MV cable testing - dc hipot - Eng-Tips

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. 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 You must be refering to outdated NETA standards.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. 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? 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. 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 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 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.
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

Not sure about the "predictive" nature of a hipot test

Predictive in the sense of finding a weakness in the groundwall insulation of the stator, or in the case of the surge test the turn-turn insulation, in the hopes of diagnosing failure during an outage and being able to replace the motor before online unscheduled failure.

These tests are "potentially destructive" in that marginal insulation may be totally compromised during the testing. I am aware of the risk/reward nature of the high potential testing, which is why I am looking for individual experience with these tests.

If you use high potential testing for predictive maintenance on motors, what criteria did you use to justify the tests?

Thanks.

Predictive in the sense of finding a weakness in the groundwall insulation of the stator, or in the case of the surge test the turn-turn insulation, in the hopes of diagnosing failure during an outage and being able to replace the motor before online unscheduled failure.

"Predictive" is usually a term used for a condition assesment test, where you can predict the time to end of life and grade equipment for priority of replacement and budgets. I would not call a hipot a "predictive" test. But all symantics, I get what you are asking, there are better ways to predict life expectancy.

These tests are "potentially destructive" in that marginal insulation may be totally compromised during the testing. I am aware of the risk/reward nature of the high potential testing, which is why I am looking for individual experience with these tests.

I agree, there are better methods and equipment to use depending on the specifics of the motors, all covered in the ANSI/IEEE specs.

If you use high potential testing for predictive maintenance on motors, what criteria did you use to justify the tests?

Thanks.

My criteria is that it is on the purchase order I am a testing guy, not an owner of equipment. Have you looked into PdMA?

The debate over whether or not to high potential test motors as a "predictive" maintenance task has proponents on both sides, to test or not to test.

I am looking for personal experiences, do you use high voltage testing (DC/AC hipot, surge testing) as part of your motor maintenance program? Why or why not?

Thanks.

wiigelec- I work for a large utility and am in the process of trying to establish a predictive motor testing program using Baker's AWA12 surge testing machine. We also purchased Baker"s PP130 power pack that would allow us to test motors that operate at volts. Test voltage for a volt motor is 2 times operating voltage plus 1,000 volts. This particular machine can perform 5 separate tests, either manually or automatically. The hi-pot test is AC and therefore not destructive. The whole idea behind this program is that over time, we can reasonably predict when a motor will fail, thus saving money because we could then budget for a rewind rather than it bite us when on line or when demand is high. Most all motor failures start out with failure of the insulation between individual coil wires or turn to turn. I in another lifetime was an owner operator of an electric motor rewind shop in Seattle. We wound motors up to 500 hp. In my opinion the only time a hi-pot tester should be used is on new motors and rewound stators if desired or required by the customer such as Navy spec. The use of hi-pot tests on older windings is very much a potential for setting up insulation failures especially in older windings that are already partly compromised by possible wet and heat affected briteleness. Although these windings may be still usable, when a hi pot is used it can cause flashovers and carbon tracking that will lead to failure. In the motor trade I dont think you will find any motorman who would use a hi pot on anything other than new or fairly new windings. We mostly shudder at the thought. Even the Navy doesnt require hi pot on anything but new windings. A surge tester is not normally an instrument that will cause any probelem and is in fact quite a good tool. Use of a good megger is actually in my opinion a better tool for the type of testing you are considering. A log of winding resistance to ground and phase to phase kept over a long period is in my opinion a much better predictor of potetial trouble than setting yourself up for failure using hi voltage testing. The "AC High Potential Test" will not give you any useful information in the context of predictive maintenance. It is a "go, no-go test," and does not provide trendable results. If the motor fails this test, you send it to the rewind shop, because the test will have destroyed the motor. If the motor passes this test, all you know is that the motor has not been destroyed yet.

A better test, if you are looking to trend results and predict failures, is the "DC High Potential Step Test." You apply DC voltage in increasing steps, until the onset of leakage current is observed. You stop the test at that point, so as to prevent actually damaging the motor. The trendable result is the voltage level at which the leakage current is observed. This value will drop as the motor ages, and a rapid drop (as compared to the last test result) is predictive of possible failure.