Induction Heating for Utlity Conductors

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sandsnow

sandsnow

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Southern California
This is something I was just curious about and thought maybe one the utility guys could shed some light on this.

Overhead wiring transitions to underground via a large steel tower (large diameter hollow pole). I don't know the voltage or if it matters. There are what looks to be six bell insulators for each conductor. Someone told me once you could figure the voltage by the number of bells.
Each conductor enters the steel tower through an individual opening in the tower.
Is there inductive heating here? If this were under 600 volts and covered by the NEC it would be a violation.
So, what's the deal??
Thanks
 
Re: Induction Heating for Utlity Conductors

I am guessing that the voltage is 69 kV but it depends on the company that is building the line. We use a string of four 10" bells for 34.5 kV and ten 10" bells for 138 kV but do not have any 69 kV on our system.

For what it is worth, we enter every transformer and switchgear through separate bushed holes. Remember the amperage is small because of the voltage, the prohibition in the NEC is for low voltage (under 600 volts).

I don't know if this is a valid argument but I think this is why it was done. I will have to leave this for Charlie B., Don G. or one of the other engineers to support of blow my argument away. :D
 
Re: Induction Heating for Utlity Conductors

I?ll support it. Magnetic fields are generated by charge in motion. That is to say, the more the current, the greater the magnetic field. The amount of voltage that created the current is not relevant. The phenomenon of inductive heating is a result of excessive magnetic fields, and the high voltage, low current systems do not have that problem.
 
Re: Induction Heating for Utlity Conductors

Thanks for your replies
I didn't mention before but the conductors (overhead) appear to be pretty large, large enough for say at least 100 amps. They are line or load conductors in or out of a substation.
So what are you saying is low amps here? Can you put a number on it.
I can understand that low amps are not a concern. Just makes me curious as to why the NEC did not put a threshold for current flow, say current flow in excess of XX then all conductors shall enter through the same opening OR
Is the phase I think I remember that "inductive heating shall be minimized" cover it?

Thanks
 
Re: Induction Heating for Utlity Conductors

Without knowing something about the electric utility and what they are using, I can't really give you much of an opinion. However, if this is a steel pole, what is the possibility that slots were cut or there is an aluminum plate that the conductors are passing through?

If these are line conductors into the substation, the amperes would be low. If the power were being taken from the substation, it may be as high as 600 amperes. There are too many variables to really make any kind of a guess from here.

As far as the NEC putting a limit, keep in mind that we are not going away. In other words, we can do it an if it doesn't work, we have to go back and fix it. An example of the NEC not being totally practical is the service for fire pumps. If I install an overcurrent device in front of the motor, it has to handle locked rotor forever. The reality is that it only has to handle LR for 20 seconds and then the controller will open up the circuit. In light of that, why can't I just use a normal circuit breaker that I can delay the trip for say 25 seconds that would trip without delay on sensing current in fault ranges? The answer, of course, is that the Code doesn't permit that type of installation. :D
 
Re: Induction Heating for Utlity Conductors

From 1947 through 1962 the rule did not apply if the current was less than 50 amperes. Then in the 1965 NEC the 50 ampere limit disappeared, but I cannot find a proposal to delete it in the 1965 Preprint.
Creighton
 
Re: Induction Heating for Utlity Conductors

Hey guys,
Keep in mind the cable going into the pole is shielded. Wouldn't that effectively reduce most or all of the inductive heating? I am familiar with the installations Sandsnow is talking about.(We are from the same area) It is very common to see them transition from overhead to underground, to get under a freeway or major road, and then back to overhead. It's most likely jacketed concentric neutral cable.
 
Re: Induction Heating for Utlity Conductors

Not sure, but I think the shields are for electrostatic purposes & do not, & are not intended to, redirect the magnetic fields that can cause inductive heating.

I think this heating is caused by (generally unwanted) current flow known as eddy currents. Believe the strength of the magnetic fields is directly related to the amount of current (in Amps).

There are methods, such as cutting slots, or using non-ferrous materials, such as aluminum, that can break the path for inductive heating.
 
Re: Induction Heating for Utlity Conductors

By Nick: Wouldn't that effectively reduce most or all of the inductive heating?
Click to expand...
The shields would only come into play if the total return current was returning on this shield. I don't think it does as with a higher voltage (above 600 volts) some current will return through the Earth.
 
Re: Induction Heating for Utlity Conductors

Originally posted by creighton:
From 1947 through 1962 the rule did not apply if the current was less than 50 amperes. Then in the 1965 NEC the 50 ampere limit disappeared, but I cannot find a proposal to delete it in the 1965 Preprint.
Creighton
Click to expand...
Interesting bit of history, thanks. It would have been interesting to know the substantiation for the proposal.


next time I talk to one of my SCE (poco) contacts I'll see if they know. I've meant to for a long time, but I'm usually working with them on another issue and forget. Also they are distribution guys and don't work on transmission lines which I think these are.
 
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