Pole Pig Fusing

[mbrooke]

Why are pole pigs fused at 200-300%? Why not at 125% if feeding a commercial facility?

https://www.sandc.com/globalassets/sac-electric/documents/sharepoint/documents---all-documents/data-bulletin-350-110.pdf?dt=637217299444528781

 

[Hv&Lv]

Is the load continuous?
transformer loading is variable depending on load diversity.
we can load a 25 to 75kVa for short periods until the temp gets too high. Winter gets us way more forgiveness than summer.
common is 200% for two to three hours, but it is all ambient temp based.

if you need 250 kVA for the majority of the day, yet need 500 for Very brief periods throughout the year, your probably gonna get a 300kVa bank built.

i have seen the wire on some some banks “barber pole” when I do IR.
we need to fix that pretty quick.

 

[mbrooke]

Is the load continuous?
transformer loading is variable depending on load diversity.
we can load a 25 to 75kVa for short periods until the temp gets too high. Winter gets us way more forgiveness than summer.
common is 200% for two to three hours, but it is all ambient temp based.

if you need 250 kVA for the majority of the day, yet need 500 for Very brief periods throughout the year, your probably gonna get a 300kVa bank built.

Near continuous for 12 hours- commercial service. Won't exceed 125%. No 1 hour (short time) 200% overloads like residential.

i have seen the wire on some some banks “barber pole” when I do IR.
we need to fix that pretty quick.

IR as in infrared?

 

[mbrooke]

I've heard to prevent nuisance blowing of fuses being one reason behind over fusing, but unsure how modern MOVs behave during a lightning strike. Any idea as to the magnitude and duration they conduct and if that weakens links below a certain rating?

 

[Hv&Lv]

its been my experience no loads are near continuous with a level curve. There are peak days that need to be covered also..
remember our discussion regarding fuse weakening due to overheating. Annealing?
fuse it high enough to keep the properties of the fuse rather than lower the fuse and weaken it with Peaking days in the summer.

 

[Hv&Lv]

I've heard to prevent nuisance blowing of fuses being one reason behind over fusing, but unsure how modern MOVs behave during a lightning strike. Any idea as to the magnitude and duration they conduct and if that weakens links below a certain rating?

http://www.arresterworks.com/arresterfacts/pdf_files/ArresterFacts_003_Understanding_Arrester_Voltage-Current_Characteristic_Curves.pdf

 

[mbrooke]

its been my experience no loads are near continuous with a level curve. There are peak days that need to be covered also..
remember our discussion regarding fuse weakening due to overheating. Annealing?
fuse it high enough to keep the properties of the fuse rather than lower the fuse and weaken it with Peaking days in the summer.

True- but some loads are more of a steady platue instead of a sharp spike with hours of light load like residential.

 

[mbrooke]

http://www.arresterworks.com/arresterfacts/pdf_files/ArresterFacts_003_Understanding_Arrester_Voltage-Current_Characteristic_Curves.pdf

In your experience do lightning strikes blow fuses? Can this be avoided? Or do you need a fast acting recloser and a "plus sized" link? I know of some POCOs that have "lightning mode" curves on their reclosers that will trip instonaously at a very low level during lightning storms. I'd imagine this is to save lateral fuses- no idea if the goal is to save pole pig fuses.

FWIW- in the past I've had several EEs tell me "the purpose of a fused cutout is to remove a failed transformer from service, not to protect it" They saw no issue with a 30K protecting a 25kva bank at 15kv class. Actually they recommended it.

 

[Hv&Lv]

In your experience do lightning strikes blow fuses? Can this be avoided? Or do you need a fast acting recloser and a "plus sized" link? I know of some POCOs that have "lightning mode" curves on their reclosers that will trip instonaously at a very low level during lightning storms. I'd imagine this is to save lateral fuses- no idea if the goal is to save pole pig fuses.

FWIW- in the past I've had several EEs tell me "the purpose of a fused cutout is to remove a failed transformer from service, not to protect it" They saw no issue with a 30K protecting a 25kva bank at 15kv class. Actually they recommended it.

the arrestor may protect the transformer, although every one you see is technically wrong according to arrestor manufacturers..
line needs to hit arrestor first, then go to bushing.

it’s aggravating to find a failed transformer behind a recloser. Fuse blows, keeps the recloser on. Customer pinpoints outage... A fuse isn’t going to protect a transformer. It isn’t near fast enough, I don’t care what size you put in there.
sometimes you just need to fuse to your coordination scheme, other times you need to fuse for load. Every circumstance and location is different. I’ve seen transformers That were too small in the first place put out Lots of amps with huge fuses until the lid blew off and caught on fire.

lightning does anything it wants to...
arrestors need to be installed on every dead end. Lightning goes down line, hits dead end, doubles on way back...

 

[mbrooke]

the arrestor may protect the transformer, although every one you see is technically wrong according to arrestor manufacturers..
line needs to hit arrestor first, then go to bushing.

Yup. But we both know no one follows the rules

it’s aggravating to find a failed transformer behind a recloser. Fuse blows, keeps the recloser on. Customer pinpoints outage... A fuse isn’t going to protect a transformer. It isn’t near fast enough, I don’t care what size you put in there.

I agree but also disagree. Tell me if I'm wrong.

Fuses start at around 200%- so yes- one could overload a pole big above 100% continuously leading to failure.

However- a fuse is quick enough to blow for a secondary short circuit (as long as the secondary run isn't so long) before the oil boils over. Thus the transformer is protected in those regards.

sometimes you just need to fuse to your coordination scheme, other times you need to fuse for load. Every circumstance and location is different. I’ve seen transformers That were too small in the first place put out Lots of amps with huge fuses until the lid blew off and caught on fire.

Yes, but its nice to have a one size fits all rule of thumb

l

ightning does anything it wants to...
arrestors need to be installed on every dead end. Lightning goes down line, hits dead end, doubles on way back...

Are you talking about reflection? What do you mean here?

 

[mivey]

You can fuse to protect transformers. Depends on economics. Some only protect when the transformer gets more expensive or harder to replace. I usually fuse to protect, even with small banks.

Long-time overload protection is usually done on the secondary side and with transformer loading reports using billing data.

High-side fuses should not be in the arrestor line-to-earth/ground path or they get damaged.

Reflection is correct. Surge voltage increases at other impedance changes also like taps and sharp turns.

 

[mbrooke]

Long-time overload protection is usually done on the secondary side and with transformer loading reports using billing data.

CSP handle good enough for this?

High-side fuses should not be in the arrestor line-to-earth/ground path or they get damaged.

I know its not best, but I frequently see that as being the case. Is weakening actually taking place or are they doing something to mitigate it?

 

[don_resqcapt19]

Because the utility wants to keep the cash register spinning, they often provide a level of protection that only prevents a transformer failure from taking out the upstream distribution OCPD. This provides little to no protection for the transformer and no protection for the secondary conductors.

 

[mbrooke]

Because the utility wants to keep the cash register spinning, they often provide a level of protection that only prevents a transformer failure from taking out the upstream distribution OCPD. This provides little to no protection for the transformer and no protection for the secondary conductors.

Maybe. The NEC lets a lot slide when it comes to protecting transformers or lack there of.

Truth be told any legit protection goes very well beyond 50/51.