DC/AC Hipot, insulation test - Mike Holt's Forum

What is the difference between and AC and DC hipot? Is DC hipot be considered as insulation resistance test as well?
We are installing a 36kV line (direct buried) and it is being tested by 10kV DC insulation test. Now the other team is requiring HIPOT test (which we dont have any info whether it is DC or AC), which from our experience is only being used with bare coppers and conductors without insulation.

So which test is not safe for MV cables with XLPE component? Is it right to tell them not to continue with their HIPOT testing?

Thanks. Appreciate any help. DC Hipot can be destructive on XLPE cable, however all the test standards (ICEA, NETA, ANSI, IEEE) that have addressed this only refer to service aged (>5 years) cable, no standard for new installations as far as I know have been revised to reflect warnings on new cable for DC hipot tests.

That being said, a DC hipot will only reveal gross installation errors and is not a very useful test. A VLF AC test would be better, tan delta or PD would be the best to establish baseline data for trending purposes if this cable will see any routine tests in the future. A DC hipot creates sustained polarization in the conductor insulation, combined with the fact that any highpot applies an overvoltage, this can lead to failure where AC might not.

That said, I agree with Zog that this is mostly a concern on service-aged gear. As a rule we won't highpot any in service equipment unless specifically requested by the customer after they understand the risk of failure.

However, if you're doing a new cable commissioning a highpot is a standard test. And if those are the only tests being performed, I would definitely prefer it over a basic 10kV megger which isn't likely to stress out any defects. We are also doing VLF together with this 10KV megger. (+ the standard sheath, armor and phase testing)
From what I understand on your replies, HIPOT and megger is not the same even if it has the same testing voltage.

With this on hand, what is the acceptable test and values to be performed with this kind of service?
Please enlighten me on this.

We are also doing VLF together with this 10KV megger. (+ the standard sheath, armor and phase testing)

Then a DC hipot strikes me as unnecessary: You're already doing a dielectric withstand test with the VLF.

From what I understand on your replies, HIPOT and megger is not the same even if it has the same testing voltage.

A high-potential test in its truest form is applying a voltage that is a multiple of the rated RMS voltage of the equipment; it's a dielectric withstand test.

10kV from a DC hipot tester is no different than 10kV from a megger.

Example: A 10kV megger can perform a dielectric withstand test on 600V equipment.

Or a hipot putting out 10kV can also perform an insulation resistance test on 35kV equipment.

But if someone calls for an actual "hipot test" on a 35kV cable likely calls for applying ~100kVDC withstand test depending on what specs I'm using.

...With this on hand, what is the acceptable test and values to be performed with this kind of service?

Often the manufacturer will have specific commissioning requirements. In absence of that or customers specs, check out NETA Acceptance Testing Standard . got it. so it all depends on the multiplier of the test voltage and the rating of the piece to be tested. If it is stressing the insulation to limits then it is considered HIPOT.

one last question, is this rule of the thumb when performing HIPOT is also applicable on cables?
ACtest=AC operating x2+
DCtest=AC test voltage x 1.4

Thanks for clearing things up with the terminologies.

Wow! Is that a current document from Southwire?? It quotes IEE 400 version, and the ICEA. Lots of things have changed since then that go against what this document says. IEEE 400 and the ICEA that are referenced advise against DC hipot of in service cables yet this document is recommending a DC hipot test as an "Advanced" test.

No mention of VLF, TD, or PD testing at all. I think this has to be an out of date document or the folks at southwire need to get it updated to current standards they are referencing. May I comment on the document provided by SouthwireSam and some of the reactions: The pages posted by SouthwireSam are from Southwire's Power Cable Manual (3rd edition)chapter 9 "Field Testing". Unfortunately, the pages do not reflect state of the art practice and are rather generic.
There are many discussion threads in this forum on the subject that you may want to research but a very brief, incomplete summary of field tests is as follows:
1. For extruded insulation (XLPE, EPR) cables, DC withstand testing on NEW CABLE ONLY (on the reel)may not hurt but it also is known not to detect much.
2. For older (aged) cables and cables installed with accessories (splices, terminations) DC withstand testing is not recommended due to the risk of damage to the insulation (older cables) or the accessories.
3. For MV cables rated 5-46kV, a VLF test (very low frequency) using an AC test voltage is the way to go. This test combines to some extent combines the economic advantages of DC testing with the relevance of AC testing.
4. For HV cables rated 69kV and higher, if a simple soak test of the installed system is not acceptable or considered insufficient, an AC resonant voltage withstand test, preferably combined with a PD test, is to be recommended. This much I know from experience, 1/0 Aluminum URD 15 KV cable 100% insulation,3 phase installation aproximatley 10 years after installation. Recloser opened on line. After verifing that there was a fault and insolating the section of line that was bad, we hypoted the cable as part of our fault locating process using a thumper. We ran the hypot up over 50 KV DC and could not get the fault to fire over. We set the recloser to one shot and hit the cable with KV AC. The recloser opened and we tried the hypot again. It still would not fire over at over 50 KV DC. The DC hypot showed the cable as being good. We hit the cable with 3 more operations of the recloser before we were finally able to get the hypot to show a fault. For a NEW cable, I would follow the manufacturer's recommendation. Period.

At present there are a lot of somewhat conflicting opinions, recommendations and even standards related to cable applied voltage testing. But very few of these sources of "knowledge" have any financial stake or liability in YOUR cable.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein) ICEA and AEIC state the DC test voltages on NEW (less then 5 years)cable is as Zog stated above. NETA uses their numbers for their specifications. Call a manufacturer of EPR (and probably XLPE) 15kV cable and they will tell you follow AEIC or ICEA test voltages. After 5 years all bets are off on what you will hear from them.

RDVETT:

One of the reasons you may not have determined the cable was "bad" was probably due to the current meter range on the thumper. Typically a thumper is defaulted to the highest current range. A unit with a 50kV hipot would most likely have a 100mA high range. If you had the opportunity to test all three phases with a basic hipot (lower current meter ranges) you may very well have been able to determine which phase was faulted.

I have seen ice in a riser pole cut the same type of cable so clean it can withstand 30kVdc hipot prior to flashing over. This same cable was then re-energized for weeks even though it was cut in two.