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Old 06-25-2015, 11:19 PM   #1
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Dear all

As far as I know there are 2 types of HIPOT test : DC and AC. Theoretically, the two methods would give same results. A couple of days ago, we were performing HIPOT test for 20kV installation and found the leakage current could reach 500mA which, in our discussion with the sub con, is too high. The subcon said that if we performed the test using DC HIPOT tester, then the leakage current must be much smaller than above value.

I guess the 'too high' leakage current was caused by the capacitance of the system. The 500mA was the sum of "real" leakage and "pseudo" leakage. Pseudo leakage (i use my own terminology) is the current that flow from live conductor to ground through system capacitance (capacitive coupling).

Let say V=testing voltage, It = current measured by tester, I = real leakage current, Ic = pseudo leakage current, and Xc = system reactance.

It = I + Ic, where Ic = V/Xc
thus
It = I + V/Xc

we know that Xc = 1/(2*phi*f*C)
thus
It = I + V*2*phi*f*C (f=operating frequency, C = system capacitance).

thus
I = It - 2*phi*V*f*C.

If I can find the value of C (using capacitance meter), then the real leakage current can be determined, and this result should be EQUAL if I perform DC HIPOT test.

I need you guys to confirm this calculation. Maybe I missed something that should be considered.

Regards

Hendra
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Old 06-26-2015, 02:04 AM   #2
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On a DC hipot we consider 100mA high, and recommend further investigation. I would think there would be something wrong if you had 500mA. Many places say that a DC hipot is a destructive test, especially to XLPE insulation. Many recommend VLF testing now, though VLF is a pass or fail test, meaning if something is wrong you will find out pretty quick and have to fix it. As far as DC testing goes, we only use DC hipot, have been for many many years. Why? I'm not sure other than that's what our engineers want. Also, our test voltage is not as high as some standards suggest.
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Old 06-27-2015, 05:09 AM   #3
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@JW Splicer : thanks for your reply. But do you think the formula I wrote o my post is correct?
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Old 06-27-2015, 08:30 AM   #4
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If you use a DC hipot, one of the advantages is that once the cable had been charged, the leakage current is in phase and closer to the actual leakage current. The VLF should be a pass or fail test correct? Either the cable held or it had not, and you shouldn't have to take leakage current measurements. Right? Sorry I haven't really dealt with VLF so as far as your formula, I'm not sure...
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Old 06-27-2015, 09:47 AM   #5
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H T A, I'm not sure what you're saying. The 500mA result is with the AC hipot test? Did you do a DC hipot test? If you have DC results then you could substitute that number as I in your formula and then solve for C to see if the theory matches the field test.
I used the 100mA number mentioned by JW Splicer and put that in your formula and came up with 1885 uF, but beware it's been waaaay to long since I last took algebra!
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Old 06-27-2015, 02:51 PM   #6
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What kind of "20kv installation" are you talking about. Is this MV cable, switchgear, circuit breaker, or what?

If it is cable a VLF Tan Delta test can be used to compare cable capacitances to similar runs of the same type and also do a proof test before energizing.

For breakers or transformers power factor testing can isolate areas of concern.

As far as any hipot test NETA pretty much says :

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


If you are testing switchgear you can always break it down by sections if you truly have concerns.

Hipot leakage current can vary greatly due to cleanliness, and ambient conditions like humidity from day to day.

Always fall back on the Manufacturer's testing recommendations if there is any doubt.
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