DC testing voltages

[tariro]

When doing DC hipot tests, what value of a DC voltage do you use relative to the rated voltage of a cable? Like for 11kV rated cables (AC) what value of DC voltage can i use? What does the BS standard say about this?

 

[candu6]

The standard maximum hi-pot value is twice the rated voltage plus 1kV. Therefore, you would test your 11kV cable at 11kV+11kV+1kV = 23kV.
Remember that hi-pot testing is potentially destructive (pass/fail).

 

[eemotor]

Please take a look at thread238-86054

 

[wareagle]

Okonite Cable specs 300 volts per mil of insualtion. For a 15 kv cable with 175 mils the test voltage would be 55 kv and for 220 mils the test voltage would be 65 kv.

 

[busbar]

 
For ANSI regions, I agree with wareagle’s numbers that are consistent with {US} ICEA standards, but only for new, yet-to-be-energized cable. Again, for ANSI practices, IEEE std 400 was overhauled in 2001 and may be worth a careful read.

 

[bmodrow]

For acceptance testing refer to the manufacturers recommendation for test voltage which could be as much as 5 times the insulation rating.

For normal maintenance of cable that has already been placed in service, I would not recommend testing the cable above its rated value unless you have made provisions to pull new cable in the event of a failure.

Since I work for a service company we ALWAYS have the customer tell us at what value to hipot exsisting cable.

As stated hipot testing is a destructive test.

 

[benlanz]

Recommendations for DC HIPOT testing should be given once the type of cable insulation and age are specified. What kind of cable insulation are we talking about? How old are the cables?

 

[tariro]

these cables have more than 20 years in service and are PILC

 

[benlanz]

A DC HIPOT is a good HIPOT for conduction type defects in PILC. If you measure meg-ohm resistance with your DC HIPOT you will get the better results. Unfortunately in your case, if you don't disconnect your load (pump), the losses you will measure in the rotating machine will eclipse any losses in the cable insulation! If there is tracking due to a lack of oil in the cable you can still have a problem, even if you measure low losses. Areas in the cable that lack oil will have partial discharge, (carbonized traking on the paper tapes) and will fail.

A HIPOT may cover you as far as you local standards but, it is a fairly useless test. The only information one knows from a successful HIPOT test is that the cable survived the overvoltage! The cable could still fail after you put the cable back in service. You must measure the cable's response during the voltage application to predict future performance.

The best recommendation for PILC cable.

1. Disconnect cable from the load
2. Perform a 5kV 'megger' test (Doble or tan delta test would be better)
3. Perform a 60 Hz PD test up to 1.7-2.0Uo.

PILC cable have a proven track record if they are built correctly. They can last 80+ years. The problem today is that there are very few contractor who know how to properly build PILC cables. The job requires a true artist with knowledge of hot oil tests (for terminations and joints)and nitrogen pressure test (for joints).

-Cheers