Power transmission lines...

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Externet
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Power transmission lines...

Post by Externet » Tue Sep 05, 2006 10:16 am

Hello all.

A plain power company high voltage transmission line, say at 100KV - AC 3 phase- as the ones commonly seen everywhere on steel towers crossing the country...

It emits an electromagnetic radiation at 60 Hz. (electrical+magnetic fields)
---> Is that radiation 60 Hz ? or is it 60, 120 and 180 Hz because of being 3 phase?

If the transmission line is energized but there is no consumption at the load end (a city) by whatever reason;
---> Is the radiated emission of the transmission line only an electric field and no magnetic field?

For the conditions of no current flowing and yes current flowing on it for constant 100KV,
---> Is that electric field emission the same intensity when there is no current as when there is current ?

Thanks,
Miguel

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Post by gerty » Tue Sep 05, 2006 12:07 pm

Here's my best guesses:
1 60 hz , just out of phase with each other.
2 no load, elctrical field, magnecteic field field comes int play when a
load is present
3 electric field same intensity

My guesses, yours may vary...

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Chris Smith
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Post by Chris Smith » Tue Sep 05, 2006 1:21 pm

The electric field and magnetic field occur with each phase or movement of the power or the electron in each wire, regardless of a load simply because there is always a load,.... and always some movement.


Empty wire doing nothing at the city still has resistance along the way, thus a drop and loss in energy,.... and in it self is a load.

IF you have a three phase system, you have three single phases in any given area.

If you measure the AREA, you have three single phases.

In them self, there is no cancellation of phase.

Tied together for work, they simply work in harmony as one.

Work and harmony are not the same thing as three empty wires accomplishing nothing, amounting to three single phases.

Apples and Oranges.

All of this is the assumption of harmony, each in phase with each other, and not separate question

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Post by Robert Reed » Tue Sep 05, 2006 1:22 pm

I agree with Gerty on his reply,but would like to add a little trivia in the event you want to plug these into calculations. Hi-Line voltage will almost always be one of three levels and that is 138 KV, 345KV, 700KV. There is never an intended no load situation on these lines as there will always be some intentional draw, even in a rare instance where all feeder loads would be shut off, reactive current would still flow due to transformer core magnetizing currents ( and there are many on Hi-Lines. As to the frequency of the feild, just get under a tower with your AM radio - definately 60 Hz interference rate.

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Post by Chris Smith » Tue Sep 05, 2006 2:35 pm

Un attached you have three separate lines and three separate signals.

Attached, you have harmony, and one three phase signal.

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Post by Externet » Tue Sep 05, 2006 3:16 pm

Hi Robert.
"definately 60 Hz "... am not sure. In a household environment, seems to be, but never tried under 3phase. There is 3 peaks and valleys per 'pseudoperiod'

Found these figures for the magnetic field strenght; but cannot find figures for the electric field at about same distances :

http://www.atcllc.com/pdf/EMF.pdf#searc ... 20lines%22

¿Do you know them?

Miguel

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Post by haklesup » Tue Sep 05, 2006 3:39 pm

What no spectrum analyzer with broadband antenna handy today?

(not quite guessing but with minimal checking of facts, my take is)
The law of superposition of waves says the net field you measure will be the sum of all the fields in the vicinity. So in a three phase t-line you would measure primarily (WRT ground) a 180Hz signal with harmonics at 60 and 120Hz. You would also measure additional harmonics caused by reflection of the primary from the tower etc.

A high voltage conductor with no current flowing would not have a magnetic field. In practice, this would never happen on a t-line but hypothetically yes, no B field (think negative ion generator). For the same voltage and distance, the E field would remain the same regardless of the B field.

That's not to say that a changing E field might not induce a current in a nearby grounded conducting structure like the tower.

See Maxwell's Equasions

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Post by Robert Reed » Tue Sep 05, 2006 6:24 pm

Exter
When I Pass under Hi-Lines and with my AM radio turned on ,it sure sounds like 60 Hertz interference although common sense would seem to dictate it should be 180 Hz interference, I am not sure why it sounds like a 60Hz rate to me, oyher than the fact that irritating noise is just that- noise. and maybe I am just not paying that close of attention to it. Two things could be occurring hear though - The resultant feild produced is coming from three conductors fairly closely grouped and with their currents not in phase and much of the time in opposite directions giving some degee of cancellation. Owing to the fact that these lines would rarely see a perfectly balanced load, maybe one phase dominates. The other thing is that obviously, my AM radio cannot pick up 60 hz EM feilds.So the 60 Hz(?) that I hear has to be caused by some aberation on the line causing a hi frequency ringing at the 60 Hz rate and sweeping the tuning range of the AM radio range. Again ,dont know if this may be more dominate on just one phase . But it sure sounds like 60 Hz . As to Electric feild strength, I can't offer any help there. Are you concerned about safety related issues?

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Post by rshayes » Tue Sep 05, 2006 7:00 pm

Adding up three 60 Hz sine waves will result in a 60 Hz sine wave of a different magnitude and phase. The addition process does not generate harmonics. If the 60 Hz waveform has harmonic content, then the harmonics will also add to produce single signals at the harmonic frequiencies. These will probably be on the order of a percent of the 60 Hz signal.

Both the electrostatic and magnetic fields tend to cancel for distances far from the transmission line. "Far" is a distance more than the order of ten times the wire spacing.

The noise on the AM radio is probably from corona discharge from the line. Thie will generate a wide band of noise modulated by the 60 Hz on the transmission line.

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Post by Chris Smith » Tue Sep 05, 2006 8:35 pm

All three lines are a single progression of one single wave front.

3 phase in one.

As to the magnetic and electro static charge, resistance is every where in this universe and these lines especially, thus resistance always exists making the other two moot.

Perhaps when gold becomes the standard and super chilling occurs in these lines every day, it may diminish,...... til then it will always exist.

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Post by MrAl » Wed Sep 06, 2006 12:34 am

Hi there,

Thoughts...

A three phase system will show a 180Hz B field, but if there is no
load that field will be much lower than if there is a load at the
other end. Why? because current will be lower.

Even though this field is 180Hz, two pieces of light steel spaced say
0.1 inches apart will vibrate at 360Hz, which is twice the frequency
of the B field. Why? Because although the polarity of the field
changes with the polarity of the currents in the lines, the steel doesnt
care what the polarity is...only what the absolute value of the
field is. Thus, the two pieces of steel will pull together for every
peak in the B field, regardless of it's polarity.
Conclusion? Some things that are acted upon by the 180Hz B field
will react more like they are being excited by 360Hz rather than
180Hz.
Real life example: Thin magnetic transformer core material used for
making transformers and chokes will give off audio frequencies of
120Hz when excited by a 60Hz voltage. Same thing here, the
steel leaves move together once for every current peak, be that peak
positive or be that peak negative. Since there are two peaks per
cycle, the steel reacts twice for every cycle.

Another interesting fact:
A 60Hz three phase line full wave rectified (using 6 diodes as usual)
produces an average dc voltage with 360Hz ripple.
LEDs vs Bulbs, LEDs are winning.

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Post by Robert Reed » Wed Sep 06, 2006 7:21 pm

Well Externet, youv'e done it now.
Your post has got me to wondering about supposedly simple things I thought I knew and always took for granite. That is until I really started thinking about it and now I can't let it go.
A series of situations popped into my mind that I am puzzling over. First consider a TV degaussing coil - a single conductor multiwound in a large loop. One current, one direction at any given time producing an alternating magnetic feild around it. No problem - easy to visualize and understand. Next we look at a peice of zip cord, the two conductor table lamp variety carrying a current in each conductor that is always out of phase with the other adjacent conductor. It would seem that the magnetic feilds produced would also have that phase relationship and cancel out. Now jump to the hi-lines that we are discussing - without going into a lot of heavy mathematics and only doing a very crude waveform analysis, it appears that at any given instant the three phase currents are each at an amplitude/phase relationship as to totally cancel out the magnetic feild as did the previous two wire example. Given most Hi-Line phases (conductors) have a fairly wide separation, it is actually small compared to the distance an object(person,vehicle) would be from it. Now I am wondering, is there actually any magnetic feild at all present(to objects down below). Jump in anybody with your thoughts, knowledge,etc.

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Post by Chris Smith » Wed Sep 06, 2006 7:48 pm

Individually any HV line would contain three separate signals.

However, all three originate from the same single generator meaning that each signal is a ramp from the one before, but when connected and used form just one single signal.

Push on one, pull on the other, and half way in between on the third.

Like a single ride on the circus dish, all come from the same point, but each one is on the way to where the last one left off. One up, one down, one in between.

If you snap shot any single one [individual] it has a pulse, times three.

But work means one is pushing exactly while the other is pulling, and the third is some where in between.

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Post by Externet » Wed Sep 06, 2006 8:16 pm

Hi Robert.
I do not see why the current nor voltage can be out of phase in conductors of a lamp zip cord. They are very in phase.
Their magnetic fields are counteracting, yes, by being parallel and with opposite current directions.

And that example is for a single phase and neutral, obvious to think the radiation is 60 Hz.
But as Mr.Al explains above, a 3 phase conductors 'should' yield a tripled frequency.
The voltage and current in the 3phase case are lagging (or/and) leading, thus the fields should too respect to each conductor. As far as I can think...

This subject comes from an attempt I did years ago to capture the radiation from very nearby transmission lines (~120ft overhead) trying to gather at least 1 Watt of power (24/7) from such radiation to charge a battery for a commercial radio repeater at the top of a mountain with no 115V in miles around.

A barbed wire fence being the antenna, and a ground rod.

I made a resonant circuit on a 12" Ø coil, a capacitor to work at 60 Hz, with inconclusive results, and now, I wonder if I should had tuned it to 180 Hz instead.

I believe the electric field is the one to fish for; not the magnetic field.

Another consideration is to know if the 3 phase transmission line is Y with grounded center or not; or delta; which I do not know what is typically used in such transmission lines and its effect on a 'tuned receiver if there is no ground. Then a 'dipole?' -2 barbed wire fences would be more convenient? But the wavelenght at 60 Hz is thousands of Kilometers, so cannot see the dipole approach being any promising...

Miguel

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Post by rshayes » Wed Sep 06, 2006 8:59 pm

A three phase system does not generate a tripled frequency. The sum of three sine waves of the same frequency is a single sine wave of that frequency. The amplitude and phase of the resultant sine wave will probably not match any of the three original sine waves.

The energy available at the ground level will probably be quite low, otherwise it wouuld increase the power loss in the transmission line. Moist soil is moderately conductive.

A wire of reasonable length that is not connected to ground will be sensitive to the electrostatic field. The open circuit voltage could be estimated by considering the capacitance of the wire to the power line conductors relative to the capacitance to ground. This will be a high impedance source, which will drop rapidly in voltage when a load is applied.

A closed loop could be used to couple to the magnetic field, but again, there will probably be little power available at ground level.

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