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Old 10-02-2017, 12:57 AM   #1
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Default Megging MV cables

Can some one clarified a reading of ohms.

I did an insulation test on a # 2 medium voltage 3 phase cables rated for 15kv.
The test results were as fallow:
5000v 5 minutes using a fluke 1550c
A 15 G ohms ( this one was steady and and going up really slow)
B 76 G ohms
C 126 G ohms


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Old 10-02-2017, 02:05 AM   #2
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need more info: (also not sure why you are testing at 5kV..we normally test at 2500V.

typically, the cable is considered acceptable when the absolute value of insulation resistance at 10 minutes is at least (X) megohms:

X = (kV +1) 1000/L
kV= cable rated voltage, in kV
L= cable length, in feet
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Old 10-04-2017, 08:34 AM   #3
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You really need more test voltage than 5 KV. There are published standards for new and in service cables or the cable manufacturer should be able to provide you with that info.
A hipot aether VLF(very low frequency) or DC and there are different standards for both is really what you need.
LC
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Old 10-09-2017, 03:52 PM   #4
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Quote:
Originally Posted by Lone Crapshooter View Post
You really need more test voltage than 5 KV. There are published standards for new and in service cables or the cable manufacturer should be able to provide you with that info.
A hipot aether VLF(very low frequency) or DC and there are different standards for both is really what you need.
LC
But every standard recommends against DC hipot testing service aged cables. VLF, Tan Delta, or PD test is what should be performed.

Megger reading is pretty much worthless for a 15kV cable, really just lets you know if it is safe to perform your VLF test.
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Old 10-10-2017, 07:27 PM   #5
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IEEE standard 43 was updated about 10 years ago. The old 1 megaohm/kilovolt rating is dead. Its now 100 megaohms for the cable described, regardless of voltage, unless its very old (pre 1970s) in which case the old standard applies. Motors are either 100 or 5 depending on design. Some issues have been raised about wound rotors in motors (WRM and synchronous) where they often barely make 1 megaohm/kilovolt so I think EASA has been slow to adopt the new standard. Second the standard is always to look at the 1 minute reading corrected for temperature which can make the readings vary by at least 3:1 between summer and winter. The test voltage depends on the service voltage and is definitely lower than the service voltage. The only reason it goes up has to do with providing enough voltage to clear cable capacitances. 2500-5000 is correct for 15 kV class.

As to using hi pots, again the standards have been updated about 20 years ago when it was found that not only do hi pots not tell you anything about the cable but they actually cause defects and the damage may not show up for a couple weeks. The standard fornhi potting is 400 and in that standard it specifically says not to use hi pots (DC) on cables over 5 years in service.. VLF is LESS damaging but works the same way. The goal is simply to shrink the size of the hi pot without being just a capacitor (its a square wave). AC hi pots using service voltages are safest but get obscenely large. Tan delta is just meggering for 10 minutes and dividing the 10 minute result by the one minute one, also recommended in standard 43. There is another one that uses the 30 second and one minute numbers because its faster. Both ratios mostly just indicate contamination on polymer insulation. You can also graph the megger as the test progresses and this can indicate cracking in insulation in motors and transformers because it looks like stair steps, and if the readings are wildly all over the place, its moisture.

There are proponents if PD too. Offline is used firing manufacturing and has been in ICEA cable specs for decades. Online tests can detect failures within a few feet in a long cable and works pretty well at finding switchgear problems as long as you're above about 3 kV (PD needs a minimum voltage). They've been using it on underground distribution systems in Europe for over a decade. A local paper mill had great success and when I tested it on substations the two that had PD also turned out to have previously unknown serious problems going on.

Offline PD is heavily promoted by one company but last time their techs looked at a cable where I worked, what they were saying about the cable that tape shields are basically ineffective was such obvious technical crap and the rest of what they said was such obvious technobabble that they convinced me that if there is something to offline PD, they need a new spokesman other than IM Corp.

Given all this and the prohibitively large size and cost of AC hi pots, I stick to megger readings or tan delta. I will use the other tests and rent equipment if the customer insists. Until somebody other than IM publishes something believable about offline PD I'm holding off on that one.


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Old 10-11-2017, 08:52 AM   #6
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Quote:
Originally Posted by paulengr View Post
Tan delta is just meggering for 10 minutes and dividing the 10 minute result by the one minute one, also recommended in standard 43. There is another one that uses the 30 second and one minute numbers because its faster. Both ratios mostly just indicate contamination on polymer insulation. You can also graph the megger as the test progresses and this can indicate cracking in insulation in motors and transformers because it looks like stair steps, and if the readings are wildly all over the place, its moisture.
Those 2 ratios you are referring to are PI and DAR, Tan Delta is a completely different test, similar to a power factor test.

http://www.versapower.pt/files/TAN-D...TESTING-07.pdf

Everything else I agree with, nice to see you on here Paul
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Old 10-11-2017, 09:47 PM   #7
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I forget the details. I just use a PDMA to get results if the customer asks. Dobles has very high dollar testers for basically "everything" too. They back it up with a huge database of test results of pass/failed equipment. They have very slick advertising but other than the occasional IEEE IAS publication not sure anyone other than certified Dobles technicians believe it either. I could go on. There is a lot of snake oil out there in the testing world.


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