Help w/ Induced Voltages from 66kV U/G to control cables

[UltraMagneticAL]

I'm having a difficult time calculating what the induced voltage would be from three separate 3-Phase Underground 66kV Cables in Duct Bank on a group of control cables running parallel above the duct bank for approximately 100 meters.

So far, I've got some formulas to use:

Faraday's Law: Emf = -NΔϕ/Δt
Ampere's Law: Σ BΔl = μ̥ I for a straight conductor

where N = number of coils
ϕ = BA = magnetic flux
B = External Magnetic Field = (μ̥ I)/(2πr) [from Ampere's Law]
A = Area of Coil (for a wire, what do i substitute here?)
μ̥ = 4π * 10^-7 Permeability of Free Space (should I be using concrete? if so, what is it?)
Δl = infinitesimally small length of B


The 66kV duct bank will be buried 1m below the control cables.

When I used Faraday's Law to calculate the induced EMF I assumed 1 coil for N since it is a straight cable. Is this a correct assumption?

Any help or direction would be greatly appreciated!

Thanks,
Allen

 

[nakulak]

can't you use shielded cables ?

 

[gar]

100718-1619 EST

nakulak:

Electrostatic shielding of cables has little effect on magnetic coupling. These would be used for electric field shielding.


UltraMagneticAL:

You have a difficult calculation problem. You need to apply broad knowledge to the question. Typically you use twisted pairs for your signal wires and this automatically cancels the induced magnetic field. For twisted wires to be maximally effective there should be no ground common between the source and destination. This means one end or the other is floated.

For an experiment: Get 100 meters of Belden unshielded 8442, twisted pair #22, and connect the two wires together at one end. Connect your meter at the other end. Lay this out in a straight line on the ground. Parallel to it create a loop of #12 wire 2 meters wide, 100 meters long, and one side of the loop 1 meter from the twisted pair. This makes the other side of the loop 3 meters away.

Energize the big loop with 20 A and measure the induced voltage, which should be very small.

In your actual application shielded wire would be a good idea to reduce capacitive coupled noise.

What kind of noise levels can you tolerate?

.