Pole transformer fusing
Pole transformer fusing
(OP)
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?
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?






RE: Pole transformer fusing
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.
RE: Pole transformer fusing
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.
RE: Pole transformer fusing
RE: Pole transformer fusing
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-distr...
http://www.cooperindustries.com/content/public/en/...
X link "slow": http://www.cooperindustries.com/content/dam/public...
K link "fast":http://www.cooperindustries.com/content/dam/public...
RE: Pole transformer fusing
RE: Pole transformer fusing
RE: Pole transformer fusing
I will give the X rated fuses a try since they are recommended for transformer protection.
RE: Pole transformer fusing
Thanks once more!
RE: Pole transformer fusing
RE: Pole transformer fusing
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.