SWBD cable insulation deterioration

[rovineye]

During a recent cleaning, we found some internal cabling that had insulation issues (see file). The cabling is from the load cables to the contactors (Cutler-Hammer), voltage level is 6.6 kV, the conactors are pretty much closed all the time so no frequent load spikes to speak of, and loading is well below capacity of these conductors. Initial assesment is low energy electrical arcing, corona discharge, and tracking as a reasult of no outer
semi conducting shield. These cables intended use is listed as temporary installation around damaged cables or for temporary power sources. They come with the cubicles, so I never gave them a second thought. Until now!

 

[ScottyUK]

Looks like PD activity caused by excessive voltage stress across a non-linearity in the insulation such as the 'rubber'-air-'rubber' interfaces. It will turn nasty at some point with an accompanying ka-boom sound.


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If we learn from our mistakes I'm getting a great education!

 

[oldfieldguy]

You caught it in time. Scotty's got the cause nailed down. Unshielded cable at this voltage will OFTEN develop these patterns as surface charge is concentrated in small areas where electrical stresses are high. That's why your pictures show it where cables touch one another. Ozone is generated by the discharge and attacks the insulation producing the characteristic white coating.

old field guy

 

[rovineye]

Thanks guys; as I thought. Yes, thankfully (luckily?) it was caught in time. Now I need to convince others to halt operationa and get theirs inspected and replaced right away. I have suitable cable identifed for replacement, but now I have to deal with extra room needed for the end terminations. OEM response = no one else is having these issues!

 

[ppedUK]

That probably ranks amongst one of the worst MV starter panels I've ever seen. As well as what the others are saying with regard to the P.D. problem, I would also be looking very carefully at the routing and fixing of the LV wiring. In your 3rd photo, the CT secondary wiring looks dangerously close to the 6.6kV terminations.

At 6.6kV, I would expect to see separated busbars on insulated pillars (not zip tied rubber insulated cable) and would also expect to see Low Voltage insulation level CTs mounted on earthed bushings.

It's almost amusing to see the MV cabling held together with zip ties. As well as the discharge between the conductors, I can also envisage that under fault there will be plenty of cables flailing around in that cubicle! Sack the company who built it. You are lucky it's lasted this long.

 

[ScottyUK]

ppedUK,

It's ok, it's 60Hz electricity in that panel. Not as dangerous as 50Hz.


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If we learn from our mistakes I'm getting a great education!

 

[jimbofitz]

Rovineye,

Out of curiosity, how long have those cables been in service at that location? I'm curious how long that corona has been attacking the insulation because that cracked piece is pretty scary looking.

Regards,
Jim

 

[rovineye]

These are standadr Cutler-Hammer Ampgards. This is their "older" version, in service only about 2 years. The worst thing about these is the LV drawer that is all wobbley when retracted and components bang into structure. That is fixed in their newer version. But that is about all that changed.

Good point about the LV wire near the termination, but know that the insulating boots have been removed from the 6.6 kV terminations as shown laying down in the first photo.

 

[rovineye]

Now that I checked, this board has been in service about 3 and a half years. Time flies...

 

[ppedUK]

rovineye,

I spotted that the red shrouds were off the terminations, but they look equally as naff as the wiring and I wouldn't trust them not to fall off. Interesting to hear the OEM's response, I wish I had £1 for every time i've heard that one, I wouldn't be sat here now! Even the big boys make some iffy gear from time to time.

Just out of curiosity, is this just the cable termination chamber shown in your photos? If so, is there an earthing truck or something that goes in there? How is the cable earthed in that gear?

Scotty,

What's that 60Hz business? Sounds like science fiction. It'll never catch on!

 

[rovineye]

This is the contactor section. The bottom stabs can be seen just behind those removed boots which is where load cables are attached. The vacuum contactor slides in and then above a fused disconnect slides in to mate thru the shutters you can see near those CTs.

That disconnect has an earthed position, and once earthed the contactor is closed to short and earth the load side. Now the part that really gets me going. The path to earth from the disconnect which is now shorted, is from the disconnect shorting bar to the panel side sheet via sheet metal screws that just happen to be there to hold the works together! And the same from that point all the way thru the switchgear structure! Unbelievable. I raised hell for so long about that (and I am sure I wasn't the only one) they finally changed that in the new configuration and have a dedicated ground wire at that shorting bar.

60 Hz just makes the work day go faster, since I use 50Hz master clocks.

 

[ScottyUK]

The circuit earth is through a contactor? So if you blow a control fuse you lose the circuit earth, and one day someone ends up dead? That sounds like either a really bad design, or I've just mis-understood your description. The earthing arrangements you describe would be comical if they didn't have scope to get people hurt or killed.

Remind me to never, ever, buy anything made by Cutler-Hammer. I'd had about enough of Eaton's sh¡t when they owned Ottermill in England, probably the worst motor control centres in British history but this stuff looks just as bad and it is working at MV.


I wonder if we have 60Hz clocks here?


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If we learn from our mistakes I'm getting a great education!

 

[rovineye]

ScottyUK,
If control power is lost, I would not be able to close the contactor to earth. If it is already earthed its OK (contactor latched).

A minor related nuisance is in the control room mimic I can't tell the difference between a contractor closed normally to the load and one closed to earth. It really doesn't matter much in my installation, but it could in other applications.

 

[jghrist]

It also looks like the cable is zip tied to a bare bus of another phase for support. This puts Ø-Ø stress across the insulation at a single contact point.

 

[rcw retired EE]

This installation is typical of 80% of the Medium Voltage Motor Controls I've seen in North America in the last 20 years. One difference is most operate at 4,160V or 2,400V and not 6.6 kV. That might explain the low awareness of corona damage by the supplier.

Older MV MCC's were less compact and did have more clearances, buswork instead of flexible cables, and separate cabling and contactor compartments. The push for smaller and cheaper resulted in these minimal space designs. (Other supplier's are worse.) An "advantage" is the equipment is front access only and can go against a wall to save valuable space.

It is unusual for ANSI equipment to have any grounding means for the outgoing cables to the motor or load as is common on IEC or BS equipment.

 

[ScottyUK]

" If control power is lost, I would not be able to close the contactor to earth. If it is already earthed its OK (contactor latched)."

I hoped you were going to say that. I just wasn't confident.


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If we learn from our mistakes I'm getting a great education!

 

[rovineye]

rcwilson,
You are right in that we are using this gear because we can place two sections back-to-back in the same depth as one breaker section.

jghrist,
I don't see that tie down to a bus, but it may be an illusion in the pic that I am seeing thru?

 

[ppedUK]

rovineye

Thanks for clearing that up. That gear really is vile stuff! When I asked the question about the earthing, I did think it looked like the cables were terminated in the starter cubicle, but I hoped I was wrong. And earthing via a contactor...........not great, even if mechanically latched! An earthing device in our 50Hz world generally has to be capable of making onto a full power fault and that generally rules out earthing via contactors!

At least the cables are secured properly in the cubicle in your last photo, but I would imagine you might still get P.D. problems even in that cubicle at the point where the cables come together through the core balance CT.

Stop talking about 60Hz, I told you, it'll never catch on!

 

[rovineye]

ppedUK,
Yes, the load cables are terminated there as well! In the 3rd photo of my first upload, you can see the black load cables terminating to the red "jumpers" just below the CTs. The only thing not in the cubicle is the supply bus, which is joind by the disconnect thru those shutters in the top.

 

[jghrist]

jghrist,
I don't see that tie down to a bus, but it may be an illusion in the pic that I am seeing thru?

On second look, I think the ties are just in front of the buses.