DC or AC Hi-Pot Testing

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mako

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What's the difference between DC and AC HiPot Testing? Is there any multiplier or percentage on a given utilization voltage to apply such test? Any advantage?
We apply DC HiPot Test on an 11KV Cable applying 22 KV and it passed but when we applied AC HiPot test it failed. The cable is 350 meters long and with 2 splices along the cable.It failed on the AC HiPot that when we were gradually increased the AC voltage,it failed at an even low 200 VAC with 12mA Leakage Current injected.Does capacitance of long cable has to do something with this?Can repeated test destroy the insulation of the cable?
Thanks.
 
Re: DC or AC Hi-Pot Testing

Mako,

The NETA MTS recommended DC test voltage is 44kV for 100% insulation level and 49kV for 133% insulation level so I would agree with Bob that your test voltages for DC was too low. Now, that said, repeated DC hipot testing for MV cables will reduce the cable life and is recommended by the IEEE that DC testing should not be performed except for a new cable installation or a suspected bad cable.

An AC cable is factory tested with AC, energized with AC, and exposed to stresses from AC so why test with DC? The answer is that the amount of energy needed to test a long run of cable requires a very large power source to charge the cable, an AC hipot to test the cable you are describing would probally need to be about the size of a VW bug. For 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!

I sounds to me like the way you are conducting your AC test, the cable is not necessarily failing, your Hipot is just not large enough and is tripping offline.

A cable the size of yours should be tested using some method of AC VLF testing (Very Low Frequency). This type of test uses an AC source around 0.1 Hz so the power requirements are less, but it is still an AC test. Example(same cable as in the example above): At 0.1 Hz, the Xc is 1.59 megohms. At 22 kV, the current needed is 14 mA. Total supply power needed is .304 kVA.

It sounds like you are in over your head a little bit and you may be ready to pull and replace a good cable. I recommend you contact someone that can do an AC VLF test on your cable. A good place to start would be www.netaworld.org and search for a NETA company near you, many of them can test this cable the right way for you, or send me a private message and I can recommend someone for you. Good luck.
 
Re: DC or AC Hi-Pot Testing

Then, we were applying too low.We are just a subcontractor and our client is insisting that we are applying too high voltage.They instruct us that we just megger the new cable( but by the way, it has two splices inbetween that long cable run, wherein the copper tape and the cable armor are electrically connected on that splice per manufacturers catalogue.)

Thanks guys for the lot of information and references you have had provided.
 
Re: DC or AC Hi-Pot Testing

If you only megger that cable before energizing I would recommend that you are about a 1000ft or more away from it, I have seen way too many cables with zero splices fail an acceptance test.
 
Re: DC or AC Hi-Pot Testing

But, by the way...Why is it that a given cable has to be Hi-Pot Tested for several minutes if the spike voltage and lightning strike will last only for seconds?
 
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