Hoover Dam Power cable and skin effect

The skin effect has a depth proportional to frequency and (IIRC) inversely proportinal to conductance. It causes problems with small wires at frequencies as a few hundred kHz -- this is why high-performance coils for 455kHz IF transformers are wound with litz wire, as well as a few high-frequency switcher coils that I've seen. Above a couple of MHz even the conductors in litz wire are too big, so you just don't bother any more.

Search around a bit and you'll find literature on the skin effect on the web -- given the 1" depth of the conductors you have, and my dim recollections from E&M theory in school I think you'll find it's on the order of 1/2" for copper at 60Hz.

------------------------------------------- Tim Wescott Wescott Design Services

For copper it is about 0.85cm (or about 1/3") at 60Hz.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
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|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
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I love to cook with wine.      Sometimes I even put it in the food.

What do they actually use? Nitrogen? Sulfur Hexafluroide? I'm guessing since they were talking about 287000 volts maybe it is SF6.

Tim.

An insulating fluid or gas filled the line. From

Transmission and Distribution.

Work on the first phase of the Ross Dam on the Skagit River by the City Lighting Department of Seattle, Washington, is now being completed. There are ultimately to be three dams, developing a total of 1,120,000 horsepower. The Ross Dam is farthest upstream of the three and will be

655 ft. over all. The cost of this particular development is $5,600,000, 45 per cent of which is PWA funds; the ultimate output will be 360,000 kw. The two lower plants will have a capacity of 240,000 kw. each. This dam will create a 3,000,000 acre-foot reservoir, equivalent to 600 days' flow of the river, giving complete storage for power and flood control.

The construction of the third 287,000-volt, three-phase line from Boulder Dam to Los Angeles is now under way, assuring an added source of power to the Pacific Coast. The line conductors are the General Cable's

1.4-inch, type-HH, hollow conductor built up of ten segments to form a smooth copper tube.

High-voltage underground cables have hitherto been insulated with paper tapes, wound spirally about the conductor and impregnated with either heavy mineral oils (called the solid type) or a light viscous oil that flows in and out through the hollow core to a sylphon or accordion-like drum, with contraction and expansion (called the oil-filled type). Such cables are, of course, enclosed in a lead sheath. A new development is the gas-filled cable. The bulk of the oil is drained from the paper, and the cable is filled with an inert gas at 10 pound pressure. Although this cable does not have the life of the oil-filled type, nevertheless it has a longer life than the solid type, and the cost is much less than that of the oil-filled type.

I don't know- this would be worthwhile to track down.

Interesting cable types, however insulated HV cables like that are only used underground or in short runs in wireway due to cost and weight. HV distribution cable on towers is uninsulated. AFIK the interlocked segmented tube design is standard for HV distribution on towers because of previously mentioned skin effect and because friction between the segments helps damp wind-induced wire oscillation thus reducing stresses on the wire. The center is not filled with anything except ambient air.

I expect that assembly of the segments is done with something very closely resembling tubing drawing equipment, with the wire drawn through and over fixed dies which squeeze the female groove over the male tounge while squishing the joints tightly.

This would be worthwhile tracking down too- a clever means of avoiding embrittlement of the copper due to constant flexing.

Filling a conductive tube with either gas or fluid cannot help "insulate" anything. However, the filler can help incooling, especially if it flows (eventually) thru a heat exchanger.