Power Lines Bounce?

Mutual Induction/repulsion from high current.

Mostly magnetic mechanical forces produced during the brief interval that the short circuit is still in place. This impulse tug then appears as the mechanical resonance of the suspended cable.

I've witnessed this on a smaller scale when visiting a radio transmission station. They had an outdoor bank of electro-pneumatic switches which switched between the amplifiers & aerials. You could quite clearly listen to the radio programmes by way of the vibrations in the switches.

Are you sure this was not due to the micro-xmtr Her Majesty had installed in your upper right molar?

The electromagnetic forces from a fault in the electrical system are a result of unbalanced current flow and can be quite spectacular. An arcing fault causes the magnetic forces surrounding the conductors to vary unpredictably, causing strong attraction and repulsion along their length.

It's similar to what you see when a fire hose is unrestrained, it whips around uncontrollably until sufficient force can be applied or the flow of water is lowered.

as stated from others. I'll share this, in my physical metallurgy class 30 some odd years ago our instructor showed us a video of the electric furnace and the cables for it. and they were swinging on a wall. At the time, I was impressed.

I would have never thought of the magnetic properties. Thanks All.

Joe

Now THAT was impressive, scary. I would not even want to be standing by with camera for that. I hope the camera was set up remote operation.

We actually built part of a substation full scale and height at a Canadian test lab to test our aluminium tubular bus bar supports and fittings. One of only a couple of places that could generate 11-500kV at 63kA for 1 second to test a fault at or near a substation.

We basically connected a bolted fault across our substation construction under test, connected it to a couple of very large transformers which were themselves connected to a local power station down the road. Stand back, turn on the power, SCARY LOUD 50Hz hum ensues and the whole structure flexes and returns to its original position due to the massive forces involved. Oh the fun we have.

One of our detectors is going to be tested soon at Sandia's Hermes facility which generates pulses of 20 megavolts at 15 terrawatts. I don't think I would want to be close to that when it fires. I believe that our detector will have to be placed several hundred metres away from the source.

Um, you know they don't have to fire at that level all the time they can dial back the voltage and current.

Ok we're ready to test that photomultiplier of yours, just go and stick it in that concrete bunker 200 feet away.

I know they can turn it down but the numbers are impressive.

I think your description of the test set up is fairly accurate. We used to supply detectors to another lab doing high energy experiments, the detectors were destroyed in every run, good business to have.

A friend had a welding shop with hangars on the wall for the welding leads. As the welder struck the arc, you could watch the leads swing as the current went up and down while he was welding. -- JHF

That seems to me to be the best "example" of what is happening. Also its totally reproducible!

Could you post a sketch of how the cables were hung and how they moved? Did they "bounce" as the OP mentioned for example?

Thanks for your trouble.

PS Your online name is perfect for such questions/answers I feel!!!

It may be of interest to you. We performed Sudden Short Circuit test on many large rating turbo generators (500 MW and like), but at reduced terminal voltage in our works. The cables from the generator terminals to the short circuit switch jump at the moment of sudden short circuit. Multiple cable per phase are grouped, one from each phase to reduce the jumping force. Initially we used to tighten the cable groups to minimize the jumping. But later it was left free to jump and we found that free groups had less magnitude of jump when compared to constrained groups.

I find that very interesting.....thanks for your post.

That's how rail guns work! Current x magnetism = force. (That's cross product, btw)

Non-magnetic causes of lines bouncing I have seen include:

1. Earthquake

2. Vehicle versus pole

3. Cow rubbing stay wire

4. Ice shedding

I wonder if it is "the motor effect"? Fleming's left hand rule.

There is the world's weak magnetic field and a large current flowing through the cable.....the ingredients for some massive movement are there I feel....

Wind!

And I don't mean the wind you get from eating beans!

Bad weather and lack of tension in the power lines, which tend to sag after a while due to gravity,plus constant expansion and contraction, allows then to move freely as a bird when the wind picks up.

The answer is to tension them if it becomes to bad.

FYI. In the UK the Grid has aluminium power lines with a steel or stainless steel inner core to give better support and prevent sagging over the longer distances between pylons

https://www.ameslab.gov/node/11152

That does not appear to follow the original post here as far as I can see, which was:-

We had the power drop off yesterday, and just as it happened I happened to be near a window, to see the power lines that feed this whole area bouncing up and down,

I've seen this in videos before when a line is getting shorted out how the wires will jump. What actually physically causes this to happen?

I have watched power lines (I pass some several times a day, usually on my bike), they hardly move except in REALLY strong wind conditions. Even then they only sway slightly in my opinion, not bouncing up and down.......that is for me something new......

Have a great day.

Andy, when was the last time that you were watching the lines AND a fault occurred? Anyone who has witnessed proof-testing of conductor support systems will attest to the massiveness of the forces involved.

You must consider that in an interconnected grid there are millions of pounds of rotating metal being pushed by millions of pounds of high pressure steam being produced by millions of pounds of fuel being burned. That's a lot of inertia, with all of that force directed to the fault, limited only by the impedances in the path to it.

To put it into perspective, a conductor less than 1.5in. in diameter can carry approximately 400MW @ 765kV. It takes a column of water 100 to 300 Feet high flowing through a penstock with a diameter of 15-45 Feet to generate that same 400MW. That's quite a compression ratio, surely you would not want to stand in the path of that much water.

Granted that there is a lot of impedance getting down to the distribution level, but the current goes up as the voltage goes down, and it's the current that produces all that force.

Are you sure that you are replying to the right person, or maybe you have misunderstood my post! Try rereading it, if you have any questions simply ask. The bold printed part is the original post from the OP by the way.....the one asking the questions!!

You should also note the post I was replaying to, who appeared not to have read the OPs first post by the person concerned.

If we are talking about how strong the effects are, I agree, I have seen fotos of when someone paralleled two alternators out of phase and one alternator was torn off its mountings, on a ship! eg. strong mountings.....a long time ago!!

Our RN training was extremely stringent, correctly so, as you could put a ship out of action for months, in a worst case situation, possibly even sink it....doing just that!

In 1970, I was in charge of main power, supply and distribution on a RN ship, then aprox. 11,000,000 UK Pounds of kit. Maybe 100 M nowadays? I was exceedingly proud of that....still am.....but I know such forces. I then had 8 "underlings" including 2 RN Petty officers.....

Which is why I was thinking that it could be heavy current and the earth's magnetic field, certainly the OP would have mentioned gale force winds.....here such winds only make the cables swing, not bounce.....

Also I have never seen such "Bouncing" cables myself, in such situations where power is dropping....I am really interested to learn more!

You might have something there. Air flowing past a cylinder (the wire) will shed vortices and, like an airplane wing, generate lift forces, which alternate in direction.

http://en.wikipedia.org/wiki/Vortex_shedding

That is a known problem for long transmission lines that cross valleys where prevailing winds can cause improperly designed lines to "gallop" in the wind, but that is not the phenomena raised by the OP.

Thank you..... Yes wind does have an effect on the cable, and NOT having the cable tensioned correctly WILL, not maybe, cause bouncing, even in a light wind. I have seen it!

Add to that high ambient temperature which causes the cable to elongate or slacken its tension, then you have all the ingredients for bouncing cables.

One of the most important jobs for the National Grid in the UK is to ensure the cables are tensioned correctly. Spacing is also important between phases, and if the cables attract each other, maybe they have insufficient spacing.

Then, assuming you are correct, then we must usually have "properly" tensioned cables here it would seem, thank God for that....

If they bounced in some places, somewhere (I am sure that some do for some reason, whether wind or whatever), I expect that this would shorten their working life considerably....possibly even be dangerous....metal fatigue etc..

Surely such problems should be mostly "designed out" by now, at least in most countries.....thanks in advance for your answers.

I like your animated diagram. Is this energy wastage taking place? Is there no way to harness the shed vortices? One might at least think that some novel trick is just waiting for the right engineer to come along and make the latest and greatest wind generator technology from things like this?

Could this be the basis for a new wind rotor, of a class not yet modeled?

Well here's just the latest example using that very principle.

http://powerelectronics.com/blog/wind-turbine-without-blades

I believe there are others. So many different wind turbine designs out there.

But again, dampers should have been fitted on lines susceptible to this phenomenon in these cases. It is a well known phenomenon in the power industry and a great deal of design actually goes in to designing and positioning the dampers (we design and sell them).

http://en.wikipedia.org/wiki/Stockbridge_damper

Any conductor carrying electric current produces electromagnetic field around it with the center of the conductor as the center of magnetic field. It is represented in the form of circular lines of forces. The direction of these concentric lines of forces depends on the direction of current through it. If the current in two conductors laying side by side is same then the magnetic field around both the conductors would be in the same direction. Similarly if the current in two conductors are in opposite direction then the magnetic field in two conductors would be in opposite direction to each other.

If the two field in any zone are in same direction then they add up and resultant field is much stronger. This causes the conductor to move away ( a repulsive force is experienced by both the conductors. If the two magnetic field are in opposite direction, then they cancel each other and the resultant magnetic field is weak (or zero if the two currents are equal). This causes an attractive force to be generated between the conductors. Higher the current higher would be the force of attraction or repulsion. If the current in conductors is of alternating nature, the force will also change its nature from attractive to repulsive every cycle.

In case of faults, magnitude of the current is very high and therefore the forces are also high resulting in conductors experiencing high force of attraction and repulsion every cycle. This causes conductors to vibrate resulting in mechanical failures of supporting systems if not designed for the forces for short circuit currents.

I liked your post and its thoughts, not TOO far from my take, except for one line I feel:-

If the current in conductors is of alternating nature, the force will also change its nature from attractive to repulsive every cycle.

I personally would expect the two lines to be part of the same supply, eg. same 3 phases and at 120° to each other.... so when one reverses, so does the other but with a 120° skew, therefore what ever effect is there due to what you said, would remain the same as both experience a "reversal".

Also, why seemingly only during power poss, why not in normal usage then? See the original post which said:-

We had the power drop off yesterday, and just as it happened I happened to be near a window, to see the power lines that feed this whole area bouncing up and down.

I am myself a bit puzzled and unclear, I am looking for a "Myth Busters" plausible explanation as well....

Transient phenomenon. Steinmetz wrote a whole paper on it. When the supply is cut, a transient is produced of short frequency AND the loads at the various ends of the line are still spinning and magnetized and they become generators. ALL of the generators are out of phase, producing more transients and the system will "ring down" if you are lucky, or it will arc somewhere and continue for some time and likely destroy some equipment in the process.

Thanks for your liking the part of my post. I tried to just explain the phenomenon as to what happens when heavy (fault) current flow through the conductors without going into the details about 3 phase conductors, phase shift etc. As regards 3 phase current system, if a phase carries peak current, then other two phases would be carrying half current in opposite direction. Actually it would depend on physical lay out of the conductors rather than phase angle etc.

As regards movement of conductors when the power is off, power off situation may be due to tripping of circuit in OP's system but the main lines may be still feeding the fault causing conductors to vibrate.

Maybe even one phase had already dropped, who knows.

One thing is, I feel we both agree, that it was a "magnetic effect" and not "wind"!

(either sort!!!)

Yes that also could be one of the possibilities. Yes it was due to magnetic field effect only. I fully agree.

"...so when one reverses, so does the other but with a 120° skew, therefore what ever effect is there due to what you said, would remain the same as both experience a "reversal"."

You made me curious, so I made a MathCad plot. The force between two conductors running parallel would be proportional to the product of the currents. For two of the three phases, 120 degrees apart, the force would alternate at twice the line frequency, with an average repulsion.

Exactly as I was thinking of when I wrote my original post, but FAR prettier. Thanks!

I do feel that the effects would be a "regular" motion, assuming of course that the cables can move at 50/60 Hz, or some harmonic of the frequency, but again, it would be regular, not "bouncing", which to my mind (possibly wrongly), implies "irregular".

Perhaps the OP could clarify that further/better.....?

Exactly as I was thinking of when I wrote my original post, but FAR prettier. Thanks!

I do feel that the effects would be a "regular" motion, assuming of course that the cables can move at 50/60 Hz, or some harmonic of the frequency, but again, it would be regular, not "bouncing", which to my mind (possibly wrongly), implies "irregular".

Perhaps the OP could clarify that further/better.....?

I agree. Absent further explanation, I'd suggest that the "bouncy" aspect comes from inability to oscillate as the harmonic of the fundamental (50 or 60 Hz) frequency. The physics of the oscillation suggest that a secondary or even tertiary harmonic might be set up by the physical constraints of the support system and span-lengths. And even proper tensioning of the cables might not be able to damp a sufficiently high-amplitude motion.

You are forgetting about the presumed fault incident that caused the grid to trip out. The period from fault current starting to breaker or fuse interrupting this current will be a one time only dramatic pulse of cable repulsion. Assuming that the fault current is at least 100 times larger than the operating current the repulsive force will now be at least 10,000 times larger than they were before. Cables that previously gently swayed in the breeze now briefly jump away, possibly violently, from each other.

Good point, thank you. I don't know why it didn't occur to me that it pretty much has to be a one-time energy input that causes the motion, since as you point out, the energy flow is going to cease very quickly after starting, if the circuit protection devices are doing their jobs. And if not, it probably won't be doing as much bouncing as something, somewhere, burning.

Calcium is non-connductive, so wont the aluminum - caclium aloy has increased resistance, increased Vdrop?

How true!

Why did you mark yourself "Off Topic"????

this was in response to post 12 which had a linnk to an experimental aluminum alloy connductor made of calium doped aluminum

This is a short circuit, direct current event, taking place in the first or last half wave from zero crossing to zero crossing.

A VFD carrier has this effect on the first turns of a motor coil if not filtered.

Magnetic fields.

During fault conditions the current reaches abnormally high levels in the conductors.

In addition to the mechanical reaction that occurs when the flow of electrons (current) is interrupted , the collapsing magnetic fields surrounding (around) the conductors also contribute to the reaction movement.

The higher the current value, the more violent the reaction.

The current interruption reaction in a conductor is much akin to opening and closing a hand operated sprayer on a fully charged water hose.

IGBT, GTO, SCR and Transistor rectifier and inverter bridges have custom on-off transition time built into the firing control as well as component "doping" specifically designed for offsetting this type of reaction during operation. If you stand near a large operating VFD or other equipment with rectification and/or inversion occurring you can hear the hammering of the busses which is being caused by interruption of current flow when the devices are being triggered on and off. It is possible to see movement using a high resolution, high speed camera to record then replay video of the busses at a slower speed. To say the least it is impressive.

"The current interruption reaction in a conductor is much akin to opening and closing a hand operated sprayer on a fully charged water hose."

Sweet analogy! When you suddenly shut off the flow the hose and the pipes "bounce" and "bang". The electrical current is doing much the same thing in a suddenly faulted sytem.

I wonder if the bouncing has at least at the Physics level, anything to do with a "Rail Gun" like this one:-

Electromagnetic-Rail-Gun

Also, in that first picture, isn't that circle the "Left hand Rule"?

Flemings_left-hand_rule_for_motors

Just a thought!!!

Well, there are two things that can cause this - acousto-mechanical resonance is one. A crop duster, or something else hitting a line and snapping it (shortly after the fireball) is another. In either case a traveling wave is set up in the lines, although the lack of damping of these oscillations in some transmission lines will make it appear to be a standing wave.

Also, large changes in current in a line will result in a collapse (or vast increase) of the local circular magnetic field around that line. This may result in some induced motion related to magnetics.

Wind.

like transformer hammering or banging before it explodes - there is an article about this somewhere ???