influence of AC power lines on steel pipelines

[mrmojo]

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

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

 

[mrmojo]

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?

 

[waross]

If an insulated object is placed between a potential source referenced to ground, and the ground plane. It will form a pair of air insulated capacitors in series. In your example the first capacitor is formed by the power line, the air between the power line and the insulated pipe and the pipe. The second capacitor is formed by the insulated pipe , the air between the pipe and the ground plane and the ground plane. If the pipe insulation has a high capacitive constant it may increase the capacity slightly.
When you bury the pipe it is on the wrong side of the ground plane for the capacitive effect.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mrmojo]

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.

 

[dpc]

AC current generally does not cause major corrosion issues. DC currents are the main culprit, at least as I understand it.

David Castor

http://www.cvoes.com

 

[rasevskii]

Buried pipelines are usually protected by cathodic protectio systems (Google that) which maintain a low DC potential on the pipe metal versus the earth. The pipe is also insulated by taping or other coating.

I don't see why you are concerned about this as the pipe (as said) is buried and the earth itself is at zero potential.

THe magnetic coupling is likely ignorable due the distance between the HV conductors and the pipe. What line currents are you dealing with in amperes? Usually we are talking about 2000A or less, some EHV lines more than that.

In Sweden some farmers were stringing insulated conductors parallel to HV lines to get free electricity by capacitive coupling...oddly we don't read much about this...

rasevskii

 

[mrmojo]

corrosion isn't the only concern. the potential between the pipe and ground can be high enough to cause health hazards if there is exposed metal or damage the coating during a fault.

 

[waross]

There is a potential gradient between the conductor and the ground. Any insulated conductor placed in this field will form two capacitors in series across the line conductor and ground. Unless the ground is very very high resistance, a buried object will not be in this field.
I used to live near a 60 KV transmission line that shared a right-of-way with an insulated gas line for at least 10 miles and I often played bridge with one of the gas service men. He told me that they had special procedures when disconnecting gas meters so as to not ignite residual gas with sparks from the induced voltage on the pipeline.
And I worked on a capacitor station close to a 500 KV line where capacitive coupling was a constant and sometimes pain full irritation. The capacitive charge was proportional to the size of the object and the distance above the ground. The capacitor support structures were mounted on vertical insulators and the capacitor units were in turn mounted on stand off insulators. There was a large number of 17000 V PFC capacitors to be connected in series parallel. Every time a worker touched a capacitor unit, he got a shock similar to a strong static shock from a door knob. The workers took to wrapping a piece of stranded #14 wire around their wrists and connecting the other end to the equipment that they were working on.
The difference:
A 60 KV transmission line runs at a relatively high current and is closer to the ground. This combination causes EMI in buried, insulated objects.
The EHV lines cause noticeable capacitive induction in elevated objects. The induced open circuit voltage is dependant on the relative distance between the HV conductor and the mean ground plane. Even with a very high ground resistance there will not be much capacitive voltage induced below the surface of the ground.
Corrosion protection. There are two methods of protection:
Cathodic protection.
and
Insulation that may be monitored to detect local insulation damage or failure.
The second method is more susceptible to electro-magnetic induction.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[veritas]

mrmojo

Further to waross:

1) The capacitive coupling experienced by the pipe is due to the resultant of 3 electric fields (one due to each phase)which are 120 degrees apart. Thus the line configuration and the exact location of the pipe with respect to the line all influences the resultant field.

2) The flow of AC current in the transmission line sets up an electric field intensity (EFI) between the line and earth (and or pipe). The strength of the EFI is not dependant on the dielectric of the insulating medium (much like the magnetic field intensity, H,F) is not dependant on the magnetic permeability of the medium)

3) The actual electric field depends on the dielectric of the insulation material (air if considering capacitance between line and ground). But if the pipe is submerged then the dielectric properties of the earth come into play. This is way, way less than air thus electric currents flow freely in the earth's surface. The result is that due to the low dielectric properties of the earth the electric field is very low - negligible (even though the electric field intensity is high). In short - it is very difficult to set up a capacitor within the earth because it will be very hard to separate the charges unless (unless like waross said the earth has an extremely high resistivity).

4) However, if your pipe is dielectric coated under the ground then yes, you will have a capacitive effect but it seems a bit of a waste.

5) The fact that the current is AC means that the electric fields are also alternating. Thus physically the fields are all dynamic. But if you consider the rms values of the fields then I suppose one could use the term "static" - but it is misleading.

6) When it comes to magnetic fields, the magnetic permeability of the air and earth can be assumed to be more or less the same UNLESS you have substantial ferrous deposits. If no iron in the ground then the magnetic field could be thought of as permeating into the earth. BUT in reality this is not what will happen as the magnetic field will induce an ac voltage in the earth's surface resulting in ground current if there is a closed path. This ground current will in turn produce it's own field which will modify the original field that caused it - the trfr effect. So a buried pipe (coated or not)will experience the effects of the magnteic field about the same as if it were above ground.

7) Again, this is a resultant magnteic field which depends again on the location of the pipe relative to the transmission line itself.

Hope this helps.