HI-Potting

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mull982

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I am fairly new to the idea/theory behind Hi-Potting power cables and was wondering if someone could help explain some results I got from a recent Hi-Pot test I had performed. The cable we tested was a 3/C SHD-GC cable rated for 5kV. We tested each of the three phase conductors up to 25kV DC stepping up the voltage in increments of 5kV and holding for a minute, and then holding the 25kV for 15min. There were two ground wires in the conductor which we tied together, and we took our positive lead from the high potter to each phase conductor (one at a time) and connected the other lead to the grounds. The cable was 2500ft long.

I'm guessing that the point of this test it to subject the cable to a voltage, and see how much current leaks (penetrates the conductor insulation) to ground? Is this correct? At 25kV the highest leakage current that we got as 135uA. Is this an acceptable reading that you would expect to see for this type of cable. What is the cut-off leakage current for detriming if the cable is bad or not? With the cable being rated as a 5kV cable, how does it allow for 25kV to be run through it, in order to perform this test?

I would appreciate any info.
 
Here I am, sorry late to the party but had a busy day.

OK, here I go again. No one, not IEEE, NETA, IEC, etc.. recommends DC hipotting service aged (>5 years old) MV cable, it had been determined to be a destructive test and may reduce the cable life expectancy. There is some debates on the value of new cable DC testing, but all groups agree that a DC test will only reveal gross installation flaws.

To try to answer your question regarding your results, (which I cant and wont), there are no set values, there are too many factors, temp, humidity, site conditions, and the type of corona supression used (Which you made no mention of in your post).

Think of a DC hipot test like this. You go see your doctor, he says get on that treadmill, and you run full speed for 10 minutes. When you are done, if you dont die, the doc says you are fine. It is a go-no-go test.

Cable testing has come a long way in the last few years and should be used as a condition assement tool by either AC hipot (This would require a power supply that would fill a semi trailer for the cable you described), VLF, Tan Delta, or PD testing. I highly suggest you do some research (IEEE 400, and NETA for starters) before performing any more of these tests, just hope that cable you tested does not fail soon.

Saying you are new to the idea/theory of hipotting cables is a scary statement, leave it to the pros or get some training and the right test equipment.
 
zog said:
Here I am, sorry late to the party but had a busy day.

OK, here I go again. No one, not IEEE, NETA, IEC, etc.. recommends DC hipotting service aged (>5 years old) MV cable, it had been determined to be a destructive test and may reduce the cable life expectancy. There is some debates on the value of new cable DC testing, but all groups agree that a DC test will only reveal gross installation flaws.

To try to answer your question regarding your results, (which I cant and wont), there are no set values, there are too many factors, temp, humidity, site conditions, and the type of corona supression used (Which you made no mention of in your post).

Think of a DC hipot test like this. You go see your doctor, he says get on that treadmill, and you run full speed for 10 minutes. When you are done, if you dont die, the doc says you are fine. It is a go-no-go test.

Cable testing has come a long way in the last few years and should be used as a condition assement tool by either AC hipot (This would require a power supply that would fill a semi trailer for the cable you described), VLF, Tan Delta, or PD testing. I highly suggest you do some research (IEEE 400, and NETA for starters) before performing any more of these tests, just hope that cable you tested does not fail soon.

Saying you are new to the idea/theory of hipotting cables is a scary statement, leave it to the pros or get some training and the right test equipment.
Click to expand...
The web site I listed above from Okonite addresses this subject.
 
Bob, that article talks about acceptance testing and maintenance testing of cables installed less than 5 years. So what about after 5 years?

Brian, the specs are outdated, we are supposed to be the experts and aware of new technologies and research. We run into this also, I tell the customer about the advantages and disadvantages of DC testing, but if thats what they want, thats what they will get. The customer is always right in the end, but I want to make sure they are aware of the new recommendations of IEEE and others so that when the cable fails, they were given the facts.
 
Zog
I used to be into Hipot tests along time ago. Its my understanding that AC test are supposed to be a better test but I do not know. If I were going to test an old cable I would not exceed the rating of the cable and would be very careful about watching the leakage current. I was responsible for a large underground network system. When I took over I wanted to begin testing the cable to get a history started hoping to be able to predict failure.
The guys in the crew talked me out of it fearing I was about to screw up the whole system.
 
If that was only 5 years ago everything you said would have been right on the money, problem with AC testing is that the power supply has to be huge (And expensive, and hard to mobilize) to charge up a cable at 60Hz. Now with VLF testing (Typically 0.1-0.01Hz) the power suppy can be small and portable, VLF is basically the same as a standard AC hipot, just lower frequency.

Tan Delta (Similar theory as power factor testing) is a condition assesment test, instead of saying a cable is good or bad you can evaluate the condition of your cables and prioritize which ones should be replaced sooner, based on importance and budget.

On line PD testing has been around for a while, people like it because it is done with the system energized, so no shutdown. Of course you have to have good baseline data and be able to interpret the data. Several companies have thier own systems (they all claim to be the best) but honestly the jury is still out on the vadillity of this method. I have seen PD testing used to correctly ID a bad termination or splice, but I have also seen iffy results. Its like arc flash, new hot topic, but the industry is just begining to learn about it, much more research is needed.
 
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