35kV Circuit Switcher
This video clip was captured at the 500 kV Eldorado substation near Boulder City, Nevada by power company engineers and maintenance staff. It shows a three phase air disconnect switch attempting to open the high voltage supply to a large three phase shunt line reactor. The reactor is the huge transformer-like object behind the truck at the far right at the end of the clip. Line reactors are inductors which are used to compensate for the effects of line capacitance on long extra high voltage (EHV) transmission lines. The utility was having difficulty cleanly disconnecting one phase on the line reactor and had set up a special test to observe and videotape the test so that they could isolate the problem.
This particular switch uses gas-filled switching elements, called "gas puffer" interrupters, which are located just to the right of the air break switches. These interrupters use switching elements housed in sealed "bottles" that are filled with a special pressurized insulating gas, sulfur hexafluoride (SF6). The SF6 gas helps to rapidly extinguish the arc that's created whenever a high voltage circuit is broken. Normally, the switcher would first open the SF6 interrupters, disconnecting the HV circuit so that the air break switches could open with no current flowing. The actual switching elements for the SF6 interrupters are hidden inside the horizontal bushings.
These particular interrupters connect two SF6 bottles in series in order to share the tremendous voltage stress that's created when the switches are opened. In this video, one of the interrupters fails to open. This places the entire switching voltage stress across the remaining interrupter. As the good interrupter tries to open, the excessive voltage stress causes the bushing of the good interrupter to flash over at the very beginning of the clip. Since this phase now remains energized (through the arc), the air break switch for this phase opens "hot", and it continues to arc as the switch rotates 90 degrees to the fully "open" position. Once the air break switch reaches the "open" position, the SF6 interrupters normally reclose. While this extinguishes the horizontal arc across the interrupter bushing, the arc across the air break switch persists - a very dangerous situation.
The arc continues to grow upward, driven by rising hot gases and writhing from small air currents, until it easily exceeds 50 feet. Switching arcs usually terminate long before reaching this size since they normally will "connect" to an adjacent phase or to ground. Once this happens, it creates a detectable fault which will trips out (disconnect) the circuit. This can be seen at the very end of the previous 345 kV air break switch video. Since it was in open air and far removed from adjacent phases, this particular arc could have persisted for quite some time. To avoid potentially damaging the equipment, the utility manually commanded an upstream Oil Circuit Breaker (OCB) to open, which abruptly extinguishes the arc. After this event, both bottles in the affected phase were damaged and are being analyzed to determine the root cause of the problem.
As impressive as this may be, the air switch was NOT disconnecting a real load. This arc was "only" carrying the relatively low (~100 amp) magnetizing current associated with the line reactor. The 94 mile transmission line associated with the above circuit normally carries over 1,000 megawatts (MW) of power between Nevada and Los Angeles, California. An actual break under normal load conditions would have been MUCH hotter and extremely destructive. Imagine a fat, blindingly blue-white, 100 foot long welding arc that would vaporize the contacts on the air break switch and then work its way back to the feeders. But, you've still gotta' admit that this "little" 50 million Volt Ampere Reactive (MVAR) arc is pretty awesome!
And, who says utility guys don't have any fun - just listen to lineman on the right whooping it up at the end of the clip!