How long does a DC hipot test take?

IC&#;Capacitive

Capacitive (or geometric capacitive) current is also called inrush current. The windings have capacitance. Current is required to elevate its voltage potential. Capacitive current typically drops to zero within seconds after the test voltage provided by the motor tester is stable.

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IA&#;Absorption

Absorption current is present during atomic and any molecular polarization of the insulation, and is the current of interest during a PI test. This current will drop to zero, or near zero, over a period of time that varies by motor. The drop can happen in seconds or may take 10 minutes or more. 

IG&#;Volume Conduction

Volume conduction current is the current that flows through the entire volume of the insulation between ground and the conductors. In good windings, this current is usually zero or near zero, and depends on the composition and condition of the insulation system. People sometimes think of this current as &#;leakage&#; current. The volume conduction current certainly leaks through the insulation, but the surface conduction current (IL) is usually the main leakage in a used motor.

IL&#;Surface Conduction

Surface conduction current is often referred to as surface leakage current. The surface conduction current runs over the end winding surfaces of the insulation.

• Surface conduction is a result of surface contamination, dirt and moisture on the windings that are connected to ground.
• As the contamination level increases, the resistance of the contamination drops, and the current increases.
• As the voltage increases, the current increases more or less proportionally with the voltage applied by the motor tester.
• For used, good motors, this current will dwarf the absorption and volume conduction currents because of the relatively lower resistance in the surface contamination.
• For new, totally clean, and dry motors this current should be zero or near zero.

View Full Version : Cable hi-pot withstanding time, why 15 minutes?

moonmoon

Hi everyone, in my instruction of cable hi-pot testing, the shield cable withstanding time is 15 minutes, un-shielded cable is 5 minutes.

It is super boring to hipot a high voltage cable, (step up time plus 15 minutes withstanding time, about half hour per cable. could we reduce the withstanding time), could somebody please explain why it needs 15 minutes?

Thank you.

madMAX

lol :D i second that! even more boring than transformer PI


It is super boring to hipot a hightvoltage cable, ( step on time plus 15 minutes withstanding time, about half houre per cable.

rgrabis

Boring, agreed. Related to cable testing, Were you instructed to raise the test voltage in 5 steps to max voltage?

JohnRaz

yes, 5 Steps.
VLF testing cable is just as much fun.

Will Ward

Normally we do an Insulation Resistance test before and after the 15 minute High Potential Test. I was wondering if this is standard or not? Thanks.

madMAX

Why would you need to do insulation resistance twice?


Normally we do an Insulation Resistance test before and
after the 15 minute High Potential Test...
I was wondering if this is standard or not?
Thanks

stevenwerynski

how were your megger readings the second time did they get worse? just curious if, and how bad the high pot damaged the insulation of the cable...

DanielTronco

Are you an owner or an hourly paid employee? haha

Wait the 15min man and plot the readings that you take every minute. You'll see the trend of the results should reduce quickly in the beginning and then start to level out at the end of the 15min period. Almost like a wave.

Sometimes we plot these on an excel file and submit them with the report. It's a good visual to show you weren't just making up numbers.

madMAX

tie all three cables together, do a gang test, and roll out early. ;)

moonmoon

I NEED TO MEGGER BEFORE HI-POT. BUT NOT DO IT AGAIN AFTER HI-POT.

THE LEAKAGE CURRENT WILL BE STABEL OFTER TEN MINUTES OFTER GOT THE WITHSTANDING VOLTAGE. SO I DON'T THINK I NEED 15 MINUTES WITHSTANDING TIME. SUPER BORRING.

DanielTronco

IEEE NETA ANSI and other standards exist for a reason. The 15 minutes has a purpose. Just test according to the spec and record your results. If your just looking to do whatever you feel like then what's the point of standards and procedures.

ARNOLD ZIFFEL

tie all three cables together, do a gang test, and roll out early. ;)

There is absolutely no reason not to hipot test all 3 phases of an MV shielded cable at one time. Neta specifies individual insulation resistance test but never states that phases must be hipotted seperately. As long as the shields are cont. tested and grounded at both ends for testing, any cable or termination fault will show itself in a gang test. And then you can test individually if needed.

SecondGen

NETA ATS- references ICEA, IEC, IEEE and "other power cable consensus standards" as acceptable means of testing and says "the selection shall be made after an evaluation of the available test methods and a review of the installed cable system."

NETA MTS- says "it is only after careful analysis of all circuit parameters between the testing entity and the cable owner that a preferred testing method be selected."


Nowhere in either NETA standard could I find where it specifically says to test each cable individually. Seems to me like it is up to the commissioning agency or cable owner to decide.



IEEE NETA ANSI and other standards exist for a reason. The 15 minutes has a purpose. Just test according to the spec and record your results. If your just looking to do whatever you feel like then what's the point of standards and procedures.

robertellis87

You can definitely tell the difference in the caliber of technicians with posts like that. Why does it matter that tech "A" has a level 4 NETA cert and and tech "B" has a level 2 NETA cert if the technicians take shortcuts like that? These certifications are a joke, especially when your pay/raises are dependent on them. What good does it do to obtain the certification if you're going to offer sub-par testing to your customer? If you hi-pot the three together and get 120 micro-amps of leakage current, but independently you would have seen 2 micro, 3 micro, and 115 micro respectively, you don't see that as a reason to investigate a little? Don't give me that "it didn't trip the test set so it passes" crap either. We all know you can spot a deteriorated cable/stress cone.

Biscuit

Just goes to show that certifications like these and pay scales based on them are bogus. I've seen guys pass the level III who are way less skilled than others who take the same test 2-3 times before passing. Some people just aren't good test takers. Everything should be based on job performance.

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Jay Knight

yup, and other people cheat


Some people just aren't good test takers.

jcisneros

Megger before hipot. This can also help identify any damage to the cables prior to hipotting.

Will Ward

I just was taught that way a long time ago but never really got a good reason why it is done... The results are usually worse by maybe 10%

Will Ward

The pre withstand test with the Megger is obvious and mandatory.... It was the post test that I question the necessity... In my area the utility will specify the gang test procedure often when they witness a cable test.... I've had that same conversation with them about not being able to identify the distribution of the leakage current.... Some listen, some don't, some listen, agree and tell me do it there way anyway... Our procedure has always been individually unless directed by the AHJ

Will Ward

You mean I can't just wave that NETA card over a broken breaker and it won't fix itself????
Some of the brightest guys I know working with the equipment we do have a hard time with NETA exams...

madMAX

nobody said anything about doing what you feel. A gang test is a perfectly viable option unless specs say otherwise.


IEEE NETA ANSI and other standards exist for a reason. The 15 minutes has a purpose. Just test according to the spec and record your results. If your just looking to do whatever you feel like then what's the point of standards and procedures.

madMAX

LMAO, so true.


You mean I can't just wave that NETA card over a broken breaker and it won't fix itself????

ARNOLD ZIFFEL

I grew up in the utility world and was taught to "gang test". Nothing in NETA specs ever told me to test individual phases only. Acceptance tests are pass/fail only. As per NETA-

"If no evidence of distress or insulation failure is observed by the end of the total time of voltage
application during the test, the test specimen is considered to have passed the test."


When contracted to perform Tan Delta VLF testing as a diagnostic test, of course you test individual conductors to assess splice, termination, and overall insulation condition when compared to the associated conductors.

And in case you are wondering I passed Level IV and did not have to resort to cheating.

ArchIsChompa

tie all three cables together, do a gang test, and roll out early. ;)
Definitely this. Saves a lot of time, granted one of your cables isn't testing bad.

benlanz

Definitely this. Saves a lot of time, granted one of your cables isn't testing bad.

Assuming we are talking about solid dielectric cable systems, you should test the phases independently with a insulation resistance test. Since the actual resistance value has little meaning the best you can do on a 3 phase system is to compare the phases. I recommend a measurement after stable voltage or <1 minute. A VLF withstand test can be performed with all three phases connected since there is no insulation performance measurement.

The major issue I have with this discussion, is that we are splitting hairs. An insulation resistance and VLF tests typically detect less than few percent of issues that do not meet the minimum performance requirements as per the cable and accessory manufacturers' QC specification. Just keep in mind that, while you can be reasonably confident that you will defect a failure (insulation rupture/short) in the cable system, these test are nearly helpless to find the vast majority of cable reliability issues. If you really need to determine a cable system's reliability, you are going to need a completely different type of test.


Ben Lanz
Past Chair IEEE 400

lovetacycle

IEEE 400 is a go, no-go test. It states, "The system is required to withstand the specified voltage for the specified time duration. This test will normally reveal gross imperfections due to improper field handling, such as excessive bending or airgaps between the insulation and shield interfaces. While performing a high-voltage dc test, it is common to monitor insulation leakage current and/or insulation resistance. The guide notes that a decrease in current with time is generally a practical criterion for acceptance." 15 minutes was chosen to make rule out improper installation, air gaps, etc.. It was deemed that in this amount of time a problem will manifest itself.

ies

Hi-potential can be performed on new cables, before it can be commissioned or put online, right. Is it still practical and or advisable to subject an old or in operation power cable to hi-pot testing? Will it not degrade or shorten it's life span?

BrickSalad

With regards to VLF hipot, I've got an excerpt from my company's literature (paraphrased of course) that might bear some light on the 15 minutes requirement:


The results of field tests on over 15,000 XLPE cable circuits show that 68% of recorded failures happened within 12 minutes, 89% within 30 minutes, 95% within 45 minutes, and 100% after an hour. (Moh[B17])

(I'm guessing here that these tests were run for an hour, hence the 100%.)

So it actually makes sense to test a cable *longer* than 15 minutes if you really want to catch all of the cable faults. There is a balance between practicality and perfection however, and it seems like most companies go for 15 minutes. Detects 3/4 of defective cables without taking an unreasonably long amount of time. We sometimes run 1/2 hour or even full hour tests on cables at my company, but usually the customer only wants the standard 15 minute test.

Hope that helps!

VallyOrent

Hi everyone..i am a new user here. As per my knowledge its true that you should wait the 15 minutes and also you should plot the readings that you take. I think you will observe the trend of the results, it will reduce quickly in the beginning and then start to level out at the end of the 15min period.

epzjamin

15 minutes because some faults wont show right away and take some time under load

mikebdub

Hi everyone, in my instruction of cable hi-pot testing, the shield cable withstanding time is 15 minutes, un-shielded cable is 5 minutes.

It is super boring to hipot a high voltage cable, (step up time plus 15 minutes withstanding time, about half hour per cable. could we reduce the withstanding time), could somebody please explain why it needs 15 minutes?

Thank you.

Man if you think that is boring you better hope you never have to test cables for the Federal Government! Spec at "No Such Agency" for 15kv cable is 4 steps at 5 minutes each for a TD reading and then 60 minute at full voltage for withstand.. It really gets to be an issue with staying awake at that point lol.

EDIT: I forgot to mention that this is PER cable.. No tying them together. Bring a book!

SuspectK

You can definitely tell the difference in the caliber of technicians with posts like that. Why does it matter that tech "A" has a level 4 NETA cert and and tech "B" has a level 2 NETA cert if the technicians take shortcuts like that? These certifications are a joke, especially when your pay/raises are dependent on them. What good does it do to obtain the certification if you're going to offer sub-par testing to your customer? If you hi-pot the three together and get 120 micro-amps of leakage current, but independently you would have seen 2 micro, 3 micro, and 115 micro respectively, you don't see that as a reason to investigate a little? Don't give me that "it didn't trip the test set so it passes" crap either. We all know you can spot a deteriorated cable/stress cone.

IEEE standard specifies the monitored withstand as a go/no go test. It either holds or it doesn't. Period. DC hipotting is dead. The results are meaningless from a VLF withstand test, which is the industry standard now.

Tan-Delta, regardless that there aren't acceptance standards from IEEE for it, is seen more and more as a requirement by commissioning agents these days.
___________________________________________
But to reply to OP's first post...

Back when I did DC testing, it was the general gambit outlined down below and 60 minutes total on DC withstand...first 15 minutes or so were incremental steps of voltage, but I still tested all cables at once if they all passed insulation resistance tests, unless the customer wanted something else that made sense...no testing cables without terminations on them. I can't believe how many times I've ran into that.

All VLF tests, the customer generally wants 60 minutes also. I thought IEEE 400.2 had an outline of the fault detection percentages broken down in withstand testing duration, 60 minutes being essentially 100% of all possible faults within the cable itself being discovered. I saw one user has had a similar thing laid out, but I'm certain this use to be in IEEE 400.2, IEC, or similar.

I remember it being something along the lines of (these are rough estimates from memory):
30 minutes- 72% Faults detected
45 minutes- 84% Faults detected
60 minutes- 99% Faults detected

I've never been requested or taught a 15 minute monitored withstand test. I've only seemed to be involved with 60 minute dielectric withstand tests for cables...most of the time from customers/clients, we're allowed to test all conductors together if they pass tan delta(which apparently is pointless with acceptance testing according to the recent Power Test discussions).

Generally, after DC stopped being a go-to method, I've done this:

Cabinet inspection- ensure LOTO before touching, check for clearances for cables during testing, and ensure no sharp edges are there to damage stress cones.
Phasing/Ring out and Shield cont- Ring out cables with shields or a jumper to check shield resistance and phasing and set numbers at the same time. (or confirm phase to phase with shield resistance measured by one end of shields being grounded.)
Ensure cables are clear, and check again for sharp edges that might be in the cabinet when I'll be fumbling around with cables later on.

I only start the tests below if the cables under test aren't manipulated after insulation resistance. Insulation resistance is to confirm the cable isn't completely screwed, but it's also to cover your ass in case it's laying up against a conductor for some odd reason.
Insulation resistance- Phase to phase and phase to ground(I've noticed no difference in measurements with shields grounded or ungrounded on the same cables, but generally prefer to have them grounded by jumpers on both ends...in case partial discharge testing is required.

Tan-Delta- generally three steps for acceptance, four for maintenance...the site I basically live on right now(for the foreseeable future) requires dielectric withstand for fourth step on accceptance tests, you can tell which techs are good on their setup and environmental control when reviewing reports with this added step. A lot of them hike up immediately on the 21/44 kV steps that we're using here....but what do you do about it? There's no absolutely no criteria regarding the fourth step.

Monitored withstand- 60 minutes, with ~25% incremental steps with all cables at once if passed Ins. Res. and/or Tan-Delta.

If any issues occur, offline partial discharge testing is performed with cables that are >260 feet(EPR) when it's determined what cables are the issues. If less than 260 feet, cables get new terminations, and rarely is it ever not just a sloppy term job. I haven't had to review or personally test a cable that was partial discharged. I don't believe it could work.... the PD signals from the far end would be overlapped by hellish noise if you tied them in series to get it long enough to catch measure the reflections.. if you didn't tie them in series, there would be no reflection.

I love partial discharge testing.... I kinda get like a dog seeing a bone when there's a hint of it on site. It's really neat and intuitive test(if you know how to interpret it).

edit- holy hell. I didn't realize I typed a novel..

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