MV cable testing - dc hipot 1

[magoo2]

Is there any additional information that one obtains by using the full test values on new cable?

Our company follows the IEEE and NETA guidelines as far as permitting higher dc test values with new cable and reduced test values with field-aged cable. It would be simpler to just go with the reduced values for all cases. It also makes it less likely to screw it up. Is there any compelling arguments either way?

I've looked through earlier threads on testing, but didn't see anything related to this.

 

[Zogzog]

Current NETA standards do not allow (reccomend) DC hipot testing of service aged cable, nor do the IECA or IEEE 400. You must be refering to outdated NETA standards.

DC hipot testing has been proven to reduce cable life expectancy (Specifically in XLPE). Not to mention a DC hipot test is nearly worthless, it wont fond anything but gross installation defects and is a go-no-go test, shouldnt really be used for new cable either on its own. A Tan Delta or at least a VLF test should be done to establish baseline data for future condition assesments or an online PD test after energization to again establish baseline data.

 

[electricpete]

I know there has been a lot of change and a lot of standards moving away from dc step voltage testing.

I remember flipping thru IEEE400 looking for clarification and it seemed like they were a little ambiguous on the subjec.t

I don't think IEEE400 prohibits dc hi-pot testing, does it?

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[ScottyUK]

Have a look for posts by member BenLanz who posted some good contributions on the use of PD in cable testing. He works (worked?) for a company who make equipment used for PD testing so he was not exactly impartial but his technical comments were usually right on the mark.


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If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

Ben is also on the IEEE 400 commitee I believe.

They dont actually prohibit anything, I believe the statement is something more like research has shown that DC hipot testing of service aged MV cables may reduce the life expectancy of the cables, mostly XLPE. Something like that.

Regardless, a DC hipot test is proven to not find installation errors or gross manufacturing defects, it is a go-no-go test and the data is pretty worthless for predictive maintenance purposes.

 

[magoo2]

IEEE Std 400, Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems, includes a cautionary note about dc hipot (sect. 5.2).

My question is not about which testing method is best for MV cable. This has been beat around before in a number of previous discussions. That's why I restricted this topic to dc testing of MV cable. I'm not interested in why VLF testing might be a better choice.

I know that Detroit Edison and CTL showed through their research that dc testing is harmful to field aged cable. Again that's not my question.

There are 2 schedules for dc hipot tests based on cable operating voltages. The one for field aged cables is on the order of 30% of the new cable values.

My suggestion is, where someone desires to do dc testing, why not just use the lower values for both new and field aged cables?

Why would someone want to do dc testing? In some cases, we have different contractors do cable work for us in substations. Before re-energizing a cable getaway circuit at a substation, there's concern about subjecting the substation transformer to a through fault if the cable were faulted. Just being able to identify through testing that you may have gross defects, as Zogzog mentioned, would be worth more than risking the life of the transformer.

Given it's shortcomings, will dc testing at the 30% level on new cable provide information on the new cable that is any different than you would get at the 100% level?

 

[Zogzog]

Not really IMO. Sure you may have a splice or termination that may flashover at 100% but not 30%, but I dont really think there would be much of a difference for the cable itself if say the insulation was damaged.

Then again, thats just my thoughts.

 

[coppertop]

IEEE 141 has a fairly good discussion on cable testing, the various test voltage levels and the relative value of the test methods.

 

[DanDel]

DC Hipot testing is destructive when voltages above the cable rating is used; it's more destructive with higher voltage. That being said, it is still recommended by NETA for acceptance testing of new cable at a destructive-level voltage, this is why it is still being used.

I believe that Hipot testing at a voltage at or below the cable rating is of some use for maintenance testing of in-service cable, similar to but better than a standard megger test. Much easier, quicker, and cheaper than Doble, VLF, or PD testing.

My company has always allowed a customer to specify test voltages different than NETA or IEEE if they want to provide their own spec.

 

[oldfieldguy]

Here we go again with the DC Hipot testing discussion.

Yes, I am aware of the more recent documents on the subject, but for dozens of years, DC hipot testing was THE method for testing cables?

Destructive? Yes, especially when you couple untrained personnel with nebulous or non-existent procedures.

With trained technicians, good equipment and a well thought out and consistently applied procedure, the DC hipot test can be used quite effectively as a PM test. I know. I applied it to a large petrochemical facility with miles of underground PILC cables dating from 1944 to the present. During the course of that testing, we NEVER had a cable fail that was on line before it came down for scheduled PM tests.

We did find incipient failures of splices and potheads, basing our determinations on comparison with previous test data. The ability to do the comparison was based on the fact that the previous test was applied in exactly the same manner as the test today, making it an apples vs. apples comparison.

We also applied similar procedures to EPR cables in the 5kV and 15 kV range.

The key to DC hipot testing is knowledge of what is to be done and the expected indications. This requires thought and training, and unfortunately, hipot testing is one of those tasks that is all too often relegated to one of the less experienced and knowledgeable individuals on a testing crew. With inconsistent application of the test, inconsistent results, up to an including failure of the cable under test, are the result.

old field guy

 

[Zogzog]

Oldfieldguy said " hipot testing is one of those tasks that is all too often relegated to one of the less experienced and knowledgeable individuals on a testing crew."

True, and why is that....

DanDel "Much easier, quicker, and cheaper than Doble, VLF, or PD testing."

Exactly! Thats the problem, it is the hacks who go buy a DC hipot on Ebay and Hipot cables without the proper training or experience. I often see the questions asked, what is the leakage spec for a 15kV EPR cable? Is there a correct answer to that? No, it depends on humidty, temp, length, age, cable type, accessories, corona suppression methods, etc.....

No one can deny that a DC test is as good as a test as Tan Delta or PD for condition assesment purposes, but until the newer methods are more accepted and the equipment is more affordable DC hipot testing will be around for awhile. However, just because it has been used for many years dosent mean it is as good as new methods.

 

[waross]

Hello Magoo2;
Maybe you will have better results if you rephrase your question.
Maybe something like this:
Cable rated voltage = xxx volts.
Hi-pot test voltage for new cable = yyyy volts.
Hi-pot test voltage for aged cable = zzzz volts.
To avoid closing into a fault, we wish to hi-pot cables prior to energization. The cables may be new or they may be aged cables that have been disturbed or serviced. We are looking for a go/no-go indication, rather than a cable quality assessment.
We deal with a mix of new and aged cables.
We propose to test both new and aged cables at the values for aged cables to keep the testing simple and avoid the possibility that an aged cable may be subjected to the higher than recommended test voltage.
Again the question is not whether to hi-pot, but a question of hi-potting new cables at the levels recommended for aged cables.
Feel free to edit and repost this suggestion to your liking and to then red-flag this post out of existence to avoid confusion if you wish.




Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

waross--

We generally pursue a regimen of DC hipot testing of new cables during the course of commissioning a new installation. since this is usually completed some substantial period of time prior to actual energization, we go back and what I term a "crowbar check" immediately prior to energizing. this is a quick application of a DC hipot or a 5 kV megger to make sure that nobody has accidentally left a tool or an undocumented ground or shorting wire in the system.

We also do this prior to re-energizing after extensive maintenance. Detecting somebody's bonding jumper left over from testing during the crowbar check is embarrassing to the guilty party, but it's not nearly as embarrassing as blowing the back off the switchgear, or as happened in one case where this was NOT done, having a 400 MW generator trip offline every time they tried to load it up.

Back to the original subject, though, here's my take: Yes, there are new cable testing technologies available, but the DC hipot test is still useful if applied and interpreted correctly. Given the choice between NO testing because of the availability/cost of the new methods and perceived shortcomings in DC hipot testing, or using what is available, a DC hipot, a good procedure, and a skilled technician, I'd go with the DC hipot.

Of course, if a cable fails under the DC test, you can always point out that if ONLY you'd have had time and money to schedule the OTHER tests...

old field guy

 

[waross]

oldfieldguy--
From my read of the original post, I think that you have supplied the information that Magoo2 was looking for.
lps for you.

Bill
--------------------
"Why not the best?"
Jimmy Carter