AC and DC Hi Pot testing

[vinod]

Sir,
What is the deferance of doing AC and DC hi-pot testing?
Why doing DC on Cables and AC on Bus bars?

 

[zog]

Sir,
What is the deferance of doing AC and DC hi-pot testing?
Why doing DC on Cables and AC on Bus bars?

MV Cables should NOT be DC hipotted, not sure where you get your info. Not really sure what your question is either, the difference is one is a DC test and one is an AC test.

 

[bob]

Check this article http://www.emersonprocess.com/home/...tfacts/maintenancefacts0310_hipot-testing.pdf

 

[zog]

Check this article http://www.emersonprocess.com/home/...tfacts/maintenancefacts0310_hipot-testing.pdf

Well Wally is a little biased (I used to work for that company also) and has $$ reasons for "Putting hipots out to pasture" but there have been many advancements (Including many since that article was published) in insulation testing, specifically for MV cables. The best method is based on type of equipment, ability to shutdown, previous testing, and budget need all be considered.

 

[mdshunk]

Well Wally is a little biased (I used to work for that company also) and has $$ reasons for "Putting hipots out to pasture" but there have been many advancements ...

I'm still trying to figure out what type of unit a pico-coloumb is.

 

[zog]

I'm still trying to figure out what type of unit a pico-coloumb is.

Well a coloumb is a unit of electrical charge of one ampere over period of one second, and pico is x10(-12 power) I think.

 

[gar]

090707-0446 EST

Marc:

Visit
http://en.wikipedia.org/wiki/Farad
Found by Google search for --- farad

A a potential difference of 1 volt across a 1 mmfd capacitor results from 1 pico-coulomb of charge to the capacitor. A short pair of small wires an inch apart will have capacitance of 1 mmfd. Cat 5E cable is about 15 mmfd (pfd) between a wire pair per foot.

And here is site with several useful comments.
http://newsgroups.derkeiler.com/Archive/Alt/alt.home.repair/2008-01/msg02441.html
Found by Google search for --- romex capacitance per foot

This reference has a discussion on stored energy in a capacitor.
http://physics.bu.edu/~duffy/PY106/Capacitors.html
Google search for --- potential energy stored in a capacitor

1000 ft of Romex is 15,000 pfd, 0.015 mfd. Charge this to 1000 V and the stored energy is 7.5*10^-3 coulombs, 7.5*10^9 pico-coulombs.

.

 

[gar]

080707-0831 EST

Last statement of my previous post, #7, is wrong. Brain disconnect.

Energy is not directly measured in coulombs, it is in joules.

"1000 ft of Romex is 15,000 pfd, 0.015 mfd. Charge this to 1000 V and the stored energy is 7.5*10^-3 coulombs, 7.5*10^9 pico-coulombs."

Should read
1000 ft of Romex is 15,000 pfd, 0.015 mfd. Charge this to 1000 V and the stored energy is 7.5*10^-3 joules, 7.5*10^9 pico-joules.

The stored charge is Q = V*C = 1000*15000*10^-12 = 15*10^6 coulombs = 15,000,000 pico-coulombs.

Maybe I got it right this time.

.

 

[76nemo]

MV Cables should NOT be DC hipotted, not sure where you get your info. Not really sure what your question is either, the difference is one is a DC test and one is an AC test.

zog, fill me in. Why NOT test cables DC wise if the cable under test is auto-discharged? Are you saying they should be AC hi-potted? For what reason????

 

[76nemo]

zog, fill me in. Why NOT test cables DC wise if the cable under test is auto-discharged? Are you saying they should be AC hi-potted? For what reason????

What is the difference between being saturated in an insulation test compared to hi-potting?

 

[zog]

zog, fill me in. Why NOT test cables DC wise if the cable under test is auto-discharged? Are you saying they should be AC hi-potted? For what reason????

We are talking MV cables here, and this has been discussed at length in this forum. Basically the IEEE std 400, the ICEA, and ANSI/NETA standards all say that Service aged (>5 years old) MV cables should not be DC hipotted because it is considered a destructive test and can damage cables and reduce thier expected life. DC testing can cause the growth of water trees into electrical trees, specifically in XPLE cables.

 

[zog]

What is the difference between being saturated in an insulation test compared to hi-potting?

Big difference, an IR test is not even considered an overpotential test. With an overpontential test (DC, hipot, AC hipot, VLF, Tan Delta) you are measuring the dielectric strength of the insulation. Again, we (I) am talking about MV cables, not LV, no need to hipot LV cables at all.

 

[rafeequepk]

In my experience, we used to do AC hipot on Bus bars, Circuit breakers, CTs etc and DC hipot on New MV cables. But i didnt understant what is the significance of following this procedure..Is there any thing wrong in doing a bus bar and CTs hipot with DC voltage apply..?

 

[zog]

In my experience, we used to do AC hipot on Bus bars, Circuit breakers, CTs etc and DC hipot on New MV cables. But i didnt understant what is the significance of following this procedure..Is there any thing wrong in doing a bus bar and CTs hipot with DC voltage apply..?

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.

 

[ramsy]

That may explain when testing most circuits my DC-Megger meter momentarily dips, before leveling off at a much higher value.

 

[zog]

That may explain when testing most circuits my DC-Megger meter momentarily dips, before leveling off at a much higher value.

Sure it is, that will be more evident in inductive circuits, like transformers and motors. When a high DC voltage is first applied the total current consists of three current components;

1.Leakage current
2.Capacitance charging current
3.Absorption current

Conduction/Leakage Current - This current passes through the surface of the insulation. The magnitude of current flow depends on the resistance of the insulation. Surface leakage is usually not a problem because it can be eliminated through external cleaning.

Capacitive Current - The insulating specimen appears, ideally, as a capacitor. As a DC voltage is applied to a capacitor initial charging current flows until the voltage drop across the component equals the source voltage. As the capacitor charges, its charging current decreases to a minimum. This is called its steady-state value.

Dielectric Absorption Current - This current also appears at the initial application of test voltage the same as capacitance current. This current is required to polarize the insulating medium. In other words it is energy absorbed by the insulating system.

 

[weressl]

We are talking MV cables here, and this has been discussed at length in this forum. Basically the IEEE std 400, the ICEA, and ANSI/NETA standards all say that Service aged (>5 years old) MV cables should not be DC hipotted because it is considered a destructive test and can damage cables and reduce thier expected life. DC testing can cause the growth of water trees into electrical trees, specifically in XPLE cables.

The testing of 5 years and older cables in dry locations are still recommended without factory consultation at 80% of the original new installation test voltages.

The DLS(dirty little secret) on this is: XLPE insulation had proven to be a bad choice. That is why nobody markets it anymore. So what do you do when you discover this water-treeing problem and you have millions of miles already installed? You can't admit that it is of fertilizer value as you would be subject to class action suit all over and most of the industry would have went bankrupt. So what do you do? They discovered that the standard DC test that WAS the industry test method would blow these tree problems fast out of water - no pun unintended. So you go ahead, revise the standard and since you can't mention that it is ONLY related to the XLPE insulation you have to be inclusive. You mention that over 5 years old cables in WET service, the user is to consult the manufacturer. So if you tell the mfr. taht your cable is EP or EPR he will tell you just go ahead, and if you tell him it is XLPE you're wondering about he will offer you a nice deal on new EPR cable.

 

[mivey]

Now that is an interesting tidbit of dirt.

 

[danickstr]

xlpe cross-linked polyethylene

epr ethylene propylene rubber

FYI

 

[zog]

The testing of 5 years and older cables in dry locations are still recommended without factory consultation at 80% of the original new installation test voltages.

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.