Pen sized hole in PEX insulation at 36 kV

[Skogsgurra]

This is way out of my field of work. Perhaps someone recognises the problem?

I visited a customer today to discuss replacement of old inverters with new ones. When we were finished the guy showed me some stumps of PEX cable where the insulation had failed and the resulting arc (impedance ground) had produced a neat perfectly circular hole with about 6 mm diameter (1/4 inch).

This has happened several times and all expertise say that these cables should withstand about 200 kV transients.

The switchgear is SF6 insulated and uses vacuum contactors. The load is a large furnace transformer for winning chromium fom chromium ore. The failures are not associated with swithing - they just happen.

Does this sound familiar? What could the reason be? Anyone seen this before? What did you do to prevent this failure?

 

[PowerfulStuff]

As a general comment Cross linked polyethylene cables have been known to develop faults when exposed to high DC voltages. These faults do not show themselves until much later when the cable breaks down. Is it possible that the cables are being DC overpotential tested after installation?

 

[Skogsgurra]

Thanks Powerful!

That is very possible. They are carrying 36 kV 50 Hz, but it is possible that they were tested with DC - in fact very probable - when installed and also during maintenance. I will ask the guys involved.

 

[bigbillnky]

Here is a link to an article with some very interesting information concerning cable testing. It seems that the accepted methods of cable testing may now be to blame for future failures.

http://www.emersonprocess.com/home/...tfacts/maintenancefacts0310_hipot-testing.pdf

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[benlanz]

I am familiar with this problem. Most likely the pinholes are due to transisents. DC testing only ecourages a naturally occurring process which 'ages' the cable. I would need to know more in order to advise you further. How old are the cables? Do you have any surge protection on the system?...

-cheers