OT: very BIG capacitor
2003-12-09 by J. Larry Hendry
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.mpg Larry Hendry