VLF vs AC HiPot

[Kalbi_Rob]

So, recently went onto a job to perform VLF testing of 2 sets of 35kV cables, when the customer threw a curve ball at me to test the 35kV breakers. I had no ac hipot to perform vacuum bottle integrity and withstand test. Our management does not want us to test with DC on vacuum bottles, as there is documentation that dc can damage vacuum bottles (haven't personally seen this documentation other than production of X-Ray radiation), so would VLF be sufficient as a AC hipot test on Vacuum bottle and withstand test for breakers?

Per manufacturer 60kVAC or 82kVDC is required to perform vacuum bottle integrity on 35kV breaker. HV60 only puts out 60kVDC or 44kVAC but NETA allows for 42kVAC for testing breakers. I'm more interested in hearing the community discussion of using the VLF for AC HiPot for or against arguments.

[ElectricalTestTech]

So, recently went onto a job to perform VLF testing of 2 sets of 35kV cables, when the customer threw a curve ball at me to test the 35kV breakers. I had no ac hipot to perform vacuum bottle integrity and withstand test. Our management does not want us to test with DC on vacuum bottles, as there is documentation that dc can damage vacuum bottles (haven't personally seen this documentation other than production of X-Ray radiation), so would VLF be sufficient as a AC hipot test on Vacuum bottle and withstand test for breakers?

Per manufacturer 60kVAC or 82kVDC is required to perform vacuum bottle integrity on 35kV breaker. HV60 only puts out 60kVDC or 44kVAC but NETA allows for 42kVAC for testing breakers. I'm more interested in hearing the community discussion of using the VLF for AC HiPot for or against arguments.

So as a brief answer, yes, I have used a VLF to hi pot a breaker in a bind. Just like you, it was all I had on site. In my eyes, a VLF was better than just a 5kV megger and DLRO. What I will speak on is "stress"... Think about why we use VLF on cables. We use it because using an AC hi pot at 60Hz is very stressful and damaging to cables (it's more technical than that explanation, but being brief). In my mind, I want to stress this breaker and make sure it won't fail on me when I rack it back into an energized cubicle. I want to stress it to make sure it wont fail when I close it and energize a circuit. That is the way I think of this topic. Its one of those statements that you make, "use the proper tool for the job" kind of deals. Use what's available to you, because something is always better than nothing

[SecondGen]

So, recently went onto a job to perform VLF testing of 2 sets of 35kV cables, when the customer threw a curve ball at me to test the 35kV breakers. I had no ac hipot to perform vacuum bottle integrity and withstand test. Our management does not want us to test with DC on vacuum bottles, as there is documentation that dc can damage vacuum bottles (haven't personally seen this documentation other than production of X-Ray radiation), so would VLF be sufficient as a AC hipot test on Vacuum bottle and withstand test for breakers?

Per manufacturer 60kVAC or 82kVDC is required to perform vacuum bottle integrity on 35kV breaker. HV60 only puts out 60kVDC or 44kVAC but NETA allows for 42kVAC for testing breakers. I'm more interested in hearing the community discussion of using the VLF for AC HiPot for or against arguments.

From what I understand the issue with DC hipotting has something to do with the rectification process and filtering. I've never had to use a VLF for vacuum bottles but I would imagine the voltage is smooth, the only thing I would be concerned with is the peak voltage exceeding my max test voltage. Also not sure what test voltage to use at that frequency since it's closer to DC than 60hz AC.

[testguy]

So, recently went onto a job to perform VLF testing of 2 sets of 35kV cables, when the customer threw a curve ball at me to test the 35kV breakers. I had no ac hipot to perform vacuum bottle integrity and withstand test. Our management does not want us to test with DC on vacuum bottles, as there is documentation that dc can damage vacuum bottles (haven't personally seen this documentation other than production of X-Ray radiation), so would VLF be sufficient as a AC hipot test on Vacuum bottle and withstand test for breakers?

Per manufacturer 60kVAC or 82kVDC is required to perform vacuum bottle integrity on 35kV breaker. HV60 only puts out 60kVDC or 44kVAC but NETA allows for 42kVAC for testing breakers. I'm more interested in hearing the community discussion of using the VLF for AC HiPot for or against arguments.

A response from our LinkedIn page:

"In my opinion, there is not a clear answer here, but in this situation I would use MFR guidelines, NETA standards, Testing company policies and procedures, in that order. If not, then use dc (at RMS AC equivalent (AC test voltage * 1.414)), VLF at RMS equivalent value. None of the other methods would be sufficient enough to “stress” (stress is not the purpose) or properly test the vacuum integrity of the bottle."

https://www.linkedin.com/feed/update...88916154368%29

[Kalbi_Rob]

A response from our LinkedIn page:

"In my opinion, there is not a clear answer here, but in this situation I would use MFR guidelines, NETA standards, Testing company policies and procedures, in that order. If not, then use dc (at RMS AC equivalent (AC test voltage * 1.414)), VLF at RMS equivalent value. None of the other methods would be sufficient enough to “stress” (stress is not the purpose) or properly test the vacuum integrity of the bottle."

https://www.linkedin.com/feed/update...88916154368%29

Thank you for the response, I was unfortunately unable to test at the values required by the manufacturer, but after debate with the customer, it was agreed to test at the NETA value of 42 kVAC with VLF. Later testing will be preformed at the prescribed value set by the manufacturer within the normal maintenance cycle as this was emergent testing. I could not find any discussion prior to this on VLF vs AC hipot, so this was very informative for not only myself but my company and associates from previous companies.

[SecondGen]

So, recently went onto a job to perform VLF testing of 2 sets of 35kV cables, when the customer threw a curve ball at me to test the 35kV breakers. I had no ac hipot to perform vacuum bottle integrity and withstand test. Our management does not want us to test with DC on vacuum bottles, as there is documentation that dc can damage vacuum bottles (haven't personally seen this documentation other than production of X-Ray radiation), so would VLF be sufficient as a AC hipot test on Vacuum bottle and withstand test for breakers?

Per manufacturer 60kVAC or 82kVDC is required to perform vacuum bottle integrity on 35kV breaker. HV60 only puts out 60kVDC or 44kVAC but NETA allows for 42kVAC for testing breakers. I'm more interested in hearing the community discussion of using the VLF for AC HiPot for or against arguments.

I talked with a senior tech at my shop about this and he says in a pinch it would be perfectly acceptable to use a VLF to test vacuum bottles, only issue might be reaching peak test voltage. His main point was that if there is a leak in the vacuum bottle it would be easily detected by the VLF.

Any time he has come across a bad vacuum bottle, it’s usually detected by the 5kV Megger first. Any strange megger values will be your first clue and can be confirmed by a VLF, it doesn’t take much to make a bottle arc with no vacuum because the contacts are so close.

He also says that X-rays are still produced at VLF frequency, and even at 60Hz some is produced but at that speed it’s so fast they are negligible. So now my question is, at what frequency do the x-rays become significantly diminished? 5Hz? 20Hz? 40Hz? I'll be looking more into this for my own curiosity.

Here is something I found regarding DC testing vacuum interrupters from GE, there is a small blurb about the rectification process I mentioned in my previous post:

GE Burlington continues to strongly recommend the use of an AC high potential machine for vacuum interrupter integrity tests. DC testing of vacuum interrupters should only be utilized if an AC tester is not available, and should be used for quick field checks only.

Our experience with DC testers over many years indicates they frequently yield false negative test results, due partially to the capacitive component of the vacuum interrupter during DC testing, and to the fact that most lightweight DC testers have a very low leakage current trip setting. They will however, reliably indicate a truly failed bottle if the voltage output is set at 50kV DC.

If using a DC tester, and a test indicates a bad interrupter, retest with the polarity of the DC test voltage reversed. If this results again in a failure, we would recommend a final AC test prior to contacting GE Post Sales Service or discarding the interrupter.

No attempt should be made to try and compare the condition of one vacuum interrupter with another, or to correlate the condition of any interrupter to low values of DC leakage current. There is no significant correlation.
After the high potential voltage is removed, discharge any electrical charge that may be retained.

CAUTION: MANY OLDER DC HIGH POTENTIAL MACHINES ARE HALFWAVE RECTIFIERS. THIS TYPE OF HI-POT TESTER MUST NOT BE USED TO TEST VACUUM INTERRUPTERS. THE CAPACITANCE OF THE POWER/VAC BOTTLE IS VERY LOW AND THE LEAKAGE IN THE RECTIFIER AND ITS DC VOLTAGE MEASURING EQUIPMENT IS SUCH THAT THE PULSE FROM THE HALF-WAVE RECTIFIER MAY ACTUALLY BE IN THE NEIGHBORHOOD OF 120KV, WHEN THE METER IS ONLY READING 40KV.

IN THIS CASE, SOME PERFECTLY GOOD BOTTLES CAN SHOW A RELATIVELY HIGH LEAKAGE CURRENT SINCE IT IS THE PEAK VOLTAGE OF 120KV THAT IS PRODUCING ERRONEOUS BOTTLE LEAKAGE CURRENT. IN ADDITION, THE X - RADIATION WILL BE OF CONCERN.

[Zog]

So, recently went onto a job to perform VLF testing of 2 sets of 35kV cables, when the customer threw a curve ball at me to test the 35kV breakers. I had no ac hipot to perform vacuum bottle integrity and withstand test. Our management does not want us to test with DC on vacuum bottles, as there is documentation that dc can damage vacuum bottles (haven't personally seen this documentation other than production of X-Ray radiation), so would VLF be sufficient as a AC hipot test on Vacuum bottle and withstand test for breakers?

Per manufacturer 60kVAC or 82kVDC is required to perform vacuum bottle integrity on 35kV breaker. HV60 only puts out 60kVDC or 44kVAC but NETA allows for 42kVAC for testing breakers. I'm more interested in hearing the community discussion of using the VLF for AC HiPot for or against arguments.

Either one is just a go-no go test of a VI. MAC testing is rapidly becoming the norm, several utilities now require it even though it is an optional test in NETA.

[Lukedl83]

So, recently went onto a job to perform VLF testing of 2 sets of 35kV cables, when the customer threw a curve ball at me to test the 35kV breakers. I had no ac hipot to perform vacuum bottle integrity and withstand test. Our management does not want us to test with DC on vacuum bottles, as there is documentation that dc can damage vacuum bottles (haven't personally seen this documentation other than production of X-Ray radiation), so would VLF be sufficient as a AC hipot test on Vacuum bottle and withstand test for breakers?

Per manufacturer 60kVAC or 82kVDC is required to perform vacuum bottle integrity on 35kV breaker. HV60 only puts out 60kVDC or 44kVAC but NETA allows for 42kVAC for testing breakers. I'm more interested in hearing the community discussion of using the VLF for AC HiPot for or against arguments.

I believe this was in the Eaton vacuum CB manual, or it may have been in NETA article, but your readings may be effected when using a DC highpot due to particles/contaminants that may be present on one side of the contact. When the voltage is applied to that side, then the leakage current may reach higher levels because of the sharp shape of the contaminants on the contact. They recommend switching the leads if the readings are out of specs, and if the readings are higher, then you know that the contact is good. This is a very general description of this potential issue.

[JeffWiley]

Hi...I have used power factor testing before, on one cable, (34.5 KV - 1500 ft, underground). They had to bring in a special piece of equipment to go with the test set, to keep the unit from tripping out. I have had that cable power factor tested several times,over the years and it has never shown a problem, but I have also had two of the terminations on that cable fail, at different times. So I'm not sure that testing was worth the cost. I still had two emergency outages, in the middle of the night. In both cases we were able to re-make the terminations, so the cable is still in use. It is XLPE and was installed in 1976. Maybe the best thing to do is just plan on replacing it, on the next turnaround.

turnkey pcb assembly

[haydensk99]

Hi...I have used power factor testing before, on one cable, (34.5 KV - 1500 ft, underground). They had to bring in a special piece of equipment to go with the test set, to keep the unit from tripping out. I have had that cable power factor tested several times,over the years and it has never shown a problem, but I have also had two of the terminations on that cable fail, at different times. So I'm not sure that testing was worth the cost. I still had two emergency outages, in the middle of the night. In both cases we were able to re-make the terminations, so the cable is still in use. It is XLPE and was installed in 1976. Maybe the best thing to do is just plan on replacing it, on the next turnaround.

Speaking from a point of ignorance here as I do not know the DLA results nor the full history but perhaps you need to consider Partial Discharge testing in conjunction with your DLA testing?.. If the cable in question is powering critical assets, it might be worth implementing High Frequency CT(s) on the cable screen with a online PD meter such as a TECHIMP PD CHECK or doing routine testing...
All that said, with a cable of that age, it might be worth budgeting for a replacement.
Just my 2 cents worth