How does a helicopter bond to a HV line?

Thread Starter

Amped_86

Joined Jun 28, 2018
10
Was watching a show where maintenance was performed on a live HV line by way of a helicopter. The tech held a rod near the line which created an arc. I believe the arc was extinguished when the rod finally touched the line. Next it looked like they had replaced the tip of the rod with a donut shaped clamp that secured the helicopter to the line while maintenance was performed.
Assuming that the helicopter and line are initially at different potentials. Wondering how the potential is equalized between the helicopter and line? Heard it mentioned somewhere that it's the capacitance of the helicopter but I don't understand that.
 
The arc occurred because the helicopter's capacitance to ground originally was at 0V so there was a high voltage arc as the capacitance of the helicopter was charged to the high voltage.
I suspect there was another arc when the helicopter's capacitance was discharged to ground as it landed.

EDIT: Why do they make a video with the background music much louder that the narrator??
 

jpanhalt

Joined Jan 18, 2008
7,867
The arc occurred because the helicopter's capacitance to ground originally was at 0V so there was a high voltage arc as the capacitance of the helicopter was charged to the high voltage.
I suspect there was another arc when the helicopter's capacitance was discharged to ground as it landed.

EDIT: Why do they make a video with the background music much louder that the narrator??
There are so many videos on that subject, I just took the first one and didn't bother screening it. Maybe the TS didn't know how to search with Google. I was busy watching LSU v. Alabama "football."
 

crutschow

Joined Mar 14, 2008
23,306
What AGa mentioned is true for a high voltage DC line.
For an AC line, as is apparent in the posted video, it's the capacitance to ground, independent of any initial charge on the helicopter..
The helicopter is one plate of the capacitor and the ground is the other.
That may not be a lot of capacitance but it's sufficient to generate a dangerous AC current for the high AC voltages involved.
 

Reloadron

Joined Jan 15, 2015
4,501
This link provides a good overview. Something to note is the worker using the wand draws an arc when both approaching the line and when disconnecting from the line. The video shows the process repeated many times as the worker replaces spacers on the line. Anyway the overview link is a pretty good read.


Ron
 
the worker
I don't know what he gets paid but it is not enough for me.
On the other hand, when I was young I built radio stations. I have a picture, sitting cross-legged on the top of a tower, putting in the last bolt. We worked on the towers while the transmitter was on. The RF was so hot that picking up a tool would shock you.
 

Reloadron

Joined Jan 15, 2015
4,501
I don't know what he gets paid but it is not enough for me.
On the other hand, when I was young I built radio stations. I have a picture, sitting cross-legged on the top of a tower, putting in the last bolt. We worked on the towers while the transmitter was on. The RF was so hot that picking up a tool would shock you.
When I was younger maybe but no way as I got older. Heck, even observation towers make me nervous. :)

Ron
 

Thread Starter

Amped_86

Joined Jun 28, 2018
10
Thanks. All replies seemed to help a bit. I'm not so smart and trying to see this from a very simplified view (maybe I can't). If I view the power line and helicopter as a circuit, the circuit is obviously incomplete. The helicopter is only connected to the line at one point. Therefore no complete path exists for current to be able to flow from the line to the helicopter even if a huge potential exists between the two. But clearly charge is moving between the line and the wand when the wand gets close enough to form an arc ( I'm also ignoring whether the line is AC or DC at this point...maybe this matters though).
Capacitance has been brought up in a few replies but I don't truly understand this. I've come to understand capacitance as an object's tendency to resist changes in voltage and it does so by drawing current whenever it's exposed to a voltage difference. So I guess I could see the helicopter acting as a capacitor when the wand creates an arc drawing or releasing charge to equalize it's potential with the line. But wouldn't this flow of charge be bad? I imagine the workers are electrically bonded to the helicopter to minimize the potential difference but wouldn't this flow of charge through the helicopter also flow through the workers? Reloadron provided a great link that said the potential is not actually equalized. If the line were AC you would constantly have current flowing back and forth (I'm not quite sure what happens with a DC line and maybe this is another part where my reasoning breaks down). Instead the use of Faraday suits are what keeps the workers safe (I don't know anything about Faraday suits and my initial intuition questions how effective they would be when dealing with 100ks of volts but again I am at the limits of my knowledge)
 

Tonyr1084

Joined Sep 24, 2015
3,572
Why don't birds get shocked when they land on HV lines? Because there's no pathway to ground or any other potential. You may see doves and such land on your back yard high voltage lines but that's only typically (but not necessarily) 2500 volts. That's what I hear my HV line is in my back yard. The transformer drops the voltage down to 240 VAC for the house.

I've never seen birds perch on VHHV lines (Very High Voltage Lines). Perhaps they too get shocked when they land on them. It may be enough to knock them to the ground - I don't know. But I've never seen a bird on an VHHV line. The helicopter in the video is bonded to the line via some sort of grounding pole, then clamped onto so the helicopter and HV line are at a common voltage whether AC or DC. One thing not to be overlooked is the static being generated (or dissipated) by the rotors. If you've ever flown commercial airlines you've seen static discharge probes along the trailing edge of the wing. Electrons migrate to the sharp point and then are stripped away by the air flow. I've also seen a C130 ramp up its engines. From one set of propellers to the adjacent motor I've seen a huge blue continuous arc. This was in the desert (dry air) conditions. I've also watched as they come in for servicing. The technician has a probe on a long fiberglass pole in which he makes an initial ground to the airplane. Then he approaches with a clamp or a plug for static discharging and bonding to earth. The fuel truck is bonded, the fuel cart is bonded, the cart is bonded to the airplane, THEN the fuel lines are connected. Otherwise the potential spark of static could ignite the jet fuel and that would make for a bad day for somebody.
 

dendad

Joined Feb 20, 2016
2,986
I know it is a bit different, but when I worked at Radio Australia years ago, occasionally a bird would land on a feeder and touch the other side. There would be a BANG and the bird would fall off, leaving its feet and legs clamped to the line. A grass fire could then start when it was dry. The transmitters were only 100KW.
But I agree with the comment above, those that do the HV line work are braver than me!
 
I know that high voltage transmission lines are AC but I do not think any are DC because then a transformer could not be used to reduce the voltage.
 

Reloadron

Joined Jan 15, 2015
4,501
I know that high voltage transmission lines are AC but I do not think any are DC because then a transformer could not be used to reduce the voltage.
Actually you have a few major DC (long distance) transmission up your way in Canada. There is something like 35 in N. America with 20 in the US. Without going too far off topic if we think about it most large wind turbines generate DC and send DC down a transmission line for later conversion to AC so with today's inverters getting from DC to AC really does not present the problems it would have a few decades ago.

We now resume bird on a wire. :)

Ron
 

Thread Starter

Amped_86

Joined Jun 28, 2018
10
Look up plate capacitor.
The helicopter is one plate and the ground is the other.
Look up plate capacitor.
The helicopter is one plate and the ground is the other.
So when the arc occurs the helicopter is charging to match the voltage of the line? Can I think of it in terms of a simple circuit: the line is the source connected in series to a capacitor (the helicopter and the ground). If so, how does the ground connect to the line?
 

crutschow

Joined Mar 14, 2008
23,306
Can I think of it in terms of a simple circuit: the line is the source connected in series to a capacitor (the helicopter and the ground). If so, how does the ground connect to the line?
Yes.
The line is referenced to ground at the generator.
 

Yaakov

Joined Jan 27, 2019
1,536
I know that high voltage transmission lines are AC but I do not think any are DC because then a transformer could not be used to reduce the voltage.
Some of the very largest transmission lines are actually HVDC. There are efficiencies to be had with DC transmission with super high voltages but you are correct that it must be converted to AC at the end to manage the voltages.

 

Alec_t

Joined Sep 17, 2013
10,355
I think the lines in the post #2 video are DC, because the arcing stops once the heli and guy have charged to line potential. With AC, wouldn't the arcing continue?
The parallel plate capacitor concept is interesting. The heli charges to the HV potntial, then when it descends the capacitance increases as the plate separation decreases, so presumably the heli potential decreases proportionally before touchdown?
 
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Tonyr1084

Joined Sep 24, 2015
3,572
I think the lines in the post #2 video are DC, because the arcing stops once the heli and guy have charged to line potential. With AC, wouldn't the arcing continue?
The parallel plate capacitor concept is interesting. The heli charges to the HV potntial, then when it descends the capacitance increases as the plate separation decreases, so presumably the heli potential decreases proportionally before touchdown?
Arcing occurs when there's a gap. Once you close the gap there's no longer an arc. I believe lightning can be classified as a DC of sorts. Though it's caused by static electricity - "Static" being the key word - electrons that are stationary are static. But once they begin to move they become dynamic and travel from the more negatively charged region to the less negatively charged region. Hence, they flow from higher potential to the lower. Directly. DC.
 
The power lines are low frequency, or indeed, possibly DC. So once the chopper is connected there is no longer a potential difference and so not much shock hazard, except from the other lines and the ground line above. The same as when I work on my power entrance cable, standing up high on my well insulated ladder. No shock when I touch a single wire. Yes, I need to be careful, but no, touching any one wire gives no shock. But I only do it in dry weather. The big thing is to be well insulated from ground.
 
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