Blown transformer Primary fuse from Secondary overload 2

[TransD]

Hi,
At work I noticed a phase loss from one of our transformers. After making some checks I noticed one of the three transformer primary fuses was blown. I am trying to figure out if it was due to a secondary overload, how much current it would take to blow the primary fuse, and why didn't it blow a secondary fuse instead? Here are some particulars that I have:

Transformer
4160/480 vac small pad mounted transformer
Primary fuses- three 65 amp fuses
Secondary fuses - three 175 amp fuses

Normally, the load on the secondary side is pretty light (just some cooling fans for another transformer). At certain times, it is used for backup power for other heavy duty equipment. Between the time there was no phase loss and one was detected, there was an instance where the transformer provided backup for that heavy duty equipment that was giving trouble. Certain members of our team say it was not related. Since it is the only abnormal event we can find I politely disagree. I am trying to prove it could happen. Thanks for any help.

 

[waross]

It may be inrush and old, partly corroded fuses.
It may be a dirty or corroded fuse holder.
It may be a combination of the two.
Examine the fuse holder for signs of overheating.


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

 

[TransD]

Inrush is definitely a possibility. The fuses and fuse holders are fairly new and inside the plant. They were in good shape. I would like to understand better the relationship between the primary and secondary current draw under a load. Thanks.

 

[Skogsgurra]

Your primary fuses should endure 560 A secondary current for a an infinitely long time. More actually - probably 600 - 700 secondary without blowing. And that should have blown your 175 A secondary fuses.

So, you either have a problem in the transformer, inrush or core saturation for some other reason. I have had malfunctioning thyristor rectifiers (missing gating pulses) that caused so much DC in the secondary winding that the core got saturated and drew a lot of primary current.

Do you have any DC equipment on the secondary? If you have that, make sure they don't misbehave.

Gunnar Englund

http://www.gke.org

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

 

[DTR2011]

The relationship between the primnary and secondary current is based on the the transformer turns ratio. Is it 4160 Delta to 480 Delta, or is there a wye involved?

 

[Skogsgurra]

That's a valid remark, to some extent.

But if there is at least one delta winding, the current relationship between secondary and primary current (at the terminals, what the fuses see) is independent on what winding configuration the transformer has.

Gunnar Englund

http://www.gke.org

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

 

[TransD]

I am not sure about the delta wye configuration. So my current fuse ratio is roughly 3 to 1 (secondary to primary) while my turns ratio is close to 8 to 1 (primary to secondary). It could be that is what the load requires for protection. So I guess the reason it blew the primary fuse instead of the secondary could be a matter of how long there was an over-current on that phase and which fuse would go first.

 

[Skogsgurra]

If you 'eliminate' the transformer by making the whole system a 480 V one, then your primary fuses will be around eight times their 4160 V value. That is around 560 A.

If you have a 560 A fuse and a 175 A fuse in series, which one blows first when there's an overcurrent?

Right. The 175 A fuse blows. The 560 A will not blow. Ever.

The only possibility that it blows before the 175 A fuse is that there is an internal fault in the transformer (or something else). Right?

Is there an internal fault in the transformer? No, it doesn't look like there is one. You can put in a new fuse and switch on again without any problems? Can't you?

So there you are - something else causes the problem. That "something else" is very often a DC saturation of the core. I have had that several times and it is very often caused by a faulty tyhristor rectifier (disturbed gating pulses, usually), be it a drive, an electrolytic plant or a heavy lifting magnet.

I repeat my question: Is there any DC equipment on the secondary?

Gunnar Englund

http://www.gke.org

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

 

[TransD]

no DC equipment. It goes to a 480 volt breaker

 

[TransD]

Let me re-state, after a 480 volt breaker the transformer goes to, I dam not sure what is next. It is something to look at.

 

[TransD]

Let me re-state, after a 480 volt breaker the transformer goes to, I am not sure what is next. It is something to look at.

 

[Skogsgurra]

OK. And that breaker was closed when the primary fuses blew?

Then there are a few reasons to see what comes after the 480 V breaker.

Gunnar Englund

http://www.gke.org

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

 

[JohnMcNutt]

Is there a possibility that the time delay of the fuses on primary and secondary were so far different that the primary blew open on some transient that the secondary held through?

 

[LionelHutz]

Knowing the fuse models would be interesting, but I find it hard to believe there would be any chance of the fuses not co-coordinating. 5kV fuses are typically very slow compared to 480V fuses and you have a primary fuse with a much higher effective current rating. Overall, I'd agree with the assessment that it is next to impossible for a simple over-current event to cause the primary fuse to blow.