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Old 12-21-2017, 03:29 PM   #1
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Default Questions about arcing

Hi, Im a brand new apprentice as a power plant operator. At the plants there are substations and switchyards so I figured I might get the answers im looking for on here. One question in my bookwork that Im having a hard time finding the answers to online and don't have any of my own reference material:

what are some of the results you would notice from arcing on:
Bus
insulators and bushings
Line

thanks
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Old 12-21-2017, 03:54 PM   #2
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Burn marks like when welding.
Black marks soot
Discoloration
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Old 12-21-2017, 04:27 PM   #3
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corona
voltage sags
explosions
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Old 12-21-2017, 04:31 PM   #4
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Lights go out and your computer goes BEEP.

Burn marks, crackling marks that has a pattern like a lighting strike. Sometimes you can actually smell the ionization of the air.

If you know what " normal" looks like, then anything that is not normal. If you hear sizzling, don't investigate where it is coming from, get out to a safe distance and let someone know right away. Don't let them enter the area either.

Look up arc flash on youtube and be safe.

Tim.
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Old 12-24-2017, 10:19 AM   #5
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Just saying "corona" doesn't say what it looks like. The most obvious is when you hear all the buzzing and crackling racket, and you see purplish glows and arcs at night.

The initial effect is that you see a whitish powder residue on the insulated cabling concentrated where two difference phase unshielded cables get too close to each other or where they get too close to a grounded metal surface. Insulated degrading starts out as whitish residue and gradually turns black as the insulation burns back and finally starts flashing over. Often hidden corona damage between phases starts with just the smallest amount of whitish ring and then when you peel apart the cables you see the black spot where it arced over deeper in.

If you do a lot of medium/high voltage work, you kind of eventually have a sixth sense about this...you can spot workmanship issues that will cause corona failures a mile away. But sometimes it is hidden such as if you have a molded termination with an internal air pocket that slowly eats the connector up...nothing easy to spot there.
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Old 12-24-2017, 11:56 AM   #6
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Quote:
Originally Posted by paulengr View Post
Just saying "corona" doesn't say what it looks like.......
i knew someone would take that and run with it.

nod,nod, wink,wink
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Old 12-24-2017, 05:19 PM   #7
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i knew someone would take that and run with it.

nod,nod, wink,wink
OK, better yet the most obvious way to tell is to look for the letters "GE" on a label on the outside of the enclosure. Is that simple enough to spot?
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Old 12-24-2017, 05:55 PM   #8
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Here is an example of a "work in progress" in terms of a cable that is rapidly working its way towards self destruction and well past the "white powder" stage.

https://photos.app.goo.gl/rbe2fMOwlGLxLLRr1http://

This was on a starter replacement job. All 3 phases were terminated exactly the same way and all 3 phases plus a ground were run in the same conduit. Strangely the shield was cut back on the MV-105 for the ground which would have drastically reduced the damage and in my experience pushed the failure out to 10+ years. Of course using MV-105 for a ground is also kind of strange but not the first time I've seen that either. At one end the cables were landed as shown in the photo on a disconnect and at the other end they were terminated onto lugs on tubular bus bar in an old school-style overhead substation. This is only 4160 by the way.

I haven't asked for permission so I won't say where I took these photos but brand new MV-105, 5 kV was installed during the summer. Instead of terminations or even at a minimum cutting the shield back, the ends were simply trimmed like a 600 V cable and then the insulation was taped up with 3M 33 or 88. After six months...I'm shocked it didn't trip anything on ground fault yet. The amount of burned cable was about a foot on both ends. We started by just opening it up to get back to good cable, then started hacking off pieces because the first 6 inches was all damaged hoping we wouldn't run out of cable. The shields were so close to the mechanical lugs that for all intents and purposes it's basically direct contact at 4160. Technically the shields were floating but that's splitting hairs. So instead of having all the voltage stress going across the EPR between the shield and the conductor, effectively it moved all the voltage stress out to between the shield and the outer jacket with no shielding at all, and really not much in the way of EPR either.

Added several hours of work to a job since we had to reterminate all 3 cables at both ends along with making a run to an electrical supply house (over an hour away) that carried crimped lugs so that we could do the whole thing right. Whenever it comes to these kinds of jobs I find I always have to budget a lot of extra time because we always seem to find something like this that adds a lot of time onto a 'simple" job.

Here is another example "courtesy" of GE which was a more severe work in progress that was another failure just around the corner:

https://photos.app.goo.gl/rbe2fMOwlGLxLLRr1

This is on a starter built by GE where they ran Exar hookup cable through a ground CT with the smallest window I think they could find at 7200 V where it was reported that it was failing megger tests on a restart. The electricians spent literally days drying everything out but it got to around 100 kohms and they could go no further in terms of insulation resistance. There were white powder residues everywhere on almost every connection because all the terminations simply had the shielding cut back or they used unshielded 15 kV hookup wire for everything but different phases were lashed together or lashed to grounded metal cable supports instead of using glastic and keeping things separated like the way this construction technique is typically done. At first all you could see at the CT was the whitish powder residue that is all over all 3 phases where they come through the ground CT. It's only once I grabbed them and pulled them out and peeled them apart that the extent of the damage became a lot more obvious.

There was a lot of cable here so we were able to pull it out and trim the bad ends off and then put it back in. If this wasn't a short term project (equipment was only going to run another year or two), the practical option would be to replace the single CT with three separate ones or replacing it with a single larger one potentially with shielded cables and running the shields back through the CT before connecting the drain wires to ground.

So just a couple examples to think about. The problems are a lot less when you get out around the GSU/stepup subyard because everything becomes air insulated so it's just a matter of inspecting insulators for damage and outdoor equipment typically has the benefit of a lot of space to work with. But indoors where everything is tightly confined, failures are common and mostly stem from shoddy design and installation practices.
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Old 03-18-2018, 05:52 PM   #9
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blackish blue color to the metal where the bad connection is.
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Old 11-24-2019, 04:37 PM   #10
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blue colour metal
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Old 11-24-2019, 05:29 PM   #11
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In a sub 25kv or higher you will hear it way before you see it which is why they inspect with ultra sound.

Normal places that you expect to find it is on CT and Pt casings (looks like a snail trail down the outside of the plastic) and on cable terminations points (arc or corona).
Also not all arcing is caused by faulty electrical equipment. I have seen more arcing/events due to foreign objects (birds, snakes, soot from burning) then from bad terminations.

Insulators pop and snap all the time especially if it hasn't rained in a while and they are dirty. They pop and snap even worse during the rain and fog.
Its hard to see how bad the arcing is until the system is shut down unless it gets to the point where you can visually see it from the ground, then its time to think about running.

Im sure they have taught you not to touch/learn against frame work while in the sub as the frame work is technical part of the system especially when it comes to dealing with arcing.

A good sign of a major arc event is when you find a 6' oil filled insulator missing and its not inside the yard.

Last edited by gpop; 11-24-2019 at 05:32 PM.
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Old 11-24-2019, 06:03 PM   #12
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Not an easy question. Depends on your situation. Ac vs DC voltages and if the system is encapsulated with SF6 to prevent arcing. Once an arc is established it is more prone to follow the path. You can clean up the effective area but pitting can occur on metal surfaces, blackening ceramic surfaces, patina color changes. At high enough voltages, it’s more economical to bypass safety interlocks an let the failure cause damage the the system to locate point of failure.
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Old 11-24-2019, 06:10 PM   #13
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Quote:
Originally Posted by paulengr View Post
Here is an example of a "work in progress" in terms of a cable that is rapidly working its way towards self destruction and well past the "white powder" stage.

https://photos.app.goo.gl/rbe2fMOwlGLxLLRr1http://

This was on a starter replacement job. All 3 phases were terminated exactly the same way and all 3 phases plus a ground were run in the same conduit. Strangely the shield was cut back on the MV-105 for the ground which would have drastically reduced the damage and in my experience pushed the failure out to 10+ years. Of course using MV-105 for a ground is also kind of strange but not the first time I've seen that either. At one end the cables were landed as shown in the photo on a disconnect and at the other end they were terminated onto lugs on tubular bus bar in an old school-style overhead substation. This is only 4160 by the way.

I haven't asked for permission so I won't say where I took these photos but brand new MV-105, 5 kV was installed during the summer. Instead of terminations or even at a minimum cutting the shield back, the ends were simply trimmed like a 600 V cable and then the insulation was taped up with 3M 33 or 88. After six months...I'm shocked it didn't trip anything on ground fault yet. The amount of burned cable was about a foot on both ends. We started by just opening it up to get back to good cable, then started hacking off pieces because the first 6 inches was all damaged hoping we wouldn't run out of cable. The shields were so close to the mechanical lugs that for all intents and purposes it's basically direct contact at 4160. Technically the shields were floating but that's splitting hairs. So instead of having all the voltage stress going across the EPR between the shield and the conductor, effectively it moved all the voltage stress out to between the shield and the outer jacket with no shielding at all, and really not much in the way of EPR either.

Added several hours of work to a job since we had to reterminate all 3 cables at both ends along with making a run to an electrical supply house (over an hour away) that carried crimped lugs so that we could do the whole thing right. Whenever it comes to these kinds of jobs I find I always have to budget a lot of extra time because we always seem to find something like this that adds a lot of time onto a 'simple" job.

Here is another example "courtesy" of GE which was a more severe work in progress that was another failure just around the corner:

https://photos.app.goo.gl/rbe2fMOwlGLxLLRr1

This is on a starter built by GE where they ran Exar hookup cable through a ground CT with the smallest window I think they could find at 7200 V where it was reported that it was failing megger tests on a restart. The electricians spent literally days drying everything out but it got to around 100 kohms and they could go no further in terms of insulation resistance. There were white powder residues everywhere on almost every connection because all the terminations simply had the shielding cut back or they used unshielded 15 kV hookup wire for everything but different phases were lashed together or lashed to grounded metal cable supports instead of using glastic and keeping things separated like the way this construction technique is typically done. At first all you could see at the CT was the whitish powder residue that is all over all 3 phases where they come through the ground CT. It's only once I grabbed them and pulled them out and peeled them apart that the extent of the damage became a lot more obvious.

There was a lot of cable here so we were able to pull it out and trim the bad ends off and then put it back in. If this wasn't a short term project (equipment was only going to run another year or two), the practical option would be to replace the single CT with three separate ones or replacing it with a single larger one potentially with shielded cables and running the shields back through the CT before connecting the drain wires to ground.

So just a couple examples to think about. The problems are a lot less when you get out around the GSU/stepup subyard because everything becomes air insulated so it's just a matter of inspecting insulators for damage and outdoor equipment typically has the benefit of a lot of space to work with. But indoors where everything is tightly confined, failures are common and mostly stem from shoddy design and installation practices.

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