FAQ - Offline Partial Discharge Testing on Motors
Offline Partial Discharge Testing on Motors Q&A
This is the transcript of the Q&A session from our webinar Offline Partial Discharge Testing on Motors held on 7/21/2023. If you would like to watch the presentation leading up to the session below please click here.
Can PD be found on new or freshly renewed motors or rewound motors?
"Yes, you can. But it's not what you’re paying for. PD can occur in a couple of different places in motor, one between the windings, which is the which is going to lead you down the road through a search failure. The other is in between stator core limitation. So, say the motor was dropped or you fill with an air gap above or something like that. But that will kill itself. Problem is you - don't know which one it is, so you got to track both.
But if I was if I was paying for a new motor or a rewound motor showed up with PD I don't exactly think that motor is one I would want to keep." - Sean McCall
How far from the motor can you apply the PD test and not be influenced by the cable run?
"Typically, anything under 1000 feet is fine. You're going to have electrical field, interference everywhere in the plant. And ideally, every time you test for the first time you want to do it from the NCC, it's easier to get to typically in an air conditioned room, not a high traffic area. Typically from the NCC, if you see a problem develop, then go down to the motor and break those connections. Yeah. From the NCC, It should be fine." - Sean McCall
How Valid is an offline PD in a 4160V Motor?
"PD happens usually in void such as air gap insulation. And we know air needs 3Kw or 2.72 Kv per millimeter to break down.
So our expectation is having a PD in one millimeter whenever we have around a 3Kv. So for 5Kv system or 4160 for sure a PD can occur. And then the question is how valid is offline testing for it- I don't like to compare offline with online because each one has advantages and disadvantages.
But the major is with offline testing you can increase the voltage while with online testing you are limited to the power system voltage. However, with online testing you're going to be more more accurate because of the pressure load and humidity condition, and its operating condition. While for offline testing, your recorded temperature and humidity might be different from the actual situation." - Shahryar Farhang
In the offline test, what is the accuracy of the failure and is it better to make the test online?
"As Shahryar was saying – online vs offline, that's more that's more a factor of what you've got available. If I'm in a motor shop, my testing is all offline. If I'm in wind towers, my testing is probably going to be easier to do online because I don't shut down much, or if I'm in a power plant during the summer, I'm not going to get a whole lot of PD testing done on it statically in a plant operating at peak hours. So, the online testing is more of a real life PD test. Not that it's better or worse, but if it's all you've got available, then the motor is seeing absolutely everything that is going to leave that PD failure down the road.
Whereas in the offline testing we're simulating something's not going the entire way, but just looking for the catch. It's not a it's a matter of where it comes in. But the fact is knowing that it exists in the first place because then you can start to plan for it. So however you happen to be able to do that dynamically or statically or comparison of the two." - Sean McCall
What frequency range does the PD pulses usually occur?
"So they are usually in the range of high frequencies of megahertz and gigahertz. PD is going to create frequencies up to around 400 megahertz if you measure them under 1megahertz you're going to probably follow IEC 6270 and you can measure them in nanocoulombs. If you pass the measurements, which is very common for multiple tests, if you measure them above one megahertz is the only way by today's standard is measuring them by millivolts usually, interestingly enough." - Swapnil Marathe
Can surge tests detect early signs of PD on motors?
"It can, but the PD itself will be pretty advanced by then for it to show up on a surge waveform. Where I've seen it is typically on the downslope of that first hump of the sinusoidal wave. On the downside, if there's PD, heavy or otherwise in there, it'll show a little peak.
I've seen that. But again, that's it's not definitive because you see that as well in certain DC motor windings set up as a natural occurrence of what that motor does or how it's wound, how connected, what have you. But you know, it could let you know there's a possibility that as far as you can go with that one." - Sean McCall