Does Partial Discharge Test Make Hi-Pot Unneccesary? 7

[FEinTX]

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

 

[StineIng]

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

 

[benlanz]

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if the economics warrant it, however, some people on this forum continue to refer to specific manufacturer who has outdated recommendation material on their website.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

 

[StineIng]

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

God day to you all!

 

[petamm]

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...

 

[olicg]

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

 

[petamm]

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.

 

[TurbineGen]

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.

 

[benlanz]

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

 

[olicg]

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

 

[petamm]

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.

 

[benlanz]

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! ;-) 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/Ma...sulation Magazine Offline PD Diagnosticsl.pdf

Best regards,


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

 

[petamm]

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.

 

[petamm]

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

 

[petamm]

Ben: I have studied the article. It is very interesting AND usefull again
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

 

[RalphChristie]

benlanz:

Very interesting!

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

Thanks
Regards
Ralph



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[olicg]

Belanz,

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

Olic

 

[benlanz]

Olicq,

We use the IMCORP DSD Technology.




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

 

[Zogzog]

Ben,

Can you define "Thermally driven Mechinisms"?

 

[benlanz]

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

 

[Zogzog]

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