Stupid question of the day....

We are indeed gifted to have so many brainiacs in here.

I looked at it, and you're right, the posting at that URL is wrong. here's another time wasting URL

Read my proof or shut up. You do not even know what "tend" intends.

-Aut

-- Tzortzakakis Dimitrios major in electrical engineering, freelance electrician FH von Iraklion-Kreta, freiberuflicher Elektriker dimtzort AT otenet DOT gr Ï "Alexander" Ýãñáøå óôï ìÞíõìá news:dcfqoa$dbo$ snipped-for-privacy@news1.zwoll.ov.home.nl...

Or of line-to-line voltage equal or above 220 kV.Therefore transmission lines of 400 kV are always designed with a double conductor, thus to reduce the corona discharge due to skin effect.

-- Tzortzakakis Dimitrios major in electrical engineering, freelance electrician FH von Iraklion-Kreta, freiberuflicher Elektriker dimtzort AT otenet DOT gr ? "John Larkin" ?????? ??? ?????? news: snipped-for-privacy@4ax.com...

Very high AC currents are much more common.The output of a moderate 300 MW alternator is 10 kA at 21 kV.A nuclear power station alternator with a voltage of 27 kV almost reaches 20kA, with a nominal power output of 1500 MVA.Always talking about balanced three-phase systems.The output of the 300 MW power-station at 400 kV transmission voltage is just 400 A.Conductors in all LV circuits are made of electroletically purified solid copper, 99,99 % Cu.In MV, HV and EHV distribution and transimission voltages respectively, they use ACSR conductors (Aluminium Conductor Steel Reinforced)that have a steel core, but an aluminium outer sheath.

-- Tzortzakakis Dimitrios major in electrical engineering, freelance electrician FH von Iraklion-Kreta, freiberuflicher Elektriker dimtzort AT otenet DOT gr ? "John Fields" ?????? ??? ?????? news: snipped-for-privacy@4ax.com...

That can happen in high impulse short circuit currents.An unfused 220 V circuit shortcircuited between live and earth, can have an impulse current of 20 kA.Properly fused with a circuit breaker, up to 50 A.In normal operating conditions, a transmission line of 150 kV operating at 200 A with an ambient teperature of 20 deg.C (65deg.F)should not exceed 50 deg.C(105deg.F)however as it operates continually at these conditions the temperature is uniform across the conductor (ACSR).

I would think that simple thermal effects would cause charge to flow closer to the surface just because that part of the conductor would be cooler, ergo lower resistance than the hotter interior.

To which Tzortzakakis Dimitrios replied:

That can happen in high impulse short circuit currents.An unfused

220 V circuit shortcircuited between live and earth, can have an impulse current of 20 kA.Properly fused with a circuit breaker, up to 50 A.In normal operating conditions, a transmission line of 150 kV operating at 200 A with an ambient teperature of 20 deg.C (65deg.F)should not exceed 50 deg.C(105deg.F)however as it operates continually at these conditions the temperature is uniform across the conductor (ACSR).

"Dimitrios Tzortzakakis" schreef in bericht news:dcl731$u0$ snipped-for-privacy@usenet.otenet.gr...

A tranismission line always has an AC element according to fourier Analysis. Some times it is superimposed on an DC element but nearly always you want to avoid this.

"Dimitrios Tzortzakakis" schreef in bericht news:dcl7ei$2f1$ snipped-for-privacy@usenet.otenet.gr...

Sometimes you have something like Aluminium inside (for the weight) and Cupper on the outside for conductivity. Due to the Skin Effect this is where the most (AC) current will flow. On some application I have even seen Cu on the inside and Au on the outside, my guess there is at least one other material between the two for obvious reasons.

Alexander (ACE, Applied Communications Engineer)

Really? The reasoning for that layering doesn't seem obvious to me, so would you mind explaining it in greater detail?

Oh boy, you have a 'couple of crossed wires' there.

"Skin effect" is the phenomenon where electric current flow is forced out from the center of a conductor due to the self-inductance in the conductor when carrying AC current. The higher the frequency, the more pronounced the current shift to the exterior. It's mostly a problem with high current situations, even if the voltages are so low that corona discharge is not a problem.

"Corona discharge" is *NOT* caused by AC or skin effect. Corona discharge is caused by a high voltage gradient in the space around a conductor. This is a combination of the voltage applied to the conductor and the effective radius of the conductor. A high voltage, or very small effective radius can increase the gradient to the point where the air is ionized. Simple proof is that corona discharge is a problem with high DC voltage systems as well as AC.

Sometimes hollow tubes are used for high frequency power conductors. This reduces the weight and cost by eliminating the central part of the conductor, where 'skin effect' has rendered the impedence high anyway. So little admittance is lost for a great savings in material/weight.

And for high voltage systems, multiple parallel conductors are used to give a larger 'effective radius', thereby reducing the corona losses.

But the two phenomenon are not related, and the two techniques used are not really related.

daestrom

An interesting point. *IF* the current density is uniform across the conductor, then the heat generated would be uniform in each unit cross-section. And a uniform heat generation in a cylindrical rod leads to a parabolic temperature profile, the highest exactly at the centerline, dropping of as you move outward along any radial line.

Of course, in an AC line, the current density isn't uniform, so neither is the heat generation. So when it comes to skin effect, it tends to lower the peak, centerline temperature.

Now, given that both copper and aluminum are excellent heat conductors, it might be interesting to calculate how big a temperature profile could be expected, and from this calculate the variation in resistivity.

I suspect the work has been done before, and that the difference is rather modest for all but the largest cylindrical conductors.

daestrom

Yes, but most of the phase conductors that I've seen from large alternators (500MW to 1200MW) to the step-up transformers are not simple round conductors. In fact, rectangular tubing is used for the conductors (at least those used in many nuclear stations). The tube is encased within an outer 'pipe' and H2 is forced down the center of the tube to the end, where it exits the tube and returns outside the tube within the outer pipe. Such 'isophase busses' are specifically designed to carry this large amount of current just far enough to reach the main step-up transformer where it rises from the nominal 25kv to 345kv or higher. The secondary is connected with 'normal' ACRS conductor to the remaining switch yard equipment.

daestrom

On Tue, 02 Aug 2005 00:45:43 GMT, "daestrom" Gave us:

For AC at this frequency there is nil skin effect.

Current in a wire will heat the wire evenly if it is of one material. a cladded conductor will disperse heat according to the thermal and electrical conductivities of the mediums that make up the wire.

A copper cladded aluminum wire (never seen it) will heat more inside as aluminum has poorer electrical conductivity and nearly identical thermal conductivities. The result will *still* be even heating for the most part.

An Aluminum cladded Copper conductor will heat evenly as the differences are so slight as to make no difference.

On Tue, 02 Aug 2005 00:45:43 GMT, "daestrom" Gave us:

It becomes more prevalent as frequency goes up, not current.

VERY high frequency. NOT AC line frequencies.

Not so. At 60 Hz, copper skin depth is about 0.85 cm.

John

Any math that results in infinity might be fundamentally flawed. Infinity is a mathematical impossibility. No mathematical functions can be performed on infinity. It cannot be divided, multiplied, added to or subtracted from. It is a non-quantity. Therefore, can it be a solution to an equation?

Just as Newton's equations where once considered mathematical law, until professor Einstein proved otherwise, Einstein's theories may also someday be shown to be incomplete.

It is possible that when an electron or photon reaches Einstein's cosmic speed limit something entirely different happens than what the currently accepted math would tell us is happening.

It may very well be that electrons can and do travel at speeds faster than C but we cannot observe nor comprehend what happens at this point.

So for practical reasons, I must agree with Fields. But for philosophical reasons, I must agree Auytm. However, none of you have the answer so it is silly to stake out some absolute ground.. Is better to dream about it.

Exuse me for now I must return to my game, rolling dice with God. It seems every time I get up on him he changes the rules.........

Herr Fields, wünsche ich, daß Sie das Verwenden solcher Geflügelsprache im allgemeinen Forum nehmen würden.

Fale não com o tounge do diabo

Define "VERY." Is that higher than Very and very?

Skin effect is measurable in cables at as low as 100 kHz.

Don