Mulling over MIKE ROSS to Steve Asher 23 Dec 2002

Hi Mike...

 SA> Does anyone know of any compelling reasons for adopting DC for high
 SA> 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 1
 MR> 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).

Thanks, I can see those advantages, & that would explain why the
interconnector I saw was of a fairly small diameter.

 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. 

I guess following on from that the insulation required for HVDC
would be less than for HVAC also, without the need to accommodate
the ac peak voltage. I have noticed that recent overhead HVAC lines 
here appear to be aluminium, probably steel cored for strength.

 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.

Yes, the DC links that I have come across are interconnectors between
states, linking what were previously isolated systems. I assume that
DC could be used to link power stations to sub-stations, if it isn't
already.

Cheers, Steve..

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