AC vs DC Hipot Test 2

[a10jp]

If I have a cable that is 375m long, and wish to do a hi-pot test, I would imagine a DC test is more suitable because AC test would induce abnormal capacitive current across the insulation, and result in a larger leakage current. Does everyine agree with this logic? Also, is there a standard what the DC hi-pot etst voltage should be? Is it alwasy SQRT(2) * AC Hipot etst voltage?

 

[Zogzog]

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.

 

[dpmac]

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

 

[electricpete]

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]

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]

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
----
"It’s always fun to do the impossible." - Walt Disney

 

[electricpete]

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]

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 2007 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.

 

[jghrist]

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.

 

[trosepe]

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".

 

[Zogzog]

Just playing catch up on this, Alehman nailed it.

 

[electricpete]

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

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[electricpete]

http://www.okonite.com/engineering/high-voltage-testing.html

In 1996, 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]

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]

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.

 

[Zogzog]

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”

 

[electricpete]

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-2001 (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]

Thanks Zog. Very helpful compendium.

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

 

[Zogzog]

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.

 

[jghrist]

In 1996, 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 that the above quote is from a manufacturer of EPR insulated cable. It is not true that EPR is not subject to water treeing as implied. Water trees can form in EPR but are difficult to detect visibly.

In DC Field Test for Medium-Voltage Cables: Why Can No One Agree?, IEEE Transactions on Industry Applications, Vol. 34, No. 6, Nov/Dec 1998, the authors indicate that the EPRI tests were limited to XLPE cable and that the recommended tests on EPR cable were stopped due to lack of funds. The lack of evidence of damage to EPR may not mean there is no damage. Does anyone know of any later tests done on EPR cable to verify that aged EPR is not subject to damage from dc hipot tests?