Re: power-grid physics

["James R. Frysinger" <frysingerj@COFC.EDU>]

On Friday 2003 August 22 16:18, you wrote:
....
> -- zero-point switching: close a switch when
> there's no voltage across it, open a switch when
> there's no current through it.
>
> The last is nothing to be sneezed at; the big
> circuit breakers at a substation are synchronous
> so as to minimize the arc when they open. These
> things are a highly amusing combination of cleverness
> and brute force: they have a syncrhonous motor
> that stays in phase with the current. The motor
> drives a disk with a dog; the dog is normally
> retracted, but when it comes time to trip the
> breaker the dog sticks out and knocks out the
> three bars that carry the current, bing, bing,
> bing. A view looking down the axis is:
>
>                Z
>
>                O----
>            Y       X
>
> where the dog is about to whack the X phase. And
> even so it takes lots of cleverness to extinguish
> the bit of arc that occurs anyway.

	I had not realized that the large substation breakers operated that way for 
their tripping operation. Thanks for posting this!

	The ones we had on the subs had no synchronization protection or interlocks. 
All three contacts closed and opened simultaneously. Of couse, arcs occured 
and the breakers had "arc chutes" to contain them. Essentially those arc 
chutes are chimneys that kept the arc from reaching the sensitive parts of 
the breaker (such as the bar motor, the protective tripping devices, etc.).

	I did see a breaker get shut horribly out of phase one time ( ~40° as I 
recollect, which resulted in our having to  shut down a turbine generator and 
conduct an extensive inspection of turbine blades and coil windings, etc.). 
The effects of this included a fireball that floated forward above a 
passageway deck and then grounded itself out on a water-tight door. The 
fireball had been preceded down the passageway by the watchstander who ran 
through the door and shut it behind him and ahead of the fireball just in 
time. It left quite a scorch mark on the paint.

....
> Already some long-distance tie-lines are high-voltage
> DC.  I'm not 100% sure what they use for getting
> power on/off these lines, but I imagine it's
> a big piece of rotating machinery plus some
> tricky and large bits of electronics.

	I think that they are still using those motor-generators. There are a few 
static inverters in commerical power distribution use but I don't know 
anything about their prevalence and reliability. An advantage that a 
motor-generator tied to a storage battery or DC line has is that it can 
double as a spinning capacitor, depending on its excitation.

Jim

-- 

James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE

http://www.cofc.edu/~frysingj
frysingerj@cofc.edu
j.frysinger@ieee.org

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