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

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

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...

 

[DanDel]

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.

 

[dpc]

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.

 

[Zogzog]

"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.

 

[electricpete]

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).


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[mpparent]

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

 

[ScottyUK]

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...

 

[mpparent]

Thanks Scotty,

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

Mike

 

[oldfieldguy]

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

 

[benlanz]

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
- 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.


Until next time… take care


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

 

[alehman]

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.

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

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

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?

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[abreug2]

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!

 

[benlanz]

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

 

[rovineye]

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.

 

[rwalker1]

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.

 

[JBinCA]

Benlanz - always good to read your explanations on PD.

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

"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