FAQ - Test Methods & Data Analysis of Offline Partial Discharge Measurements on Power Cables
Test Methods & Data Analysis of Offline Partial Discharge Measurements on Power Cables Q&A
This is the transcript of the Q&A session from our webinar Test Methods & Data Analysis of Offline Partial Discharge Measurements on Power Cables held on 6/17/2022. If you would like to watch the presentation leading up to the session below please click here.
How does partial discharge typically occur?
"So typically, partial discharges occur when either small voids are left over from the manufacturing process and those trapped gas pockets can become weak points that as the cable ages can can develop into faults. Also, poor work practices during splices terminations and other installations can also cause voids and moisture ingress that can develop partial discharge faults.
So typically, partial discharges occur when either small voids are left over from the manufacturing process and those trapped gas pockets can become weak points that as the cable ages can can develop into faults. Also, poor work practices during splices terminations and other installations can also cause voids and moisture ingress that can develop partial discharge faults." - Glen Wahl
Do we consider VLF an insulation resistance test?
"Well, really, the VLF test is like any other hipot test. It's an AC hipot test at a very low frequency. So it's not really insulation resistance test. However, as you probably know from the old style of DC hipot test testing on especially on paper systems, straightforward oil and insulating cables, you could run a leakage current test at the same time.
So you would see what how much leakage could you have? Why are you doing the hipot test? And that would give you a good indication of whether the cable has a good insulation or bad insulation. Now, when we when we do this now on it on extruded cables, we don't want to use DC anymore, so we have to use an AC hipot test.
That's where we use the VLF and the VLF is only chosen because the power requirement is to do a 60 hertz AC hipot test is humongous on the cable because of the reactive power required. So that's why the VLF was invented. And with the VLF, if you have a sinusoidal wave shape, 2.1 hertz, which is one of the common wave shapes, obviously you cannot really get the leakage current anymore because you're constantly charging and discharging.
So you don't have a steady state. However, with the other accepted VLF technology, the cosine rectangular technology, you can measure leakage current because you have basically a combination of a 60 hertz transition between the two polarities and then you have a 5 second hold plateau. And at the end of the 5 second you can measure leakage currents. So those are the differences." - Henning Oetjen
Which waveform is the best for detecting PD?
"So the different waveforms kind of depend on when I was saying the installation or the acceptance testing or maintenance testing. So as far as maintenance testing goes, you typically want to use DAC, AC, which I would highly recommend. If you do not have the availability to do so, then use sine wave.
As far as installation goes or acceptance, I would use cosine, rectangular as you wanted to, to fully stress the cable to see if there's any kind of workmanship, errors or apparent manufacturer defects before you turn it over to the customer or to yourself to be energized." - Joseph Aguirre
What is the average life expectancy of a cable?
"That's kind of a tricky question from a manufacturing perspective. Most medium voltage shielded cable manufacturers usually give an average of somewhere between 20 to 30 years of life expectancy. But there are a lot of factors that can kind of lessen that that can be kind of predictable. As far as, you know, what are the conditions as indirect barriers have been conduit?
is it submerged in water or is it dry? Is it frequently close to 100% of its rated current carrying capacity or is it, say, 50%? So there's a lot of factors that can affect the overall life expectancy out of those cables. But from a manufacturers perspective, they usually say about 20 to 30 years." - Glen Wahl
How do you test low voltage copper cable that does not have concentric neutrals?
"So that right there, since you do not have a concentric neutral, the only thing that you can do is an insulation resistance test, which a lot of people refer to as a Megger test. So on these, you can only do phase to phase and then phase to ground because there's no other way to check it all the way directly to ground as far as concentric neutrals are concerned, because concentric neutrals wrap all the way around the insulation, which if there was a direct short from the center conductor to there, it would be more apparent.
But the only other way that you can do it is just check phase to phase and then phase to whatever ground that you’re by." - Joseph Aguirre
What dielectric grease do you use? Is it used only for testing and wiped clean after?
"So the dielectric grease is usually we will go off whatever the manufacturer of that termination is. So if it's a load, break, elbow or a t body, usually with that kit they have a manufacturer recommended grease to use with it.
The point of that grease is when that low break elbow or t body is sitting on that pushing for long periods of time. If that grease isn't on there, it's just a thin layer- they can kind of tend to almost adhere to each other over time and they dry out and can be very difficult to remove and push them back on afterwards.
And so that grease is there to keep it from seizing. It's usually just to make it easier for the operator of the equipment and the installation removal of those connections. As far as what to use, we always go back to the manufacturer's recommendation." - Glen Wahl
How often does your cable need to do PD testing or when does the cable need to have a PD test?
"So that all depends on your maintenance interval. The first time to do PD tests, of course, is whenever you do installation or acceptance testing, so the very first time that you're running your VLF testing, then you'll want to do a PD test as well to get your nice baseline on that as well.
And then after that, how often do you do PD testing? You'll want to do that whenever your maintenance intervals require. So if you do, if you do your VLF and tan delta testing every 3 to 5 years after installation, that's when you start doing your PD testing." - Joseph Aguirre
What is the maximum recommended distance for cable segment in a single shot? Is it equipment dependent?
"So it is going to be equipment dependent and also test specimen dependent. And we've got test sets capable of- at least on underground cables, testing segments up to 20 kilometers with the right conditions. So it is largely dependent on the size of the conductor, the installation of the of the specimen under test.
There's a lot of factors that go into that, including also the waveform. The waveform that you’re using can also give you a longer cable distance that you can test on." - Glen Wahl
How high can you go for your test voltage? Does it depend on the cable type?
"So that doesn't necessarily depend on the cable type more than the installation rating on the cable. So whenever you talk about cable type like your EPR, XLP. So each one is going to have its own strengths and weaknesses as far as insulation goes. But if you look on the jacket, if you do have a jacketed cable with the concentric, look on the jack in there will tell you, Hey, this is a 15 KV Cable, 25 cable, 35 KB Cable.
And basically the test voltage that you can go is maximum of two times U0, which is the one I mentioned earlier. The U0 means operating voltage. So if you're testing at 8.6, U0, which is 12 peak, so double that right there, which can be 24 kV on there. But that's only for new installation acceptance testing. But if you do your maintenance interval, the maximum that you want to do is 1.7 times that number" - Joseph Aguirre
In buildings, should the testing done before it was installed in the conduit?
"No. You want to make sure you do it after it's in its final position. You can do the testing before, basically like, on real test, if you want to do that, that is perfectly fine. But since this could be improved through the conduit, could be stress, or could be multiple elbows which can overbend the radius, making the installation weak at that point, any kind of cuts or nicks on the cable as it's being pulled.
So you want to make sure that it's going to be in its final position before you start any testing. Some people would test it right then, and then test it again right after they made sure all the terminations and splices are complete as well." - Joseph Aguirre
Why is the ground table four times the duration of test after the VLF?
"So that's generally a good rule of thumb for cables that have very long distances where you can build up a high capacitive charge. So when you build up that high capacitive charge, it can take time for it to bleed off once the cable has been de-energized and you put it in the ground, because you don't want to partially discharge the cable and then have the technician get potentially shocked afterward. So we typically recommend as kind of a catch all- obviously on shorter cable runs, it's not going to take as long, but I'm very, very long cable runs you do want to, for safety's sake, make sure that the cable has been effectively discharged subsequent to the test." - Glen Wahl
Why do we select the air insulated for underground cables?
"Okay. Well, the insulated setting has nothing to do with the fact that the cable is an underground cable, that has to do with a termination, where the termination is connected to the switchgear.
Is this air-insulated termination, or is it an SF6 termination, or is it an oil-insulated termination, that is based on the type of termination that you use on an underground cable or even on a cable that goes up to a pothead. Okay. So that's what designates this type of style of termination, because obviously if you have air insulated, you could have corona like Joe was talking about, you know, so it's important to know what type of termination you have, especially when you find PD at the termination. In that regard, you certainly want to know what you have." - Henning Oetjen
How important is the velocity of propagation cable parameter when setting up cable information? Does it really change among cable brands?
"Okay. Good question. Thank you, Michael. So propagation velocity is very important when we are setting up these cables because it is what's going to give us our accurate distances. It is going to set the value that the machine is going to use to calculate those distances out for where those partial discharges are occurring.
We don't have that set correctly. All of the measurements will be off. And can it change from manufacturer to manufacturer? It can slightly, but the big things that are going to affect that propagation velocity are going to be the conductor diameter and the insulation thickness. So we want to make sure that we get the most accurate propagation velocity that we can for those measurements because we want to really pinpoint where those partial discharge events are happening on a cable." - Glen Wahl