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Question on line sag - September 12 Frisco outage

R

Rowbotth

Jan 1, 1970
0
I read the report on the incident on Sept. 12 that shut down about 1 M
people's lives. One of the things in the report was that there was no
power from an adjacent area (North, maybe?) because due to the load, the
line dropped 25 feet and contacted a tree, causing a line loss.

I am curious about this. I think the outage only lasted a couple of
hours; and so I'm wondering how much current there would have to be in a
line to lengthen a span by 50 feet (25 feet down; then another 25' back
up. Let's ignore any catenary characteristics for now.)

First, how long of spans would you expect to see here, anyway? And
secondly, I'm still struggling with the vision of a 25' drop in line
clearance. Where can I learn more about this?

I have heard of this happening at other times, but I have no frame of
reference for this. I'd appreciate any help, please.

HR
 
W

w_tom

Jan 1, 1970
0
Major high voltage, long distance transmission lines get
warm when under heavy load. A warmer metal becomes longer -
droops. These transmission lines have a maximum summer and a
maximum winter current rating. Since these lines can remain
cooler in winter, their current rating is higher.

To appreciate the concept, learn how First Energy of Ohio
all but intentionally created the NE blackout. Among the
violations included raising the summer and winter spec ratings
of some transmission line so that the summer current rating
was even as high as winter's (this reeks oF MBAs changing
specifications). Letting trees grow too high so that the
sagging Hanna Jupiter power line shorted to those trees (see
testimony from employees of Robert's Tree Service who saw it
and fortunately were not under that line when the fault
occurred). First Energy apparently even chopped down that
tree below ground level hoping that investigators would not
discover this (and other) violations.

Due to a large NE blackout created by numerous divisions of
First Energy, industry standards were changed. I believe laws
were created as to what electric companies must do because of
why and how these electric lines sag when under heavy currents
- especially in hot weather.

Pictures showing those lines and how lines sag are included
in NERC reports at www.nerc.com/~filez/blackout.html
including Gerry Cauley's Presentation and the final report of
5 April 2004.
 
A

Andrew Gabriel

Jan 1, 1970
0
I read the report on the incident on Sept. 12 that shut down about 1 M
people's lives. One of the things in the report was that there was no
power from an adjacent area (North, maybe?) because due to the load, the
line dropped 25 feet and contacted a tree, causing a line loss.

I am curious about this. I think the outage only lasted a couple of
hours; and so I'm wondering how much current there would have to be in a
line to lengthen a span by 50 feet (25 feet down; then another 25' back
up. Let's ignore any catenary characteristics for now.)

First, how long of spans would you expect to see here, anyway? And
secondly, I'm still struggling with the vision of a 25' drop in line
clearance. Where can I learn more about this?

I have heard of this happening at other times, but I have no frame of
reference for this. I'd appreciate any help, please.

A similar problem was contributory to the large outage in the US
last year(?). The overloaded lines coincided with a lack of tree
pruning under them, so there was less clearance than there should
have been to start with.
 
R

Rowbotth

Jan 1, 1970
0
w_tom <[email protected]> said:
Major high voltage, long distance transmission lines get
warm when under heavy load. A warmer metal becomes longer -
droops. These transmission lines have a maximum summer and a
maximum winter current rating. Since these lines can remain
cooler in winter, their current rating is higher.

To appreciate the concept, learn how First Energy of Ohio
all but intentionally created the NE blackout. Among the
violations included raising the summer and winter spec ratings
of some transmission line so that the summer current rating
was even as high as winter's (this reeks oF MBAs changing
specifications). Letting trees grow too high so that the
sagging Hanna Jupiter power line shorted to those trees (see
testimony from employees of Robert's Tree Service who saw it
and fortunately were not under that line when the fault
occurred). First Energy apparently even chopped down that
tree below ground level hoping that investigators would not
discover this (and other) violations.

Due to a large NE blackout created by numerous divisions of
First Energy, industry standards were changed. I believe laws
were created as to what electric companies must do because of
why and how these electric lines sag when under heavy currents
- especially in hot weather.

Pictures showing those lines and how lines sag are included
in NERC reports at www.nerc.com/~filez/blackout.html
including Gerry Cauley's Presentation and the final report of
5 April 2004.

I do understand the concept of line sag, and I've done a few calcs
myself. I am just really curious about how you'd get an extra 50' of
conductor between spans. (Of course, all we use is Drake or Linnet, but
still...)

Thanks for the link to the pictures, but I was actually more looking for
some info on the physics of so much expansion in the conductors. What
might these folks have been using - a 4/0 conductor? This sort of
information...

HR.
 
S

SQLit

Jan 1, 1970
0
Rowbotth said:
I read the report on the incident on Sept. 12 that shut down about 1 M
people's lives. One of the things in the report was that there was no
power from an adjacent area (North, maybe?) because due to the load, the
line dropped 25 feet and contacted a tree, causing a line loss.

I am curious about this. I think the outage only lasted a couple of
hours; and so I'm wondering how much current there would have to be in a
line to lengthen a span by 50 feet (25 feet down; then another 25' back
up. Let's ignore any catenary characteristics for now.)

First, how long of spans would you expect to see here, anyway? And
secondly, I'm still struggling with the vision of a 25' drop in line
clearance. Where can I learn more about this?

I have heard of this happening at other times, but I have no frame of
reference for this. I'd appreciate any help, please.

HR

Before an intelligent conversation can start, the voltage of the line needs
to be known. Voltage will and terrain will dedicate span lengths.

I seriously doubt that a single span lengthen 25 feet with out a cable
failure.

I have not strung any lines in some time. The last towers I worked on were
345kv lines and the towers were ~800 feet apart. I have run some 15kv over
a mesa in Arizona where the poles were ~350 feet apart.

I have seen lines "reverse sag" in winter storms. Which is pretty scary.

Lines are tensioned between supports. Some times the tension can be measured
in tons.
 
R

Rowbotth

Jan 1, 1970
0
I don't remember the line voltage, but I'd bet at least 240kV,
considering the population and the area. I'm used to 300 m line lengths
(300/0.3048 = feet) and that is sort of why I am wondering about the
quoted line sag.

Hence the posting....

HR.
 
C

Charles Perry

Jan 1, 1970
0
Before an intelligent conversation can start, the voltage of the line
needs
to be known. Voltage will and terrain will dedicate span lengths.

I seriously doubt that a single span lengthen 25 feet with out a cable
failure.

I have not strung any lines in some time. The last towers I worked on were
345kv lines and the towers were ~800 feet apart. I have run some 15kv
over
a mesa in Arizona where the poles were ~350 feet apart.

Child's play. In eastern Kentucky, I know of 15kV spans over 1000 feet in
length! Transmission spans over 1500 feet are not unhead of.

Charles Perry P.E.
 
S

SQLit

Jan 1, 1970
0
Charles Perry said:
Child's play. In eastern Kentucky, I know of 15kV spans over 1000 feet in
length! Transmission spans over 1500 feet are not unhead of.

Charles Perry P.E.


Feather Mesa in AZ is not all that steep but there were no roads, just
bloody rugged. Holes for the poles were blasted into the granite. Then we
assembled the poles in a field and had them flown in with a helicopter. Only
thing that could get to the locations was a crawler, which I refused to ride
on. Looked like it was going to tip over all of the time.

I did a transmission line over the edge of one of the mines near Globe, AZ.
Span was just short of 3400 feet. Took us almost 6 months to build both
sides of that one. Flew the wire over with helo.
 
S

SQLit

Jan 1, 1970
0
snipped=====================
First, what is reverse sag.

I have seen wind take the wire from its natural state, drooping toward the
earth. To the same position vertically. Gets pretty scary when the lines
are not in sync. Arcy sparky time, with the poles or towers rockin and
reelin.



snipped
 
D

daestrom

Jan 1, 1970
0
Rowbotth said:
I read the report on the incident on Sept. 12 that shut down about 1 M
people's lives. One of the things in the report was that there was no
power from an adjacent area (North, maybe?) because due to the load, the
line dropped 25 feet and contacted a tree, causing a line loss.

I am curious about this. I think the outage only lasted a couple of
hours; and so I'm wondering how much current there would have to be in a
line to lengthen a span by 50 feet (25 feet down; then another 25' back
up. Let's ignore any catenary characteristics for now.)

Well, that's where you went awry. The conductors don't have to grow by
anything near 25' to get a 25' sag. When they are strung 'tightly' between
towers, a relatively modest growth of just a couple of inches can make for a
noticable sag. The tighter the original line, the less change is needed to
get a given amount of sag.

Think of it in reverse, how tightly do you have to tension a line to get it
perfectly straight? (impossible) How much tension to get the line 'pretty
straight' versus 'droopy'. Now, how much expansion in the length of
conductor to 'ease' that tension from the 'pretty straight' value to the
'droopy' tension? Not very much.

To get into all of it you need sinh and cosh functions, but 'salmonegg's
approach is not too bad an approximation.

Not intuitive, I know. Kind of like how many feet do you add to a ring
encircling the world in order to expand it to encircle the world at a height
of 3.14 feet (answer, 1 foot).

daestrom
 
O

operator jay

Jan 1, 1970
0
Not intuitive, I know. Kind of like how many feet do you add to a ring
encircling the world in order to expand it to encircle the world at a height
of 3.14 feet (answer, 1 foot).

daestrom

Reverse those? Still neat, though.

j
 
D

daestrom

Jan 1, 1970
0
VWWall said:
Oops! Do the math again: PI(D+(2*3.14)) - PI(D) = PI(2*3.14)

At a height of 1 foot: PI(D+2) - PI(D) = 2PI = ~6.28 feet

Yeah, you guys are right.

Like I said, 'not intuitive' ;-)

daestrom
 
W

w_tom

Jan 1, 1970
0
Just remembered: the CA blackout that week in September was
in LA. Are you confusing a San Francisco power problem with
one that occurred in LA on that same date?
 
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