AC and DC Hi Pot testing

[rafeequepk]

To charge a MV cable of any signifigant length with a 60hZ AC Hipot you need a huge power suply, the Xc of the cable @60Hz is very low, thats why VLF testing is done.

Xc= 1/(2(pi)fC)
The lower the frequency, the higher Xc (capacitive reactance).

The higher Xc (or resistance across the power supply output), the lower the current/power needed to apply a desired voltage.

60Hz Example:
At 60 Hz. a 1 μF cable has an Xc of 2.65 kOhms
At 22 kV, it requires 8.3 amps of current to test
Total power supply rating must be 183 kVA

0.1 Hz VLF Example:
At 0.1 Hz, the Xc is 1.59 megohms
At 22 kV, the current needed is 14 mA
Total supply power needed is 0.304 kVA

At 0.1 Hz, it takes 600 times less power to test a cable, or any other high capacitance load, than at 60 Hz. At 0.01 Hz, 6000 times higher capacitive loads can be tested than at 60 Hz.

Zog,
It was really a worth explanation from u for the question why we usually we prefer DC hipot on New MV cables than AC Hopt. Thank you very much. I know that DC Hipot is more destructive than AC hipot. But what kind of destruction it can make on bus bars and CBs?

 

[zog]

Zog,
It was really a worth explanation from u for the question why we usually we prefer DC hipot on New MV cables than AC Hopt. Thank you very much. I know that DC Hipot is more destructive than AC hipot. But what kind of destruction it can make on bus bars and CBs?

Not any more than an AC hipot, used properly DC hipot is the choice for bus and breakers. Just make sure you use the proper test voltages and procedures.

 

[weressl]

By whom? Please provide reference.

You are right on the mark about your DLS. Big, oops. I like the XLPE-TR effort, that didnt work too well either.

Kerite and they seem to claim that they just reproduced the IEEE table.

http://www.kerite.com/catalog/catalogfiles/DC_field_testing_power.htm

Mind you Kerite cables stand head above shoulder and conservative in their approach.....

XLPE-TR, Phuuuulleeeese...... the perfect example when a scientist refuses to believe he is wrong regardless what it costs....

 

[zog]

Kerite and they seem to claim that they just reproduced the IEEE table.

http://www.kerite.com/catalog/catalogfiles/DC_field_testing_power.htm

Mind you Kerite cables stand head above shoulder and conservative in their approach.....

XLPE-TR, Phuuuulleeeese...... the perfect example when a scientist refuses to believe he is wrong regardless what it costs....

Read the fine print
"Refer to IEEE Standard 400 dated 1991 or later for additional details"

Thats a 17 year old standard, before we knew about the XPLE problems.

I stand by my no DC hipot statement I made earlier, wet or dry.

 

[weressl]

Read the fine print
"Refer to IEEE Standard 400 dated 1991 or later for additional details"

Thats a 17 year old standard, before we knew about the XPLE problems.

I stand by my no DC hipot statement I made earlier, wet or dry.

Or later is the key-word. My point is that Kerite AS OF TODAY sees no problem of testing their cables at 80% if they are in a dry, aboveground location.

When you go to the Okonite website they talk about past 5 year service testing as well.

The treeing problem with XLPE has been recognized at least 30 years ago. I have been staying away from that cable that long.

 

[zog]

But a manufactures OK is not the same as a standard. ANSI, NETA, IEEE, and ICEA are all recogonized standards and they all same the same thing about DC testing of service aged MV cables.

 

[weressl]

But a manufactures OK is not the same as a standard. ANSI, NETA, IEEE, and ICEA are all recogonized standards and they all same the same thing about DC testing of service aged MV cables.

So who are you going to follow? The manufacturer who makes the cable and who has liability assigned to their recommendation in the use of their product or the 'organization' who has to accomodate all sort of manufacturers with their products and are free from liability by the individual customer?

I will go with the individual cable manufacturer recommendation, whatever they might be. The cable manufacturer may tell me that I should not even go with the IEEE/ICEA rates, but recommends that I do not go above 65%.

 

[zog]

What liability? You think the manufacture will pay for all of the damaged equipment after a cable failure when they find out the cable what tested against internationally recognized standards? Maybe they give you a replacement cable but good luck with the labor and other affected equipment.

Me, I follow International standards, if you are my customer and demand a DC test (No idea why you would want it anyways) I will do it and have you sign a release of liability to me, the guy (company) testing the cable because you are asking me to go against the standards, it is my butt on the line, not the cable manufacturer.

 

[weressl]

What liability? You think the manufacture will pay for all of the damaged equipment after a cable failure when they find out the cable what tested against internationally recognized standards? Maybe they give you a replacement cable but good luck with the labor and other affected equipment.

Me, I follow International standards, if you are my customer and demand a DC test (No idea why you would want it anyways) I will do it and have you sign a release of liability to me, the guy (company) testing the cable because you are asking me to go against the standards, it is my butt on the line, not the cable manufacturer.

You would not be testing my cables, rest assured.

 

[zog]

You would not be testing my cables, rest assured.

Why, because we do it the right way?

We dont do DC testing of MV cables because it is destructive. And I have tested many, many cables at BASF facillities (and transformers, breakers, etc.), VLF and Tan Delta. We do it right, not the cheap easy way.

 

[ramsy]

I see a difference between cable-failure tests and checks for normal operating limits. If "Hi pot" is a stress check for equipment failure, such STD's or factory spec's are a defect check during manufacturing, perhaps not for field operators or installation contractors.

Should consumers verify Mfg. stress checks "Hi pot" before cable failure or transients are suspect?

If Megger checks are less destructive shouldn't field operators be satisfied with the minimum voltage test required to check the cable's operating integrity?

 

[zog]

I see a difference between cable-failure tests and checks for normal operating limits. If "Hi pot" is a stress check for equipment failure, such STD's or factory spec's are a defect check during manufacturing, perhaps not for field operators or installation contractors.

Should consumers verify Mfg. stress checks "Hi pot" before cable failure or transients are suspect?

If Megger checks are less destructive shouldn't field operators be satisfied with the minimum voltage test required to check the cable's operating integrity?

A Megger test dosent tell you much about the condition of a MV cable, it is just essestially a go-no-go test. If the cable didnt fail with system voltages and stresses, it wont fail from a Megger test.

Todays preference in most power MV systems is either Tan Delta or Pardtial Discharge, both are condition assemnet tools, rather than a simple spot test.

Condition assessment tests allow data to be compared to a huge database of similar equipment from all over the word and to assign that equipment a grade.

Example:You have 40 MV cables at your facillity that get Tan Delta tested. Those 40 cables are compared to test results of 5000 cables of the same type, length, and age. You have 34 Cables that get a Grade of "A", meaning thier results compared to 90% of the others in the database. 4 cables get a "B" grade, compared to the worse 10% of all cables in the database, 2 cables get a "C" (Maybe worse 5%) and 2 Get an "F".

The bigger the database of test results, the more accurate the condition assessment.

The ones that were graded "F" should be repaired ASAP, thats the easy part. Now you have a little money show up in your budget, replace the grade "C" cables, or test them more often.

Condition assessment is a much better tool than a simple spot test.

 

[weressl]

A Megger test dosent tell you much about the condition of a MV cable, it is just essestially a go-no-go test. If the cable didnt fail with system voltages and stresses, it wont fail from a Megger test.

Todays preference in most power MV systems is either Tan Delta or Pardtial Discharge, both are condition assemnet tools, rather than a simple spot test.

Condition assessment tests allow data to be compared to a huge database of similar equipment from all over the word and to assign that equipment a grade.

Example:You have 40 MV cables at your facillity that get Tan Delta tested. Those 40 cables are compared to test results of 5000 cables of the same type, length, and age. You have 34 Cables that get a Grade of "A", meaning thier results compared to 90% of the others in the database. 4 cables get a "B" grade, compared to the worse 10% of all cables in the database, 2 cables get a "C" (Maybe worse 5%) and 2 Get an "F".

The bigger the database of test results, the more accurate the condition assessment.

The ones that were graded "F" should be repaired ASAP, thats the easy part. Now you have a little money show up in your budget, replace the grade "C" cables, or test them more often.

Condition assessment is a much better tool than a simple spot test.

I agree that these tests are superior. Of course these tests are more costly than others. No wonder YOU like them

The criticality and value of the equipment will also guide what test procedures will I select. I think permanently installed, on-line PD sensors is the most cost effective way to go.

 

[zog]

I agree that these tests are superior. Of course these tests are more costly than others. No wonder YOU like them

Any monkey can go buy an old Hipot test set on ebay and go blow up, uh I mean, test cables. Thats not what we do or is it our client base.

The criticality and value of the equipment will also guide what test procedures will I select. I think permanently installed, on-line PD sensors is the most cost effective way to go.

Now you are talking sense, 20 years from now I think the installed PD sensors will be pretty standard, we install those in alot of the MV switchgear we build.

 

[weressl]

Any monkey can go buy an old Hipot test set on ebay and go blow up, uh I mean, test cables.

OK Gimme my BANANAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

 

[ramsy]

A Megger test dosent tell you much about the condition of a MV cable, ..Todays preference in most power MV systems is ..Condition assessment tests.. cables are compared to test results of 5000 cables of the same type, length, and age.

I recently replaced some 120v Knob & Tube wiring, insulated with cloth, the copper was green with oxidation underneath, and cooked in a hot attic for about 80 years. After witnessing this stuff still operating perfectly, the concept of cable maintenance would have never crossed my mind.

Of course the advantage of separately run Knob & Tube conductors was that copper only reacts with air and ceramic holders, no ground wire or conduits, kind of like a transmission line, where neither insulation or covering affects operation.

Many thanks for the education in MV equip. The concept of perishable conductor life was not shared in my studies of 600v equipment. I now see the value in preventing inevitable operating failure before it happens, with MV equipment.