Half Mega Volt Switch

My day gig

From "Stooge Larry’s 'Wise Guy' Synthesizer page"

http://www.wiseguysynth.com/larry/day.htm

The photo above will link you to an amazing video (~1.5 Mb) of a ½ million volt switch failing to interrupt the arc when operating. Special thanks to Old Crow for hosting this popular video. If you are interested in some light technical analysis of what you are looking at, see the text below this photo.

When I am not messing around with modular synths or shipping parts for synth builders around the world, I have a regular job. Like most musicians, I call it my day gig. I have worked for an electric utility company in the Midwest, USA for over 27 years. I am what is called a "high-voltage specialist." So, while my synth circuit abilities are somewhat limited, I am well versed in the operation of high voltage AC circuits.

I was not given the details of this clip when it was sent to me. However, based on what I do know about the equipment in the video and what I see, I offer the following info: The file name Lugo suggests a large California substation that is populated with 500 KV and 230 KV switching devices. This one is clearly a 500KV (I can tell by the size) three-phase switch, probably rated at about 2000 amps of normal current carrying capability.

This type of switch typically is used at one end of a transmission line, in some cases in conjunction with or instead of a circuit breaker for a variety of different configuration reasons that vary greatly from one utility to the other. Or, it may be used to connect a large transformer to the system.

In this case, the switch is being used to connect a special kind of transformer. The 3 single-phase transformers can be seen behind the truck. I say transformer, but as you can see, they have leads going in, but not coming out. These are actually single winding inductors connected from phase to ground and are commonly called "shunt reactors." These inductors are installed to offset the capacitive effects of un-loaded transmission lines, When a long 500 KV or 765 KV line is energized from one end, its inherent capacitance causes an unacceptable voltage rise on the open end of the line. The "shunt reactor" is installed to control that open-circuit voltage. Where current into the capacitor component of the line impedance leads voltage by 90 degrees, current into the shunt reactor lags voltage by 90 degrees.

     The switch being opened is called a "circuit switcher." It consists of two series SF6 gas puffer interrupters (kind of like a circuit breaker) and an integrated center-break disconnect. The interrupters are to the right of the switch blades. They just look like gray porcelain insulators. At 345 and 500 KV these types of switches typically have two interrupters per phase in series in order to withstand the open circuit voltage encountered when de-energizing a line or transformer. They rely on synchronized opening of the two interrupters and voltage even distributed across the two interrupters by "grading" devices (typically lots of series capacitors or resistors).

     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 originally 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) per mile. However, we are actually looking at the shunt reactor current which is inductive and lags the voltage by 90 degrees. So, I should have said "very big inductor."

     The switch operation you see in this video in my opinion is a failed attempt to interrupt that inductive current. The failure appears to be that the far right interrupter does not open or the grading device has failed. The voltage across the remaining open interrupter exceeds the rating and it flashes over (you can see the first arc develop across one 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. Or, perhaps the connected line has tripped out of service allowing the arc to extinguish. 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.

I hope you enjoyed the show.