Generator core problems 1

[TurbineGen]

We are having problems with insulating the core of our 200MW generator and are running out of options. During our loop tests we noticed several hot spot, most of which were fixed via mica inserts. However, there are two hot areas which we cannot easily access to insert mica as they are back behind the packs and at the bottom of the generator.

The problems lie on the splits (the generator was made in three separate 120 degree sections). We have tried acid etching (with phosphoric acid) with limited success. We are now grinding away beind the packs in the hot area and are then acid etching to remove the ground material. We are finding limited success with this as well.

Does weeping epoxy work? If so, how should we go about applying it since the problem is in the lower part of the generator. Do we need to loosen tension on the laminations? Are there any other methods to eliminate hot spots other than restacking?

 

[byrdj]

Have you tried a second consultant or just the OEM?

 

[edison123]

What are the hot spot and average core temperatures ? In extreme cases, a 15 deg C difference is allowed.

Hot spots in large areas would require core dismantling. In my experience, weeping epoxy/varnish do not work well in deep seated hot spots.



*Why make the same mistake twice when there so many new ones available*

 

[TurbineGen]

We have consulted two outside sources as well as the OEM. The hot spots are in the back of the unit and go about 7 inches high and span about 3 inches wide. We have noticed a differential tempurature of 15 degrees celcius initially. With grinding and etching the metal we got it down to around 9 degrees. Because this is used as a motor as well the requirements are little more strict. Unfortunately we have done all the grinding possible and can go no further. Is there anything else we can do? Restacking isn't an option right now due to time constraints.

 

[edison123]

If you have got it down to 9 deg C, it should be ok. The normal allowance is 10 deg C.

Is there any winding or core RTD near this hot spot area to monitor the temperature trend during operation ? Or better yet, can you possibly install a RTD in this area ?

Is this a pumped storage scheme ?

*Why make the same mistake twice when there are so many new ones available ?*

 

[electricpete]

For large motors (but smaller than your generator), typical repair options beyond grinding or restack include:

cracking by mechanical agitation and spreading and perhaps spray with sodium silicate during the process.

Needless to say, these techniques work better when the core clamping system is loosened.

I am not exactly sure what is the mechanism why mechanical agitation improves hotspots. I picture there may be high-spots/burrs on the edges of the laminations that are broken off during movement.

These techniques are discussed briefly on page 5 here:

http://www.lexseco.com/Files/Lexseco_WhitePaper.pdf

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[dpc]

Maybe the basics of core testing/repair would be a good topic for a FAQ - there seems to be a lot of expertise represented in these forums.

In your spare time, of course....

 

[wolf39]

For properly commenting on your problem you should give us more details. From the information available we can assume that we deal with a vertical synchronous machine of 200 MW output installed in a pumped-storage plant.

Please let us have the following information:

1. Are we dealing with a completely new stator core or is this an old core (how old?) loop tested after removal of the stator winding and prior to the installation of a new winding?

2. Was the stator core loop tested with 1 Tesla or less? What frequency?

3. Loop test duration after which the 15 or 9 degrees temperature difference was recorded?

4. Approximate core dimensions (bore diameter, axial core length, slot number)?