Enough to set the transformer on fire, yes. Is this a trick question?

 

[davesparks]

I'd hazard a guess that it probably won't be possible to draw enough current from the secondary to blow those fuses due to the windings melting first. This is based purely on the primary/secondary ratio being 1:25 so to see 200A on the primary you'd need 5000A on teh secondary which is going to equate to a couple of MVA.
Which is a ridiculously rough estimate as it makes no allowance for losses or efficiency or power factor or anything else I don't even know about.

But my knowledge of those kind of transformers doesn't extend past being able to point one out in a substation so I wouldn't trust my word on it!

 

[Lucien Nunes]

I'd call that a fairly accurate rough estimate. You could probably blow the fuses if you were quick about it though.

 

[davesparks]

Well I was assuming that the primary fuses wouldn't be the fastest acting fuses in the world, just using a bit of common sense (although I think that's been banned by the EU recently)

 

[Knobhead]

750KVA @ 11KV = 39.3A.

Giving a bit of leeway for inrush and available fuse sizes 60A is what I would fit.

Now comes the BIG question, how do you know what size fuses are fitted? Have you looked in the switch?

I’m going to take a guess, you been looking at something like this

And seen this

Don’t interfere with things you don’t understand.

 

[DPG]

Yorkshire switchgear, what more could you want Daz

 

[Knobhead]

Something that doesn't have a live switching embargo.

 

[Bobster]

Unlike some of the Long and Crawford gear. That sometimes has been found to explode when switched live!

 

[Knobhead]

Ooo, nasty!
We used Long & Crawford on 3.3 as isolators, never switched live.

That Yorkshire gear was OK once:
The oil leaks were fixed.
The trip bar pivots replaced.
The trip latch replaced.
There were other faults, I’ve still got the report somewhere.

 

[Bobster]

There's an embargo on a few models of the long and Crawford gear. Almost exclusively to the models fitted with striker fuses.

Most of the gear they built was actually top notch.

 

[Richard3009]

750KVA @ 11KV = 39.3A.

Giving a bit of leeway for inrush and available fuse sizes 60A is what I would fit.

Now comes the BIG question, how do you know what size fuses are fitted? Have you looked in the switch?

I’m going to take a guess, you been looking at something like this

And seen this

Don’t interfere with things you don’t understand.

I simply looked at the drawing of the circuit in the substation

 

[Richard3009]

is that medling erm no

 

[Knobhead]

Have another look at the drawings then and tell us what type of switch it is.

 

[ipf]

750KVA @ 11KV = 39.3A.

Giving a bit of leeway for inrush and available fuse sizes 60A is what I would fit.

Now comes the BIG question, how do you know what size fuses are fitted? Have you looked in the switch?

I’m going to take a guess, you been looking at something like this

And seen this

Don’t interfere with things you don’t understand.

Especially when there's a 'TYKE IIA' involved

 

[Knobhead]

Especially when there's a 'TYKE IIA' involved

Anything to do with tyke’s is bound to go wrong.

The only good thing I could find in Yorkshire was the A623 back home.