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

[mbrooke]

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.

Spacer cable varies greatly depending on the utility, service territory and desired outcome. Generally, areas which have trees see far more spacer cable where as, for example, in mid western utilities spacer cable is non existent. Spacer cable does cost more, however depending on a variety of factors it can be worth the initial cost.


Large trees and exceptional ice accumulation will take down spacer cable, but with the right design mild to moderate events are handled with higher reliability. Limbs and vegetation can come closer to spacer cable then conventional construction, but large limbs at risk of falling should be removed. In terms of handling severe events like a large tree coming down it is often advised that the messenger be sized such that it will break first before the pole snaps at the base, and likewise where severe ice accumulation may occur the messenger be large enough and the pole thick enough to handle it. The messenger size and pole size is weighed against cost vs a contingency probability analysis. A word of caution though, In applications where the spacer cable is likely to broken, twisted, or encounter very serve conditions repair cost goes up since fixing the spacer cable becomes a complex job with more parts involved.

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.

In that case nothing will hold up. BTW, not to nit pick but its killo-volts or kv rather then kva. 115kva (115,000 va) is not a lot of power.

 

[mivey]

Limbs and vegetation can come closer to spacer cable then conventional construction, but large limbs at risk of falling should be removed.

That would be nice but the spacer cable I've seen has tree wire and is threaded through a small window cut in the tree canopy. The folks would chain themselves to the tree if you tried to cut large limbs. They don't believe in V cuts like reasonable people.

Don't cut trees but don't blink the lights and we want cheap power. It's crazy, really.

 

[iwire]

Are you guys talking about this kind of installation?

If so it is used extensively in this area and we do get ice and snow.

It seems to be used often when multiple sets need to fit on the poles.

 

[kwired]

Spacer cable varies greatly depending on the utility, service territory and desired outcome. Generally, areas which have trees see far more spacer cable where as, for example, in mid western utilities spacer cable is non existent. Spacer cable does cost more, however depending on a variety of factors it can be worth the initial cost.


Large trees and exceptional ice accumulation will take down spacer cable, but with the right design mild to moderate events are handled with higher reliability. Limbs and vegetation can come closer to spacer cable then conventional construction, but large limbs at risk of falling should be removed. In terms of handling severe events like a large tree coming down it is often advised that the messenger be sized such that it will break first before the pole snaps at the base, and likewise where severe ice accumulation may occur the messenger be large enough and the pole thick enough to handle it. The messenger size and pole size is weighed against cost vs a contingency probability analysis. A word of caution though, In applications where the spacer cable is likely to broken, twisted, or encounter very serve conditions repair cost goes up since fixing the spacer cable becomes a complex job with more parts involved.





In that case nothing will hold up. BTW, not to nit pick but its killo-volts or kv rather then kva. 115kva (115,000 va) is not a lot of power.

My mistake there, I meant kV not kVA. Whether 115,000 VA is a lot depends on your perspective. Many dwellings it is quite a bit, but to a POCO it typically is just pocket change so to speak.

 

[mivey]

Are you guys talking about this kind of installation?


If so it is used extensively in this area and we do get ice and snow.

It seems to be used often when multiple sets need to fit on the poles.

Yes. Imagine that buried in a tree canopy.

I do recall seeing it used for multiple feeders. Either pictures or somewhere I visited. Wish I could remember but it all blurs together after a while.

 

[mbrooke]

That would be nice but the spacer cable I've seen has tree wire and is threaded through a small window cut in the tree canopy. The folks would chain themselves to the tree if you tried to cut large limbs. They don't believe in V cuts like reasonable people.

Don't cut trees but don't blink the lights and we want cheap power. It's crazy, really.

I agree, you cant have everything for nothing. Spacer cable sure cuts down on blinks (you still have to trim because continuous tree contact will erode the covered wire via corona) and reduces tree trimming but it does come with a cost. However some POCOs are actually now footing the bill for trunk lines since service continuity has become so lack luster in some areas that customers have noticed.

Are you guys talking about this kind of installation?

Yup, that it!

If so it is used extensively in this area and we do get ice and snow.

It is very popular in the North East. National Grid is is using this on the majority of new trunk lines and municipal utilities like Groton, Sterling, Littleton, Hudson MA and Wallingford CT use it almost exclusively. New Jersey utilities have a great deal as well from over the decades. While it is certainly more expensive, the added reliability is often justified in heavily treed areas which are common in New England.

It seems to be used often when multiple sets need to fit on the poles.

Where space needs to be conserved (such as alley ways or multiple circuits on a single pole) spacer cable wins. Because the conductors are covered (insulated is not used as a term here because conductors must still be treated as live for various reasons) they can be placed closer together without violating NESC code rules which apply to bare conductors.

 

[mbrooke]

Yes. Imagine that buried in a tree canopy.

I do recall seeing it used for multiple feeders. Either pictures or somewhere I visited. Wish I could remember but it all blurs together after a while.

Here are some pics of multiple circuits on a pole:


http://www.utilityproducts.com/arti...for-distribution-infrastructure-upgrades.html


http://wpcore.wpe.s3.amazonaws.com/...ille-Substation-close-up-of-1-pole-30-per.jpg


And a good technical paper on the advantages:


http://www.massmunichoice.org/resources/Bouford-IEEE-paper-on-spacer-cable.pdf

 

[mbrooke]

And this less technical one on ice storms:



http://www.massmunichoice.org/wt120609.html

 

[mivey]

Thanks for the links mbrooke. Interesting.

 

[mbrooke]

Thanks for the links mbrooke. Interesting.

Welcome, and thank you your input