DC/AC Hipot, insulation test

[Cloud]

What is the difference between and AC and DC hipot? Is DC hipot be considered as insulation resistance test as well?
We are installing a 36kV line (direct buried) and it is being tested by 10kV DC insulation test. Now the other team is requiring HIPOT test (which we dont have any info whether it is DC or AC), which from our experience is only being used with bare coppers and conductors without insulation.

So which test is not safe for MV cables with XLPE component? Is it right to tell them not to continue with their HIPOT testing?

Thanks. Appreciate any help.

 

[zog]

DC Hipot can be destructive on XLPE cable, however all the test standards (ICEA, NETA, ANSI, IEEE) that have addressed this only refer to service aged (>5 years) cable, no standard for new installations as far as I know have been revised to reflect warnings on new cable for DC hipot tests.

That being said, a DC hipot will only reveal gross installation errors and is not a very useful test. A VLF AC test would be better, tan delta or PD would be the best to establish baseline data for trending purposes if this cable will see any routine tests in the future.

 

[junkhound]

DC Hipot can be destructive on XLPE cable

How so? What is the theory behind that, or is the 'destructive' part due to probability of test procedure errors?

 

[big john]

A DC hipot creates sustained polarization in the conductor insulation, combined with the fact that any highpot applies an overvoltage, this can lead to failure where AC might not.

That said, I agree with Zog that this is mostly a concern on service-aged gear. As a rule we won't highpot any in service equipment unless specifically requested by the customer after they understand the risk of failure.

However, if you're doing a new cable commissioning a highpot is a standard test. And if those are the only tests being performed, I would definitely prefer it over a basic 10kV megger which isn't likely to stress out any defects.

 

[Cloud]

We are also doing VLF together with this 10KV megger. (+ the standard sheath, armor and phase testing)
From what I understand on your replies, HIPOT and megger is not the same even if it has the same testing voltage.

With this on hand, what is the acceptable test and values to be performed with this kind of service?
Please enlighten me on this.

 

[Cloud]

And what is the difference with HIPOT and megger using the same test voltage.
Thanks for all your help.

 

[big john]

We are also doing VLF together with this 10KV megger. (+ the standard sheath, armor and phase testing)

Then a DC hipot strikes me as unnecessary: You're already doing a dielectric withstand test with the VLF.

From what I understand on your replies, HIPOT and megger is not the same even if it has the same testing voltage.

A high-potential test in its truest form is applying a voltage that is a multiple of the rated RMS voltage of the equipment; it's a dielectric withstand test.

10kV from a DC hipot tester is no different than 10kV from a megger.

Example: A 10kV megger can perform a dielectric withstand test on 600V equipment.

Or a hipot putting out 10kV can also perform an insulation resistance test on 35kV equipment.

But if someone calls for an actual "hipot test" on a 35kV cable likely calls for applying ~100kVDC withstand test depending on what specs I'm using.

...With this on hand, what is the acceptable test and values to be performed with this kind of service?

Often the manufacturer will have specific commissioning requirements. In absence of that or customers specs, check out NETA Acceptance Testing Standard 2013.

 

[Cloud]

got it. so it all depends on the multiplier of the test voltage and the rating of the piece to be tested. If it is stressing the insulation to limits then it is considered HIPOT.

one last question, is this rule of the thumb when performing HIPOT is also applicable on cables?
ACtest=AC operating x2+1000
DCtest=AC test voltage x 1.4

Thanks for clearing things up with the terminologies.

 

[jtinge]

Megger is an insulation resistance test. Do a search on "Megger-A Stitch in Time" to find a nice pocket guide on megger testing.

IEEE provides guides on cable testing:

IEEE 400 - IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems Rated 5 kV and Above
IEEE 400.1 - IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage
IEEE 400.3 - IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz)
IEEE 400.3 - IEEE Guide for Partial Discharge Testing of Shielded Power Cable Systems in a Field Environment