Excused the OT, but I think you will all enjoy the link. A little
entertainment I'll call "another day at the office" in the utility business.
Actually, the link below will take you to a truly amazing video (about
1.5Mb). You will be looking at a 1/2 million volt AC arc which I am certain
exceeds 50 feet in length.
I have not yet uncovered all the facts about this video that was sent to me
today. But, I will fill in the details I know from my observation of the
event. The file name Lugo suggests a large California substation that is
populated with 500KV and 230 KV switching devices. This one is clearly
500KV (I can tell by the size). The switch being opened is called a
"circuit switcher." It consists of 2 series SF6 gas puffer interrupters
(kind of like a circuit breaker) and an integrated center-break disconnect.
The way they are supposed to work is the interrupters both trip, grading
capacitors or resistors cause the open circuit voltage to split evenly
across the two interrupters, the switch blades open with no current flow,
and the interrupters close as the switch reaches the full open position.
I titled this "very BIG capacitor" because that is what unloaded
transmission line looks like. The parallel wires have a huge capacitive
effect between ground and each other. On a 500KV line like this the current
(leading the voltage by 90 degrees) required to energize this capacitor is
approximately 2 amps per mile of line per phase. That's 2 amps per phase at
500KV, or about 1.7 MegaVars (million volt amps reactive).
The switch operation you see in this video in my opinion is a failed attempt
to interrupt that charging current. I am going to take a SWAG guess and say
we are looking at ~ 25 amps (I've seen a few of these in a smaller scale),
or about 7 Mega Vars. Funny that it is only called "apparent" power, heh?
The failure appears to be that the far right interrupter does not open or
the grading device has failed. The voltage across the open interrupter
exceeds the rating and it flashes over (you can see the first arc develop
across the interrupter). Therefore, the switch blades are left to interrupt
the current (not designed to do that) as they open. As the interrupter
closes you can see the arc across it go out. However, the arc across the
switch gets as tall as a 3 story building before the arc resistance is
sufficient for the ionized gas to quit conducting. This is the only failure
I have ever seen where the arc lasted so long and grew so large without
first going phase-to-phase or phase-to-ground taking the circuit out of
service. It just keeps growing straight up where it contacts nothing.
Check out the guy on the right by the truck. The taller the arc gets, the
more he crouches down behind the truck.
Here's the link
http://www.wiseguysynth.com/larry/misc/LugoSWR.mpgLarry Hendry