Inductance from nearby 25kV line
Inductance from nearby 25kV line
(OP)
Hello everyone,
I am doing a study on the inductive effects of a 25kV underground transmission line. The purpose of this is to mitigate any induced currents on a power cable running perpendicular to this line. From what I assume, there will be no inductive effects considering the physical layout but nonetheless I will do a calculation anyways. I am assuming to use this formula:
M = [10^-7]*(Iphase)*[2ln(D/d)]
for the mutual inductance.
Then I will use this formula:
V = dM/dt to find the electric field strength at the induced cable surface and thus able to find induced current flowing.
Now, these cables are perpendicular so by maxwell's equations there should be zero induced current in the cable, but just to be safe, what type of grounding methods do you think would suffice?
I am thinking a faraday shield grounded at one end (to eliminate current flow in the cage). This shield will be similar to a substation ground mat placed parallel to grade.
Any thoughts?
Thanks for your time
I am doing a study on the inductive effects of a 25kV underground transmission line. The purpose of this is to mitigate any induced currents on a power cable running perpendicular to this line. From what I assume, there will be no inductive effects considering the physical layout but nonetheless I will do a calculation anyways. I am assuming to use this formula:
M = [10^-7]*(Iphase)*[2ln(D/d)]
for the mutual inductance.
Then I will use this formula:
V = dM/dt to find the electric field strength at the induced cable surface and thus able to find induced current flowing.
Now, these cables are perpendicular so by maxwell's equations there should be zero induced current in the cable, but just to be safe, what type of grounding methods do you think would suffice?
I am thinking a faraday shield grounded at one end (to eliminate current flow in the cage). This shield will be similar to a substation ground mat placed parallel to grade.
Any thoughts?
Thanks for your time






RE: Inductance from nearby 25kV line
RE: Inductance from nearby 25kV line
Thanks
RE: Inductance from nearby 25kV line
If you ground both ends, current in a parallel line (or the parallel component of a line that is skewed) will induce a current in both the conductor and the shield. The current induced in the shield will induce an opposing current in the conductor, thereby acting to shield the conductor from induced currents. I doubt that the shielding effect for a 25 kV line would be significant even if the cables were parallel instead of perpendicular.
RE: Inductance from nearby 25kV line
Do your calcs despite the theory.
Sounds like a student post to me.
RE: Inductance from nearby 25kV line
been out of school for years. Yes, 25kV is technically a distribution voltage but the nomenclature isn't important, just the solution. You think this type of question would arise in an undergrad power course? Think again.
I ask about grounding methods because I have never had this specific issue arise before, and I do not know the typical method to mitigate induced currents in adjacent conductors.
Here's where I got the basic mutual inductance calculation:
h
jghrist: What I was thinking was a typical grounding mat you see in substations near the junction boxes and hand operators for the disconnects. Those are non-insulated aluminum meshes. Like I said before I am sure the induced currents will be near zero, but as it is non-shielded control cables I need to make sure there is zero interference. I was thinking the Faraday shield arrangement and just did not know the method of grounding the shield mat.
Any insight is appreciated.
Thanks
RE: Inductance from nearby 25kV line
RE: Inductance from nearby 25kV line
Yes, there is a shielded power cable but I am more worried about the non-shielded control cable running along the same line. And yes, I was also wondering how you ground something that is continuously grounded, seeing as I would think one end bonded to copper clad rods will have a lower ground impedance. And yes, now that I do a little more reading on it, zero induced currents would require a continuous closed loop of conductive shield.
What I mostly need to figure out is the proper formula to calculate the induced current in the control lines (but I have mostly found scalar equations).
Thanks for your input though,
Regards,
RE: Inductance from nearby 25kV line
RE: Inductance from nearby 25kV line
"Mutual inductance calculations between underground power cable with earth return and underground telecommunication Cable"
I think I did once a telecommunication cable of 3600 pairs 0.3 mm conductor copper, foam polyethylene insulated. I don't think one need to waist so much copper instead of fiber glass, and fiber glass is not sensible to magnetic field.
Let's you have an old fashioned cable not shielded [nevertheless usually the pairs or quarts are individually shielded and an overall shield is provided too] and you have to protect it against a parallel laid medium voltage [single core cables, I presume].
The solution indicated by jghrist to put the control cable through a steel conduit is very good but it is too late as the cable is already laid on, I understand.
You may try to use the very sophisticated theory from the above article in order to calculate the induced voltage. In my opinion you can calculate the induced voltage for a single control pair and if it is negligible that means the problem is not in this corner.
You may, also, provide a shield as a thin reinforced concrete envelope connected both ends to the grounding system-the connection has to be insulated.
But, since the 25/33 KV cable must be shielded the electric field is confined so only concern it is the magnetic field which depends on current intensity only. If short time of short circuit is also a concern, you can take this in you calculation.
The induction voltage depends also upon the clearance between power conductors and number of parallel conductors and the distance to the control cable.
As the return path through the ground [See Carson theory] is involved, the calculation could be a bit more complicated.
You may use the theory for overhead line against static wire for instance or for induced voltage in a pipe line. For instance see:
htt
For two parallel loops:
L12=miuo*Length/4/pi()*ln(((Dp-h)^2+b^2)/(h^2+b^2)*((Dp+h)^2+b^2)/(h^2+b^2)) [H/m]
Where:
h-vertical distance b=horizontal distance if miuo=4*pi()/10^7 H/m then Length=m
Dp=658.5*sqrt(roxf) Carson formula for the earth return 'image' conductor. f=50-60 Hz ro=earth resistance ohm.m