Uneven Stator Winding Temperature

[Equest]

I've recently been reviewing 2 years of data on a relatively new hydrogenerator. I've noticed that one of the stator winding RTDs is consistently reading higher than the other 5 RTDs by about 3-5 degrees C. At what point does the temperature difference become a concern?

 

[itsmoked]

RTD's have offsets.

Temperature readers have offsets. Stress or strain from mounting will shift RTDs too.

If this wasn't a sudden offset I would not worry.

 

[Equest]

Thanks for the help.

It does look like it has been this way for quite some time. This machine has experienced numerous problems in it's short life so I'm investigating every anomoly I find.

It's nice to know that there is at least one part I don't have to worry about.

 

[QCE]

Some possiblities:

It could actually be due to where the RTD is placed on in the winding.
Could be line end coils well others are not.
Could be closer or farhter away from the surface mounted coolers.
Depending on how the winding is configured could effect the temp.
If you have airgap monitoring check to see if the airgap has any "sucked in" or "pushed out" areas. this could cause heating.
Could possibly be partial discharge in the area.

What is the max output of the generator and what temp is the winding usually running at? How many RTDs are being monitored?

 

[QCE]

I would expect 5C to be more then a RTD error. They should be much more accurate then that. Have you talked to the supplier or are you the supplier?

 

[Equest]

Thanks, QCE.

To answer your questions....

This particular RTD is located close to the neutral leads. It is right in a gap between the surface coolers.

We do have an airgap monitoring system and this RTD is located in an area of small airgap. This was my initial concern - small airgap causing unacceptable heating in this specific area.

The winding temp varies significantly with the load. e.g. 50 deg C @ 10MW and 100 deg C @ 90MW.

I haven't discussed this with the supplier yet as I wanted to make sure this was a real concern and not just an offset.

 

[QCE]

Something maybe up.

I would expect that the hottest part of the stator would have the largest airgap. As the iron is heated it should expand outwards. Making it the largest airgap. Maybe the soleplate key is stuck.

If it is near the neutral it could be a line end coil which may run hotter. Usually you monitor all the line end coils from each parelle path and they are all similiar temp. But if you monitor random RTDs maybe some are line-end coils and some are not.

If the RTD is reading hot but the iron is not expanded it could be because it is only very localized heating coming from the coil. Not a good sign. I'm assuming that this is a RTD that is wedged between the top and bottom bar.

 

[Equest]

Yes, the RTD is located between the top and bottom bar. I'm going to search through the commissioning reports and see what I can turn up regarding the offset and accuracy of the RTDs when they were installed.

I will also check if it a line end coil.

Thanks for the tips...I'll keep you posted on my findings.

 

[QCE]

A 5C offset is just not acceptable. There should be a calibration report from the supplier as well as in field test prior and after installation. If it was 5C out it should not have been installed. I doubt this is the problem. a good thing to check would be what the temperatures look like when the unit has been offline for a while. If the RTD reds 5C high when the stator is cooled then you have a problem. I would hope that if the core coils to 20C that all RTDS would read very close to 20C. with a 100ohm RTD i would expect less then 0.5C error. It is usually closer to 0.1C error.