Conductor Gallop 1

[Bigbrain]

First post on this forum. I'm a distribution design engineer for a power company in west Texas. Linemen like to call me big brain because I went to college and I seem to have an abnormally large head.

Here's why I'm posting. I've volunteered to give a presentation on the phenomenon of line galloping at our company's anual engineering conference. I'm in search of more information regarding this topic. Primarily, I'm looking for information relevant to engineers: the physics behind what is happening, a little math maybe, what we can do when designing a power line to prevent it, etc. I already have some cool videos to peak their interest.

I have found this to be a relative topic considering just south of where I live in another company's territory, several miles worth of distribution power line was completely destroyed due to line galloping during an ice storm. This happened last week. I marveled at the amount of money that was probably spent seeing 30+ linemen crews working to repair it.

I know a some things about line galloping. It's caused by the wind when the ice forms on the power lines. Although I haven't read this anywhere, I've noticed the tear drop shape of the cross-sectional area of the ice resembles a wind turbine blade. Intuition tells me that the horizontal force from the wind causes an upward force on the power line, thus causing the power line to vibrate sometimes to the point of failure. I've found some decent power point presentations about line galloping online that got my brain working but the information was vague since most of the material was probably spoken by the speaker. Some of it mentioned how twisting of the conductor has an affect on galloping.

My main question: does anyone have any material on line galloping from an engineer's prespective that might help with my presentation? Besides what's on Wikipedia.

There are some things we are doing at our company to help prevent line galloping:
-Sometimes we install the dampeners between the phases, although thats rare and can be expensive over a 10 mile power line.
-We typically use class 2 poles and grade B 10 ft fiberglass crossarms. Poles are typically loaded to 15% their maximimum strength this way, 40% maybe with heavy equipment.
-When building over a long distance, we stagger the span lengths. Example: Span 1: 300 ft, Span 2: 310 ft, Span 3: 290 ft.

Another question I have: Is there any evidence showing that staggering the span lengths helps? I have been told it works but it seems no one can produce any evidence.

Thanks for the help,

Bigbrain

 

[magoo2]

If you do a search on aeolian vibration, you'll probably find some material available on the web.

I think Southwire has an unsymmetrical or not-round conductor designed to resist galloping.

 

[crshears]

Torsional vibration dampeners shaped like dumb-bells are often installed fairly close to towers to "de-tune" the conductor spans, reducing the amount of twist and metal fatigue in the conductor material itself; much less expensive than insulated inter-phase dampeners.

You may already know this bit, but the problem of conductor lift due to the airfoil shape of ice build-up is compounded by the fact that as it lifts the angle of attack is altered, leading to an aerodynamic 'stall' with consequent loss of lift. The cyclic lift & stall causes the galloping, which should perhaps be called bucking; a galloping horse covers ground, but a bronco unwilling to be ridden stays in the same place while trying to toss of its rider...then again, you'd know that, being from West Texas and all...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[zeusfaber]

The wikipedia article on vortex shedding, though short, might lead you some useful places.

A.

 

[rterickson]

Preformed Line Products acquired Dulmison Inc. a while back, and have several line motion control products and good literature in their catalogs, available on their website.

 

[ghostbuster7]

I.E.E.E. has hundreds of tutorials and papers on this subject over the last 50 years.Also keywords such as spacer dampers may help.

 

[GallopingSolutions]

Snow, ice and freezing rain combined with winter winds cause galloping of over-head conductors which often result in costly maintenance and operation issues. This air-foil of frozen precipitation causes aerodynamic lift resulting in clashing or flash-over of over-head conductors. A combination of ice, flat terrain, wind, and a line section running perpendicular to the direction of the wind create a situation prime for galloping.

The most effective means to reduce or eliminate galloping amplitude is to change the frozen, uniform air-foil along the span length with relationship to the wind. This can be accomplished by (1) forcing the snow or ice to freeze in a non-uniform pattern (T-2 conductor) or (2) by rotating the iced conductor with strategically installed damper units and altering the angle of attack of the wind with the leading edge of the conductor. This rotation "stalls" the conductor cancelling aerodynamic lift. This is no different compared to an airplane at take-off where at a critical ground speed the airplane achieves lift. Likewise, at a critical wind speed the iced conductor achieves lift and galloping occurs.

Inter-phase spacers are utilized to separate the conductors, but do nothing to cancel the aerodynamic lift that is present. Staggering span lengths also does not address the aerodynamics; galloping amplitude is a function of conductor sag where longer designed span lengths are more likely to clash or flash-over.

Let me know if I can help further.

GALLOPINGSolutions

 

[Bigbrain]

Got a lot of good info from the leads on this thread. Thanks a lot.

but a bronco unwilling to be ridden stays in the same place while trying to toss of its rider...then again, you'd know that, being from West Texas and all...

Despite popular belief, not everyone from West Texas rides broncos . But yes, I do know what you mean.

 

[prc]

Let me share my experience as a transformer engineer.It was in the year 1984. A Generator step up transformer bank of 600 MVA commissioned near to sea coast. The connection to 220 kV HV Bushing was by ACSR conductor connected to a catenary of ACSR conductors. The connection was at the middle of catenary.With wind or with fault current flow, the catenary oscillated or galloped giving pull on the transformer bushing terminal. With in couple of months, two bushings failed with cracking of outer porcelains. Since these bushings were dry type (RIP Resin impregnated Paper Condenser bushing) with out oil,there was no arc flash over and consequent fire, sure failure mode if it were OIP (oil impregnated paper condenser) bushings.

Then the bushing connection to catenary was made at the end of catenary and for the last 30 years there were no bushing failures.

 

[EP007]

Galloping:

https://www.youtube.com/watch?v=ko4goyw1Q84

 

[crshears]

Despite popular belief, not everyone from West Texas rides broncos . But yes, I do know what you mean.

I never said you RODE them, just that you would know what I meant, it being a reasonable assumption that you'd see it on TV at some point, even if only in passing...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]