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AC vs DC Hipot Test

2

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AC vs DC Hipot Test

AC vs DC Hipot Test

a10jp

(Electrical)

(OP)

5 Jul 09 04:03

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?

Replies continue below

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RE: AC vs DC Hipot Test

Zogzog

(Electrical)

5 Jul 09 10:38

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.  

RE: AC vs DC Hipot Test

dpmac

(Electrical)

5 Jul 09 11:04

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

RE: AC vs DC Hipot Test

electricpete

(Electrical)

5 Jul 09 12:59

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?

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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RE: AC vs DC Hipot Test

electricpete

(Electrical)

5 Jul 09 13:01

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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RE: AC vs DC Hipot Test

alehman

(Electrical)

5 Jul 09 14:12

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.

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

RE: AC vs DC Hipot Test

electricpete

(Electrical)

5 Jul 09 14:34

Quote:

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:

Quote:

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.   

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

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RE: AC vs DC Hipot Test

a10jp

(Electrical)

(OP)

5 Jul 09 15:23

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.

RE: AC vs DC Hipot Test

jghrist

(Electrical)

5 Jul 09 16:24

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.
 

RE: AC vs DC Hipot Test

trosepe

(Electrical)

6 Jul 09 00:51

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".

RE: AC vs DC Hipot Test

Zogzog

(Electrical)

6 Jul 09 08:49

Just playing catch up on this, Alehman nailed it.  

RE: AC vs DC Hipot Test

electricpete

(Electrical)

6 Jul 09 10:05

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

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RE: AC vs DC Hipot Test

electricpete

(Electrical)

6 Jul 09 10:11 http://www.okonite.com/engineering/high-voltage-testing.html

Quote:

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.

Note tables are provided for installation testing and maintenance testing.

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RE: AC vs DC Hipot Test

electricpete

(Electrical)

6 Jul 09 10:15

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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RE: AC vs DC Hipot Test

Zogzog

(Electrical)

6 Jul 09 10:25

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.  

RE: AC vs DC Hipot Test

Zogzog

(Electrical)

6 Jul 09 10:31

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"
 

RE: AC vs DC Hipot Test

electricpete

(Electrical)

6 Jul 09 12:51

Quote:

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?
 

Thanks. It will be helpful to look at the standards. I have at my fingertips only one of them.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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RE: AC vs DC Hipot Test

alehman

(Electrical)

6 Jul 09 13:08

Thanks Zog. Very helpful compendium.

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

RE: AC vs DC Hipot Test

Zogzog

(Electrical)

6 Jul 09 13:22

• http://files.engineering.com/getfile.aspx?folder=36ec----f4

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.

RE: AC vs DC Hipot Test

jghrist

(Electrical)

6 Jul 09 15:03

Quote:

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.

The company is the world’s best vlf ac hipot tester supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

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 , 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?
 

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, Vol. 34, No. 6, Nov/Dec , 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?

RE: AC vs DC Hipot Test

Zogzog

(Electrical)

6 Jul 09 17:42

I was involved in the EPRI MV cable working group last year and they are moving away from DC testing of any service aged cables, regardless of insulation type. EPRI seems big on the TD testing, had some TD guys involved in the commitee may be part of the reason.  

RE: AC vs DC Hipot Test

trosepe

(Electrical)

7 Jul 09 00:08

Same Okonite article:

"It is, however, the specter of mechanical damage, or substandard splicing and terminating that could cause the engineers responsible for continuity of service to desire a field applied proof test to establish the cable's serviceability. The time-honored methods of proof testing in the field involve high potential direct current (dc). The advantage of the dc test is obvious. Since the dc potential does not produce harmful discharge as readily as the ac, it can be applied at higher levels without risk or injuring good insulation".  

RE: AC vs DC Hipot Test

Zogzog

(Electrical)

7 Jul 09 09:02

I tend to follow industry recognized standards over 13 year old manufactures marketing info. There has been a ton of research the last few years on MV cable testing methods, we have built a better mousetrap.  

RE: AC vs DC Hipot Test

dpmac

(Electrical)

7 Jul 09 14:44

ZogZog,

The AC/DC/VLF/AC DC debate is well over 40 years old. NETA is just beginning to catch up.   This debate has always been driven (or at least very well fueled) by manufacturers.  The influence of test equipment manufacturers is huge. Who do you think started the "MV Testing Wars" in the first place?  Its all about getting you to buy one manufacturers equipment over another.    

RE: AC vs DC Hipot Test

Zogzog

(Electrical)

7 Jul 09 14:56

dpmac,

Agreed, even PD has been around for a long time. I have seen the influence of the test equipment manufactures first hand, basically if you build it, it is the best method. But, NETA is lloking at this issue from outside the box, as a group of experienced testing people around and not influenced by a manufacturer (I hope).

The NETA conference was all focused on MV cable testing, thats about all we heard about for a week, every seminar I went to I walked out thinking that was the best method. Then I went to the next one......

Lets see what we can all agree on, I will start.

1. There are many different methods for MV cable testing, some methods are better for certian applications than others, there is not one method that is better for all cases.

2. DC testing can reduce the life expentancy of service aged cables, XLP types for sure, maybe all types.

3. MV cable testing should only be done by experienced (Hopefully certified) people that understand not only the test equipment and procedures but also cable theory and construction.

4. TD and PD are much more valuable as condition assessment tests than AC/DC/VLF testing.

I don't think any of those statements are debatable, can anyone add more to the list?

RE: AC vs DC Hipot Test

ausphil

(Electrical)

7 Jul 09 19:03

Zogzog,

Agree with all statements, just a few notes to add.

All high voltage testing reduces the life of any equipment that it is applied to.  It may only reduce the life of the insulation by a few minutes or days, which won't be seen in the service life of the equipment, but there is definitely some reduction of life whenever you overstress insulation.

TD testing gives you the average power factor of the whole cable under test.  This is fine if you know that it is all the same type, or has all aged or degraded the same way.  You may have small sections of very poor cable in a long length, which averages out to give you a good reading.  Or you may have a section of poor XLPE cable in amongst predominantly paper cable, which you may interpret as the whole cable being in reasonable condition, because you don't expect that the paper cable TD to be very low.  The user must be aware of what the TD reading is actually telling them.

PD can be used as a very localised method of determining cable problems.  This can only be done if the discharge is sufficiently large enough to be able to reach the measuring point, as well as reflecting off the far end to give the user a location, all without falling into the noise floor of the measurement.  Manufacturers of PD location equipment don't tend to mention this point much in their presentations, but it is a reality they know about.  Most systems can't measure discharge locations on cables longer than about 3 km (usually on XLPE), but it can be as short as 1km if the conditions are wrong, or it is paper cable.  It all depends on the attenuation of the discharge pulse in the cable.

I suppose all this supports your point number 3, in that the test people that you get should be experienced in the testing methods, theory and cable construction.

ausphil

RE: AC vs DC Hipot Test

cgrodzinski

(Electrical)

9 Jul 09 11:24

Gentlemen, all "withstand" tests are of "kill or survive" type tests. They give very little (if any) information about any cable conditions. The discussion among cable engineers and scientists is ongoing for decades. Fortunately, with the development of power electronics we have a better chance to use testing equipment in the field that was once restricted to labs because of its size and weight.

PD and tdelta measurement (or fingerprints) can give some inside view of the problems. Beside the DC/AC/VLF there is another method - damped oscillation system. The equipment is small enough to be used in the field and store results for future comparison. However, even PD detection (or not) cannot give a full evaluation of a cable. The PD can develop in gaseous voids but moisture penetration does not result in PD. So, even the best method cannot give a full picture. It should be combined with the DC test. All depends on type of cable and accessories.

As any test, the cable evaluation test should be done by a person who can suggest a method, level, and who can properly evaluate the results. I do not think that unless information coming from test is obvious for anyone the test results from one test can be a ground for an immediate action. It is known that many cables, paper insulated, showing enormous magnitude of PD have been working for years without problem.

The very important statement came from the last ausphil's statement.... "the test people that you get should be experienced in the testing methods, theory and cable construction."

Just my 2c.

Chris

RE: AC vs DC Hipot Test

dpc

(Electrical)

9 Jul 09 11:49

 

Just to kick an already dead horse, I think it is also prudent to follow manufacturer's recommendations for the specific type of cable under test. Performing tests not recommended by a manufacturer lets them off the hook, regardless of the technical merits of your arguments or what IEEE working group said what.

"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)

RE: AC vs DC Hipot Test

Zogzog

(Electrical)

9 Jul 09 12:25

Good point DPC. But, sometimes the owner of the equipment requires a specific test. Sometimes they have a reason and/or know what they are talking about, other times they are clueless. As the testing company we are off the hook if the owner specs a certian test, but always try to educate them if possible.   

RE: AC vs DC Hipot Test

a10jp

(Electrical)

(OP)

11 Jul 09 20:27

I apologize for adding to a discussion so late.  

Here is the story.  We pulled the cable (L=350m), and we pulled it in 2 sections, via the same manhole.  It was fairly successful pulling on both sides of the cable except where the cable enter the manhole in the middle, the MH opening dia is only 600mm or 2-ft, which is less than min bend radius = 500mm.  One of the customer's inspection we need to test the cable due to potential damage of the shielding because we exceeded the bend.  We cut off the remaining section of the slack for a sample, cut open sheath and expose the shieldng, and tried to bend it really hard, and it does not appear the shiedling lap buckles.  My question is, what the customer inspector wants is to perform additonal testing to test for failure due to the bends, but our Japanese contractor refuses to conduct addiitonal testing, becasue:
1. DC testing (because of cable length) is acceptable in Japan for new acceptance test for cables.  
2. There is a licensed engneer on site, a liason with the local utility power company who dictates this test, and reaffirms this test is acceptable.
3. The testing company refers all my questions regardng the value of DC testing to this license engineer.
4. None of the contractors nor the licensed engineer would contact the cable manufacturer to discuss whether DC and the test voltage (DC=V for a 6.6kV cable) is acceptable because it is not in theri culture to inquire more information expecially because the test is a testng requirement (stahdard requirement in construction practices, like the construction law) in Japan, and it is counterproductive for this kind of questions.

I am really at lost.  Is DC test truly sufficient in this case?  The customer inspection is an American who may not understand all these political interests.   

RE: AC vs DC Hipot Test

electricpete

(Electrical)

11 Jul 09 22:06

I don't think even any of the chicken littles in this thread said that dc test was destructive for new cables.

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RE: AC vs DC Hipot Test

electricpete

(Electrical)

11 Jul 09 22:09

My apologies. Should've put a smiley. I was merely expressing the fact that I don't share the same level of concern as some of the others.

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RE: AC vs DC Hipot Test

trosepe

(Electrical)

12 Jul 09 02:25

On the sample you removed, did you inspect the semi-conductor layer over the XLPE insulation? If the shield is OK and the semi-conductor is smooth,no waves, it will be OK.  The main reason for the min radius is to prevent the semi-conductor layer from seperating from the insulation. Air voids from even small separations will create an uneven electric field in the insulation and could breakdown the insulation over time. The bending would have to be severe to cause this type of separation on XLPE cable. Take a few samples and perform a 'peel' test on the semi-conductor just to be sure and give everyone involved peace of mind.

 

RE: AC vs DC Hipot Test

a10jp

(Electrical)

(OP)

12 Jul 09 04:07

Thanks.  In the first test we did, which was done very roughly, we bent hard, and cut open the sheath, and the semi-cond tape just under the copper metal sheilding appears ok.  

But I think you gave me a very good idea.  I will take a few samples.  I will perform the test for the manhole situation (dia=600mm, or 24") we have, which we have done and appeared no problem, and also what will be the minimum bend radius and find out at what point the shielding will fail.  I think this will be easy to do, than performing any other kinds of HIPOT test for which the test setup may not be readily avilable.

 

RE: AC vs DC Hipot Test

a10jp

(Electrical)

(OP)

12 Jul 09 08:10

In Japan, the AC and DC test voltages (for V) are as follows:

AC test voltage = x 1.15/1.1 x 1.5 = V AC
DC test voltage = AC test voltage x 2 = DC

Can anyone relate to this?  To provide further info, a typical cable (150mm2 equiv to 300KCM) is usually tested in the factory at 17kV AC for 10 min.  

At the vesy least, we know the dielectric withstand test voltage is below 75% of the maximum breakdown voltage.

RE: AC vs DC Hipot Test

trosepe

(Electrical)

12 Jul 09 13:11

If you tested the cable for 10 min. at 20.7KVDC you should be fine and I would not think you would need to do more. The DC test voltage for 5KV, 100% rated cable is 25 KV so your 20.7 KV test voltage is a bit conservitive but fine. The test voltage has only to do with the insulation, not the conductor size.

What is a good idea is to perform a 'proof' test after all splices, stress cones, elbows have been made but before connecting the cables to the equipment. The general rule of thumb for the DC proof test is TWO TIMES CABLE RATING PLUS 1,000. This is used on service aged cable when a new splice or termination is installed to 'proof' the system before re-energizing.  Very little stress but will detect many cable termination errors.  

RE: AC vs DC Hipot Test

benlanz

(Electrical)

29 Jul 09 05:17

I am coming to the discussion late too.  I have thoroughly enjoyed reading all of your posts. I am glad to see such a lively discussion about this often misunderstood subject.  It is exciting to hear so many writing in with fairly accurate, up-to-date information.  The word is getting out and that is good news for the industry.  I would like to add a perspective that I did not notice in your discussion.  We as engineers tend to like the science behind the technologies we use but, sometimes it is difficult to consider the business case.  Cost wise the tests per unit can be generally ranked in following order.  DC<AC (including 0.1Hz VLF)<TD< on-line PD<off-line PD.  However, accuracy and value of the test follows nearly the same ranking.  DC<AC<on-line PD<TD<Off-line PD.   I have clients ask me all the time, "What test should I use?."  My answer is question.  What does a failure of a your cable cost, in outage cost, repair cost, political cost, ....  ?  If you can give me a cost figure and the application, I will tell you the best test to use.  The cost of a failure is not something you are likely to ever hear discussed with IEEE, IEC, ICEA, AEIC,  or even little NETA.  Let's keep this simple:

New cable with a potential failure cost  <$10k,
I recommend DC megger  or a VLF <operating voltage for few seconds to make sure someone didn't leave the grounds on the other end and turn it on

New cable with a potential failure cost  >$25k
Repeat the factory PD test (50/60Hz off-line PD test) to locate any workmanship errors while the installer is on site and require them to repair the system as necessary and learn from their mistakes.

A few (<10) aged cables (>30yrs) with a potential failure cost < $10k ea
I don't recommend any testing.  If you own a TD set you can take a look at them to help get some kind of prioritization information but keep in mind the significant potential for measurement error. Replace cables as funds become available

Many (>20) aged cables (>30yrs) with a potential failure cost of whatever or a few high critical aged cables with a potential failure cost >$25k
Repeat the factory PD test (50/60Hz off-line PD test) to locate any insulation defects and use the information to develop a repair/ replace strategy.  .

I would like to review some of the data we have collected over last decade:

I agree that the DC withstand test is not good for aged PE cable systems
I agree that the DC withstand test can not detect most defects in extruded systems (detects approx. less then 1% of defects)
Our client's experience indicates at the AC (including 0.1Hz) withstand test can not detect most defects in new extruded systems (detects up to approx. 3x more than DC, less than 3% of defect)
Our client's experience indicates that a 0.1Hz AC withstand test will typically fail any where from 10 to 40% of critically aged xlpe systems.  An in some cases the cable systems have higher failure rate after the testing.
Cable and accessory manufacturers use the PD test as the final insulation quality check for their products
I agree that TD is a good insulation property test but, it can not detect where the losses are coming from.  
Recent TD studies show that most of the losses measured by typical field TD tests come from termination and splices which can have 10, 100, times more losses than cable insulation and still perform reliability.
Almost all TD tests performed in the field today do not employ guard circuits at the terminations and thus are susceptible to substantial leakage measurement error.
An on-line PD test detects approx. up to 3% of defect in cable systems.
An off-line PD test detects on the order of 99% of defect in cable systems if.... administered properly.  TDR mapping, sensitivity assessment, pC calibration, and measure cable response at the IEEE thresholds.

A10jp,  as far as the DC test, I am sure you can cover your political basis with a DC test but, don't count on it to assure reliability.   The test voltage values are nearly inconsequential.  I do recommend an IR reading as you go up to operating stress level to see if there is a tip-up in the curve.  Some times you get lucky and the defect has enough conduction to show up even if it does not fail.  

Cheers,

Hello to all,I am coming to the discussion late too. I have thoroughly enjoyed reading all of your posts. I am glad to see such a lively discussion about this often misunderstood subject. It is exciting to hear so many writing in with fairly accurate, up-to-date information. The word is getting out and that is good news for the industry. I would like to add a perspective that I did not notice in your discussion. We as engineers tend to like the science behind the technologies we use but, sometimes it is difficult to consider the business case. Cost wise the tests per unit can be generally ranked in following order. DC$25kRepeat the factory PD test (50/60Hz off-line PD test) to locate any workmanship errors while the installer is on site and require them to repair the system as necessary and learn from their mistakes.A few (<10) aged cables (>30yrs) with a potential failure cost < $10k eaI don't recommend any testing. If you own a TD set you can take a look at them to help get some kind of prioritization information but keep in mind the significant potential for measurement error. Replace cables as funds become availableMany (>20) aged cables (>30yrs) with a potential failure cost of whatever or a few high critical aged cables with a potential failure cost >$25kRepeat the factory PD test (50/60Hz off-line PD test) to locate any insulation defects and use the information to develop a repair/ replace strategy. .I would like to review some of the data we have collected over last decade:I agree that the DC withstand test is not good for aged PE cable systemsI agree that the DC withstand test can not detect most defects in extruded systems (detects approx. less then 1% of defects)Our client's experience indicates at the AC (including 0.1Hz) withstand test can not detect most defects in new extruded systems (detects up to approx. 3x more than DC, less than 3% of defect)Our client's experience indicates that a 0.1Hz AC withstand test will typically fail any where from 10 to 40% of critically aged xlpe systems. An in some cases the cable systems have higher failure rate after the testing.Cable and accessory manufacturers use the PD test as the final insulation quality check for their productsI agree that TD is a good insulation property test but, it can not detect where the losses are coming from.Recent TD studies show that most of the losses measured by typical field TD tests come from termination and splices which can have 10, 100, times more losses than cable insulation and still perform reliability.Almost all TD tests performed in the field today do not employ guard circuits at the terminations and thus are susceptible to substantial leakage measurement error.An on-line PD test detects approx. up to 3% of defect in cable systems.An off-line PD test detects on the order of 99% of defect in cable systems if.... administered properly. TDR mapping, sensitivity assessment, pC calibration, and measure cable response at the IEEE thresholds.A10jp, as far as the DC test, I am sure you can cover your political basis with a DC test but, don't count on it to assure reliability. The test voltage values are nearly inconsequential. I do recommend an IR reading as you go up to operating stress level to see if there is a tip-up in the curve. Some times you get lucky and the defect has enough conduction to show up even if it does not fail.Cheers,

Benjamin Lanz
Past Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

RE: AC vs DC Hipot Test

ScottyUK

(Electrical)

29 Jul 09 07:47

Welcome back.
  

benlanz,Welcome back.

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: AC vs DC Hipot Test

Zogzog

(Electrical)

30 Jul 09 10:11

Thanks for the input Ben, always like to hear what your thought are on MV cable testing.  

RE: AC vs DC Hipot Test

unclebob

(Electrical)

14 Jan 10 12:18

sorry to bring that subject back to page 1, but I think it's important to realize that VLF Testing is a destructive test if defects are presents in the tested cable:

Please read the attachment (page 3).


Is the VLF test destructive?
....

But my cable might fail during the test?
....

So, if you plan to VLF cables, plan to repair them too.

• http://files.engineering.com/getfile.aspx?folder=434acaf3-3caa-4c78-8cc0-97

Hello all,sorry to bring that subject back to page 1, but I think it's important to realize that VLF Testing is a destructive test if defects are presents in the tested cable:Please read the attachment (page 3).Is the VLF test destructive?....But my cable might fail during the test?....So, if you plan to VLF cables, plan to repair them too.

RE: AC vs DC Hipot Test

cgrodzinski

(Electrical)

15 Jan 10 08:43

inclebob, every voltage AC, DC, VLF (technically it is AC) ... that is applied to the insulation will have some detrimental effect on it. If the voltage is high enough to break the insulation ... this is the outcome of the test.

The purpose of the "HI-POT" is to check if the cable insulation is capable of surviving the rated voltage. If it is not .... it MUST fail at any test.

The discussion still remains which of the test reveals cable weak points without killing a good one(to be simple).

Chris Grodzinski

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News

Very low frequency (VLF) withstand testing is the application of an AC sinusoidal waveform, generally at 0.01 &#; 0.1 Hz, to assess the quality of electrical insulation in high capacitive loads, such as cables. During the test cables are subjected to a test voltage significantly higher than what they experience during normal operating conditions. The higher test voltage allows for weak points or pre-damaged areas within the cable to breakdown during the test, rather than while they are in service. Essentially, the VLF withstand test is a &#;go or no-go&#; test, otherwise known as a &#;pass/fail&#; test.

What are the advantages of VLF over power frequency?

A good basic equation to know is: P = 2πfCV2 

where f = applied test frequency, C = capacitance of the test object, and V = applied test voltage.

Already from this equation it can be seen that decreasing the applied test frequency will decrease the power required to apply a voltage, and since P = IV (where I = current), the amount of current required to apply a voltage is also lowered.

From the above relationship, when comparing VLF 0.1 Hz to power frequency (60 Hz), there is a 600x lower power and current requirement when testing a cable at the same test voltage. This has the added benefit that the size of the test instrument can be significantly reduced to allow for a very portable high voltage tester. An example of which is the Frida VLF Tester, which can generate test voltages up to 24 kVRMS or 34 kVpeak and weighs only 22 kg (48 lbs).

Frida VLF Tester

Due to the sinusoidal waveform, VLF can also be used for tan delta diagnostics and partial discharge diagnostics.

What are the causes of cable failure during a VLF withstand test?

When a cable is subjected to a considerably higher test voltage than what it typically sees in service, any defects in the cable will see higher stress levels that may grow within the insulation. This is a phenomenon known as &#;treeing&#; and these trees arise at stress enhancements where there are voids, protrusions, contaminants, or water trees. The term &#;treeing&#; stems from their branch-like structure resembling a tree.

Electrical trees are channels of carbonization that arise from partial discharge activity within the insulation. Once an electrical tree grows big enough and bridges the electrodes of the cable system, a breakdown of the cable insulation is created. Water trees are tree-like structures that form from the electrochemical interaction of the electric field and water ingress within the cable. Their growth is extremely slow, but they act as stress enhancements, which can help to initiate an electrical tree. Below is a photograph of a water tree growing into an electrical tree.

A water tree that is growing into an electrical tree.

What are the recommended test voltage levels and testing times?

The IEEE Std 400.2-: IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz) was introduced to give an easy to interpret guide for conducting VLF withstand tests on shielded power cables rated 5 &#; 69 kV. Below is an overview of the recommended voltage levels that should be applied during installation, acceptance, and maintenance testing of medium voltage distribution cables depending on the cable system rating (phase to phase voltage). Generally, VLF withstand testing calls for testing the cables up to 3U0, where U0 is the rated phase to ground voltage.

VLF testing times should last between 15 and 60 minutes, depending on the age of the circuit and what type of test is conducted. For example, a minimum test time of 30 minutes is recommended for aged cable circuits. Extending the time to 60 minutes should be considered for particularly important circuits, such as feeder circuits. For installation and/or acceptance tests, the minimum recommended time is 60 minutes.

The times recommended for VLF withstand testing stem from studies conducted on tree growth rate on partial discharge defects in XLPE cable systems. According to IEEE Std 400: IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems, differences can be seen in the channel tree growth rate between power frequency, VLF sinusoidal, and VLF cosine-rectangular, and the fastest tree growth rate is achieved by applying VLF sinusoidal. A channel tree growth rate with a 3U0 test voltage at 0.1 Hz VLF sinusoidal on field-aged XLPE cables is 10.9 &#; 12.6 mm per hour (mm/h). A typical 15 kV medium voltage cable in USA has an insulation thickness of 0.22&#; (5.6 mm), and therefore, during a VLF test time of 30 minutes all defects within the cable should grow to failure. When comparing this figure to power frequency AC or 0.1 Hz VLF Cos-Rectangular, the tree growth rate is only 2.2 &#; 5.9 and 3.4 &#; 7.8 mm/h, respectively. As a result, 0.1 Hz VLF sinusoidal is the ideal frequency and waveform for cable withstand testing. Potential failures should happen during the actual test so that repairs can be made immediately. Failures in the cable during service result in higher costs for the utilities and are a nuisance for the power consumer.

Why is withstand testing with DC voltage not recommended?

Testing of cables was traditionally conducted with DC voltages in the past, which was also known as DC hipoting. Once polymeric cables (XLPE, EPR, etc.) were introduced into the electric grid and readily being tested with DC, a considerable rise in premature failures occurred. These failures were being attributed to trapped space charge within the defects of the insulation due to enhanced charge migration in one direction (DC electric field). Once testing was completed and AC power frequency was reapplied in service, a significant field enhancement could occur at these defects leading to a fast-growing electrical tree and subsequent failure. An illustration of the space charge effect after DC testing a XLPE cable is shown below.

Space charges created in voids of polymeric cables during DC testing.

HVT&#;s Range of VLF Testers:

For more information, please visit vlf tester manufacturer.

Frida Viola PHG 80 Portable

• Voltages up to 24 kVRMS
• Ideal for testing voltage class cables up to 20 kV
• Ideal for maintenance testing of 25 kV cable systems Weighs only 22 kg (48 lbs)

• Voltages up to 44 kVRMS
• Ideal for testing voltage class cables up to 35 kV
• Ideal for maintenance testing of 46 kV cable systems 2 part design for enhanced portability

• Voltages up to 57 kVRMS
• Ideal for testing voltage class cables up to 46 kV
• Can be placed on wheels or mounted in a van