DC hipot of 15kV cable

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coulter

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We are installing a bunch of 15kV shielded, 3/c - 4/0 to 500kcmil. I've been looking over the test data and have a bunch of questions. DC hipoting is not my area of expertise. So far I've read IEEE 400, (IEEE 400.1 is not available to me), IEEE 576, IEEE std 4, NETA MTS, NETA Acceptance Criteria, maybe some other stuff NEMA 54? and 73?) - these are all real page turners (yawn).

IEEE 400 says 56kV max, and lists a max current dependent of the insulation type, insulation thickness, temp, voltage, length. The acceptance criteria is listed as the 56kV current at 1 minute greater than the 56kV current at 15 minutes.

What I am seeing, is up to 30kV the current is linear with respect to the voltage and the curve is through the origin (0kV,0uA). Around 35kV the slope changes up significantly. The ends are terminated, clean and bagged and the RH is 40% to 50%, ambient is 58F to 70F. Between 50kV and 56kV, the current takes another slight jump above a linear approximation (least squares) of the 35kV-50kV data.

The cables meet test spec.

Here is what I think I am seeing. Up to 30kV the I'm just seeing the cable insulation resistance. Above 35kV I think I am seeing corona discharge. Above 50kV I think the insulation is being degraded slightly.

The electricians are pretty good. They seem to have a lot of experience in doing the tests and doing the terminations. The terminations look good.

So, who has some experience to give me some clues?

Is it corona I'm seeing above 35kV?
If it is, what are some methods of raising the voltage where it starts?
Are we gaining any additional information about the cable integrity by testing to 56kV over say 35kV?

Like I said, the cables passed, I am just trying to gain insight on what I am seeing - interpretation of test results

I'm planning on sending my data to the cable mfg and calling him next week. Too many rats left to kill to do it this week.
 
You are correct in saying they passed, since the test in question is considered a "go or no-go" test. Beyond that, the Hi-pot test is designed to only detect gross imperfections due to mishandling or improper installation.

I'm not sure what could be gained from trying to interpret the data resulting from the tests, especially since the intent of the test is not to gather data.

If you are interested in specific information, then performing a Time Leakage Test, and Partial Discharge Test might be in order.

"Maximum Profit Usually Comes at the Brink of Disaster"
 
Depending on the insulating material (XLPE), you may not want to do DC testing at all. It will artificially age the cable. In my opinion, DC hi-potting is a ?no-go/don?t know? test. That is, if it ?passes? you still really have no idea whether it?s good or not. However, unless there is good reason to believe the cable was mishandled during installation the likelihood of it not being good is extremely low if it is purchased from a reputable source.

If at all possible, get the manufacturer?s recommended acceptance test procedures for both initial and long term in service installations. For the most part, the ?weak? points are the terminations or splices, if any, and they need the closest scrutiny.
 
All

I appreciate you taking the time to answer. I believe I have the test part figured out. IEEE 400 and IEEE 576 are clear and the company specs match the IEEE. The choice to use or not use DC hipot is not one I'll be making for this job.

The questions I am asking have to do with interpreting the data for insight into the physics. Maybe kingpb and zog are right and there isn't any insight available from this data. Bummer. I have enough samples to be away from "statistical averages of one" issues. Too bad I can't use this for an opportunity to learn some science.

Maybe the factory rep ....
 
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