Effect of AC powerlines on nearby pipeline

[mrmojorizing]

Hi,

I've been reading papers about the influence of high voltage overhead AC power lines on steel pipelines that run parallel to the power lines in the same right of way (that is near the power lines). The pipelines are coated in a dielectric coating to prevent corrosion and can be above ground or underground.

Based on the papers I have read there are 3 ways in which the power lines can influence the pipeline:

1) through magnetic induction due to the AC current in the power lines
2) through conduction during a fault where a large current enters the ground causing a rise int he ground potential relative to the pipe metal, due to the dielectric coating separating the pipe form the ground
3)through capacitive effects -- which i believe is the influence of the E field from the power lines on the pipe, although I'm not clear on this. Apparently it's supposed to be due to electrostatic effects, but i don't see how this can be when you have an ac (changing) field.

So anyway, my question is: Do I understand the capacitive effects correctly? Capacitive effects are due to the E field of the EM waves from the power line?

Also the papers mention that: "a [capacitance] potential is not normally induced on a buried pipeline since the capacitance between the pipeline and Earth is negligible,even when dielectric bonded coating sare used." and "However, during installation, a voltage can be produced by the influence of a strong electrical field on an insulated pipe when located above and insulated from the ground."

So my additional questions are: Why is it that voltage induced by the AC magnetic field is a concern on buried pipelines while the voltage induced by the electric field from the power lines is not? why is the capacitive voltage primarily a concern during above ground construction?

Some papers on this topic are at the links below.

Thanks

or wait, I think I see where I misunderstood. 60 Hz is too low to produce any EM waves, so we are really only doing with "electrostatic" influences for that reason -- even though the E field changes?


http://www.elkeng.com/pdfs/11-Basic Concepts of Induced AC101994.pdf
http://www.dipra.org/pdf/ACpowerLines.pdf
http://ie.utcluj.ro/Contents_Acta_ET/2009/Number4/Paper11_Purcar.pdf

 

[Mbert]

In terms of electric field, perhaps an easy way to see this is to put an unearthed infinite cylinder below an infinite cylinder at some given HV potential and to put these cylinders on top of an infinite plane at potential zero. The electric field generated by the HV line would create a potential in the conductor below. However, if the cylinder is buried, it is at ground potential so there is no potential coming from the E field of the line. In practice, ground has some conductivity as well, so the pipe is part of a surrounding conductor with varying conductivity, but the potential might be negligible for good conductivity soils. Hence, if some part of the pipe is under a line above ground, it will get some potential, while the rest of the pipe which is not under the line will be at another potential. The gradient of potential will create a current density, since the conductivity is different than zero.

On the other hand, whether buried or not, conductors that are submitted to time-varying magnetic field (B) will pickup induced voltage that will create induced current.

Hope this helps,

M.

 

[mrmojorizing]

But if the pipe is coated with a dielectric coating, it's not at ground potential, so I don;t understand why below ground pipes don't suffer from capacitive effects -- there will still be a capacitance between the pipe and the ground since they are at different potentials.

 

[uart]

But if the pipe is coated with a dielectric coating, it's not at ground potential, so I don;t understand why below ground pipes don't suffer from capacitive effects -- there will still be a capacitance between the pipe and the ground since they are at different potentials.

Because the capacitance between the ground an the pipe is very large (comparatively) and the capacitance between the line and the pipe is very tiny.

 

[Mbert]

Actually, not only that, but if the surrounding Earth is at 0 potential, where would the electric potential come from?

M.