Surface conduction at 60 Hz

On Tue, 28 Feb 2006 03:55:36 GMT, "Don Kelly" Gave us:

Didn't you know? Pops considers himself to be "expensive equipment".

On Wed, 01 Mar 2006 00:58:44 GMT, ehsjr Gave us:

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.

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.

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.

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.

On Wed, 01 Mar 2006 00:58:44 GMT, ehsjr Gave us:

It CAN be and IS negligible in low power applications or where the conductor diameter is smaller than the skin depth times two.

When the cable size is great enough that the skin depth can be a factor, it WILL be a factor, and IS a factor on large diameter high voltage high tension power lines.

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.

Ed

Enough. already. Google is your friend! ;-)

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

On Wed, 01 Mar 2006 15:43:55 GMT, ehsjr Gave us:

They used to be ALL steel. We used to cherish our copper a lot more in the days of full metal jackets, and we went through 3 wars conserving it monetarily and industrially as well. The losses are very low when one considers the voltage. At 100,000 Volts a couple hundred volts lost is a very low number. It DOES add up though, so yes, using copper clad cables reduce those losses considerably.

The losses I can't stand is where in Ohio, I could walk past a high tension line and NOT hear leakage, and that is a high humidity part of the world. Here in California, I can walk past nearly ANY high tension insulator and line, and hear it leaking badly. I'd say that california loses 20% more juice to leakage than any other state I have ever been in. They just don't service their lines.

WE PAY FOR THAT... so they don't care! That is the OTHER reason why privately held utilities suck big time. The bastards are worse than the worst NYC landlord that ever lived. Rent keeps going up... while quality of service keeps going down. Major LAME.

The criminals of Enron and Sempra energy ought to be lined up at a wall and stoned to death with very small stones... thrown very hard, like from a paintball gun(s). Either way, I want have my payments over the last seven years back. The bastards!

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

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

On Thu, 02 Mar 2006 16:56:00 GMT, ehsjr Gave us:

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.

;-)

On Thu, 02 Mar 2006 18:31:32 -0800, "Paul Hovnanian P.E." Gave us:

Hahahaha... at least I am not out of my depth, despite what some have stated.

If I am so slow, why then is my skin so thin? :-]

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).

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.

Salmon Egg schrieb:

Hello,

the steel core is used to carry the mechanical tension.

Bye