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Old March 28th, 2009, 01:39 PM
Doug Kerr Doug Kerr is online now
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Default Electrical transmission gear - part 1

Inspired (or provoked) by the comments on my recent photo of the voltage regulator, I thought I would exhibit some shots I took on a similar errand about two years ago - again to come to understand some apparatus I had seen but not understood.

The shots were taken at the White Rock electric substation in East Dallas. Carla and I had gone there to photograph a colony of Monk Parakeets that had built an elaborate community among the ironwork at the substation. I'll post those shots a little later (although we do get to see the little critters here).

While I was there, I was interested in some apparatus that was standing on the ground, evidently ready to be installed. I took some shots of nameplates and such, and after a bit of research came to understand the story. I'll summarize it for you here as I show the pictures.

***********
The disconnects used to isolate paths in high-voltage transmission lines essentially use a "knife switch" concept. They cannot be opened (shut of) while the line is energized. In such a situation, when they begin to open, an arc jumps the initial gap between the blade and the jaw. This arc creates a plasma path (conductive superheated air), which will gladly extend the entire distance of the gap to which the switch eventually opens, keeping the current flowing and generating enormous heat that can well destroy the switch and its supporting infrastructure.

Here we see two installations of these at the White Rock substation. The ones on the right are the traditional kind. Those on the left have a special attachment I will be describing shortly (the set to the right would have the same thing shortly, which is why there were pieces of apparatus on the ground). These power lines operate at a nominal voltage of 138 kV (phase to phase; the lines are three-phase).



Here we see one disconnect (the older type) close-up. We also see an outpost of the Monk Parakeet colony, evidently undergoing some reconstruction. The second insulating bushing from the right is turned to operate a mechanism at its top that lifts the blade to open the circuit.



The matter of a plasma arc if such a disconnect switch were to be opened with the line energized is amply illustrated by this video.

http://vids.myspace.com/index.cfm?fu...ideoID=1587733

This test was conducted on a disconnect on a 500 kV line. One leg was intentionally left "live". As we can see, as the disconnect opened (this kind opens to the side), the plasma arc formed, and continues as the switch reaches its fully open position.

The plasma arc rose into the air as a result of the hot plasma being lighter than air. This might have eventually actually extinguished the arc, but not before severe damage would have ensued.

(In fact for operation at only a few hundred volts, we depend on that behavior to solve the problem.)

The phenomenon was terminated by the test conductor by tripping off an interrupter at the source of the line (several miles away). Otherwise, both the test switch and the surrounding infrastructure would have probably been completely destroyed. One observer estimated the total path length of the arc just before extinction at over 100 feet.

[continued in part 2]
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Old March 28th, 2009, 01:42 PM
Doug Kerr Doug Kerr is online now
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Default Electrical transmission gear - part 2

[continued]

As the network management plans of power distribution companies evolve, it often becomes useful to have a certain switching station, formerly used only to rearrange the network with parts of it "dead", be able to open paths while they are energized. (There may well have been a dual path to the loads at the time.)

In the past, this would have mean replacing the "knife switch" disconnects with actual circuit interrupters. At the voltages and currents involved, these are gigantic pieces of apparatus, housed in large transformer-like cases and weighing many tons.

However, a recent development allows existing disconnect installations to be retrofit so they can be opened under load.

The interrupter component that is added has an electrical contact that can only be opened to a modest gap (several inches). It is housed in a cylinder filled with sulfur hexaflouride gas under pressure. This is an evil substance with wondrous electrical insulting properties. Even a very high voltage is unable to start an arc through even a modest gap filled with this gas. And if a contact immersed in the gas opens under load, the initial arc is quickly extinguished by the gas.

In an "SF6" (based on the chemical formula for sulfur hexaflouride) interrupter of this type , the contacts and circuit paths do not carry the any current under normal conditions. They only have to do that for a brief time during an "interruption" event. Thus they can be made relatively small, with the result that each interrupter module is of reasonable size (the ones we will see here are about the size of the average Cherokee copy editor.)

Here we see one in place, attached to an otherwise-conventional disconnect switch. (It is actually a new switch, as the company wanted to upgrade these while the interrupters were being added.



Here we get a better view of the interrupter.



The "B" terminal of the interrupter contact is connected to the jaw side of the disconnect switch. The "A" contact is reached via a swinging conductive arm extending from the "head" of the interrupter; this arm also mechanically operates the interrupter contacts.

With the disconnect switch closed, the interrupter operating arm rests on a little roller rest, electrically connected to the bottom of the interrupter and thus to the disconnect jaw, so there would be no voltage flow through the contacts. With the arm in this position, the contacts inside the interrupter are closed.

When the disconnect switch begins to open, a "lifter" roller (conductive), mounted on the blade, contacts the interrupter operating arm from beneath. But no substantial current yet flows through the interrupter, as the disconnect blade is still in contact with its jaw.

But as soon as the disconnect blade comes clear of the jaw, the full load current passes from the disconnect blade through the lifter into the interrupter arm, into the interrupter, though its contacts (still closed) and back to the jaw frame, to go on along the transmission line. Because of this alternate path, no arc at all forms at the tip of the disconnect blade.

As the disconnect blade moves upward, the interrupter operating arm moves as well, cocking a spring inside the head of the interrupter. At a certain point in the movement of the arm, a mechanism is tripped allowing the spring to rapidly open the interrupter contacts. This opens the circuit path to the downstream part of the transmission line (the real object of this exercise).

The presence of the sulfur hexaflouride means that only a small arc ensues, which is quickly extinguished.

A very clever solution to a practical need.

Best regards,

Doug
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Old August 10th, 2009, 07:50 PM
RoyVarley RoyVarley is offline
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Thanks for that, Doug. Nice shots. Great explanation. I was wondering why they couldn't just use an SF6 interrupter alone instead of in combination with the switch - safety issue? Certainty of disconnect? Also, I checked the msds for SF6 (curious) and it's a non-toxic asphyxiant - so extremely dangerous in confined space but essentially harmless if released to atmosphere - not too evil, then?
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Old August 10th, 2009, 08:29 PM
Doug Kerr Doug Kerr is online now
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Hi, Roy,

Quote:
Originally Posted by RoyVarley View Post
Thanks for that, Doug. Nice shots. Great explanation. I was wondering why they couldn't just use an SF6 interrupter alone instead of in combination with the switch - safety issue? Certainty of disconnect?
Doing that would require the SF6b interrupter to be able to carry the full load current on a continuous basis, which would then require the current-carrying members and the contacts proper to be larger in cross-section, which would then increase the bulk of the assembly, and thus require it to be filled with a greater volume of SF6, etc. etc.

Quote:
Also, I checked the msds for SF6 (curious) and it's a non-toxic asphyxiant - so extremely dangerous in confined space but essentially harmless if released to atmosphere - not too evil, then?
Oh, indeed!

Thanks.

Best regards,

Doug
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Old August 31st, 2009, 01:39 PM
Tom Robbins Tom Robbins is offline
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Doug,

This post haunted me as I happened to look upward while photographing something totally unrelated in a rural location today. My photographs aren't as dramatic as yours, but the disconnects were noticed only because you took the effort to point out their existence earlier.



One of three lines, each mechanically ganged together. The mechanical linkage terminates in a locked cabinet at the base of one of the supporting poles.

Tom
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Old August 31st, 2009, 02:22 PM
Doug Kerr Doug Kerr is online now
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Hi, Tom,

Quote:
Originally Posted by Tom Robbins View Post
Doug,

This post haunted me as I happened to look upward while photographing something totally unrelated in a rural location today. My photographs aren't as dramatic as yours, but the disconnects were noticed only because you took the effort to point out their existence earlier.
Thanks for the nice shot.

Best regards,

Doug
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