Pole transformer fusing

[Mbrooke]

Any good rule of thumbs for fusing pole transformers?

For some time Ive been fusing 50kv single phase units on a 10K or 15K fuse links, which Im not sure if are even recommended for transformer use. This gives enough room for a 200% overload that usually runs for about 3 hours or a 300% overload for about 1 hour. However, because of hot load inrush, (such as monetary outage during the summer causing refrigeration compressors to stall) the fuse link is often over sized relative to transformer damage curve, up to a 20K on a 25kva if needed.


Im thinking about lower rated X links or "cordination" links, that will give inrush/load restraint while being closer to the wanted maximum current limit, ie a sustained 400% overload would blow the link within 10 minutes.


How are transformers normally fused?

 

[magoo2]

The general rule of thumb that most utilities follow is to pick a fuse rating that is about 2 X the full load current rating of the transformer. This usually gets you out of the inrush damage area.

You didn't mention the voltage rating, but let's take a 50 kVA 7.2 kV distribution transformer for example. The full load amps is 6.94 A and 2X that gives 13.9 A. So a 15 A rating or higher would be fine.

I think of the function of a transformer fuse primarily as a fault isolation device, not one to provide overload protection. It offers some of both, but I don't like the idea of extreme overloading.

 

[Mbrooke]

My mistake, coil voltage is 16kv. That gives a full load amperage of about 3.2 amps. 6.5 amps at a 200% overload.

A bolted fault at the transformer will blow even a 20K near instantaneously but a 400% or above overload could go unnoticed indefinitely. Especially downed LV primaries with a moderate impedance or far enough may no clear fast enough.

In theory the units could be run at at major overload with a life trade off or increased risk of violent failure but that is not so much the goal. Longevity is more the concern then a 20 year in service expectation.



In general residential dwellings with gas heat are permitted about a 300% overload is done since this is usually seen in the evening and morning with lights and electric cooking appliances briefly; then the load will rapidly drop off often to just a few amps during the day.

Electric heat, motor load and commercial service banks are specd with less overload capacity (110% to 150%) since they run for longer periods near the transformer kva rating. The coils have less thermal buffer since the oil is already near the desired temperature limit, hence the up size theory.

Both end up with the same fusing though, an over rated K link.

 

[squeeky]

I agree with Magoo2, the fuses cannot really protect the transformer and are an automatic isolation device. How long should you sustain the disturbance on the network before it is isolated is the question. The next one would be, if there was a catastrophic failure, what would be the result? Can the installation contain the fire, explosion, dangling live conductors?

 

[Mbrooke]

My thinking is a fuse closer to the kva rating would extend life (detect overloads above the desired can loading) and disconnect failures quicker such as a fault in the LV.


A Kearney type X fuse link or an S&C KSR link are both far more inverse over a K link, so at a lower rating could in theory withstand inrush and hot load inrush exceptionally well while blowing on quickly on long time overloads.


A 5 1/2 X to 7 X or very slow link fusing a 50kva unit looks like it could do a better job over a 15K. I guess my real question is what is an appropriate fuse link speed for protecting a transformer?

http://www.sandc.com/products/fusing-outdoor-distribution/positrol-fuse-link.asp

http://www.cooperindustries.com/con...ystems/products/fusing/fuse_links/edison.html

X link "slow":

http://www.cooperindustries.com/con...ystems/resources/library/Kearney/K41000AB.pdf

K link "fast":

http://www.cooperindustries.com/con...ystems/resources/library/Kearney/K31000AB.pdf

 

[stevenal]

While protecting the transformer from overloads, the closer rated fuse ensures more frequent service interruptions and call outs.

 

[txlineman]

7.2kv and 14.4 kv conventional single phase transformers 1/3 and 1/5th rule will get you close. For example 50 kva single phase @ 7.2kv = 16.7 amps would get a 15 amp link fuse. this applys to K type fuses. Hope this helps. JB

 

[Mbrooke]

Thanks for the replies!

I will give the X rated fuses a try since they are recommended for transformer protection.

 

[Mbrooke]

Just to add, thanks as well for the rules of thumb, it turns out the units weren't being over fused as much as I previously had thought.


Thanks once more!

 

[stevenal]

Okay, keep it clean folks. Not sure that X rated fuses are a suitable discussion topic. Or maybe you meant X type?

 

[Mbrooke]

That was a good one

I meant X type. Its a fuse offered exclusively (I think) by Cooper power designed to protect transformers. Either that an S&C coordinate fuse that has similar characteristics. Basically Im going from a fast blow to a very slow inverse fuse link.