DC Hipot test voltages for new cable

[rdvette]

I am required to do a hipot acceptance test of newly installed 15KV EPR copper cable. I have heard that a standard rule would be to Hipot at 3 times the rated cable voltage or 45KV DC. Is this a good rule to follow?

 

[rbulsara]

Refer to NETA-ATS (Acceptance Testing Specs). Available at

http://webstore.ansi.org/

Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

Your test voltage depends on the conductor size and insulation level (%).

I would hope someone doing acceptance testing of MV cables would know what the test voltages should be. I also recommend you read this thread.

http://www.eng-tips.com/viewthread.cfm?qid=248906

 

[dpc]

No, it is not a good rule. A good rule would be to test to the acceptance voltage recommended by the CABLE MANUFACTURER who made the cable.

 

[SouthwireSam]

Please See Table 9.1 and 9.2 for HiPot Voltage testing standards.

 

[Zogzog]

Wow! Is that a current document from Southwire?? It quotes IEE 400 1991 version, and the 2000 ICEA. Lots of things have changed since then that go against what this document says. IEEE 400 and the ICEA that are referenced advise against DC hipot of in service cables yet this document is recommending a DC hipot test as an "Advanced" test.

No mention of VLF, TD, or PD testing at all. I think this has to be an out of date document or the folks at southwire need to get it updated to current standards they are referencing.

 

[dpmac]

A quick visit to the Southwire website indicates they are involved with the latest testing methods.

http://www.southwire.com/transmission/onSiteUndergroundTesting.htm

 

[Zogzog]

Yep, that must have been an old standard that was posted.

 

[wareagle]

Okonite specs 300 volts per mil of insulation. Check site below.

http://www.okonite.com/engineering/high-voltage-testing.html

 

[HSKabel]

May I comment on the document provided by SouthwireSam and some of the reactions: The pages posted by SouthwireSam are from Southwire's Power Cable Manual 2005 (3rd edition)chapter 9 "Field Testing". Unfortunately, the pages do not reflect state of the art practice and are rather generic.
There are many discussion threads in this forum on the subject that you may want to research but a very brief, incomplete summary of field tests is as follows:
1. For extruded insulation (XLPE, EPR) cables, DC withstand testing on NEW CABLE ONLY (on the reel)may not hurt but it also is known not to detect much.
2. For older (aged) cables and cables installed with accessories (splices, terminations) DC withstand testing is not recommended due to the risk of damage to the insulation (older cables) or the accessories.
3. For MV cables rated 5-46kV, a VLF test (very low frequency) using an AC test voltage is the way to go. This test combines to some extent combines the economic advantages of DC testing with the relevance of AC testing.
4. For HV cables rated 69kV and higher, if a simple soak test of the installed system is not acceptable or considered insufficient, an AC resonant voltage withstand test, preferably combined with a PD test, is to be recommended.

 

[oldfieldguy]

HSKabel--

In the years that I've tested cables, one thing that is a common find on DC tests of new cables is NOT a "bad" cable, but rather a poorly executed termination (stress cone) or splice.

old field guy

 

[pie314]

A 15 KV system is normally tested at 34 KV hipot voltage. The peak voltage of 34 KV is 47.9 KV. Since you are testing the cable with DC then 45 KV sounds reasonable.

 

[Zogzog]

"A 15 KV system is normally tested at 34 KV hipot voltage."

per what standard?

NETA/ANSI Acceptance test voltages are max of 56kVDC for 100% insulation rating and 64kVDC for 133% insulation.

Maintenence test values are max of 18kVDC and 20kVDC. These are for up to 5 year old cable, after 5 years no values are given.

THe ICEA has the same values.

 

[rdvette]

This much I know from experience, 1/0 Aluminum URD 15 KV cable 100% insulation,3 phase installation aproximatley 10 years after installation. Recloser opened on line. After verifing that there was a fault and insolating the section of line that was bad, we hypoted the cable as part of our fault locating process using a thumper. We ran the hypot up over 50 KV DC and could not get the fault to fire over. We set the recloser to one shot and hit the cable with 7200 KV AC. The recloser opened and we tried the hypot again. It still would not fire over at over 50 KV DC. The DC hypot showed the cable as being good. We hit the cable with 3 more operations of the recloser before we were finally able to get the hypot to show a fault.

 

[dpc]

For a NEW cable, I would follow the manufacturer's recommendation. Period.

At present there are a lot of somewhat conflicting opinions, recommendations and even standards related to cable applied voltage testing. But very few of these sources of "knowledge" have any financial stake or liability in YOUR cable.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[dpmac]

ICEA and AEIC state the DC test voltages on NEW (less then 5 years)cable is as Zog stated above. NETA uses their numbers for their specifications. Call a manufacturer of EPR (and probably XLPE) 15kV cable and they will tell you follow AEIC or ICEA test voltages. After 5 years all bets are off on what you will hear from them.

RDVETT:

One of the reasons you may not have determined the cable was "bad" was probably due to the current meter range on the thumper. Typically a thumper is defaulted to the highest current range. A unit with a 50kV hipot would most likely have a 100mA high range. If you had the opportunity to test all three phases with a basic hipot (lower current meter ranges) you may very well have been able to determine which phase was faulted.

I have seen ice in a riser pole cut the same type of cable so clean it can withstand 30kVdc hipot prior to flashing over. This same cable was then re-energized for weeks even though it was cut in two.

 

[Zogzog]

DPC, I agree that following manufactures recommendations is prefered, but if they are not available, then you turn to industry standards like ICEA, NETA, or ANSI.

 

[pie314]

Distribution Transformer standards call for 34 KV AC hipot for 15 KV systems

 

[Zogzog]

OK, but this is a cable, different test voltages than transformers.

 

[dpmac]

I was assuming the OP is testing a 15kV feeder properly terminated with no equipment (transformers, switchgear) attached in the USA.

The test voltages would be 55kVdc/65kVdc for 175mil or 220mil respectively per IECA/AEIC/NETA. These specifications also may make mention not to exceed test ratings for installed equipment. This would include elbows and terminations. Do keep in mind 15kV load break elbows may or may not be able to withstand the higher DC field test voltage depending on manufacturer and style (test point).

I believe ANSI/IEEE 386 requires a 200A elbow to be able to withstand a factory test of 53kVdc for 15 min and 34kVAC, 60Hz for 1 min. Field Testing 15kV cables with elbows at these levels will of course require proper preparation of the elbow by using a proper parking stand in good clean condition. Most terminations seem to hold up well with DC testing if installed properly, kept clean and tested with proper corona suppression.