Ice on transmission lines. 1

[itsmoked]

They are airing a "perfect disaster" show on the boob-tube that is based on an ice storm in the Montreal area. They are discussing the results of a major ice storm. They are focusing on the power transmission lines, 8? that serve the city. As they each get too much ice on the lines, eventually a line will snap. The sudden shock causes the towers to all fail.

My question is if a transmission line design has to take into consideration the I^2R losses to prevent over temperature lines from over sagging why don't they just crank up the current to keep the lines above freezing? Even if Montreal can't use the power can't they import then export it out the other side or play some other game?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Warpspeed]

Perhaps, but what if it is -40 and blowing like a hurricane ? The heat transfer into the moving airstream will be huge, far in excess of the conduction loss. The Ice only has to remain just below freezing to remain happily in position.

In the Antarctic I have seen ice build up on wire radio antennas as thick as a man's arm. That becomes extremely heavy over a long span, and it certainly does tend to break things.

 

[waross]

Hi Keith;
I remember an ice storm bringing down a major transmission line in western Canada years ago. The line was 800 or 900 miles long. The area affected by the storm was a valley about 90 miles long and the hardest hit section was the eastern half of the valley. The capacity of the line was 2000 amps at 500,000 volts. I don't think that heating was an option.
respectfully

 

[itsmoked]

I remember reading something like you could end up with the wires being 200 degrees. Seemed to me that you could still reach 32 somehow in a hurricane. But I figgured it would be something not directlky related, like half the line is in the desert or as you say waross the cold is localized. Or you just have no way to load up the line (no A/C going) at the time, etc.

Thanks for the thoughts.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jghrist]

It's best to design the lines to withstand the ice load. During a major ice storm, distribution lines will be down and many customers will be off line. If you based the integrity of your transmission lines on putting a lot of load on them, the plan may fail just when you need it. Distribution outages may prevent you from loading the transmission line.

High winds are not usually a problem with ice loading because high winds blow the water off the conductors and prevent ice from building up.

 

[bacon4life]

This article talks about using short circuit forces to deice bundled conductors.

http://www.ewh.ieee.org/reg/7/canrev/canrev37/landry_eng.pdf

There has also been talk about installing power electronics to circulate DC currents.

 

[waross]

Hi bacon4life
Thanks for the link. Very imformative.
respectfully

 

[alehman]

The National Electrical Safety Code (U.S.) has design criteria for ice and wind loading on overhead lines. The design ice thickness aries from 0 to 1/2" in comibination with specified wind loads, both dependent on the region.

Ice-laden trees falling on lines causes the vast majority of the damage in these storms. It's hard to design for that.

 

[rmw]

I was taught in college that shipping VAr's across transmission lines would help heat them up. I don't know if I believed it then, or if I believe it now.

rmw

 

[davidbeach]

Shipping VArs across a line will heat it; there are amps associated with those VArs and those amps interact with the resistance of the line to cause I²R heating. Deucedly inconvenient to ship VArs independent of Watts as you need to use a voltage difference between the two ends to control the VAr flow.

 

[tinfoil]

The ice storm of january '98 had some freakish weather conditions. It had been seasonally cold for a while, so every object was cooled well below freezing.

Then, the upper air was warmed up to above freezing, while the air near the ground was below freezing. The low-pressure rain-bearing clouds were stalled for a week by neighboring high-pressure zones, and a LOT of rain fell in a short period. As the rain passed down through the cold air, it was chilled, and some experts say that is was even supercooled (ie. below 0°C yet still in liquid phase).

So this 'rain' made contact with cold objects, and stuck, freezing instantly. There was enough 'cold' in the ground to provide for lots of freezing. The accumulation rate was very high ( as much as 100mm+ of ice in 3-4 days).

It would have taken a lot of heat to stay ahead of the problem. Even if you do, a 100mm 'shell' on the structures weighs a lot more than the designers allowed for. The minimum 'standard' was to allow for 12mm (0.5inches) of ice accumulation, and most structures were designed for 25mm ( a fourfold weight safety factor). In my area, the ice built up to over 75mm, which is 36 times more ice than the minimum standard. Even without the ice on the lines, some structures were toast.

 

[unclebob]

Hi all!

I live near Montreal and I once visited the IREQ (an institute
where Hydro-Quebec does some research) and they work really hard on the subject. Although de-icing the lines with helicopters does the job, it is somewhat dicey...

They (H-Q) finished a new line 2 years ago. It seems the only solution they come up with is puting a bigger, beaffer tower
between a series of 4-5 normal ones to stop the cascading
mouvment...

 

[itsmoked]

unclebob; That makes the most sense. Then all you would need would be a few extra kitted towers in the yard. Would help with terrorist garbage too.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com