What are those barbells on the power-lines!!?? 2

[pnjunction]

Hello everyone,

Just a longstanding curiosity of mine:

On the power lines near-by; the lines running alongside interstate roads...not those feeding homes...... Near each insulator I see what I can only describe as a small "barbell" shaped thing attached to the line.....

From a distance, it looks like a small barbell which is attached (from the center of the bar) to the electrical line. The ends of this barbell do not connect to anything, and it's oriented such that the bar runs parallel to the wire to which it's attached. It just looks as if somebody decided to hang a barbell from the wire like a Christmas ornament...

WHAT IS THIS THING?

 

[alehman]

Vibration dampers. Wind can cause aeolian vibration in taught cables. The weights prevent the vibration.

 

[Skogsgurra]

They work by absorbing mechanical energy from the cables so that wind induced standing waves (that would slowly crack the insulators) do not have a chance to build up.

A question in return: Why are the top lines isolated? Are they not grounded? Why then isolated?

 

[busbar]

Search "Stockbridge damper" / “Aeolian vibration”.

 

[pnjunction]

Reply to skogsgurra:

I doubt the top lines [are] isolated. I don't think THEY have these devices on them. The fact that they were NOT on the ground wire- let me to believe they were some sort of lightning/surge suppression attachment. (Don't ask how I expected it to work- it was all I could come up with)

I guess the only part I had right was the 'suppression' angle.

To everyone else:

Thanks for the replies, guys. One less thing I have to obsess over.


Johnny

 

[mersin33]

skogsgurra,

Just asking to learn:

you mentioned the vibration dampers are used so that they avoid the crack of insulators. I thought they were used only for protecting the conductor against aeolian vibration.

Thanks in advance,

 

[IRstuff]

see:

http://www.uh.edu/engines/epi1653.htm

TTFN

 

[Skogsgurra]

I loved that one, IR! So, you can actually measure wind speed that way. Interesting.

Saltan, the main problem is the ceramic insulators. The cable is attached to the insulator and that means a very high mechanical impedance so that the wave that is coming along the cable snaps the insulator every time it hits it. The insulator is a chain of ceramic (or glass) elements with steel members that keep them together. Long time exposure to vibration starts micro cracks in the ceramic and these cracks grow with time and do eventually initiate a flash-over if the insulators are not changed. The dampers are placed close to the insulators in order to protect them. But, of course, the wire is protected from fatigue as well.

If you wanted to protect the wire itself from fatigue, I think that the dampers should be placed where the vibration is strongest - and that is not close to the insulators.

And pn, yes the top lines are insulated here in Sweden. Not all, but on 400 kV lines, definitely. And I do not understand why. And, yes. The top lines usually do not have dampers. I am still confused there.

 

[rbulsara]

see the attached link with pictures..and confirm this what you were referreing to. It supports skogsgrra's views.

http://users.castel.nl/~berka01/hvinfo.html

 

[mersin33]

skogsgurra,

thank you for your reply.

you mention the ceramic insulators in your answer. but we know that vibration dampers are used at the lines with composite insulators too, which are not as brittle as ceramic or glass ones.

the conductors are never connected directly to the insulators, there is always some hardware between the clamp of the conductor and ball of the insulator (or last disc), which mechanically provides free movement for the conductor in both up-down axis and left-right axis. The aeolian movement of the conductor is not directly transferred to the insulator.

the reason the dampers are put close to the insulators is not to protect the insulators, but because this area forms a node for the oscillating frequencies, thus the most effective location for the dampers.

in answer to top lines: if I am correct that top wires are actually shieldwires. there are some applications which require the shieldwire to be insulated from the tower. Sometimes shielding wire is used instead of the power conductor for PLC applications. However this is a very old and very questionable method, thus very hard to see one in operation today.

shieldwires are generally steel or steel alloy, which has got much more strength than acsr conductors. therefore they can bear more tension. damper requirement is generally determined by the percentage of the actual tension of the conductor with respect to its ultimate tensile strength. 20% is generally considered as the limit, i.e. higher tensions require dampers, lower ones not. figures can change based on the acsr type, avarage wind, etc. although it is common to see shieldwires damped when the conductors are damped, it may be possibe that a very strong shieldwire is chosen for the application and does not require dampers.

regards,

saltan

 

[Skogsgurra]

Thanks saltan,

My knowledge in the insulator/cable field seems to be somewhat outdated. Personally, I never seen dampers on composite insulator lines. I have always thougt about the dampers as protection for the insulators. I now know that there are two reasons to use them.