To follow-up the "High Power Arcing" thread, here's yet another
impressive video clip. Below is a description of what you're about to
see. Listen closely at the very end of the clip and you'll hear a big;
"WHOOOO!!!" from one of the other maintenance workers in the yard. It
seems this event even impressed him!
Steve
**************************
This video clip was captured by Neil Brady, the maintenance foreman of
the 500 kV Eldorado Substation near Boulder City, Nevada at the time of
the event. It shows a three-phase motorized air disconnect switcher
attempting to open high voltage being supplied to a large three phase
shunt line reactor. The line reactor is the huge gray transformer-like
object behind the truck at the far right at the end of the clip. Line
reactors are large iron core coils (inductors) which are used to
counteract the effects of line capacitance on long Extra High Voltage
(EHV) transmission lines. Internally, this line reactor has three coils,
one for each phase in the three-phase system. 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. The substation
maintenance crew set up a special test so that they could videotape the
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 (circuit breakers). 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 ("quench") 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 while still
"hot" (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. Since the 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 the upstream circuit
breakers 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 SF6 gas inside one of the interrupter bottles is
suspected as the root cause of the initial 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 94 mile long 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, melting and vaporizing them along
the way. Still, you've got to admit that this "little" 33 MVAR arc is
certainly an awesome sight!
http://www.stevezeva.homestead.com/files/500kV_switch.mpeg
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Louis Hart
Sent: Thursday, July 19, 2007 9:48 AM
To: [log in to unmask]
Subject: Re: [TN] NTC - I wonder how much this job pays?
The IEEE Spectrum magazine had an article with some pictures a few years
ago about this work and high voltage lines in the American west. Aahne
mentioned the helicopter's capacitance. It may be low, with respect to
Earth, since it is up high. Still, it looked like a lot of charge was
being transferred at the hook up. One thing interesting to me was the
arcing when the copter came back to pick him up. Looks like a lot of the
charge on the 'chopper' was dissipated while it apparently flew around
to the point where it picked up the lineman, since some big sparks flew
when it hooked up again. Maybe when it gets back to base, it's safe to
just step off. But I would still want someone with a voltmeter to come
out and check before I got off.
Louis Hart
Technology Transfer Specialist
National Technology Transfer Center
316 Washington Av
Wheeling, WV 26003
304-243-2553
[log in to unmask]
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