Cable Testing

[timm33333]

A question about the testing of medium voltage copper cables: what should be the minimum megger test volatge for a 30kV cable? Can a 30kV cable be tested by a 2.5kV megger?

 

[dpc]

You should do a search of this forum regarding hi-pot testing of the medium voltage cable. It has been discussed quite a few times. A 2.5 kV Megger will find gross problems in the cable, but not tell you much about the condition of the insulation.

 

[collies99]

For acceptance testing, megger st 5 kv is recommended as per NETA in North America. Megger IR values is just a qualifier for the dielectric test with either AV or VLF.

collies99

 

[timm33333]

Thanks dpc, I did the search for hi-pot. The company standard says that all medium voltage cables shall be tested by 5kV megger, which does not make any sense to me. Because I think testing a 30kV cable with only 5kV megger will not give much information and this test will definitely pass unless there is something desperately wrong. Am i correct? Thanks for help!

 

[dpc]

Well, the megger test should be done prior to the high pot test. High pot testing of solid dielectric cable is controversial. My suggestion is to contact the cable manufacturer and get their recommendations for cable acceptance testing.

 

[ScottyUK]

You might find the following document of interest for DC cable testing:

http://www.spenergynetworks.co.uk/n... Electrical Insulation Testing of HV Equ.pdf.

Cable tetsing starts at Page 12.

You might also want to investigate low frequency AC cable testing as an alternative type of test for polymeric cables.

 

[timm33333]

ScottyUK, the document you gave mentions 5kV Megger for the sheath of 33kV cable, which makes sense to me. But it also mentions 5kV Megger for insulation resistance test of 11kV cable.

If the standards say that use 5kV Megger for insulation resistance of 11 -33kV cables, we will do it that is not a problem. I just want to understand the theory that how will a Megger test voltage of only 5kV measure the insulation resistance of 11-33kV cable. Please let me know comments!

 

[Zogzog]

The Megger test is just done prior to a dielectric strength test to ensure you won't damage the cable. The VLF/PD. or Tan Delta test is what tells you the conidtion of the insulation.

 

[ScottyUK]

That test is basically to check that the cable is clear of earths etc. As far as I know the British DNO's generally aren't using a pressure test unless there are specific reasons for doing so, perhaps a long cable with a joint rather than a single length or following a repair. The DC pressure test for 33kV is 30kV phase-earth, which is implicit in testing a screened cable. I expect Marmite will chip in at some point, maybe ppeduk too.

 

[waross]

We use a 5 kV megger test to check for shipping damage when cables reels are received. More testing to other standards may be done after the cable is installed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[collies99]

IEEE 400.2-2013 has just been approved for AC testing of extruded cable installation. The major differences to the 2004 issue is the recommended testing time to 30 minutes from the previous time of 15 minutes. This is the final results from years of study of cable testing.

DC cable testing is now frown upon as it can accelerate insulation defects which leads to earlier failure of cables both new and service aged.

 

[ppedUK]

tim33333

Don't dismiss an I.R test on a cable as useless. A P.I. test carried out using a Megger, followed by a little careful analysis of the results can give you a fair bit of confidence (albeit a go/no go decision) as to whether or not to energise a circuit, although I agree that 2.5kV on a 30kV cable seems low. Insulation resistance is the same value whether you test at 2.5kV or 5kV, it's just the stress that you put the cable under that is different. Personally I would probably be looking for a 10kV I.R. test on that voltage, but I'm not a cable testing expert. I personally am more comfortable making decisions about energisation when I have a P.I. and resistance reading, rather than a leakage current value from the HiPot set, but that's just me.

The situtations I am usually involved in (with respect to cabling) are testing of new cables prior to first energisation and testing cables after a trip. For new cables we use I.R. (P.I. test) and HiPot, after trips obviously just I.R. I have never been involved with VLF testing for condition assessment.

I would also say that you are almost always testing a cable connected to a piece of equipment (usually via the circuit spouts) and regardless of what any standard tells you, you have to observe the equipment manufacturer's recommendation. For example, some old 11kV switchgear that I regularly come across has a manufacturer's recommended maximum DC test voltage of 18kV, so that's what we HiPot the cables at (even new polymeric cables). That value then set the cable DC test voltage standard of 18kV across the site for 11kV cables, even on new switchgear.

Sorry I can't give you an absolute value to use for your cable, but the standard that Scotty posted might give you a better clue.

 

[timm33333]

Sorry I hate to expand on it, but does it mean that the value of insulation-resistance (measured by Megger) of a 30kV cable will be almost the same no matter whether you use the Megger test voltage of 5kV or Megger test voltage of 30kV?

5kV Megger is used for the insulation testing of low voltage cables as mentioned at the following link:

http://www.nexans.us/eservice/US-en_US/fileLibrary/Download_540198890/

There are several cable tests but most of them (like tan δ test and partial impulse test) are done during the manufacturing of cable. But I am basically only concerned about the after-installation Megger test of medium voltage cable.

 

[Zogzog]

Tan Delta and PD (VLF Also) are tests you do after installation, every standard you will find on testing MV cables tells you that. A Megger test alone is not an acceptable test. Perhaps you should hire a testing company that does this type of test.