Meggering 15kV cables 1

[tommom]

A contractor recently told me he was going to hi-pot some cables some time before he energizes them. I suggested to him that he megger them at the same time, get a base line reading, then megger them again prior to re-energization and compare the readings to verify there was no damage or water infiltration into the cables.

Was this bad advice? Thanks for any help you can offer.

 

[waross]

I have always understood Hi-potting to be the high voltage equivalent of meggering.
The highest voltage megger I have seen was 2500 volts. Standard seems to be 500/1000 volts. Hi-pot sets start at about 1000 volts and go up from there.
respectfully

 

[dpc]

I would check with cable supplier before hi-potting any cable in the field. I believe IEEE now does NOT recommend high-pot testing of MV cables in the field, due to risk of insulation damage.

If I was going to hi-pot, I would megger BEFORE the hi-pot. I agree that a second megger just prior to energization would be a good idea. Who knows what might be hanging out there.

 

[Zogzog]

AC hipot is OK, DC may damage the cable. If you can provide cable details I can give you some recomendations.

 

[tommom]

Okay, I found out a little bit more about this:

In the Okonite engineering handbook (p.52), it says that XLPE insulation is damaged by hi-potting and therefore dc testing was not recommended any more (after 1996), but that EPR insulation is not damaged by this testing (I only spec EPR unless otherwise required by the client).

The cables the contractor is supplying are Okonite. Okonite gives guidelines for field hi-pot testing of their cables, but suggests that hi-potting after splicing/terminations could damage those components.

BTW, meggering has been used to identify imminent failure of high-voltage cables by establishing a baseline and then meggering on a schedule, and meggering is non-destructive. I read an IEEE Indusry Applications article about this several years ago.

Thanks for your responses.

 

[waross]

Hello tommom
It looks like I have an opportunity to learn something today.
At what voltage would you megger a 15kv cable?
Thanks
respectfully

 

[tommom]

According to Okonite's guide, it's 300 volts per mil, so at 15kV class for acceptance testing:

100% (175 mil) = 55kV
133% (220 mil) = 65kV

I haven't seen 173% insulation in any catalogs for some time, and never had a reason to use it.

For maintenance testing for cables less than 5 years old:

40 kV

Testing frequency is also given in the Okonite engineering handbook. I got mine free from the local rep.

 

[waross]

Thanks tommom
Same words, different meanings.
I would call that a DC hi-pot at 40kv. At that level I agree that a baseline reference is a good suggestion.
I guess I did learn something today.
Thanks.
respectfully

 

[rbulsara]

Refer to NETA- ATS (Acceptance Testing Standards). You can buy them from

http://www.netaworld.org.

 

[benlanz]

Depending on the protocol the DC HIPOT and the insulation resistance test can be different or the same. According to IEEE 400 both tests are useless at finding defects on new cable systems. However, if you do not have a diagnostic test, I recommend an insulation resistance test to prove that the system is not shorted. If someone left the grounds on the cable it would be a terrible shame is someone got hurt and it could have been prevented with a simple resistance test!

Cable manufacturers give recommendations for a DC test knowing full well that it is completely useless to find 99% of defects. They would never use a DC test to pass cable through their quality control department! So why do they still recommend it in the field? Soon they will not be able to make this recommendation because it is being dropped from the standards.





Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

 

[tommom]

Ben Lanz,

So what IS recommended (besides an insulation resistance test) for an acceptance test? Is there such a thing?

I was thinking of the megger more as a baseline for future maintenance. I have seen this discussed in IEEE IAS articles (I don't have citations.)

Thanks.

 

[benlanz]

IEEE 400-2001 still allows for a simple AC high potential withstand. However, with this test you will miss many types of defects, you can not assure the reliability of your system, and you may unknowingly do more harm than good.

If you want to assure reliability and find defects before the contractor warranty has ended IEEE 400-2001 states:

"If the cable system can be tested in the field to show that its partial discharge level is comparable with that obtained in the factory tests on the cable and accessories, it is the most convincing evidence that the cable system is in excellent condition."

So what are the manufacturers' standards for cable systems? These standards have over the 40 years of expericence behind them!

IEEE 48 Terminations No PD > or = 5pC up to 1.5Uo
IEEE 404 Joints No PD > or = 3pC up to 1.5Uo
IEEE 386 Separable Connectors No PD > or = 3pC up to 1.3Uo
ICEA S-94-649 MV Extruded Cable No PD > or = 5pC at < 4Uo

Let's not reinvent the wheel or make things complicated. Let's use the proven industry standards.



Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants