Winding Failure 3

[petronila]

Dear All

We have to repair two wind generators, we found many grease(old grease) in contact with the winding, We think the the amount of grease on the windings over years (for 4-5 operation ) cause this failures.The questions are : How the grease week the insulation? Have someone good paper about this problem?Thanks for the inputs

Best Regards

Carlos

 

[electricpete]

We had a 100hp 2-pole horizontal motor where a large amount of excess grease expelled from cavity into the winding and eventually failure occurred. We thought the cause was overheating due to air flow restricted by the grease.

I don't know about other adverse effects. I would think vpi winding would be relatively immune to other than thermal effects.

What votlage is the machine?

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(2B)+(2B)' ?

 

[wolf39]

What kind of repair activities are required? Which components failed?

Wolf

http://www.hydropower-consult.com

 

[electricpete]

The three possible ways grease can lead to winding failure from most likely to least likely imo:

1 - thermal failure (due to disruption in cooling airflow)
2 - chemical failure (interaction of grease chemicals)
3 - mechanical failure (grease facilitates movement of windings in presence of other vibratory forces, by reducing usefeul restraining friction forces at locations of blocks and ties, and possibly in the slot... movement results in failure.)

I don’t know of any paper. I found a very cursory mention of these items as discussed below.

EASA Motor Failure Root Cause Analysis
Chapter 3 – Winding Failures.
Contamination can come from within the motor. Overgreasing has led to contamination of the windings which in turn created thermal problems. [photo included]

EPRI 1000898 – Random Wound Motor Failure Investigation
2.2.1 Stator Failures
Typically stator windings will fail in the slot, at the point where the coils exit the core, or out in the end turns. Over—greasing can lay a coating of grease on the endwinding which can hamper heat trasnfer from the endwinding surface to the cooling air.

Electrical Insulation for Rotating Machines by Greg Stone, Ian Culbert, et al

8.10 CHEMICAL ATTACK
Chemical attack describes the deterioration of the insulation that can occur if the insulation is
exposed to an environment in which chemicals such as acids, paints, and solvents, as well as
oil and water, are present....

8.10.1 General Process
Most types of older insulation systems are prone to chemically induced degradation due to
the presence of solvents, oil, water, or other chemicals....


8.10.2 Root Causes
The root cause of chemical attack is the presence of oil, water, solvents or other reactive
chemicals......

Windings in motors and small generators with grease-lubricated bearings can be contaminated
with this lubricant if grease relief plugs are not removed during regreasing. If this is not
done, then grease is forced between the bearing cap and machine shaft and enters the stator
end-winding region.

EASA Motor Failure Root Cause Analysis
Chapter 3 – Winding Failures....
Hence, many turn-to-turn and ground failures are actually
caused by winding movement over time which breaks down
the insulation system. This can occur in the slot, end turns
or connections. Winding movement can be controlled or lessened by the
use of coil bracing.....[note there is no mention specifically of grease, but imo common sense dictates that a lubricant can contribute to undesired movement, along with loose blocking, loose coils, excess forces etc]

Can you share any further details of the failures you have seen on these generators?


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[petronila]

Thanks for the comments,

Pete good information, Attached you will find photos.

Regards

Carlos

 

[TheBlacksmith]

From the pictures it does not appear to be VPI insulation, so I'm betting on chemical deterioration of the insulation leading to shorts.

 

[starkopete]

Hi, from what I can see in the pictures - I'm thinking an overload (pretty severe). Is there any protection on these generators?

 

[rhatcher]

starkopete, I agree that generator #2 looks overloaded. It can probably be rewound with no problem.

Generator #1 looks like insulation failure due to grease. I would think that the failure most likely started phase-phase and then progressed to phase-ground in the slots. Keep in mind that there is another phase winding in the bottom of the slots that cannot be seen from the photos. The #1 generator will need new stator laminations plus a rewind.

 

[electricpete]

I would like to add a fourth (less likely) item to the list:
1 - thermal failure (due to disruption in cooling airflow)
2 - chemical failure (interaction of grease chemicals)
3 - mechanical failure (grease facilitates movement of windings)
4 - electrical/conducting failure (grease forms conducting path between pre-existing "pin-hole" defects in magnet wire enamel insulation

This is based in part on the following:

Electrical insulation for rotating machines


8.3.1 General Process
Poorly impregnated random-wound stators are much more likely to fail due to dirt, pollution,
oil, and moisture that can be partly conductive. It is not unusual for the magnet wire insulation
to have small pinholes or cracks as a result of the rigors of manufacturing. If the impregnation
or dipping cycle is poor, then these imperfections in the magnet wire insulation, combined
with a partly conductive contamination, leads to shorted turns and, thus, failure.

I don't know to what extent grease can be conducting. I do know that some non-negligible current conduction occurs through extrremely thin films of grease in a bearings undergoing with electrohydrodynamic lubration in bearings.

With possibility for contaminants and decomposition products, I would suspect the conductivity of grease can vary.

Fwiw, I think the failures shown certainly could have begun as turn-to-turn faults. Turn to turn faults often result in significant amount of melted copper, and continue until they go phase-to-phase or phase-to-ground to finally cause a trip.

The windings look unfamiliar to me. Certainly not lap wound. I guess they are concentric wound? I didn't see anything resembling phase separator paper that I could pick up, although I'm not sure where to look. If no phase separator paper, then it certainly could begin directly as phase to phase fault because the phases are in intimate contact in the endwindings and see a higher voltage than the interturn voltages.

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[rhatcher]

electricpete, it looks like a concentric (basket wound) three phase winding. This will have three layers. In any of the photos the layers that are apparent are the two top layers.

Although phase paper could be used on a concentric winding, is is not used here. Nice (lps) observation. This is why I suggest a phase-phase winding failure with the (not visible) winding that is in the bottom of the same slot.

This suggestion is because of the idea that any degradation of insulation will cause failure at the point of weakness. In this case, the highest voltage stress occurs phase-phase where the windings of different phases lay over top of (or under) each other in the three different layers of coil end turns.

I will add a suggestion for an alternate failure mode for #1 generator. If the failure is mostly confined to the one, top layer and is truly a turn-turn failure, then a voltage surge may be the cause. If this is the case, the failed coil group will be the lead pole group in the phase winding. Specifically, there will be a line lead attached to this beginning group. If this is the case, then a voltage surge is a possible cause for #1.

I stick with the idea that #2 is an overload and a separate type of failure than #1. The 'burned from the inside out' look cause by the various holes in the winding are the key indicators for me.

 

[electricpete]

Great point - now that you mention it...makes perfect sense that a concentric winding needs three layers.... one phase in each layer. You can see where the 2nd layer passes directly under top layer with no paper in sight at that location.

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[electricpete]

actually I'm not sure how the phases are distributed (maybe you can explain that Ray?). But no paper is either between layers or betwen coils in a given pole.

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[dArsonval]

The winding Failure in this post has all the attributes of a Voltage Transient, spike, lightening strike, etc.
The grease is an entirely different issue.

Fix it, and get the thing running again!

Always enjoying this forum,

John

 

[petronila]

Hello all

Thanks again for the answers, thsi failures are happening in wind generators, our investigations point to a winding failure due to grease degradation.

We have two scenarios: One with a earth fault ( "Hence, many turn-to-turn and ground failures are actuallycaused by winding movement over time which breaks downthe insulation system. This can occur in the slot, end turnsor connections. Winding movement can be controlled or lessened by theuse of coil bracing.....[note there is no mention specifically of grease, but imo common sense dictates that a lubricant can contribute to undesired movement, along with loose blocking, loose coils, excess forces etc "]

and two Insulation breakdown ("EASA Motor Failure Root Cause AnalysisChapter 3 – Winding Failures. Contamination can come from within the motor. Overgreasing has led to contamination of the windings which in turn created thermal problems. [photo included")]

The winding is not in 3 layers, if you take a look of photo you can see one winding on other, so its a two speed two windings.

Regards

Carlos