This is a 3 phase 500KV motor operated disconnect that is NOT intended to be opened under load. There is a switcher that is designed to open under load in the circuit and then the disconnect opens. In this case, one phase of the switcher failed to open, resulting in one phase of the disconnect opening under load. The resulting arc was truly spectacular. The reason that this event was caught on camera is because this particular switcher had failed to open in the past so every time the switcher was opened, a camera has been set up. This time they got the failure on tape.

Bert Hickman at supplied the following description of the events: This is NEW the record holder for the world's biggest Jacob's Ladder!

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 switcher attempting to open the high voltage being supplied from a 94 mile long section of transmission line to a large three phase shunt line reactor. The line reactor is the huge transformer-like object behind the truck at the far right at the end of the clip. Line reactors are large iron core coils which are used to compensate for the effects of line capacitance on long extra high voltage (EHV) transmission lines. Internally, each phase of the reactor is connected through a large coil to ground. Each coil within the reactor can provide 33.3 Million Volt Amperes of compensating inductive reactance (MVAR) at 290 kV between each phase to ground . The power company had previously encountered difficulty interrupting one of the three phases when trying to disconnect the line reactor. Their power engineers set up a special test so that they could videotape a switching event, and they made arrangements to "kill" the experiment, if necessary, by manually tripping upstream circuit breakers.

This particular switcher uses gas filled switching elements, called "gas puffer" interrupters. These are located just to the right of the rotary air break switches. The actual switching elements of these interrupters are hidden inside the gray horizontal insulators (bushings). The switching elements are housed within sealed "bottles" filled with a special insulating gas (sulfur hexafluoride, SF6) under high pressure. SF6 helps to rapidly extinguish the arc that's created when the high voltage circuit is broken. During normal operation, the switcher will first open the SF6 interrupters which disconnects the HV circuit so that the air break switches can open with no current flowing. Once the air break switches completely rotate to the "open" position, the SF6 interrupters then reclose. Normally, this sequence insures that the air break switches operate de-energized and arc free.

These gas puffer interrupters use two SF6 bottles that are connected in series, since it takes two switches to withstand the high voltage stress. In this video, one of the pairs of SF6 interrupters is defective and it fails to open. This places the entire voltage stress across the remaining good interrupter. As the good one valiantly tries to open the inductive load, it creates a high voltage surge that causes the bushing of the good interrupter to flash over. The initial flashover can be seen arcing across the horizontal interrupter bushing at the very beginning of the video clip. Since the affected phase remains energized (through the flashover arc), the air break switch begins to open "hot" (still energized). It continues arcing as the switch rotates 90 degrees to the fully "open" position. Once the air break switch reaches the fully open position, the SF6 interrupters then reclose. Although this extinguishes the horizontal arc across the good interrupter's bushing, the arc across the air break switch persists, continuing to grow and creating a potentially dangerous situation.

The arc stretches upward, driven by rising hot gases and writhing from small air currents, until it easily exceeds 100 feet in length. Switching arcs usually terminate long before reaching this size since they normally flash over to an adjacent phase or to ground. Once this happens, the phase-to-phase fault current will cause an upstream circuit breaker to trip, disconnecting the circuits. A phase-to-phase arc can be seen at the very end of the previous 230 kV air break switch video, just before the resulting short circuit trips an upstream Oil Circuit Breaker (OCB). Since this 500 kV arc was in open air and was sufficiently removed from adjacent phases, it could have persisted for quite some time. To avoid risking further damage to their equipment, the utility manually commanded an upstream circuit breaker to open, abruptly extinguishing the arc. After this event, it was determined that both SF6 switch bottles in the affected phase had sustained permanent damage. The bottles were sent back to the manufacturer for analysis to determine the root cause of the problem. Loss of pressurized insulating SF6 gas inside one of the interrupter bottles is suspected as the root cause of the switching failure.

As impressive as this huge arc may be, the air break switch was really NOT disconnecting a real load. This arc was "only" carrying the relatively low (about 100 amps) magnetizing current associated with the line reactor. The transmission line associated with the above circuit normally carries over 1,000 megawatts (MW) of power between Boulder City, Nevada (from the generators at Hoover Dam) to the Lugo substation near Los Angeles, California. A break under load conditions (~2,000 amps) would have created a MUCH hotter and extremely destructive arc. Imagine a fat, blindingly blue-white, 100 foot long welding arc that vaporizes the contacts on the air break switch and then works its way back along the feeders, vaporizing them along the way. Still, you've got to admit that this "little" 33 MVAR arc is certainly an awesome sight!

And, who says utility guys don't have any fun - just listen to lineman on the right "whoop" at the end of the clip!

Thanks to Sylvia Gallus for also refering me to this information.

This is part of the fun of this site. I also receive an update from Neil Brady who was on the scene.

You're right about the Lugo Switcher on our 500KV line having problems. This was our first time trying to record the problem though. The 500KV line was isolated from the system and we fed from our 500KV bus in order to have control of de-energization by circuit breakers, without any interruption of power to the 500KV grid and jeopardizing the system. I was the maintenance foreman at Eldorado Substation, Boulder City, NV. when this ocurred and this is my video that I shot. The maintenance crew was at the truck (no linemen) and one of them is yelling at the end because the arc was so spectactular.

P.S. We HAD to open the 500KV CB to de-energize the arc. It would not extinguish by itself.

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