11KV Paper insulated screened cable faults

[ItAintMe]

Has anyone had experience with paper insulated cables at 11kV failing due to damage of the metalised paper screen? I know screening isn't mandatory for 11kV cables, but if you have it, have you had any experience of faults arising from imperfections or tears in the screen?

thanks

 

[oldfieldguy]

Since nobody else has weighed in, I'll take a shot.

I'm assuming that your "screen" is the same as my "shield", i.e., a conductive layer outside the main insulation of the cable.

The only paper-insulated cable I've used has been paper insulated, lead cased (PILC). Since this package consists of three phase conductors, each with its own shiled, enclosed in a lead sheath, the individual conductor shields are protected.

In dealing with a system using several miles of this cable at 13.8 kV, the only failures I've seen are directly traceable to failure of the lead jacket to keep moisture out of the oil-impregnated paper insulation of the cable. Our problems were usually located at a splice point.

I'm interested in seeing what you find out.

old field guy

 

[isquaredr]

Screened cables of this type in the UK were more widely used on systems of 33kv and above. Having located many faults on this type of cable oldfieldguy is correct that water ingress plays a large part. One other problem is drying out of the papers due to the cables being laid on a gradient, this leads to voids and eventual failure.
How do you know that the screens are damaged ? If inspected after failure what was the condition of the rest of the papers ? How old is the cable ? Was there any signs of treeing ?

 

[ItAintMe]

Yep we're all talking about the same type of cable - PILC. I know it's normally only screened/shielded at 15kV or above, and belted below that, but screens permit a higher rating, which we have been using. We have had a number of failures in this cable due to either damage during installation (knife nicks to both metalised layer and paper insulation) or just to the screen, and some due to manufacturing defects (as the metalised paper is very thin). Just curious to see if anyone else had experienced similar problems not related to water ingress.

My concern is about the damage leading to localised stress concentrations, which lead to eventual breakdown, as happened in the cases I mentioned above.

We have used MIND cables to minimise impregnant migration, and it seems to have worked well - we have cables up to 70 years old still working well, although we now routinely test the papers whenever we open one up, they are mostly indicating they have many years still to go. Wish new cables were this reliable!

thanks

 

[oldfieldguy]

ItAintMe--

You point out correctly the big problem with PILC: installing it is a specialty skillset. My client used a specialty contractor for this work and had a very rigorous testing regimen using high-potential DC.

Testing a new installation required three tests: first, when the cable was pulled into position, second, when the first termination (pothead) was installed, and third when the second pothead was installed. Additional tests were used when splicing was involved. Easch splice or pothead was stamped by the splicer who performed the work.

It was costly, but his premature failures on new installations was proactically zero. Also, his test regimen was so structured as to allow quantitative interpretation of results, giving him a certain amount of predictive maintenance capability on his distribution system.

old field guy