Surface conduction at 60 Hz

[Don Kelly]

----------------------------

...

No. It is true to all but the most extreme purists of the world who will
waste much time and energy on things that are not discernible in the real
world. At 60 Hz and much, much higher frequencies, it will take some very
expensive equipment to even theorize the still negligible effects of it.
People who think like this are of no practical use for such areas as this
thread; it's been pretty well pointed out.
You sound like the kind who will claim your weight also varies because
you go from the first to the second floor of a building; true, but of no
use to anyone but an extreme purist or anecdotal collector of trivia, just
as your weight "changes" when you walk into a tunnel but don't change your
relation to the center of the earth. interesting but useless information
to the real world.

Pop

If you had cared to read what I said. you would have seen that I indicated
that for most conductors, the effect is negligable at 60Hz. That doesn't
mean that it isn't there, it simply means that it can be ignored.
By the way, how does it take expensive equipment to "theorise" this effect?
Rudenberg and others did it with pencil, paper and brains in the 20's and
30's.

 

[Don Kelly]

----------------------------

I was thinking a bit more about this and have resolved the paradox in my
mind.

For a given power capacity of a line, the voltage required is going to be
proportional to V^2/Zo. By putting multiple conductors in parallel, (per
unit length) the inductance is decreased while the capacitance is
increased.
This lowers the characteristic impedance of the line. Thus, the voltage
required to transmit the original power is dropped, and the likelihood of
corona is diminished.

Bill
-- Ferme le Bush

That too- and it is a fact. Single conductor lines may have a Zo of the
order of 350-400 ohms while a 3 or 4 conductor bundle line will have Zo
nearer to 250-300 ohms.
Of course, spacing and height have an effect as well.

Reduction of corona does occur with bundling but not due to any reduction
of voltage at any power level. A voltage is chosen that is economical for
the power level and the line design follows from that. Corona is only one
concern involved.

A lot of concepts that are extremely useful in communications are of more
limited use in power transmission (well under 1/4 wavelength without
sectionalization and compensation). One such is impedance matching which is
simply not done. --

Don Kelly @shawcross.ca
remove the X to answer

 

[Bud--]

He didn't explicitly mention steel clad copper.

But he may be referring to bus-work used in substations. For short runs
between circuit-breakers, lightning arrestors, disconnects and other
components, you will often see hollow tubing used for the conductors.
If the same cross-sectional area of material were in the form of a solid
bar, it would have higher resistance owing to the skin effect. By
simply reshaping the material into a tube with a larger OD, more
material is in the region near the 'skin', providing lower resistance.

(the tube also has more structural strength than a solid bar of the same
material cross-section).

daestrom

I thought large diameter tubing conductors in HV substations was mostly
for corona reduction.

bud--

 

[daestrom]

OK, I'll rephrase and say what I meant, politeness aside since you're so
thick: Idiots who depart substantially from the post topic in order to
attempt a shot at some sort of infamy whether it be good or negative in
nature, and which serves to do nothing but confuse and create a sense of
information based on misinformation, and under the guise of being a know
it all. If you were a fraction of what you say you are, you'd have a lot
more things to do than puppet around these groups.

Nice try fool.

Topic is surface conduction at 60 hz.

It is an issue with transmission line sized conductors. It can change the
effective resistance of the conductors by several percent. It is often
mitigated by the use of hollow conductors.

ACRS has a steel strands for strength. Because skin effect makes the center
area less useful for conducting electricity anyway, ACRS is built with the
steel strands in the center.

Your point? You seem to be ranting about things and upset because several
others have told you you're wrong, and you just can't handle it.

There's no future in discussing anything further here as I consider you to
be an intentional moron in this area. Too bad you never went to a real
school.

You'll never know how wrong you are....

daestrom

 

[ehsjr]

I hope this statement is not attributed to me because I never considered
steel clad copper.

It says, if you read it, *by implication*.
If you did not consider steel clad copper, then you realize
that the larger diameter is created for the strength that the
steel core offers. Therefore, since the diameter is already
increased for strength, the reason for increasing the diameter
*cannot* be skin effect. In other words, skin effect is negligible.
The resistance of the steel core, however, cannot be neglected.
That is why it is copper clad. Nothing to do with skin effect.

Central copper in large conductors is wasted! Skin effect can be
significant. Thin copper cladding on steel is often used for antennas where
the skin effect would make thick cladding very wasteful indeed.

Of for goodness sake. We are talking about *60 Hz*. We are
*not* talking about antennas and hardline.

Enough, already. Post something real that demonstrates that
skin effect often cannot be neglected at 60 Hz.

 

[Don Kelly]

----------------------------

On 2/27/06 8:22 PM, in article weQMf.78742$H%4.46124@pd7tw2no, "Don Kelly"



Although power lines ordinarily do not run with impedance matched loads,
you
still have forward and backward traveling wave on the lines. Each of these
two waves individually does have voltage/current = the characteristic
impedance.

Bill

-- Ferme le Bush

True but not really of use in analysis of a power line's performance. One
goes from distributed parameters ("telegrapher's equations") to a lumped
model for a given line. Consideration of a standing wave can be used for
estimation of mid-line voltages where needed but even this can be avoided
by simply using multiple pi sections. For switching and lightning surges,
the travelling wave model is used where necessary. Dr. Hermann Dommel did a
lot of work on this and the models have become quite sophisticated.

 

[Don Kelly]

----------------------------

It says, if you read it, *by implication*.
If you did not consider steel clad copper, then you realize
that the larger diameter is created for the strength that the
steel core offers. Therefore, since the diameter is already
increased for strength, the reason for increasing the diameter
*cannot* be skin effect. In other words, skin effect is negligible.
The resistance of the steel core, however, cannot be neglected.
That is why it is copper clad. Nothing to do with skin effect.



Of for goodness sake. We are talking about *60 Hz*. We are
*not* talking about antennas and hardline.

Enough, already. Post something real that demonstrates that
skin effect often cannot be neglected at 60 Hz.

---------------
I just looked at a wire table:
There are or were AAC conductors and the AC/DC resistance for a 1 inch
diameter conductor(approx) is about 1.02. 2% isn't much. However, for a 2
inch diameter conductor the ratio is 1.31 and I don't think that 30% is
negligable.
For ACSR
Kiwi (44mm diameter) the AC/DC resistance ratio is 1.11
For Drake (28mm diameter) the ratio is 1.01
Compared to all- aluminum conductors- the AC/DC ratio is lower because, even
without any skin effect, the current flows mainly in the lower resistivity
aluminum.

Of course, none of these conductors are used for house wiring.

The point is that there are situations where 60Hz skin effect does have some
significance and there are lots more where it doesn't. It also happens
that design for strength or rigidity such as hollow tubes also mitigates
skin effect.
Bill's comment regarding coil construction in large machines is true- heavy
duty litz wire to try to get a more uniform current distribution in a coil.

 

[ehsjr]

NOT TRUE! First off, there are no steel clad copper cables. There
is no reason to place the poorer conductor on the outside of a cable,
and the CORE of a cable MUST have a higher tensile strength than the
jacket or the cable will fail to work as designed.

Exactly right. You seem to be the only one who understands
what is going on. Salmon egg seemed to imply that there might
be such a thing as steel clad copper cable.

The thin steel CENTRAL "carrier strand" is just that... A CARRIER.
It bears the weight of the cable, NOT the current. It is the
mechanism for providing a strong tensile link between towers. It is
clad with copper not to make the cable bigger, but to make it conduct
better.

Exactly right.

A steel cable WILL work fine at the voltages carried on those towers.
The benefit from having the copper cladding is that nearly ALL of the
current will be IN the copper, and SKIN EFFECT IS the reason the
current will be there, and IS the reason the industry clads steel
cables in copper for HV power transmission lines.

Here we may disagree, not in the facts, but in the rationale.
I think the disagreement is probably a nit, based on your
last paragraph, below. I'll address the apparent disagreement:
The steel cable already must be some specific diameter,
for strength. While steel would work, it doesn't work
well enough for the power utility economics. The steel
is too lossy. So they use copper clad to lower the resistance.
The reason it lowers the resistance is that copper is a better
conductor than steel.

So, yes, the cable has a lower resistance, but much of that lower
resistance is due to skin effect, as it relates to the characteristic
impedance of a given segment of high tension cabling.

That's the key. You understand the issue. What would
be nice is some real world numbers. I assume that in
designing the cable, skin effect is taken into consideration
as one of the factors to determine how thick to make the
cladding. I suppose they could use a standard - make a steel
core big enough that could be clad with 9+ mm of copper -
but I doubt that's real world.

Ed

 

[Keith Williams]

Of for goodness sake. We are talking about *60 Hz*. We are
*not* talking about antennas and hardline.

Enough, already. Post something real that demonstrates that
skin effect often cannot be neglected at 60 Hz.

Enough. already. Google is your friend! ;-)
http://en.wikipedia.org/wiki/Skin_effect

At 60Hz the skin depth in copper is about 8.5mm; not a big cable to
power companies.

 

[ehsjr]

Enough. already. Google is your friend! ;-)
http://en.wikipedia.org/wiki/Skin_effect

At 60Hz the skin depth in copper is about 8.5mm; not a big cable to
power companies.

Exactly. The cable core already has to be large (relatively)
diameter for strength. One does not need to make it larger
to ensure that the copper cladding is not wasted expense.
Skin effect has no bearing on the cable dimension. Whatever
amount of copper is added as cladding will not be wasted.
The reason copper is clad onto the steel is not because skin
effect exists. Copper is clad onto the steel because it (copper)
is a better conductor.

I suspect that this dispute was caused by my initial post that
sounded too absolute. I get the sense that people think, based
on that post, that I am claiming skin effect does not exist at
60 Hz. The bone of contention here is not the existence of skin
effect. It is the rationale in the power line example used to
support the idea that skin effect must often be considered at
60 Hz. The rationale does not stand up to examination, and
even if it did, a single example does not provide evidence of
"often". It gets discouraging when an example in Mhz or higher
is offered.

Ed

 

[daestrom]

Exactly right. You seem to be the only one who understands
what is going on. Salmon egg seemed to imply that there might
be such a thing as steel clad copper cable.


Exactly right.


Here we may disagree, not in the facts, but in the rationale.
I think the disagreement is probably a nit, based on your
last paragraph, below. I'll address the apparent disagreement:
The steel cable already must be some specific diameter,
for strength. While steel would work, it doesn't work
well enough for the power utility economics. The steel
is too lossy. So they use copper clad to lower the resistance.
The reason it lowers the resistance is that copper is a better
conductor than steel.



That's the key. You understand the issue. What would
be nice is some real world numbers. I assume that in
designing the cable, skin effect is taken into consideration
as one of the factors to determine how thick to make the
cladding. I suppose they could use a standard - make a steel
core big enough that could be clad with 9+ mm of copper -
but I doubt that's real world.

Skin effect is also a problem in the short runs of bus-work used in
substations. These do not have a steel core (no need for excessive tensile
strength), but the bus-work is *not* solid aluminum or copper rods. It is
aluminum tubing with a large ID. The tubing wall isn't more than about 1/2
inch thick (close to 9mm). For larger ampacities, larger diameter tubing is
used, not tubing with different wall thickness.

daestrom

 

[Paul Hovnanian P.E.]

I know! At 1 Mhz we can truly call it "skin effect". At 60 Hz, we
should call it "clad effect". It even sounds thicker. Thickerer.

How about 'current density depth gradient'? That'll keep most of the
know-it-alls tongue-tied.

;-)

 

[Don Kelly]

----------------------------

Exactly. The cable core already has to be large (relatively)
diameter for strength. One does not need to make it larger
to ensure that the copper cladding is not wasted expense.
Skin effect has no bearing on the cable dimension. Whatever
amount of copper is added as cladding will not be wasted.
The reason copper is clad onto the steel is not because skin
effect exists. Copper is clad onto the steel because it (copper)
is a better conductor.

I suspect that this dispute was caused by my initial post that
sounded too absolute. I get the sense that people think, based
on that post, that I am claiming skin effect does not exist at
60 Hz. The bone of contention here is not the existence of skin
effect. It is the rationale in the power line example used to
support the idea that skin effect must often be considered at
60 Hz. The rationale does not stand up to examination, and
even if it did, a single example does not provide evidence of
"often". It gets discouraging when an example in Mhz or higher
is offered.

Ed

How many power lines now use copper?
I recall some relatively low voltage distribution lines using steel with a
copper cladding but any "real" transmission line over the order of 30Kv uses
ACSR (Aluminum Cable, steel reinforced. With such cables, the 60Hz skin
effect is much smaller than with an all aluminum or all copper cable because
the current will flow in the outer Aluminum layer even at DC (serendipity?).
However, any table of parameters for such cable shows that AC resistance is
higher than for DC- In essence the effect of skin effect, however slight, is
accounted for in the stated resistance- so you don't even have to think
about it.

For example "Joree" has an AC/DC ratio of 1.05+ at 25C Mind you, the change
in resistance in going from 25C to 75C is 15%.

This is a very large conductor and is rarely, if ever, used now- but I have
seen larger conductors in the supply from a hydro plant to a nearby
aluminum smelter (at 13.8KV).

 

[Don Kelly]

----------------------------

The skin depth in aluminum is somewhat greater than it is in copper
because
Al has higher resistivity than Cu does. I don't know how economics factors
in, but the price of Cu has greatly increased the last few years. What
probably clinches the deal for Al is that its lower density allows for
larger diameter conductors. In turn, that allows higher conductance for
the
conductors while also allowing increased separation between support
towers.

Bill

-- Ferme le Bush

Actually, ACSR (Aluminum outer layers steel core) started to replace copper
in the 1940's and at present, the use of copper, except for distribution
(even there ACSR is common), ended long ago. You are right in that the
economic/engineering constraints dictate the choice but that balance between
copper and ACSR was won by ACSR a long time ago.

 

[Uwe Hercksen]

Skin depth at 60Hz in copper is about 6 mm. While that is large compared to
typical wire radii in house wiring, it often cannot be neglected. That is
why why power transmission lines often are fabricated with steel cores clad
with copper or aluminum. Aluminum is preferred much of the time because of
its low density and greater skin depth.

Hello,

the steel core is used to carry the mechanical tension.

Bye