AC vs DC Hipot Test - Eng-Tips

Zogzog

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NETA, ANSI, IEEE, ICEA, all agree that a DC hipot is a destructive test for service aged cables and should nto be done. AC testing at power frequencies for that length of cable will require a huige power supply so you should look at VLF testing your cable.

For a better condition assesment of your cable do a TD or PD test.

There are standards for test voltages but they depend on the type of test, the cable rating, the insulation rating(%), and the cable type and size. Upvote 0 Downvote

dpmac

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More information would be helpful. Cable voltage rating, insulation type, (XLPE, EPR,PPP, PILC, etc), age, terminations installed. Upvote 0 Downvote

electricpete

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I am not up to date on latest standards. I recall we have had similar discussions before but I just want to check my understanding.

IEEE95 still specifies high pot test for motors (although test voltage for motor hi-pot is lower than for cable hi-pot). There is no requirement to disconnect the cable.

As I recall the concern of treeing was unique to XLPE cables, right?

And as I recall the water treeing concern related to cables that were exposed to moisture such as those run in undeground vaults that might flood occasionally,.

If I have a non-XLPE cable which runs indoors, do you have a sound basis for telling me that dc hi-pot is inappropriate?

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electricpete

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As a preemptive comment, I'm not interested in hearing about other test equipment - just would be interested to hear the basis for condeming dc testing.

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alehman

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DC testing is only discouraged for aged cables (particularly XLP) because it can cause premature failure of an otherwise healthy cable.

DC is still accepted by IEEE and recommended by some cable manufacturers for new installations. The value of DC testing is questionable. I think many agree that PD tests are better, but it can still be hard to find testing firms that are familiar with PD or have the correct equipment.

Search this forum for several other threads on this subject.

Alan
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"It’s always fun to do the impossible." - Walt Disney Upvote 0 Downvote

electricpete

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Is it alwasy SQRT(2) * AC Hipot etst voltage?

As I remember, the dc test voltage for cable is typically 3 times the ac test voltage. (for motors it is 1.7).

I was looking for someone to provide substantiation or qualification of the blanket statement:

NETA, ANSI, IEEE, ICEA, all agree that a DC hipot is a destructive test for service aged cables

I believe appropriate qualifiers may include the type of cable and whether it is exposed to moisture.

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a10jp

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Hi, the installation is in Japan. This is a new cable, XLPE, at 100% insulation. After I submitted this post I realized there were also other discussion on this subject before. In short, it seems most have discouraged using DC hipot as it does not give you meaningful result as alehman has said, other than just pass/fail qualification. It also recommends other testing procedure like Zogzog stated. When I read the NETA ATS it gives a DC test voltage for DC-hipot in Table 100.6, kind of lead me to believe DC test is just as acceptable as AC test, even though it may give less indicative information of other potential defects the cable insulation might have. Upvote 0 Downvote

jghrist

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Voltage acceptance tests on cable are usually dc for reasons that you stated or very low frequency ac to avoid problems with damage to aged cables. 50 Hz or 60 Hz ac test equipment for long cable lengths would be very expensive because of the capacity required.
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trosepe

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DC overpotential testing is the test most typically performed on new cable after installation. If performed properly; regulated power source, proper corona suppression devices, proper test voltage etc., the test is not destructive for new cable. Improper test procedures can damage good cable. A good example is not properly teminating the far end of the cable under test. A flash over at the far end while the cable is charged can create a reflected standing wave which can generate upto three times the applied DC voltage. Another is using the test voltage recommended for the cable and not considering the rating of the termination device, elbow, stress cone etc. You need to hire a real testing company to perform the tests.

P.S. Underground cable will always be "exposed to moisture". Upvote 0 Downvote

Zogzog

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Just playing catch up on this, Alehman nailed it. Upvote 0 Downvote

electricpete

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Please provide the reference where NETA, ANSI, IEEE, ICEA say that dc testing is destructive for all cable types.

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electricpete

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In , the insulated conductor industry determined that dc withstand testing of the plastic (XLPE) insulation systems either in the cable factory as a routine production test or after installation as the higher voltage proof test was detrimental to the life of the insulation and therefore discontinued recommending dc testing. Medium voltage EPR insulating systems are not subject to the same aging characteristics and, therefore, can be dc tested as required in accordance with the tables below.

Note tables are provided for installation testing and maintenance testing.

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electricpete

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I was not disagreeing with Alehman. Just pointing out that the earlier statement from another poster suggesting about blanket condemnation of dc testing is not reality.

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Zogzog

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I said for service aged cables, it wasn't a blanket statement if you read it again. DC testing is still accepted for new cables. Upvote 0 Downvote

Zogzog

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NETA MTS:
Does not specify what testing but rather refers to several other specifications (IEEE and ICEA)
Allows for Direct Current, Alternating Current, Partial Discharge or VLF Testing
Provides the “standard” DC High Potential Test warning for Cables over 5 years in service.
Provides guidelines for maintenance frequencies based on criticality and equipment condition.

ICEA:
After Installation
Provides values for DC Proof Testing during Installation

In Service
Allows for testing at reduced voltages for first 5 years
Will only discover “gross” problems
Not expected to reveal deterioration
DC Testing after 5 years is “not recommended”
Evidence that DC Testing can lead to early failures

IEEE 576
Acceptance Testing
Provides values for DC Acceptance Testing
DC has no harmful or cumulative effects on insulation
As long as voltage is not high enough to break down good insulation – it has no deteriorating effects.

In Service
Allows for testing at reduced voltages for first 5 years
Does not provide for testing after 5 years in service

IEEE 400
Provides an overview of techniques for performing electrical tests in the field on shielded power cable systems from 5 – 500kV
Provides a summary and warnings about DC high potential testing.
States “even massive insulation defects cannot be detected with DC at the recommended voltage levels”
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electricpete

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Thanks. It will be helpful to look at the standards. I have at my fingertips only one of them.

NETA MTS:....Provides the "standard" DC High Potential Test warning for Cables over 5 years in service.

I am not familiar with the "standard" warning. Can you share it?
My copy of NETA MTS- (don't know if that's the most recent) talks about reducing the voltage for service aged cables. There is nothing resembling "DC hipot is a destructive test for service aged cables and should not be done". Does a more recent revision say something different?


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alehman

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Thanks Zog. Very helpful compendium.

Alan
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Zogzog

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Current NETA specs (MTS) step out of the MV cable testing "wars", ANSI?NETA say to llok at all the options and to carefully choose the methods. However they only provide DC test voltages for the 1st 5 years after installation, no DC test voltages are available for after 5 years. That is thier way of staying "politically correct" on the subject until more testing is done.  http://files.engineering.com/getfile.aspx?folder=36ec----f4fecdbd&file=NETA_cable.png
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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.