Annealing 6

[riensche]

Can someone provide me with a good, technical definition of "conductor annealing" (as it relates to overhead transmission/subtransmission lines)?

 

[rbulsara]

riensche:

Welcome to Engineering tip forum. However, it is advisable to do a internet search using google etc. for the information you are looking for, before you post a question here.

So look up 'annealing' in the web search, it gives you thousands of hits. Look for links that deal with heat treatment of metals.

 

[riensche]

So, I join an engineering forum to get a specific answer to a technical question, and the response I get is to "Google" it?! What kind of a forum is that? The advantage of posing a question in a forum is that you don't have to wade through dozens of junk search results unrelated to what you're looking for; supposedly you can obtain a quick and direct response from an industry colleague. Thanks for nothing.

 

[rbulsara]

riensche:

All of us here are volnteeers. Some senior members may be can explain this better than I , but it is not the purpose or intent of this forum to hand ready made answers to those who do not want to take a trouble to find the answers themselves. This forum is not here to foster lazyness or reward lazy people. But rather exchange views and if you are nice enough to ask, get a peer review on you specific issues.

If you think you are entitled to an answer on a platter, you have joined the wrong forum.

 

[49078]

this is definitely a google promoting forum. No problem with that though, google holds many answers.

 

[itsmoked]

http://www.ponderaengineers.com/publications/PonderaSagandTensionWhitePaper.pdf

?

 

[jghrist]

I don't have a good technical definition off-hand, but in relation to overhead lines, the main concern is that if the temperature is allowed to exceed the annealing point, you will lose strength in the conductor. This is a concern for single material conductors like copper, all-aluminum (AAC), all-aluminum alloy (AAAC), and normal ACSR. Steel-Supported Aluminum Conductor (SSAC) does not depend on any aluminum strength and was developed to allow operation above the annealing point of aluminum.

 

[riensche]

Thank you, jghrist. We have some 110kV subtransmission lines in a loop configuration on our system, that consist of some 795AAC and some 336ACSR. Our dispatch department likes to load up the lines as if the whole thing were 795. As a junior protection engineer, you can imagine that I'm concerned about possible annealing of the 336 conductors. Your information was helpful -- thanks for your response.

 

[jghrist]

Even if the conductor temperature does not exceed the annealing point, if it exceeds the design temperature, you may have dangerously low ground clearances when the conductor sags more than it was designed to.

Lines sagging into trees can cause problems. Ask First Energy, whose lines contacted trees and started the 2003 northeast blackout (although this was because of poor tree trimming not overloaded lines). The Western Power Grid blackout of 1996 started with overloaded powerlines sagging into trees.

Ask your dispatchers if they want to be the cause of the next big blackout.

 

[cuky2000]

Annealing is the process of heating and slow cooling of metals. For conductors, this is an undesirable process because annealing decreases the tensile strength of the metal. Annealing is a cumulative process and is very dependent on the type of parent metal, alloys if present, and the amount of previous cold reduction.

Overheating of a conductor, causing it to expand, sag, weaken, and sometimes break. Usually caused by a combination of too much current passed through the conductor and high temperatures in the air surrounding the conductor.

If the temperature of a line is permitted to reach the annealing point, the conductor may be damaged. Fortunately, before the annealing point is reached, vertical clearance requirements may be the limiting factor. IEEE studies showed that the majority of TL in the USA use SCSR with a few rare exceptions, actual line-specific thermal limits is caused by clearance limits and not by annealing.

 

[bacon4life]

jghrist points out “Lines sagging into trees can cause problems…Although this was because of poor tree trimming not overloaded lines” In fact, poor tree trimming caused all three of the critical 345 kV phase 3 outages. Loading was only 44%, 88% and 93% of the ratings

Again in 1996, it was poor tree trimming, not overloaded lines that caused the intial problems. Overloaded lines sagging and faulting did not occur until the system was already collapsing.

For utilities that do a better job with vegetation management, or that are in non vegetated areas, the dispatchers may be able to have more current due to 3 reasons.

1. The temperature rating is specified with a number of factors that do not always occur simultaneously such as low wind and high temperatures. However, the line must be designed to ALWAYS meet clearances. The following is an example of a conductor temperature calculation input to IEEE 738 in PLS-CADD. Just because the current is higher than rated doesn’t necessarily mean annealing has occurred.

PLS-CADD Version 7.30+    5:48:28 PM Thursday, August 11, 2005
IEEE Std. 738-1993 method of calculation
Air temperature is 85.00 (deg F)
Wind speed is 2.00 (ft/s)
Angle between wind and conductor is 90 (deg)
Conductor elevation above sea level is 200 (ft)
Conductor bearing is 270 (deg) (perpendicular to solar azimuth for maximum solar heating)
Sun time is 12 hours (solar altitude is 71 deg. and solar azimuth is 180 deg.)
Conductor latitude is 42.0 (deg)
Atmosphere is CLEAR
Day of year is 172 (June 20) (day of the year with most solar heating)
Given a maximum conductor temperature of 167.0 (deg F),
The steady-state thermal rating is 829.9 amperes

There are a number of devices that claim to measure the temperature of the conductor by either measuring sag, measuring tension, measuring the surface temperature using infrared or by measuring both the current and the weather conditions. Dispatchers could then (at least theoretically) run it up to the design temperature all the time, not just on the hottest, sunniest, and also calmest day of the year. Without monitoring, they may be in code violation. And of course, it could be even hotter and calmer than the condition you pick.

2 Lines may be designed to more than NESC clearances and may include addition construction buffers. For example, our sub transmission was typically designed about 8 ft higher than NESC requirements to allow for survey and construction tolerances along with future underbuild or road elevation changes. We are currently doing LiDAR surveying and PLS-CADD modeling to check how addition capacity can be realized from actual condition vs. design conditions.

3. If there is a calculated clearance violation, and there is not a fault, is it still a clearance violation? There is clearly a different response between a) an operator seeing a 50 year old drawing with a limit that has been exceeded regularly in the past with nothing bad happening and b)power lines sagging so low they actually touch something or the public notices. Without a detailed survey/study of the power line and an inquizative engineer, it will not be obvious there is a problem developing. It sounds like you may have your work cut out for you getting the money to fix this problem.

If it has been operated above design limits, damage could have already occurred. What temperature is the 795 line designed for? Does this happen regularly or is it irregular system configurations? Annealing damage is due to both the temperature and the duration, with temperature relationship being nonlinear.

In addition to the conductor itself, many terminals & clamps could also be damaged by high temps. It might also pay to check if things like buswork, transformers and breakers are overloaded.

 

[jghrist]

Can anyone explain this in light of FirstEnergy's poor tree trimming starting the 2003 NE blackout. In T&D World from a May 5, 2005 FirstEnergy press release:

FirstEnergy, on behalf of its seven electric utility operating companies, has been named a Tree Line USA utility by the National Arbor Day Foundation in cooperation with the National Association of State Foresters. It is the seventh year in a row that FirstEnergy has received this award for excellence in vegetation management practices.

 

[stevenal]

Outages don't seem to be part of the equation, but maybe they should be.

http://www.dnr.state.oh.us/forestry/Urban/treeline.htm

Urban forests are motioned. Was the tree in question urban or rural? And evidently topping is bad according to the Foundation. If a tree cannot be removed, topping is better than shaping.

NESC: "Trees that may interfere with ungrounded supply conductors should be trimmed or removed."

This can be handled either by limiting the loading to avoid the interference, or by clearing the vegetation to allow the desired loading. It would seem ST Energy did neither. The lines were overloaded for the conditions present even if the conductor itself could handle more.

 

[jghrist]

O.C. Seevers, in Power systems Handbook

The proper way to trim a tree that endangers an electrical circuit is with a chain saw at the ground line. If this is not feasible, move up a little.

 

[stevenal]

Damn, I need that edit button. I meant to say "mentioned" not "motioned" I was thinking about quoting Seevers too. Great read, I recommend the book.

 

[bacon4life]

FE’s UVM [utility vegetation management] program, and program attributes are consistent with what we would expect to find at a similarly sized utility. It is not a best-in-class program, nor would we consider it to be substandard. ... While the Tree Line USA designation does nothing to indicate how effective a utility is in reducing tree related outages, it is a basic measure of the utility’s commitment to various important UVM functions (public education, worker training, and quality work).

From the UVM final report at ftp://

http://www.nerc.com/pub/sys/all_updl/docs/blackout/uvm-final-report(trees).pdf.

It contains detailed analysis of tree related aspects of the August 2003 blackout. Note that you will have to cut and paste the URL because it contains parenthesis.