Hoover Dam Power cable and skin effect

[David Lesher]

So a friend brought me a souvenir from a trip west.

It's a 3" section of old Dam->LA power cable, surplused when some
of the towers were moved for highway construction.

It's of interesting design... it's ~1" in dia, copper, and hollow.
The NPS brochure has some mumble-jumbo about interacting fields,
etc. making the center pointless.

1) It's been a while, but the only think I could recall was skin effect,
and my remaining feeble brane tells me was nil until the UHF region.

2) It's built out of a number (~8) of flat pieces with tongue and groove
edges, interlocked and twisted. I'm still pondering how they made it...

3) While looking, I found:
<http://www.ieee.org/organizations/history_center/oral_histories/transcripts/c-smith.html>

 

[Fred Bloggs]

So a friend brought me a souvenir from a trip west.

It's a 3" section of old Dam->LA power cable, surplused when some
of the towers were moved for highway construction.

It's of interesting design... it's ~1" in dia, copper, and hollow.
The NPS brochure has some mumble-jumbo about interacting fields,
etc. making the center pointless.

An insulating fluid or gas filled the line. From
http://encarta.msn.com/sidebar_461501187/1939_Electrical_Engineering.html

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.

 

[Tim Wescott]

So a friend brought me a souvenir from a trip west.

It's a 3" section of old Dam->LA power cable, surplused when some
of the towers were moved for highway construction.

It's of interesting design... it's ~1" in dia, copper, and hollow.
The NPS brochure has some mumble-jumbo about interacting fields,
etc. making the center pointless.

1) It's been a while, but the only think I could recall was skin effect,
and my remaining feeble brane tells me was nil until the UHF region.

2) It's built out of a number (~8) of flat pieces with tongue and groove
edges, interlocked and twisted. I'm still pondering how they made it...

3) While looking, I found:
<http://www.ieee.org/organizations/history_center/oral_histories/transcripts/c-smith.html>

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.

 

[Jim Thompson]

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.

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

...Jim Thompson

 

[Tim Shoppa]

the cable is filled with an inert gas at 10 pound pressure

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

Tim.

 

[Glen Walpert]

An insulating fluid or gas filled the line. From
http://encarta.msn.com/sidebar_461501187/1939_Electrical_Engineering.html

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.

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.

 

[Fred Bloggs]

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

Tim.

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

 

[Fred Bloggs]

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.

 

[Robert Baer]

An insulating fluid or gas filled the line. From
http://encarta.msn.com/sidebar_461501187/1939_Electrical_Engineering.html

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.

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.

 

[B Perry]

I too, have a 6" section of the cooper conductor from Hoover Dam as I was involved with replacement a number of years ago. One basic question and several related: How was it made (had to be in the '30's)? How long were the sections? How was it installed and connected?