PT100 low insulation in generator

[pbeigbeder]

Hello

I have a PT100 probe in a generator stator (20kV). This probe have an insulation level (between probe and phase) lower than 200kOhms.
What can I do with such problem. Is there a possiblity that this probe create an stator insulation failure?

Does anyone have idea

Thanks

 

[edison123]

Yes, the IR value is low. For personnel safety, it is better to remove this RTD leads from the measuring and control circuits and ground it to the stator frame.

No, it wouldn't endanger the stator winding insulation.

* I would go green if only I were not yellow *

 

[electricpete]

Am I understanding there is 200k between the rtd lead and the stator winding?

That seems like a scary condition to me.

If you ground the rtd lead, then you have a ground on the stator during operation which is 200k.

And at what voltage was this 200k measured? Since the rtd insulation is probably low voltage I'm guessing something like 500kvdc.

And how will you measure the insulation resistance of stator itself? With the RTD floating I would assume since otherwise you never get above 200k. Then the stator insualtion should be well above 100 megaohms as we know. I have a hard time imagining why the RTD which comes only in contact with the coil at a few locations has lower resistance to the coil than the grounded stator core which contacts at many places. In the slot section there should be semicon to maintain that ground potential. At the core exit past the semicon, there should be some spacing. In spite of all this, it would appear the rtd contacts the surface of the coil at some point that is not grounded and the I.R. from that contact to the coil is 200k.... oodles less than the entire core contacting the coils over a very large area. Seems to me perhaps abnormal positioning of the rtd lead may itself be cause of degradation of insulation at that point.

If forced to operate the machine without addressing the condition, , I think I would be inclined to leave the RTD lifted and floating (ungrounded) during operation with some kind of suitable controls to prevent anyone from entering the rtd terminal box.

I welcome any comments if I am looking at this wrong.

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[electricpete]

Correction: "Since the rtd insulation is probably low voltage I'm guessing something like 500vdc."

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[electricpete]

I don't think that leaving the RTD grounded is a good solution for equipment reliability reasons discussed above... it is a 200kohm ground (possibly measured at 500vdc) which would remain during operationg.

But now that I think about it more, leaving the RTD floating is not a great solution from personnel safety... even if you determinate that one RTD, you doubtless have other RTD leads passing through same terminal box which will leave the terminal box to plant computer and you don't want to determinate all of those (you would lose all temperature indication). So now there would be some possibility for a fault to the lifted /ungrounded RTD lead which could then short to other leads and leave the terminal box. This seems kind of remote, but still it is a scenario to consider.

I mentioned that improper RTD lead positioning may be leading to local degradation of the coil insulation.

It may be similar condition (more severe) to slide 2 here:

http://home.comcast.net/~electricpete/eng-tips/Presentation.ppt

That was a 13.2kv air-cooled motor. In that case RTD was routed too close to the coil in the coil extension area resulting in partial discharge. More details here thread237-200716 . Of course conditions for partial discharge are different in an air-cooled machine than a hydrogen cooled machine (is yours hydrogen cooled?).

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[pbeigbeder]

Hello Electricpete

Thanks a lot for your informations.
Yes the generator is hydrogen cooled.
The generator manufacturer installed 12 PT100. 4 probes for each phase.