Hi MIKE,

On 23-Dec-02, MIKE ROSS wrote to Steve Asher:

 SA>> My understanding was that DC had lost out to AC a century ago
 SA>> (Thomas Edison) for power distribution due to resistance losses &
 SA>> inefficiencies. Does anyone know of any compelling reasons for
 SA>> adopting DC for high voltage transmission?

 MR> Indeed there are a couple of reasons. One is financial and the
 MR> other is compatibility between networks. For the first reason, as
 MR> you know Alternating Current exhibits a skin effect so that a
 MR> flowing current in a wire cable doesn't go much deeper than about
 MR> 1 skin depth of the surface. DC on the other hand will go right to
 MR> the core so the cable can be thinner (less copper).

I'd forgotten that one...

 MR> For 60 Hz AC the skin depth is about 1/3 of an inch. The greater
 MR> part of the current flows in this outer area but the total current
 MR> flows out to about 5 skin depths, less and less as it goes deeper.
 MR> Below this 5 skin depth no current flows and the wire may as well
 MR> be hollow or made out of steel, which is exactly what is used to
 MR> make the cables strong.

Yes, most of the UK national grid backbone uses 4 wires for each phase
(treated as one cable with X shaped spacers at regular intervals) of
steel cored aluminium. The 4 wires is to get more surface area _and_
it gives better cooling.

 MR> The other reason is distant networks won't need to be synchronized
 MR> with each other if they are using a DC link between them to
 MR> exchange power back and forth.

Got that one...
Some others as well (see my reply to Steve).

George

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