Generator rotor and TG bearing to ground isolation

[Duncan2000]

What are the standards for isolating the turbine and generator bearings? It has been my thought that all the bearing pedestals should be isolated from ground. Is that correct?

 

[byrdj]

Experanced with utility size TG. Never seen or heard of any standards (ASME IEEE)I've never seen an insulated turbine bearing, only one of the generator bearings. I alway though the one insulated generator end (bearing Oil deflector, etc) was to prevent induced circulating current. the static generated in the the steam turbine is normaly taken to ground (grounding brush)so its potential to arc through the oil film is reduced. The potential on the generator inslulated end can get to a couple hundred volts.

 

[Duncan2000]

Thank you for your reply. You are certainly correct on all accounts. I was thinking that all the generator bearings were isolated; but having recently encountering grounding problems, I discovered that only the exciter pedestal and outboard bearing were isolated. GE also ran grounding tests on the rotor I believe. I should have left with some procedure or criteria for the testing and am trying to recoup.

Another question: I have had difficulty determining the difference, on one occasion I should say, between a fatque failure and an arcing failure in a steam turbine bearing. The bearing babbet failed several times on a 20 year old machine. Of course the root cause could have been arcing which preceeded a fatique failure. We addressed the problem as if both could have occurred. What has been your experience?

 

[metengr]

I have had difficulty determining the difference, on one occasion I should say, between a fatigue failure and an arcing failure in a steam turbine bearing. The bearing babbit failed several times on a 20 year old machine. Of course the root cause could have been arcing which preceded a fatigue failure. We addressed the problem as if both could have occurred. What has been your experience?

I have seen both contact fatigue failures and surface pitting from arcing of large T/G bearings in service. Typically, contact fatigue failures of babbit will exhibit rather abrupt or distinct fracture surfaces or breakout of babbit material with no surface pits or local arcing damage. Babbit failure can also occur in regions that were puddle repaired, if not done correctly. Normally, with electrical arcing you will see a localized region or band of surface pits that are distinct and can become deep over time.