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]

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.

 

[trosepe]

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".

 

[Zogzog]

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.

 

[dpmac]

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.

 

[Zogzog]

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 2007 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?

 

[ausphil]

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

 

[cgrodzinski]

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

 

[dpc]

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)

 

[Zogzog]

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.

 

[a10jp]

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=20700V 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.

 

[electricpete]

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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[electricpete]

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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[trosepe]

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.

 

[a10jp]

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.

 

[a10jp]

In Japan, the AC and DC test voltages (for 6600V) are as follows:

AC test voltage = 6600 x 1.15/1.1 x 1.5 = 10350V AC
DC test voltage = AC test voltage x 2 = 20700 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.

 

[trosepe]

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.

 

[benlanz]

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<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, 1000 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

 

[ScottyUK]

benlanz,

Welcome back.


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

 

[Zogzog]

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

 

[unclebob]

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.