Ground clearances for transmission lines

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Why are ground clearances so large for 765 kV towers?

I did a little research on this and even if the switching surge factor is 3 and you have a factor of safety of 1.5, in air (3 kV/mm), it corresponds to 1.62 m of maximum arc length, which doesn't explain why towers are so large.

According to "Power System Analysis and Design," by J. Duncan Glover et al., "Line height is selected to satisfy prescribed conductor-to-ground clearances and to control ground-level electric field and its potential shock hazard."

From what I have read the effect of low frequency EMFs on humans is pretty minimal. Has anyone actually been shocked by a power line 10 m away? If so, can someone explain the physics behind this? Does anyone know the calculation the NESC committee uses to come up with minimum clearances?

Thanks a lot.
 

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  • #2
Baluncore
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The voltage you have calculated determines the separation of the wires and the length of the insulators.

The current and magnetic field in a balanced 3-PH line sums to zero, as does the electric field. The fields induced in the earth are minimised by making the towers high relative to the wire separation. The wires swing in the wind and sometimes carry an ice load.

But what happens during a fault, say when a phase is shorted to ground by lightning during a rainstorm ?
 
  • #3
anorlunda
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Ground clearance also has to factor in sag when conductor temperature is highest, sag when ice loaded, sway of conductors and the nearest trees in high winds, snow depth, and the possibility of vehicles driving underneath.

Rain, fog, or salt spray in the air may also modify the calculations.

Also, as @Baluncore mentioned, there is the issue of line-to-ground fault currents.

All these considerations are not unique to 765 KV. I also suspect (but I'm not sure) that IEEE standards for clearance are uniform worldwide and not customized to local conditions such as salt spray or snow pack.
 
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Thanks for the help, but I am still confused. I used 765 kV as an example, since I live in California and there are no 765 kV lines due to the size. My main question is why does an increase in voltage correspond to a massive increase in transmission tower size?

As for when one line is shorted to ground in a rainstorm, that would be dangerous even at 4 kV.

For sagging in extreme weather, I assume that that would also be similar at 230 or 500 kV.

For arcing in weather, do you know the dielectric strength of humid, salty air?

I also assume that the main concern is the electric field perpendicular to the line, since magnetic field depends on current. Also if EMFs were a huge concern, wouldn't you see a lot linemen with cancer, which I haven't heard about.
 
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anorlunda
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My main question is why does an increase in voltage correspond to a massive increase in transmission tower size?
For all the reasons mentioned before. Volts per meter is the gradient of concern. More volts needs more meters to hold the V/m ratio constant. But there are also minimum heights for low voltage lines, so constant V/m might not hold for them.

@berkeman , this crossing always stands out in my mind. It seems shockingly low. It is Skyline Drive in Shenandoah National Park, a mountain ridge. The mountain slope is 30-45 degrees. In the foreground is a 115 KV line and behind is a 230 KV line. Vehicles up to 12 feet tall drive on that highway. Maybe snow blowers too.
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  • #7
Tom.G
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Yeah, volts-per-meter at the ground can be a concern. Many years ago a few of us went out in the California desert to see the new Solar Power plant. We decided to drive up one of the surrounding hills to get a good overview and take some photos.

We parked under some high voltage transmission lines and got out of the car. We soon noticed that the car and us were at different voltages. Not a lot, probably a few hundred volts and, fortunately, a very low current (at least thru our shoes, we didn't try barefoot!).
 
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  • #8
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Why are ground clearances so large for 765 kV towers?

I did a little research on this and even if the switching surge factor is 3 and you have a factor of safety of 1.5, in air (3 kV/mm), it corresponds to 1.62 m of maximum arc length, which doesn't explain why towers are so large.

According to "Power System Analysis and Design," by J. Duncan Glover et al., "Line height is selected to satisfy prescribed conductor-to-ground clearances and to control ground-level electric field and its potential shock hazard."

From what I have read the effect of low frequency EMFs on humans is pretty minimal. Has anyone actually been shocked by a power line 10 m away? If so, can someone explain the physics behind this? Does anyone know the calculation the NESC committee uses to come up with minimum clearances?

Thanks a lot.
When transmission voltages are deep in EHV region and start approaching UHV ballpark then the answer is YES: you can be shocked even at that distance from power line when touching bigger insulated conductive objects or simply by touching grounded objects. E-field induction can be quite serious when massive boundled conductors and very high AC voltages are employed...
 
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  • #9
dlgoff
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Also it wouldn't hurt to mention that 3 kv/mm figure (refered to by OP as breakdown field for normal air) is good only for small air gaps with uniform field. Average breakdown field in wire-plane and wire-wire gap configuration is much lower than that. Especially for positive surges in very long air gaps.
 
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  • #11
anorlunda
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Especially for positive surges in very long air gaps.
Correct. Also especially with contaminants in the air. Salt spray. Rain. Smoke and ashes. Snow blower fountain.
 
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