Hi-Potting existing 5kV cable

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lielec11

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Electrical Engineer (PE)
I designed a job and just got the hi-pot results for all 5kV cables. Most of them were brand new, but one section was spliced into an existing cable. Our specs call for 35kV DC test voltage for all 5kV cables. The testing company only raised the test voltage to 9kV for the cable spliced with existing. A couple of questions for those that do this testing on a regular basis:

In your experience, is 9kV sufficient?
If not, in your experience is there a high chance of damaging the existing cable by raising the test voltage to the 35kV range?
After the test, the cable was drained, however, after 60 seconds there was still a residual current voltage around 3-4kV on each phase. IS this a problem, or should this be expected on longer sections of cable? All other new cable sections went down to 0 in the same time period.

All input is appreciated. Thanks.
 
Sounds like the 9kV test voltage was high enough to inject charge into the insulation of the existing wire, creating pockets of charge.
When you drained the cable, this would have the effect of a series resistor and a charged capacitor, maintaining current for a prolonged time beyond what the capacitance of the cable itself would sustain.
What the consequences of this situation are in terms of the condition of the old insulation and whether additional damage was done by the testing are both beyond my experience.

I thought that DC tests of this sort were discouraged for a variety of insulation types because of the high risk of damage? (e.g. water trees)
 
lielec11 said:
Does anyone know where these test voltage levels come from? The commissioning agent mentioned NETA...
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IEEE Std 400, 400.1, 400.2 and 400.3, NETA Maintenance Testing Specifications (MTS) and NETA Acceptance Testing Specifications (ATS) having test voltage levels depending upon the type of test (DC Hi-Pot, VLF, partial discharge) and whether it is testing of in-service cables or acceptance testing of new cables.

My facility has stopped using DC hi-pot and now uses VLF exclusively for maintenance and acceptance testing of cables. If we have to replace a section of existing cable with new, we revert back to maintenance test levels of in-service cables after the repair.
 
jtinge said:
IEEE Std 400, 400.1, 400.2 and 400.3, NETA Maintenance Testing Specifications (MTS) and NETA Acceptance Testing Specifications (ATS) having test voltage levels depending upon the type of test (DC Hi-Pot, VLF, partial discharge) and whether it is testing of in-service cables or acceptance testing of new cables.

My facility has stopped using DC hi-pot and now uses VLF exclusively for maintenance and acceptance testing of cables. If we have to replace a section of existing cable with new, we revert back to maintenance test levels of in-service cables after the repair.
Click to expand...

Thanks for the info..Never heard of VLF testing, will look into it.
 
lielec11 said:
I designed a job and just got the hi-pot results for all 5kV cables. Most of them were brand new, but one section was spliced into an existing cable. Our specs call for 35kV DC test voltage for all 5kV cables. The testing company only raised the test voltage to 9kV for the cable spliced with existing. A couple of questions for those that do this testing on a regular basis:

In your experience, is 9kV sufficient?
Click to expand...
No DC hipot test is sufficient but if you use DC I wouldn't go any higher than that. 9kV is the old max test voltage for 5kV (100% insulation level) per the old NETA standard which came from the ICEA standards for cables in service less than 5 years. However, DC hipot is considered a destructive test (and at 35kV would be very destructive) by all of the current standards.

lielec11 said:
If not, in your experience is there a high chance of damaging the existing cable by raising the test voltage to the 35kV range?
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Yes
lielec11 said:
After the test, the cable was drained, however, after 60 seconds there was still a residual current voltage around 3-4kV on each phase. IS this a problem, or should this be expected on longer sections of cable? All other new cable sections went down to 0 in the same time period.
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Expected on long runs

lielec11 said:
All input is appreciated. Thanks.
Click to expand...

Here is a summary of what each syas about DC testing.

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.

Modern test methods recommend for MV cables are AC (Power frequency, but requires a huge AC hipot for anything but short cable runs), VLF, PD, or Tan Delta.
 
IEEE 400.1 and NETA ATS both indicate 28 kV dc, phase-to-ground, as the maximum DC test voltage for new 5 kV cables (assuming 100% insulation level). NETA shows 36 kV dc, phase-to-ground, as the maximum for 133% insulation level.
 
jtinge said:
IEEE 400.1 and NETA ATS both indicate 28 kV dc, phase-to-ground, as the maximum DC test voltage for new 5 kV cables (assuming 100% insulation level). NETA shows 36 kV dc, phase-to-ground, as the maximum for 133% insulation level.
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For new cables yes but the OP was refering to 9kV being used on the existing cable so the MTS test voltages should apply.
 
jtinge said:
Agreed. I skimmed over "existing" in the OP.
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I appreciate both responses because I was actually looking for the answer to both new and existing. The insulation is 133% so glad we spec'd 36kV and I can only seem to find NETA ATS online. I'd have to pay for NETA MTS.

What section in ATS indicates 36kV DC for 5kV, 133% insulation, I have it open and cannot find it?
 
lielec11 said:
I appreciate both responses because I was actually looking for the answer to both new and existing. The insulation is 133% so glad we spec'd 36kV and I can only seem to find NETA ATS online. I'd have to pay for NETA MTS.

What section in ATS indicates 36kV DC for 5kV, 133% insulation, I have it open and cannot find it?
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I found it in Table 100.6, Medium-Voltage Cables Acceptance Test Values, page 206, ANSI/NETA ATS-2013. If you have the 2009 edition, Table 100.6 is on page 210.
 
For maintenance test values, NETA MTS-2011, Table 100.6.1 lists Maintenance DC Test Voltages for 5 kV cables at 9 kV for 100% insulation level, and 11 kV for 133% insulation level.

For laminated cable (PILC), NETA MTS-2011, Table 100.6.2 lists the Field DC Test Voltage for 5 kV cables at 23 kV. No insulation level is mentioned but I assume 100%. For 8 kV cables (5 kV at 133%), the DC test voltage is 29 kV.
 
lielec11 said:
Thank you. Can you briefly describe the VLF testing?
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If you do an internet search on "high voltage VLF" you can find some good general descriptions of the test method, much better than I can describe. You will also find several of the VLF test equipment mfrs.


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