Utility transformer sizing and thoughts on across-the-line motor starts

[mbrooke]

Depends what the OP intends to do. Started sounding like he wanted to make his own recipe.

Or doing what most POCOs do, getting the most out of anything which require "what if" scenarios. Personally I dont think the voltage drop upon inrush will be as bad as the OP thinks. Ive seen 25kva banked pigs run store fronts with many large AC roof top units as well as sewage lift stations no problems. The low Z of most utility transformers especially pole pigs is no accident as these are frequently overloaded into oblivion.


Edit: the OP is asking about a padmounted unit, but those are not exempt from this philosophy either.

 

[fastline]

As a point of reference, I am curious of the price differences in a 50kva pole pig vs pad mount? Since I have to foot the bill, I am able to decide UG or OH service.

 

[mivey]

As a point of reference, I am curious of the price differences in a 50kva pole pig vs pad mount? Since I have to foot the bill, I am able to decide UG or OH service.

Pad-mount is roughly 50% more than pole mount. UG supply line will be more expensive as well.

 

[meternerd]

As a point of reference, I am curious of the price differences in a 50kva pole pig vs pad mount? Since I have to foot the bill, I am able to decide UG or OH service.

You're fortunate there...we and and many other utilities I dealt with do not offer overhead unless UG is impractical. Greenies like the look of no OH wires. So do I. Plus, OH is much more subject to weather problems. Wish they had put UG in my subdivision, but back then (1970's) it was more difficult and more expensive to do. Now it's too late to convert. Gas, sewer, water, etc. are already buried and power must be on the bottom, so not an affordable option. Snow is the biggie....trees and snow unloading from lines is a constant headache.

 

[kwired]

You're fortunate there...we and and many other utilities I dealt with do not offer overhead unless UG is impractical. Greenies like the look of no OH wires. So do I. Plus, OH is much more subject to weather problems. Wish they had put UG in my subdivision, but back then (1970's) it was more difficult and more expensive to do. Now it's too late to convert. Gas, sewer, water, etc. are already buried and power must be on the bottom, so not an affordable option. Snow is the biggie....trees and snow unloading from lines is a constant headache.

What kind of winter weather conditions do you get. Snow alone generally isn't much problem here, it is when you get rain/drizzle with conditions right about at freezing point that are a problem. Even that isn't real bad until some wind comes up, and it don't need to be all that hard of a wind either to get a conductor swaying that is normally 1/2 inch diameter that is now encased in ice with a total diameter of 2 or 3 inches. Takes down a whole mile of distribution real fast.

 

[mbrooke]

What kind of winter weather conditions do you get. Snow alone generally isn't much problem here, it is when you get rain/drizzle with conditions right about at freezing point that are a problem. Even that isn't real bad until some wind comes up, and it don't need to be all that hard of a wind either to get a conductor swaying that is normally 1/2 inch diameter that is now encased in ice with a total diameter of 2 or 3 inches. Takes down a whole mile of distribution real fast.

I think a well though out spacer cable design would take care of this?

 

[kwired]

I think a well though out spacer cable design would take care of this?

probably so. What I described happened here two years in a row, but had been 15-20 years since it last happened and that time wasn't nearly as severe. It has now been about 10 years with nothing even close to that happening, and who knows when it will happen again. I do believe they have made some improvements though that will help withstand more of that kind of thing. But by the time such an event happens again who knows how old components will be and how much that will factor into what happens.

 

[meternerd]

What kind of winter weather conditions do you get. Snow alone generally isn't much problem here, it is when you get rain/drizzle with conditions right about at freezing point that are a problem. Even that isn't real bad until some wind comes up, and it don't need to be all that hard of a wind either to get a conductor swaying that is normally 1/2 inch diameter that is now encased in ice with a total diameter of 2 or 3 inches. Takes down a whole mile of distribution real fast.

The snow we get most of the time is heavy and wet, known as Sierra Cement. It coats the bare distribution lines until they're several inches in diameter. Many of our single phase taps were installed vertical and when the snow unloads, the lower neutral wire flies up into the phase. Spacers help, but that still doesn't save us from tree branches falling across the lines. Also, in the summer, squirrels tend to commit suicide on cutouts, switches and transformers. Substations all have squirrel guards on the bussing, but they still seem to find a way to blow themselves up. That's why we love UG. Plus, it's beautiful compared to a pole line filled up with power primary, cable TV, phone, fiber, secondary, etc. And...when you get a car/pole accident, there's a LOT of stuff to repair. Nasty.

 

[mivey]

I think a well though out spacer cable design would take care of this?

Seems to me like it would add more space where ice could build up, at least in the configurations I'm picturing.

 

[mivey]

The snow we get most of the time is heavy and wet, known as Sierra Cement. It coats the bare distribution lines until they're several inches in diameter. Many of our single phase taps were installed vertical and when the snow unloads, the lower neutral wire flies up into the phase. Spacers help, but that still doesn't save us from tree branches falling across the lines. Also, in the summer, squirrels tend to commit suicide on cutouts, switches and transformers. Substations all have squirrel guards on the bussing, but they still seem to find a way to blow themselves up. That's why we love UG. Plus, it's beautiful compared to a pole line filled up with power primary, cable TV, phone, fiber, secondary, etc. And...when you get a car/pole accident, there's a LOT of stuff to repair. Nasty.

Have y'all had any sucess with swapping loads to overload tie lines to melt ice or does the ice get too overwhelming?

I can see where on radial taps and that much ice you would just have to up the design strength or save that money for repair work.

 

[kwired]

Seems to me like it would add more space where ice could build up, at least in the configurations I'm picturing.

Spacers would help keep lines from touching one another, but at same time do provide more places for ice to build.

What meternerd described does happen here sometimes - especially on single phase distribution with only two conductors installed with one directly over the other. Lines get a lot of sag from the weight of ice, when ice is released from bottom line it springs back up and touches the still sagging upper line, resulting in operation of overcurrent devices. POCO employees have to come out and knock ice off the upper line before they can reset.

 

[GoldDigger]

Isn't ice a reasonably good insulator?

 

[kwired]

Isn't ice a reasonably good insulator?

Must not be good enough @ 7200 volts.

 

[mbrooke]

Seems to me like it would add more space where ice could build up, at least in the configurations I'm picturing.

THe NESC changed how its calculated (let me double check however its been a while), but spacer cable can easilly be designed to handle the added weight plus extra.

 

[mivey]

but spacer cable can easilly be designed to handle the added weight plus extra.

So could other construction, but I guess there is a big cost difference.

 

[mbrooke]

So could other construction, but I guess there is a big cost difference.

Yes fiberglass arms would be cheaper, but if trees are involved spacer cable kills 2 birds in one stone.

 

[mivey]

Yes fiberglass arms would be cheaper, but if trees are involved spacer cable kills 2 birds in one stone.

The spacer cable I have seen is used for threading through trees, making the ice storm impact worse. I have seen pics of ally-arm type construction on the road side that might help the situation, much like we do with vertical construction.

 

[mbrooke]

The spacer cable I have seen is used for threading through trees, making the ice storm impact worse. I have seen pics of ally-arm type construction on the road side that might help the situation, much like we do with vertical construction.

I disagree, spacer cable when designed properly holds up to ice storms better then any other method. The real support and load carrying ability comes from the messenger. Ice and small to medium branches are a breeze requiring less restoration effort. Further spacer cable has fewer blinks and nuisance fuse blowing.

 

[kwired]

You see some spacer cable in cities/towns here but never in the rural distribution systems. Something tells me it cost more then what it may save in situations described here. The lines hitting each other, is something that doesn't happen a lot, but does happen. Occasionally you will see some spacer devices in select areas where maybe wind, ice, maybe both seem to be more frequent of a problem, but is not widespread use of such devices.

The ice storms we had about 10 years ago would have taken down spacer cable. It wasn't just conductor that came down in those storms, it was many miles of poles broken off near the ground.

Even several miles of 115kVA transmision line on two pole structures went down, it wasn't so much broken cross arms, insulators, etc. the poles broke off near the ground and once it got started kept going like a domino setup.

 

[mivey]

I disagree, spacer cable when designed properly holds up to ice storms better then any other method. The real support and load carrying ability comes from the messenger. Ice and small to medium branches are a breeze requiring less restoration effort. Further spacer cable has fewer blinks and nuisance fuse blowing.

Disagree if you want but when the tree comes down it takes the line/poles with it. Spacer cable not threaded through trees would be different.