MV Cable Testing

[softlynx]

We are conducting MV cable testing. Length of cable is 7.2 Km. It has 14 joints. It was laid down almost one year back and was not put into service due to some reasons. It was lying on ground.
Now when we wanted to put this cable in service, we started its testing. For insulation, we tested it at 5 KV for one minute, but we do not get stable value, it is all the time fluctuating between 200 G ohms to 800 G ohms. Fluctuation is not understood.
What may be the reason for fluctuation? What remedial measures can be taken? Can we go ahead for Dc withstand test or AC test? And if the cable clears these tests can we put this cable in service.

 

[boboelectric]

We look for Megs,not Gigs. What is your voltage level?

 

[ohmhead]

Well heres how we test dont know what you voltage is ?

HV cable high pot dc or ac start at 500 volts for one minute watch you voltage display on tester watch your micro current meter on tester also at the same time after one minute run it up 500 volts wait one minute watch meters do this until you reach 3 times the rating of that cable or use the manufactures test instructions for voltage on that cable .

If you do this for every 500 volts and watching the volt meter & micro current meter the meters should go up and be stable your current meter should never run away up or down with just a small charging or discharging if its running away shut down your tester . cable or your connections are bad or your not sealed up the ends enough with sandwich wrap air is you worst problem on taps or joints water in terminations is also a issue dry them up .

Most electricians cant read a megger or do a hi pot test the correct way its not always the cable its the test person so are you megging or hi potin sounds like your meggin which is not good enough for mv cable

 

[zog]

We look for Megs,not Gigs. What is your voltage level?

Huh? MV cables should be in the Gigohm range.

 

[zog]

We are conducting MV cable testing. Length of cable is 7.2 Km. It has 14 joints. It was laid down almost one year back and was not put into service due to some reasons. It was lying on ground.
Now when we wanted to put this cable in service, we started its testing. For insulation, we tested it at 5 KV for one minute, but we do not get stable value, it is all the time fluctuating between 200 G ohms to 800 G ohms. Fluctuation is not understood.
What may be the reason for fluctuation?

Your megger is too small to charge the cable and is causing the fluctuation.

What remedial measures can be taken? Can we go ahead for Dc withstand test or AC test? And if the cable clears these tests can we put this cable in service.

OK, I assume these are >5 years old. In which case you do not do a DC test!!. It is a destructive test on service aged MV cables.

So you do an AC hiopot, not with a cable that long, you will need a huge power supply, don't even think an AC hipot exists large enough to do a power frequency AC test on that long of a cable.

So, you are left with the 3 best choices.

Partial Discharge
Tan Delta
VLF

Of those 2 the most common are TD and VLF. VLF is like a normal hipot test, you only a leakage current reading. If all you care about is if the cable if safe to use or not a VLF is a good choice.

TD is much more in depth, it is a condition assesment test and a better indicator of your cable health. If you plan on doing regular cable testing as part of a PM program , a TD test is better for trend analysis and condiction assesment.

 

[ohmhead]

Well Zog i agree i had to look up kilometers thats miles of cable good point and yes dc is bad news on old cables but it would be nice to say what type of insulation and voltage hes using and its on the ground exposed to what ever ? dc or ac at that length no way .

 

[zog]

For the people that have recommended a DC hipot test, here is a summary of what all the available national and internationally recognized standards say about DC hipot testing MV cables.

2007 ANSI/NETA MTS:
Does not specify what testing but rather refers to several other specifications (IEEE and ICEA).

Allows for Direct Current, Alternating Current, Partial Discharge or VLF TestingProvides the "standard" DC High Potential Test warning for Cables over 5 years in service.

Provides guidelines for maintenance frequencies based on criticality and equipment condition.

ICEA:
Allows for testing at reduced voltages for first 5 years
Will only discover "gross" problems
Not expected to reveal deterioration
DC Testing after 5 years is "not recommended"
Evidence that DC Testing can lead to early failures

IEEE 576
Allows for testing at reduced voltages for first 5 years
Does not provide for testing after 5 years in service

IEEE 400
Provides an overview of techniques for performing electrical tests in the field on shielded power cable systems from 5 ? 500kV
Provides a summary and warnings about DC high potential testing.
States "even massive insulation defects cannot be detected with DC at the recommended voltage levels"

Every MV cable standard says something about DC hipot test being destructive, some allow it for acceptance testing to find "gross installation errors" but recent tests by IEEE have shown this to be false also and every standard will be adopting changes on this.

All the standards say not to DC test a MV cable after 5 years. This is because a DC test will cause the minor PD sites, water trees to grown and the cable to fail prematuely.

 

[ohmhead]

Well how old is that cable Zog ? Heres how the power company test they turn it on if it blows out they replace if it holds they are done .

But yet electricians must test there cables and conductors .

What cable can be dc tested and be safe ?

What cable can be ac tested and be safe ?

Whats the best method to test a 15 kv cable?

Is all insulation the same at rated voltage or can a test method effect that insulation when new ?

 

[zog]

Well how old is that cable Zog ? Heres how the power company test they turn it on if it blows out they replace if it holds they are done .

But yet electricians must test there cables and conductors .

What cable can be dc tested and be safe ?

What cable can be ac tested and be safe ?

Whats the best method to test a 15 kv cable?

Is all insulation the same at rated voltage or can a test method effect that insulation when new ?

To answer this would require about an 8 hour seminar. You should know better, there are not simple answers to these questions, there is no one best method. there are many dofferent factors. Cable testing is a specialized feild these days, not just some guy with a hipot. There is a good conference in Ponte Vedra beach next month that will be talking alot about MV cable testing, you really want to learn, go attend it.

 

[ohmhead]

To answer this would require about an 8 hour seminar. You should know better, there are not simple answers to these questions, there is no one best method. there are many dofferent factors. Cable testing is a specialized feild these days, not just some guy with a hipot. There is a good conference in Ponte Vedra beach next month that will be talking alot about MV cable testing, you really want to learn, go attend it.

Well i learned about testing in the Navy electricians school you know but in 1969 they didnt have vlf testers they had a hi pot a hv motorgenerators so thats what we used i dont need school Zog were getting old and gray i just asked the question to see what answers that i could get free online .

 

[softlynx]

MV cable testing

MV cable testing

Rated voltage is 11 kv and it is not five years old cable. Infact it is a new cable. It was laid down about one year back, but was not put in service. Now it is needed to be put in service. After successful testing it will be burried in the trench.

 

[ohmhead]

Rated voltage is 11 kv and it is not five years old cable. Infact it is a new cable. It was laid down about one year back, but was not put in service. Now it is needed to be put in service. After successful testing it will be burried in the trench.

Well your fine the hi pot is not good for mv cable over 5 years of service but years ago it was ok .

Its all about the way you test it do it in steps slowly . Ask you cable manufacture what test voltage but normally its 3 x top out

Check current running up or down as stated and run your voltage up in steps the last step test is 15 minute at full voltage if it lasts current stable and voltage stable your fine .



I think testing is kinda a test but thats it.

If that cable checks out it doesnt mean it still will not blow out in a month so why do we test to make us feel better .


The problem with old cable and lets just use simple words and terms is its old and its not know what stress its been thur so if you put a dc hi pot or ac hi pot test it could blow the cable out .

If you test with the vlf its not going to blow it out but gives you a good test its safer on cables.

I look at it from the old way if your cable cant take the voltage it was rated for and pass a hi pot its not worth putting it in service now watch the comments but thats my way high voltage is what it is turn it on heat it up if its good its good .

 

[Pierre C Belarge]

The real answer to the OP's question

The real answer to the OP's question

The real answer to the OP's situation most likely cannot be had here. The responses to the question are limited in answer due to conditions not known, and the complexity of the situation.

With that said, my best experience with situation tells me to let my ego take a rest and hire the best people I know for the testing.

Buying good test equipment, taking the classes offered by the test equipment manufacturer is a good start, but not enough for this situation.

One needs experienced people with the proper equipment. Hence my answer is relatively simple. Get yourself some good references and call those companies. Most likely there are not too many of these companies available, so it should not take long to locate them.

Not to solicate for him, but Zog's company sounds to me like they have the proper equipment and well trained individuals...maybe he can help.

I bet that cable cost more than a days pay.

 

[zog]

Not to solicate for him, but Zog's company sounds to me like they have the proper equipment and well trained individuals...maybe he can help.

I bet that cable cost more than a days pay.

Thanks, but we don't do cable testing. Last few companies I worked for did but not this one. I still do seminars on cable testing at conferences and such and we do on line PD testing but the OP needs either a TD or VLF test, we don't do either one. I just want to make sure the OP does the right test and does not damage or reduce the life of this very expensive cable.

 

[zog]

Rated voltage is 11 kv and it is not five years old cable. Infact it is a new cable. It was laid down about one year back, but was not put in service. Now it is needed to be put in service. After successful testing it will be burried in the trench.

What the insulation %? What is the insulation type? conductor size? Sheilded?

These all make a difference. The thing is, a DC hipot, while allowed for by some standards (As of today, that will change on the next revision) is not recommended by others. There is a small grey area on what tests are allowed because some contractors are fighting it because they only have DC hipots and limited training and knowledge of MV cable systems.

Here are some facts you cannot dispute.

1. A DC test will only find some gross installation errors, it is not a assesment tool and may not find some installation issues which could cause cable failure upon energizing.

2. A DC test will degrade the cable and reduce it's life expantcy.

3. This cable is very expensive, has been sitting for a year, and if a failure occurs, there will be someone looking for someone else to replace it. Then the questions of the types of tests done, to which standards they followed, and the qualification and experience of the testing company will come into play.

If you need to be asking questions, you may be in over your head. I highly recommend you get someone qualified to do these tests before you damage a cable that costs more than you earn in a year. If you decide to do the test your self, follow industry standards and test methods. I will be happy to help you out when you provide the information about the cable.

 

[softlynx]

MV cable testing

MV cable testing

I checked with bigger size meggar and IR value was stable.

 

[zog]

I checked with bigger size meggar and IR value was stable.

Good, now all that tells you is that it is safe to do a overpotential test. Do a VLF or TD. Are you going to provide the additional info about your cable so I can give you some real guidance?