Refurbished cable testing questions
Refurbished cable testing questions
(OP)
I have a few questions for the MV cable experts out there regarding compliance testing of shielded MV cables. We have a mobile substation in the U.S. with 9 - 500MCM - 15 kv shielded cables, 3 per phase. These are fed from the mobile substation by 6 bare copper (2 per phase) 500MCM cables. Cables are EPR with stress cones on either end. After less than 2 months in the field, one of the stress cones on C-phase had failed on the far side and another stress cone also on C-phase at the mobile sub end was very near failure. We saw a significant amount of adhesive (mastic?) coming from several of the other stress cones. The manufacturer has agreed to replace all of the stress cones on the cables.
Here's my question: What type of compliance testing should we be doing to ensure that these cables will not fail again?
The vendor says that they do an AC HiPot test to 31.5kv for 5 minutes and a DC 31.5kv HiPot withstand test for 30 minutes. I did the AC HiPot on the cable that was near failure and it passed, thus I feel that this test is inadequate. Looking at several references they suggest only a HiPot and a PD test. I'm not too certain we have the capabilities to perform the PD test.
Thank you in advance.
Here's my question: What type of compliance testing should we be doing to ensure that these cables will not fail again?
The vendor says that they do an AC HiPot test to 31.5kv for 5 minutes and a DC 31.5kv HiPot withstand test for 30 minutes. I did the AC HiPot on the cable that was near failure and it passed, thus I feel that this test is inadequate. Looking at several references they suggest only a HiPot and a PD test. I'm not too certain we have the capabilities to perform the PD test.
Thank you in advance.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.






RE: Refurbished cable testing questions
http://www.eng-tips.com/viewthread.cfm?qid=248906
RE: Refurbished cable testing questions
I am having issues with stress cones overheating and then failing. The cable insulation is ok. The only thing that I can see is to do an off line PD test. Is this my only other option?
The vendor has mentioned using a thermal camera during the AC HiPot tersting and looking for a hot spot. Do you think this would be sufficient? I am highly uncertain of this test.
Unfortunately we do not manufacture cables here at this facility and have no experience doing PD testing on cables. We also need to get this unit back in service in 2 weeks.
Thanks again.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Refurbished cable testing questions
Www.imcorp.com
Www.advtest.com
RE: Refurbished cable testing questions
RE: Refurbished cable testing questions
RE: Refurbished cable testing questions
We sent the stress cones out for evaluation and I have not heard back yet. Attached are the photos taken from the job site. I'd like to hear your comments.
Thanks again
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Refurbished cable testing questions
In our case, no amount of testing would have found the problem when the cable was put into service, as the faults developed only after several weeks of running.
In our case there was nothing wrong with the termination kit, but we concluded that constant force springs don't like circulating currents and we changed the method of termination on later installations.
RE: Refurbished cable testing questions
The panel frames at the load end have to be grounded according to code obvoiusly at some point, but the cable shield/braids should not be that ground path. A separate grounding cable may be an option instead from one end to the other.
We had a lot of trouble with an REF protection once on a 6.6Kv system, only resolved by lifting sheath gronds at the load end of cables (about 50m distance).
rasevskii
RE: Refurbished cable testing questions
So now the next question: Do we ground the shields at just one end and keep the other ends all tied together (but not grounded) or do we only tie the three shields on the same phase together on the other end?
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Refurbished cable testing questions
I suppose this is a mining application where special codes rule.
rasevskii
RE: Refurbished cable testing questions
RE: Refurbished cable testing questions
I believe the OP stated in his original post that the cables were tested with 31.5 kvac for 5 min. and the cables still failed in service.
Are you saying they should be tested at a higher voltage or a longer time or both?
RE: Refurbished cable testing questions
In terms of circulating currents, it doesn't matter whether your screens are grounded at one end or both. If the screens of all phases are tied together at both ends, as in your installation, you will get circulating currents whether grounded or not. To eliminate circulating currents, you will have to isolate the screens from each other and from ground at one end. In our installation we couldn't ground at one end only due to the long runs and we accepted that we would have circulating currents, but the constant force springs were not up to the job of carrying circulating currents of that magnitude (around 100A in our case).
The decision of whether to ground at one end or both is determined by calculation of the potential rise of the isolated end of the screen under phase fault conditions. The potential rise limits are usually dictated by local regulations or may be determined by the additional voltage stresses on the insulation.
Incidentally, text books will tell you that if single core cable screens / armours are tied together and grounded at both ends, then installing the cables in close trefoil formation will virtually eliminate circulating currents in the screens / armour. In the real world, you still get significant circulating currents even when installed in trefoil formation throughout their entire length.
RE: Refurbished cable testing questions
Thank you so much for the help. This was a mystery to me up until now.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Refurbished cable testing questions
Your post made sense until the silly and erroneous comment "If a cable passes a proper AC withstand test, who cares what the pd is, it will last for years.".
I guess from your handle you are in the VLF market - if you have any expertise to contribute on this technique I'm sure people would be interested - it doesn't get the discussion and recognition it should.
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If we learn from our mistakes I'm getting a great education!
RE: Refurbished cable testing questions
Current induced in cable screens. Opening gnd connection in one end cured it. One solid gnd connection installed.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Refurbished cable testing questions
We have started our inspection of the cables as we are refurbishing the whole lot. A few of the cables have heating right where the semiconducting layer meets the insulator. It looks like the stress barrier wasn't applied properly. In other cables the hetin iriginated from the band where the shield is connected.
So it seems we had two problems: Poor cable termination and circulating currents. It's no wonder they lasted only a couple of weeks.
Thanks for all the explanations.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.