High Voltage-Ouch! This stuff really can kill you!

This is a 1.6 MB movie showing what happens when someone at TVA pulled the plug on PlanetChristmas. Click here to see the movie!

Real Story:

The video clip shows a three phase air disconnect switch attempting to open the high voltage supply to a large three phase 33 Million Volt Ampere Reactive (MVAR) shunt line reactor. The reactor is the huge transformer-like object behind the truck at the far right at the end of the clip. The clip was believed to be part of the 500 kV Lugo substation near Los Angeles, California. Line reactors are large 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 the line reactors and had set up a special test to videotape, and hopefully isolate, the problem. If you look carefully, you can see evidence of previous arcing – notice the blackened horizontal bushing (insulator) just behind the bushing that arcs at the beginning of the MPEG.

Normally, pressurized sulfur hexafluoride (SF6) gas “puffer” interrupters, just to the right of the air break switches, will first de-energize the circuit so that the air break switches can the open with no current flowing. The actual switching elements for the interrupters are hidden inside the horizontal bushings. However, as the interrupters open, a high voltage switching surge causes one of the interrupter bushings to flash over. Since this phase remains energized, the air break switch for this phase opens “hot”, and it continues arcing as the switch swings to the fully “open” position. The arc continues to grow upward, driven by rising hot gases and writhing from small air currents, until it easily exceeds 50 feet. Long arcs usually terminate before by connecting to an adjacent phase or to ground, causing a detectable fault which then trips out the circuit. This particular arc could have persisted for quite some time, but the utility manually commanded an upstream Oil Circuit Breaker (OCB) to open, abruptly extinguishing it.

As impressive as this may be, the air switch was NOT disconnecting a real load. The arc is “only” carrying the relatively low (perhaps ~20 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 California. An actual break under normal loading conditions would have been MUCH hotter and extremely destructive. Imagine a fatter, 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 have to admit that this “little” 10 million Volt Ampere reactive arc is still pretty awe inspiring!

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