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!
Find more details and some pretty
amazing videos by
clicking here.
