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Prospero (Electrical) (OP)
24 Sep 04 7:55
I want to know if I can joint an old 11kV cable that's 50yrs old? The cable is an armoured with a 150mm csa, about 0.5 mile long and goes under a stream. It also passes through ground that has injected sewage sludge in it, which I'm told can deteriorate the physical integrity, so . . .

i)  What cable tests/checks should be perfomed?
ii) . . and how reliable are the results?







DanDel (Electrical)
24 Sep 04 8:28
Testing old MV cable is the first step, and you have several options, ranging from the (inexpensive and simple) megger/PI test, to Doble Power Factor, Hipot, and (more expensive and detailed) Partial Discharge.  
The PI test will give a quick, cheap indication if you should go further.
I wouldn't recommend hipot testing at any value above the cable rating, since this is a destructive test, and is very likely to fail(damage) a cable that is marginal.
The Doble PF test and Partial Discharge tests are much more detailed, but more expensive (especially the PD). Of these two, the PD is probably better than the PF.
If you are planning to splice into this cable, some of these tests should be performed first, and if testing indicates a good cable, testing should then be performed after the splicing as well.
Helpful Member!  ausphil (Electrical)
26 Sep 04 21:08
I,d start with a serving (sheath) test first, which should tell you whether or not your cable has had any real moisture ingress or any damage that can be shortening the life of your cable.  We've had cables that run through some disgracefully contaminated areas that have had the servings dissolved in some areas, but the conductors and main insulation are OK.  This is OK at the present, but eventually moisture and other chemicals will get in and make the main insulation fail.

We've jointed many cables that are that old, and as long as the original cable is OK, then there hasn't been a problem.

If your sheaths are down at any point, you can do a fault location on them, and you may find that the problem lies either in the water or on dry land, which could influence your decision either way.

As DanDel points out, power factor tests are not that great, only because they average the condition of the whole cable.  You may have a section of 100m that is really bad, and the rest is good, and the pf test will tell you that the whole cable is in reasonable condition.
busbar (Electrical)
26 Sep 04 22:38

What is the cable dielectric — paper. varnished cambric, polymeric or something else?

It may be wise to ‘start with the small guns‘ and get a more immediate idea of condition comparing phase-to-ground leakages at 5kVDC.  That may save getting out more sophisticated equipment.  Subjectively, workmanship 50 years ago will probably affect its present condition most.
  
Helpful Member!  benlanz (Electrical)
27 Sep 04 13:42
I completely agree with busbar.  A 11kV cable that is 50yrs old is most likely a laminar cable (paper insulated). I will proceed under this assumption.

In general laminar cable fail by conduction.  Either there is a loss of oil and the paper is carbonized by internal tracking or, more likely, water penetrates the shield causing localize conduction leading to thermal runaway.

What tests to perform?:
I agree with busbar. The cheapest test you can perform is the 5kV DC megger. This will give some indication if there is serious conduction.  A PF or tangent delta dielectric spectroscopy test will give you a more accurate handle on the water penetration.  PD will only locate areas that lack oil and there is carbonization.

How reliable are the results?:
Don't expect more than 85-90% accuracy after performing a combination of PD and tangent delta.

If you are splicing a paper cable you better have the job done by an expert team.  Demand that a paraffin (hot oil) test be perform during the jointing and a nitrogen pressure test be performed on the lead wipe when they are done.  There are very few outfits that can build a good lead wipe and prevent moisture ingress during the jointing.  Fewer yet can perform a valid nitrogen pressure test.

-cheers




Prospero (Electrical) (OP)
28 Sep 04 4:21
Thankyou all for the high quality advice.

Hot oil test, nitrogen test. . . what are they?
benlanz (Electrical)
28 Sep 04 9:54

Again, I am assuming your 50 year-old cable is the laminar type of cable with a lead sheath (PILC Cable, Paper Insulated Lead Covered Cable). Please check to make sure or this advice is useless!

I have not performed a pressure test myself. Most of my experience is in field testing cable and dissecting failed samples. If you want expert advice in building you may contact Joseph Zimnoch. He is a retired Okonite Engineer and is an expert with paper insulated cable.

Joseph Zimnoch
(845) 357-8923
Joseph Zimnoch (joezimnoch@aol.com)

Nitrogen pressure test:
A nitrogen pressure test is used to insure the lead sheath has been properly sealed. Even a microscopic pin hole will allow moisture to enter the joint and eventually fail the cable.  During the joint sheath installation the jointer must install a port with a valve.  After the lead wipe is completed and set, nitrogen is injected into the joint through the port.  The outside of the joint is soaped.  The presence of bubbles signify a leak in the wipe.  

Hot oil test:
A hot oil test, or paraffin test, checks to make sure the paper in cable being jointed and the paper being used to build the joint are free from moisture.  Silicon oil is heated to 160 degrees Celsius.  The paper tapes used to make the joint are dipped into the oil. If moisture is present anywhere on the tape, white bubble will clearly define the region.

You may want to consider using a transition joint. These joints allow one to joint PILC with extruded cable.  You can ask Joe what he recommends.  Joe isn’t cheap but, if what the job done right, he is the guy.

Can you see why there was such a push towards extruded cables in the 60's? Building a PILC joint is an art but, once made properly it can last 90 years+!

-Cheers

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