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Does Partial Discharge Test Make Hi-Pot Unneccesary?
6

Does Partial Discharge Test Make Hi-Pot Unneccesary?

Does Partial Discharge Test Make Hi-Pot Unneccesary?

(OP)
Hello,

     We are building a wind farm with a 34.5 KV collection system.  The contract calls for hi-pot testing prior to energization of all circuits.  In addition to the hi-pot, the customer has recently decided to have the entire collection system tested by a reputable firm using Partial Discharge equipment/analysis.

Question (1):  Is there any added value to doing the hi-pot test, or would this now be unnecessary?

     I've been told that a PD test will catch everything that a hi-pot will, and more, but need to be sure before deciding to skip the hi-pot.

Question (2):  Are there any potential risks to doing a 1.5X hi-pot test on brand-new cable?

     I am aware that hi-potting can lead to premature failure on cable with early-stage water penetration, but I'm not sure if there are risks if the cable is newly laid.

Thanks for any clarification you can offer!

FEinTX

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

I cannot agree that PD makes Hipot unnecessary. Nor does Hipot make PD unnecessary. PD detects voids in the insulation system that causes ozone generation and premature failure in windings. Dry type transformers are a prime example. Hipot tests withstand voltage - and that is quite another thing. I would go for both. But if I was allowed only one pick, that pick would be Hipot.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Have a search for posts by BenLanz using the 'Search by Handle' option. It should pull up about 50 posts which Ben has answered. He is a little biased toward PD testing but he makes some very sound arguments in favour of it. Well worth a read if only to help make up your own mind.
 

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  Sometimes I only open my mouth to swap feet...

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

HiPot testing of new cable is still the standard. It's the best way to determine that the cable is OK to energize.

PD testing is a great alternative to maintenance HiPot testing of cable in service, especially since it (theoretically) does not require a shutdown. In reality, the installation of the monitoring devices for PD testing may involve risky exposure to live connections without a shutdown.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

I would get testing recommendations from the cable manufacturer and make sure that these are followed.  If this is XLPE insulation, they may have very specific recommendations regarding hi-pot testing.

Cable testing standards are a moving target right now.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

"Cable testing standards are a moving target right now."

Boy, you can say that again. There is also VLF and Tan Delta methods to consider. I would recommend a VLF hipot test and then an on line PD test after energizing to establish baseline data for future PD tests.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Question 1-  Speaking for motor montoring (not cable), I think PD test does not eliminate need for hi-pot.    The PD and Hi-pot get at two different things.  

1A - Hi-pot is more suited to an acceptance test.  In theory it demonstrates you have margin between breakdown voltage and operating voltage which provides some limited measure of assurance of reliable operation.  
1B - PD is more suited to a condition-monitoring test than a acceptance test.  PD is not at all suited as an acceptance test because it relies more on rate of change than on absolute level.  Additionally, it is not destrutive (hi-pot can be).

Question 2 - I think 1.5X operating voltage should certainly be ok on dry new cable.  In the old days, the dc hi-pot levels for cable ranged up above 4x operating voltage (although dc testing of cables has become more controversial of late).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

I agree with the others...use DC Hi Pot as an acceptance test for NEW cable.  For older cable..don't use it.  Use online PD testing or something like VLF Hi Pot if you have short cable runs (it's been awhile, I think that's how VLF goes).

Mike

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Mike,

VLF is good for longer runs because the capacitive reactance of the cable at low frequency is high making it easier to pick out the resistive element.
 

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  Sometimes I only open my mouth to swap feet...

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Thanks Scotty,

Right,...I must be thinking of AC Hi Pot.

Mike

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

I have never been bashful about weighing on this subject.

The DC hipot test WAS the standard for years.  I have seen it badly done and in such cases you can end up with ruined cables.  I have seen it properly applied by qualified technicians using a structured approach, and in those cases it returned valuable, trendable data over a period of years.

In my own experience with a client who had such a specification, I never saw a cable fail under DC hipot test.  I did see a cable fail which had previous tests which were approaching a "failing" reading three years before, but that cable (5 kV PILC) failed  when it was being megged at 5 kV as one of the steps in the client's hipotting procedure.  I tested hundreds of cables in the 5 kV - 15 kV range for this client, in lengths from 20 to 8000 feet.

Due to this experience, I am not ready to write off the DC hipot for maintenance testing.  However, I find myself going against the tide here.

The hipot test has the advantage of needing less expensive equipment.  I am not "hands-on" familiar with the PD testing for cables, so I cannot address the training issues.

old field guy

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

6
At the risk of repeating myself, I would like to address the topic of commissioning tests for this forum once again.  

In the spirit of full disclosure, I work for a company that builds and provides services with PD diagnostic equipment.  ScottyUK is absolutely correct. I am biased but, for very good reasons.  Why don’t I work for a company that makes withstand (HIPOT) equipment?  There is no future in it.  Modern extruded cables do not work like the old paper insulated systems.  Please do not compare paper insulated, transformers, motors, cables or any other electrical components with modern extruded components. The failure mechanisms are completely different.

FeinTX,

The Bottom Line:
- DC is no longer supported by IEEE as an acceptance test.
- High potential withstand tests can not assure future reliability.
- A standardize PD test is the test that manufactures have used to prove electrical integrity of extruded components for the last 40 years!  Hmmm…. Maybe they are on to something smile
- A standardized PD acceptance completely eliminates the need for a withstand test. (Basically, if you could get inside a defect while a HIPOT is ‘failing’ it, you would see the material being pitted away and PD all over the place.  PD is almost without exception a precursor to extruded system failure)
- No problem with going to 1.5, 2, or even 3x on new systems!! Cables and Accessories are required to withstand routine withstand voltages in excess of 3x the operating voltage for hours. A PD test is only for a few seconds. Again, if you can prove there is no PD and you can be assured that there isn’t an active failure mechanism.

Dandel,

Contractor recommendations:
Yes, many contractors still use a DC HIPOT for commissioning wind farms and would say that it is ‘standard practice’.  Unfortunately, what most of them don’t know is that a 110V extension cord will pass a DC HIPOT, yet we all know that it will blow up with medium voltage AC applied.  Most contractors who have used a DC HIPOT on extruded cable systems over the last 40 years have developed a false sense of security.  This is because nearly all defects will pass the test!  What has saved our industry is good design and workmanship practices, not the DC HIPOT.  Please help the industry and get the word out.  If you want to do a HIPOT, at least do an AC HIPOT.  Although the AC HIPOT test can not assure future reliability, at least it creates that conditions that could lead to a failure under test!  A low frequency HIPOT (VLF) is the best choice in the destructive, withstand class of tests (IEEE Type 1 test).  The caveat with destructive withstand tests is, What if I grow one defect from 10% of in the insulation to 100% and grow another from 5% to 95%?

A true story:
We just did some consulting for one wind farm.  They just had a cable failure a few months after the DC HIPOT that took out several turbines. After the wind farm owner lost around $400,000 in revenue and the contractor paid around $80,000 for repairs, what do you think they did, DC HIPOTed the repair and let system go again? I don’t think so.  They did a PD diagnostic acceptance test which used the IEEE standards.  A proper field PD test repeats the factory PD test on the cable and accessories. They found three other defects on the collector system with the PD test (failures just waiting to happen).  


When to use the PD Diagnostic Acceptance Test:
If the world was ideal we would do the same test in the field that the manufacturers use at the factory.  However, I know that a standardized PD test isn’t always the right choice for an acceptance test. If you have a URD cable feeding ten customers, so what if it fails?  You reconfigure the loop, repair the cable, and move on.  However, when you are talking about industrial and commercial plants, one failure can pay for all the PD diagnostics for a whole fleet of plants.  Let’s make sure we keep the financial impact in mind when we are making recommendations.

Ask the manufacturer:
I agree with dpc.  Check with the manufacture of your joints, terminations, and cable.  However, don’t ask them what to do in the field, ask them how they prove the electrical integrity of the components they sell you!  That right, if you are talking to a reputable manufacturer they will tell you that every product they sell is 100% tested with a PD test in the factory to IEEE 404, IEEE 48, and ICEA S-94-649 PD specifications.  If they are confident with their product they will welcome a PD acceptance test.  If the recommend a DC withstand hmm….

IEEE 400-2001 states that if: (once again for this forum)

If the cable system can be tested in the field to show that its partial discharge level is comparable with that obtained in the factory tests on the cable and accessories, it is the most convincing evidence that the cable system is in excellent condition.

An on-line PD test, although it may be useful in some cases, does not fulfill this requirement.

Again, please do not compare paper insulated components with modern extruded components. A few mils of extruded material will withstand a DC test. Ever wonder why you can DC test a cable system without a stress cone at the termination, yet we all know that the cable would fail in service without it? Sure you won’t hurt a new cable with DC but, you won’t find the problems either. So what is the point in going through the exercise of a DC test on an extruded cable system? The only reason I recommend a DC test, if other options are not available, is to assure that someone didn’t leave the other end grounded. smile


Until next time… take care

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

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

I'll add that I have had trouble getting a testing firm to do PD testing. A while back I asked for a quotation from one of the major national firms in the U.S. At first they didn't know what I was talking about. Then they said they could only do an on-line test. I pointed them to IEEE. A few weeks later I received an outrageously high quotation. My point is that PD testing is apparently not yet well understood or accepted by some and more education is needed.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

I learned a lot from this thread. I am not much involved in cable testing any more. But if I am allowed one pick - I will pick PD before Hipot (yes, I changed my mind). And that is not because Ben said so - it is because he gave me good reasons.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Quote (benlanz):


DC is no longer supported by IEEE as an acceptance test.

Quote (IEEE 400-2001):


5.4 Performing HVDC tests
Equipment for producing these voltages is typified by rectification of an ac power supply. Output voltage is
variable by adjusting the ac input voltage. Output current, i.e., current into the cable systemunder test, may
be measured on the HVDC side or ratio transformation of the ac input. For the latter case, the test equipment
leakage may mask the test current and the interpretation of results. Apply the prescribed test voltage for the
specified period of time. IEEE P400.1 provides guidance for the selection of test voltage and time.
The following three general types of tests can be conducted with this equipment.

5.4.1 DC withstand test
A voltage at a prescribed level is applied for a prescribed duration. The cable system is deemed to be acceptable
if no breakdown occurs. This may be categorized as a “pass/fail” or “go-no go” test (Type 1 Field Test).
5.4.2 Leakage current-time tests
Total apparent leakage output current is recorded as a function of time at a prescribed voltage level. The
variations of leakage current with time (rather than its absolute value) provide diagnostic information on the
cable system.
5.4.3 Step-voltage test or leakage current tip-up tests
The voltage is increased in small steps while the steady-state leakage current is recorded, until the maximum
test voltage is reached or a pronounced nonlinear relationship between current and voltage is displayed.
Such departures from linearity may denote a defective insulation system.


5.5 Summary of advantages and disadvantages
Some of the advantages and disadvantages of dc testing are listed below.
5.5.1 Advantages
- Relatively simple and light test equipment, in comparison to ac, facilitate portability.
- Input power supply requirements are readily available.
- Extensive history of successful testing of laminated dielectric cable systems and well-established
data base.
- It is effective when the failure mechanism is triggered by conduction or by thermal consideration.
- It is effective on interface problems of joints and terminations and surface problems of terminations.
- Purchase cost is generally lower than that of non-dc test equipment for comparable kilovolt output.
5.5.2 Disadvantages
- It is blind to certain types of defects, such as clean voids and cuts.
- It may not replicate the stress distribution existing with power frequency ac voltage. The stress
distribution is sensitive to temperature and temperature distribution.
- It may cause undesirable space charge accumulation, especially at accessory to cable insulation
interfaces
- It may adversely affect future performance of water-tree-affected extruded dielectric cables.
- Leakage current readings may have wide variations due to atmospheric conditions and lack of
control of charges at termination lugs.
 

I didn't interpret this to mean that IEEE said that DC testing was "no longer supported".   I saw advantages and disadvantages listed as if the user is supposed to decide which matches his situation best. I did see that IEEE P400 has not yet been issued, which doesn't tell me anything about whether dc is endorsed or not endorsed.  I can go back to previous standards to find test levels and durations.  Is there a relevant excerpt or standard I have overlooked?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Well enough said on both types of tests and I agree and have learned a few things.

I work for a petrochem company in the central engineering group and our philosophy is to DC Hipot cables that are new before we put into service. In addition we also conduct PD test so that we have a base line to compare in the future as we recommend PD testing for predictive maintenance.

So yes we are getting away from DC Hipot of cables older thab 5 yrs. However, as many have indicated DC HIpot is still a good go/no go test!

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

electricpete,

I understand where you are coming from. You are correct. I should have been a little more cautious and specific in my statement. IEEE 400-2001 still has the DC test listed as one of many tests the can be applied to cables. However, FEinTX has a specific application. FEinTX needs acceptance test for extruded cables.  I addressed this issue in thread238-125873.

So again,

IEEE 400 definition of a shielded power cable acceptance test:
"A field test made after cable system installation, including terminations (see IEEE 48) and joints (see IEEE 404), but before the cable system is placed in normal service. The test is intended to further detect installation damage and to show any gross defects or errors in installation of other system components."

The DC HIPOT withstand test no longer fulfills IEEE 400’s definition of an acceptance test. IEEE 400, section 4.2, states:
    
"Furthermore, from the work of Bach  [Bach, R., et al, “Voltage Tests to Assess Medium Voltage Cable Systems,” Elektrizitaetswirtschaft, Jg. 92, H. 17/18. pp 1076-1080, 1993.], we know that even massive insulation defects in extruded dielectric insulation cannot be detected with dc at the recommended voltage levels."

If the purpose of an acceptance test, according to the definition above, is to detect any gross (massive) defects or errors in installation of cable system components, then, according to IEEE, the DC HIPOT is not suitable for extruded cable installations.  Therefore, it would be unwise for cable owners who require a high reliability to depend on a DC test which (by definition) is no longer supported by IEEE as an acceptance test for extruded systems.

alehman,

You are correct about the cost. As I stated in the “When to use the PD Diagnostic Acceptance Test: “ section, in my comments above, at this time it does not make sense to call on a reputable firm to perform a standardize off-line PD acceptance test for every little piece of cable you put in the ground.  However, when the stakes are high, the cost benefit of a meaningful acceptance test can be tremendous.  Some day standardize PD acceptance test will become common place but, until then they will be used in high risk areas and where ‘high volume production line’ testing can be performed at a considerably lower cost.



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

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Everyone has addressed the cable PD issue. But I am not sure if cable is all FE is asking about. His testing may go beyond the cables. For instance if the gererators are involved they should record a polarization index that can be trended.

 

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

My only input would be, that while hi-pot testing is definately being readdressed (now considered destructive) it is still a very cost efficient method to expose early construction problems.  As stated earlier, PD testing is still considerably more expensive than hi-potting, and while the hi-pot test might not give you all that the PD does, it still has value.  It is typically (at this time) more of a local on site type of test than PD and can make sure the system is ready for PD testing.  Much the same as meggering cables prior to paying someone to hi-pot.  

You will be able to segement the testing as construction proceeds and pick up typical failures (terminations typically) prior to testing the whole system.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Benlanz - always good to read your explanations on PD.

I have to challenge you on one of your points though:

Quote:

"Ask the manufacturer:
I agree with dpc.  Check with the manufacture of your joints, terminations, and cable.  However, don’t ask them what to do in the field, ask them how they prove the electrical integrity of the components they sell you!  

1. Why would the manufacturer be a good reference for a "factory" test, but not a "field" test?

2. Economics factor into testing decisions just as much as they do application / purchasing decisions.  XFMRs, gens, motors, SWGR, BKRs, etc are all subjected to more stringent tests in the factory than they regularly are for field acceptance.  Why would cable not follow suit?

Best Regards,

JB

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Hi

I also have some questions for the PD test:

Are there any guidelines for what level of pC are acceptabel for cables, joints or other accessories ?

What method is most used for PD testing ?
OWTS, AC 50-60Hz resonans test, or others??

Aren't there any startup tests before you charge the cable to 2xUo or more?

The IEC 62067 recommends you do a DC oversheat and an AC insulation test. Is that recommendation only for HV and not MV.. can't remember..

- Stine

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

JBinCA,

Thanks for your support. I am glad to answer your questions as best I can.

*****1. Why would the manufacturer be a good reference for a "factory" test, but not a "field" test?

Good question. I would say most manufacturers recommend a partial discharge test in the field if the economics warrant it, however, some people on this forum continue to refer to specific manufacturer who has outdated recommendation material on their website.

*****2. Economics factor into testing decisions just as much as they do application / purchasing decisions.  XFMRs, gens, motors, SWGR, BKRs, etc are all subjected to more stringent tests in the factory than they regularly are for field acceptance.  Why would cable not follow suit?

You are correct. Factory tests are typically more stringent and sometime not achievable in the field. However, where technically possible and economically feasible, I think the industry should strive toward the goal of comparing apples with apples. I don't think anyone can argue with this goal. An example of what you are talking about is the ICEA/AEIC requirement for a factory MV cable insulation test.  ICEA recommends an approximate stress in Volts/mil of insulation which equates to about 4Uo.  This is impractical in the field so, a good trade off is somewhere between 2 and 3Uo.  One would not expect to have transients on a cable system greater than 3Uo so this is a fairly conservative tradeoff.


StineIng,


*****Are there any guidelines for what level of pC are acceptable for cables, joints or other accessories?

Good question. Yes IEEE 400 recommend used the factory levels for the field. So… here you go.

IEEE 400-2001 General Guide                       Shielded Power Cable Testing
IEEE 48-1996 Terminations                     No PD ? 5pC at ? 1.5Uo
IEEE 404-2000 Joints                         No PD ? 3pC at ? 1.5Uo
IEEE 386-1995 Separable Connectors                  No PD ? 3pC at ? 1.3Uo
ANSI/ICEA S 97-682-2004–HV/EHV Extruded Cable         No PD ? 5pC at ? 2.0Uo
ICEA S-93-639-2000 MV Extruded Cable              No PD ? 5pC at ? 4.0Uo
IEEE 400.3-2007 Guide to Partial Discharge Testing         General description of PD test

*****What method is most used for PD testing ?
OWTS, AC 50-60Hz resonans test, or others??

I assume by ‘test method’ you really mean what voltage source is used the most. Factories almost always use a 50 or 60 Hz series resonant voltage sources.  Series resonant voltage sources are also widely used in the field.  Most of the statistically significant PD data which has been presented is derived from 50/60Hz work.  However, where the use of a series resonant voltage source is not practical for  PD tests, very low frequency AC and variable frequency resonant voltage sources are used.  Pulsed/ oscillating wave voltage sources are used less frequently. One of the problems with changing the frequency too much from 50/60 Hz is that PD activity behaves differently at different frequencies, at which point, we can no longer compare directly to the standards listed above.







*****Aren't there any startup tests before you charge the cable to 2xUo or more?

Typically the voltage is ramped up to operating voltage first to make sure that that nothing active at or below operation before proceeding to higher voltages.


*****The IEC 62067 recommends you do a DC oversheat and an AC insulation test. Is that recommendation only for HV and not MV.. can't remember..


In my opinion IEC 620607 (Power cables with extruded insulation and their accessories for rated voltages above 150 kV up to 500 kV) is a very lame standard which at the end basically says… “Do what and this standard will cover you.”

Section 14 Guidance for “Electrical tests after installation”:

“For installations where only the oversheath test according to 14.1 is carried out, quality assurance procedures during installation of accessories may, by agreement between the purchaser and contractor, replace the insulation test.

The a.c. test voltage to be applied shall be subject to agreement between the purchaser and the contractor. The waveform shall be substantially sinusoidal and the frequency shall be between 20 Hz and 300 Hz. The voltage shall be applied for 1 h, either with a voltage according to table 10 or with 1,7Uo , depending on practical operational conditions.

Alternatively, a voltage of Uo may be applied for 24h.”

Therefore, you have 3 options.  None of which can give you an assurance of reliable future performance.
1) do nothing except inspection during construction
2) perform a 24-hour withstand at Uo (soak test)
3) perform a test for 1 hour at 1.7Uo (hipot test)

In my opinion IEC is far behind IEEE in these matters.

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

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Thank you very much benlanz! Just what I needed to know, your posts have ben very helpfull...

God day to you all!

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

IEEE 400-2001 General Guide                       Shielded Power Cable Testing
IEEE 48-1996 Terminations           No PD ? 5pC at ? 1.5Uo
IEEE 404-2000 Joints                No PD ? 3pC at ? 1.5Uo

I have 3 things:
1)unfortunately these values are mostly not detectable in the field during PD test
2) speaking about XLPE cable: I agree PD test is a good method for afterlaying test - the only thing you can find testing a new cable is a pure workmanship (joints, terminations,laying process). The cable itself has been already tested in the factory with values like 1 pC! I would definitely avoid DC Hipot testing! AC (VLF) Hipot is OK as an additional information.
3)There is different situation regarding IN OPERATION XLPE cables.In this case I would combine both - PD Test and VLF test - for example if there is a water contained in the cable, there are no PD and it this case VLF can help you - of course, help to DAMAGE the cable - it is a destructive methode. An alternative is a VLF tan delta measurement or other polarisation/depol. methodes...   

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

This is a great discussion mostly focussed on cable testing.

There are few areas which should be dicussed as well:

1. How close will the PD results from factory and those of field in harsh noisy environment match and how can they be compared with each other? To me PD testing and evaluation is still evolving and will take some time before it gets accepted. IEEE has issued some draft guides in this regard.

2. Unlike HiPot where only current sourcing capability is to be considered for various equipment (like cable, transformer, switchgear, etc), different PD detection and evaluation techniques are recommended for various equipments. I am not saying that these tests serve the same purpose. I just mean to say that perhaps non-standard and varying PD techniques are the reason people are sticking to HiPot as acceptance test.

3. Finally we have to see it from the EPC company's perspective who has to keep the cost to minimum. Mindset of consultants are to be changed also.

4. I would also like belanz to comment on different techniques of PD detection like electrical, acoustical, etc and noise rejection philosophies based on narrow band, wide band detection, etc.

Thanks,
Olic

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

1. How close will the PD results from factory and those of field in harsh noisy environment match and how can they be compared with each other? To me PD testing and evaluation is still evolving and will take some time before it gets accepted. IEEE has issued some draft guides in this regard.

According to my experience you can not compare factory and field results. The factory limit is 1-2 pC.This is an acceptance test. You can not practicaly measure these values in the field. If there is a problem in the field, you can see hunderds, sometimes thousends pC.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

We recently rewound a generator in which a PD test concluded that the unit was at the end of its life.  We Hi-potted the unit to failure, which happened at 80 kV.  The generator is a 13.8kV unit.

Honestly, I'm no expert in PD testing.   Perhaps it's because the tests were conducted poorly or incorrectly.  I don't know.    

On the other side, however, we had a failure of a unit which had passed the Hipot test a year earlier.  PD testing on this unit was not done to my knowledge.  We found debris had been inside the insulation and eventually caused a failure.  So the Hipot test isn't necessarily perfect.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

StineIng.

You are very welcome.

Please note that the question marks are an error created when my text was converted to HTML.  All of the standard recommendations should read (for example)

IEEE 48-1996 Terminations           No PD [greater than] 5pC at [greater than or equal to] 1.5Uo


Petamm

You raise a good point. If one can not achieve factory comparable sensitivities in the field they are probably not using state-of-the-art technology.  With state-of-the-art equipment and good methodology, 10pC can be achieved on approximately 90% of all tests.  Notice IEEE use of the word comparable. We do not live in an ideal world.  For example if one can only achieve 10pC sensitivity in the field, statistically significant studies show that 95% of defects can still be detected in cable systems.  Is a 95% confidence level still comparable to the factory test? Experience shows that it is.

You are correct.  Tangent delta and other loss factor measurements can are useful for significantly aged systems (greater than 20 years old).  However, water is not specifically the problem.  When there are impurities in the cable and they are aged over decades of time water trees can form.  However, according to large scale field studies, water trees do not fail cable directly.  The vast majority of the time an electrical tree forms first and then over a period of years a carbonized fault channel grows.  This is why an off-line PD is so effective with aged cable systems.  Please refer to the August 2006 issue the IEEE Insulation magazine for more information.

Please remember that pC magnitude is only a measure of a PD test’s sensitivity and reliability, not a defect’s severity.   

Olicg

Please note that IEEE does not officially issue draft guides for the general public.  IEEE 400 has been approved and published since 2001!  IEEE 400.3 is approved and will most likely be published this year.

It would take quite some time to discuss all the differences of PD tests but here are some of the basic facts:

traditional narrow band systems <500kHz (factory test systems) are only useful in shielded rooms.
Modern wide band can detect more than a narrow band systems and locate defects if the noise is filtered.

Acoustic PD testing is an on-line method, which means it can not prove reliability (absence of PD/standards) but, it can find some (PD sites) issues which are physically accessible.  If these issues are corrected, the cable system’s reliability will improve.  

As far as filtering techniques, although a fascinating topic, I think it is beyond the scope of this practical forum.



TurbineGen

Please be careful not to compare mica and paper insulated rotating machines with extruded cable insulation. They are vastly different.

Also be careful when comparing a PD test with a high potential withstand.  PD is associated with an erosion process that can take decades to go to failure. An AC break down test does not simulate what years of erosion at a significantly lower voltage will do.

Kind regards to all,

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

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

belanz,

thanks for the reply.

Like you i am also a big supporter of PD testing. Over the years i have been involved in PD testing of rotating machinery using commercial instruments from Adwel, Power Diagnostix and Lemke. I agree with what someone has written above that PD testing is more beneficial as Preventive Maintenance Tests although it is more than useful as acceptance test also. To me online test gives important information provided you can distinguish accurately between noise and PD of interest.

You are right filtering techniques discussion may not be a feasible discussion at the forum but would be quite interesting if done in a seperate thread.

Olic

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

BenLanz: as usually, your writting is very interesting. I have one notice - you mentioned that water trees do not damage the cable directly. Of course it is true. But I am not sure if you are right claiming that after changing the water tree to electrical tree it takes a peiod of years to the fault in the cable. Some studies says that after this change(WT to ET) the process to fault is quite fast.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

You are welcome Olicg.  I agree with you.  Separating noise and signal is very difficult while testing on-line.  I know of several case studies where external signals were mistaken for PD activity in the cable.

Petamn.

I am glad you find my comments interesting. I hope you find them useful too! winky smile  I understand your confusion with the water tree/ electrical tree issue.  If you asked me this question 10 years ago I would have said the same thing as you.  If you look at a vast majority of the research on water tree/electrical tree interaction you find that the work was mostly done in the laboratory.  This has only served to confuse our industy.  In the largest and first field study of its kind only about 20% of the electrical trees went to failure in the first year.  Watching these electrical tree sites over the last 8 years, the study found that a water tree is both a curse and a blessing. The water tree creates a high stress area which can lead to the formation of an electrical tree. However, once the electrical tree (carbonized partial fault channel) starts growing, it grows toward the water tree and is slowed because it cannot grow through its low stress interior and must grow around it!  Growing around the water tree can take years.  In the afore mentioned study less than 50% of the cables with electrical trees have failed in 8 years.

Again, for more information please review the August 2006 IEEE Insulation Magazine article.  As you seem to be interested in the fundamentals of diagnostics, I think you will enjoy this ground breaking research article.

http://www.imcorptech.com/Papers/Mashikian%20Electrical%20Insulation%20Magazine%20Offline%20PD%20Diagnosticsl.pdf

Best regards,

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

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Back to the discussion regarding HiPot and PD test. Excuse me for the simplyfied way I am trying to explain my feeling about: You can not really compare Hipot and PD tests because it is not the same method, the target and result is not the same as well. It is a mistake thinking both can find the same or both can lead to the same result. Simply - Hipot is a "brutal" destructive method, like someone kick you to your mouth - the realy pure teeth will probably break, sometimes even good teeth as well, some future problems possible too. But if you have and discover a tooth decay (PD) in one tooth-it really takes a lot of time till big defect. But this does not mean it is OK. And it is not neccessary every tooth with tooth decay will break after kicking down. Means not every cable with PD must fail during Hipot test. Not every cable passing Hipot test is PD free. PD test is "gentle" not destructive methode, which gives you usually time enough to prevent the break down, plan your repair,know your equipement condition.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Ben: of course it is not only interesting but usefull too smile Thank you for the article link, I am going to study it.

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Ben: I have studied the article. It is very interesting AND usefull again smile
In that study, WT seems to be like a protection for ET growth.
My opinion is that most of the mentioned examples had PDIV above Uo(p.u.), that is why it took so many years till defect.
Different situation would be if PDIV was at 1Uo or less - then the process leading to the fault would be fast or faster. Do the authors have any examples with PDIV=Uo?
Some studies says the 0,1Hz 60 min 3Uo withstand test is enough to pass the cable insulation in case some insulation problems, I agree sometimes it can only speed up this process to the fault. May be the WT really slows the speed of the ET growth. The problem is how to found out if it is separate ET or ET associated with WT... The fact it takes years does not mean it is OK - you know what I mean - that´s why we do PD measurement (to predict not to damage).
As mentioned in the article - when WT changes to ET after some stimulation - the defect comes quite soon. This is exactly the example I wanted to describe in my previous post - you are mostly not able discover the WT areas using PD test in case they are not associated with ET.That is why I think it is good to combine PD and VLF tests.
There are more things to discuss and not time enough - sorry, next time smile   

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

benlanz:

Very interesting!

You might consider putting it together in the FAQ-section.

Thanks
Regards
Ralph

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RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Belanz,

I would like to know which equipment do you use for PD testing? Does your company make test sets?

Olic

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Olicq,

We use the IMCORP DSD Technology.


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

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Ben,

Can you define "Thermally driven Mechinisms"?

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Zogzog,

My definition of a thermally driven mechanism is: a means of dielectric failure which is created by elevating the cable insulation temperature beyond its rated thermal limits. For example, ~>80C for PILC, ~>90C for XLPE and some EPRs, and ~>110C for new specially designed EPRs.  I am curious why you ask?

Regards,

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

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

It was listed as an advatage of DC hipot testing per IEEE 400 in a presentation at the 2006 NETA Conference. As in "able to find defects from thermally driven mechinisms". I just wasnt sure what they refering to, but makes sense.

So an overload condition for a long period of time may damage the insulation, and a DC overpotential test (might) find that specific type of failure. Would you agree with that statement? OF course by the time you test it, the cable has probally already failed smile

RE: Does Partial Discharge Test Make Hi-Pot Unneccesary?

Zogzog,

I believe I understand your question much better now. Your question is a good one.  No test can predict if a cable is going to be mistreated and over heated. However, some conduction types of defects can produce localized heating.  This is a very low probability event in extruded insulation (plastic and rubber) but, there is a substantial probability in paper insulation. With the data I have seen, I would say that the chance of detecting conduction defects with a DC test in extruded cable systems (such as the systems at wind farms) is less than 1%.

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

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