Failure of 11kV Cable 1

[briand2]

I've just had a failure of an 11 kV cable in the connection box to the rear of a VCB. The cable is three-core, XLPE, armoured. The failure occurred after the point at which the cores had been separated. It looks to me like one of the cores was bent at too tight a radius; this is the only core for which the insulation was blown apart, at the bend. Does anyone have comments as to whether or not a tight bend is likely to damage the primary insulation and / or semi-conductive layer around the individual core, leading to failure? The duty is an indoor substation, so environmental conditions are not likely to have played any part in the failure.

Thanks in advance for you comments.

Regards,

Brian

 

[jezz1]

hi briand, most xlpe failures are down to poor termination of the semi conductive layer.
the semicon layer needs to be removed with special care and use of correct tooling .
whats the history of the cable ?

 

[busbar]

Depending upon interest level, the cable manufacturer and termination-kit manufacturer may be contacted to obtain installation recommendations that could be used to demonstrate incompetence in cable installation.

 

[cuky2000]

To protect the insulation integrity cable manufacturer recommend not exceed the "Minimum Bending Radii". For armored MD cable or individual conductor this radio is several time the cable diameter. For specific recommendation, see the enclose information

http://www.okonite.com/engineering/bending-ratios.html

 

[Rodmcm]

Yes we have seen this problem. If the join is not dressed correctly then a corona starts between phases that leads to the breakdown of the cable. Generally due to the split of the single cores not being dressed with enough clearance.

 

[briand2]

Thanks to all of you for your input.

It turns out that although the bending radius was indeed a little tight, the main cause of the failure was that the installing contractor "nicked" the primary insulation when preparing the connection!

Regards,

Brian

 

[jbartos]

Comment: Actually, the bending radii of conductive and semiconducting surfaces or geometries are subject to corona if they are too sharp.

 

[INNJunEER]

Well that would explain it. I know that radius is very important but the termination quality would be so by a much higher factor.

What is the warranty from the contractor? This is very exspensive for someone to eat.....

 

[briand2]

We normally call our electricity supplier ask him to explain why he's caused us a problem, this time we had to ring him and admit that we'd been the cause of his problem!! No warranty, as the cable had been installed for two or three years!

Regards,

Brian

 

[busbar]

Aside from the widely circulated industry buzz, I believe an experienced tester birdogging leakage current with reasonable caution during DC-overpotential testing will detect medium-voltage shielded-cable dielectric nicks in a heartbeat.

 

[busbar]

But, argue the “safe” service-aged procedures until the cows come home.

 

[jbartos]

Suggestion: Reference:
Dr. Professor Dean C.L. Wadhwa "Electrical Power System," John Wiley & Sons, 1991, Chapter 6 Corona, pages 135-149.
Method of Reducing Corona (Loss)
1. Large diameter conductors,
2. Hollow conductors, and
3. Bundled conductors.

 

[benlanz]

The only test that is going to detect workmanship errors of this nature is a partial dicharge test. You can cut half way through the insulation on a XLPE cable and it will clear a DC HIPOT test. If you apply a PD test it will be simple to locate.

 

[briand2]

Benlanz,

Thanks. That's what I was thinking of doing for the other cables that were installed at the same time as the one that failed...I'll post results when I have them!

Regards,

Brian