Megger vs VLF tan delta testing

[powerpete69]

I am electrical engineer at big industrial site with over (40) 5KV feeder cables. 40 substations obviously. Some cables are long runs, in the 2500 ft range.
Older site, 1940's. Most cables have been replaced, some are on their way out again,

Northeast, Ohio. Decent amount of rain so cable sometimes get submerged in water when sump pumps fail.

We are in process of replacing a couple that have already shorted out.
We are also replacing some that are in the 150 to 300 megaohm range or even lower.

My question is this. If "meggering" tests the insulation, what benefit does VLF tan delta testing provide?
It's expensive and if "meggering" already tests the insulation and the insulation is bad, why do I want to test the insulation further at a much higher price?
Please enlighten me and others.

 

[zbang]

(I know I'm not answering the question directly)

Most "meggers" only go to 1000 volts, so not enough to test a 5k cable; what's he actual test voltage? Also, IIRC most cable manufacturers will tell you not to test MV cables with DC anyway, at least for modern insulations (see below). A 1kv megger will tell you that the cable is bad, but it won't tell you that it's really good for the working voltage and conditions. The common wisdom is to have NETA-certified company do the tests to prove the cable is good unless you're experienced and have the right test gear.

A good source is https://netaworldjournal.org/accept...sting-medium-voltage-electrical-power-cables/, especially the section "High Potential Testing with Direct Current (DC)" and also https://netaworldjournal.org/change...testing-specifications-in-ansi-neta-mts-2023/

If you search these forums for "mv cable testing", there are some useful discussions, also at
https://hvinc.com/knowledge-center/ has some papers about how tan delta works if you want to get into the guts of the process
and
https://www.okonite.com/media/wysiwyg/Engineering Technical Center/EHB 2010.pdf
(Okonite has a bunch of info about testing).

 

[powerpete69]

(I know I'm not answering the question directly)

Most "meggers" only go to 1000 volts, so not enough to test a 5k cable; what's he actual test voltage? Also, IIRC most cable manufacturers will tell you not to test MV cables with DC anyway, at least for modern insulations (see below). A 1kv megger will tell you that the cable is bad, but it won't tell you that it's really good for the working voltage and conditions. The common wisdom is to have NETA-certified company do the tests to prove the cable is good unless you're experienced and have the right test gear.

A good source is https://netaworldjournal.org/accept...sting-medium-voltage-electrical-power-cables/, especially the section "High Potential Testing with Direct Current (DC)" and also https://netaworldjournal.org/change...testing-specifications-in-ansi-neta-mts-2023/

If you search these forums for "mv cable testing", there are some useful discussions, also at
https://hvinc.com/knowledge-center/ has some papers about how tan delta works if you want to get into the guts of the process
and
https://www.okonite.com/media/wysiwyg/Engineering Technical Center/EHB 2010.pdf
(Okonite has a bunch of info about testing).

Ok, interesting stuff.
I would agree that you didn't quite answer my question.
We have roughly 40 feeder cables on site. These cables have been meggered the past 50 years and like I mentioned, when we start getting to around 150 Megaohms, we know trouble is coming soon.

We do our preventative maintenance every 3 years. So roughly test 13 feeder cables a year.

The cables have been neglected by guy before me. His motto was we will deal with it when the time comes. Well, we had two shorted cables last year. Luckily they had back up feeds, but eventually back up feeds run out.

We have 3 cables that we are replacing right now.

Looking forward to the next 5 years, per the megger testing, it look like we need to replace 5 more additional cables.

During our shutdown in July this year, I have the option of VLF testing on the 13 cables. I can obviously do this the next two years as well.

I guess the real question is this:

Should I spend an extra $15,000 to test these 13 cables per year over the next 3 years? I can sort of fit it in the budget but I would like to know if its REALLY going to give me info.....such as, yes this cable meggered bad, but its really OK per the VLF test.

$15,000 (per year) sounds high to me, but maybe not. Please keep in mind these cables will be de-energized and disconnected for megger test already.

If the megger reads bad, but VLF says cable is good, than the $15,000 investment seems wise. Replacing these underground cables is super expensive.

 

[jim dungar]

Part of the issue with MV table testing is developing a trend plot rather than doing a one time test with improper procedures.
At the least you should have been doing High Potential (Hipot) testing instead of LV megger. But the current philosophy is that Hipot can cause more damage than it detects.

What is more costly, performing a scheduled test or having an unscheduled outage?
But you are correct, if the cable fails with a simple test, why bother with more diagnostics, just schedule a cable replacement.

 

[powerpete69]

Thanks Jim. Good points.
We definitely doing LV "meggering"....not HIGH POT.


I just chatted with experienced (non involved) VLF guy.
He says if the cable meggers bad at 150 Megaohm, the cable is also going to fail VLF test.
BTW, The tan delta test is included in the $15,000.

That being said, he said you can get a much more detailed description of the cables health with the VLF/ tan delta testing.

Also, VLF/tan delta testing is much easier on your cables then megger/high pot.

He did recommend doing the test over the next 3 years to get a complete scope on whole site. And no, he is not the one who would be doing the testing.
So maybe it is a good idea....still not 100% sure.

 

[zbang]

$15,000 (per year) sounds high to me, but maybe not. Please keep in mind these cables will be de-energized and disconnected for megger test already.

If that's for 13 cable sets, it doesn't sound high to me. And you can trained personnel and (presumably) calibrated equipment doing the tests. Seems like a small price to know that something is good and get the records.

Look at it this way- would you rather rebuild an automatic transmission with simple tools and the shop manual or take it a transmission shop that does them every day?