VLF-Very Low Frequency AC Hipot Cable Testing 2

[jburn]

I would like to have some discussion on using VLF-Very Low Frequency AC Hipot to test MV power cables. Is anyone on this forum using it and what is your experience with this technology. How often do you test your cables? How long have you been using this technology? Etc.
What are the pro and cons from your experience.

I have XLPE cable (15KV) and I keep being asked to test our cables. So far I have refused to do any DC hipot testing because it damages the cable. From what little I have read about VLF-Very Low Frequency AC Hipot, it sounds like it resolves the problem with cable damage that the DC testing causes, and therefore I may want to consider using the VLF AC Hipot test. Most of our cables are approaching 20 years of age.

Thanks

 

[electricpete]

I have no experience whatsoever in this technology.

IEEE 433 gives info on VLF testing for motors. I couldnt find any comparable standard for cables. Likewise EPRI doc's don't give any recognition to VLF testing of cables that I could find.

I did see A 1992 Doble article which appeared to be written by an equipment manufacturer (HDW Electronics). It listed several disadvantages for dc and power frequency ac hi-pot testing of cables. The punchline was "At this time, the VLF test set seems to be the only device which permits testing of in-service cables with solid dielectric insulation such as PE and XLPE efficiency without adversely affecting the cables life expectancy." He goes on to say test set weighs about 375 pounds - lighter than ac hi-pot. I'm not sure whether his company might make the test boxes (in which case I am skeptical of his claims).

 

[electricpete]

It seems like power factor and tipup tests might be good if cable is short enough to be tested without tripping the set. Have you tried that?

 

[jburn]

Electricpete

Thanks for the information. What little I have read about VLF, is on High Voltage, Inc's web site. Several pages of info. Interesting reading. I had not heard about "VLF" AC Hipot testing before yesterday, only AC Hipot, which has always required heavey, costly equipment, which was hard to drag out into the field. Some one on another thread had mentioned VFL, but he did not go into any details.

http://www.high-voltage-hipot.com/vlf40prd.htm

Jack

 

[jburn]

Electricpete

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.

 

[jbartos]

Suggestion: There is another posting in this Forum dealing with similar topic, namely:
HV cable pressure testing
thread238-4571
wowski (Electrical) Jan 30, 2001

 

[jburn]

jbartos

Thanks. I have been reading that posting threads. I am looking for more information just on using VLF-Very Low Frequency AC Hipot. So far I have not found many folks who know much about it. I had not heard about it until last week and I have been doing Electrical maintenance in industrical plants for 32 year.

 

[jbartos]

Small encore: Visit

http://www.high-voltage-hipot.com/vlfcbldg.htm

for more info (There is also an article to be downloaded.)