Generator refurbishment 1

[wbd]

Hello,
I have an older (1950s) 1000 kva, 2400V, generator from a hydro unit that has been stored outside for a year before being moved inside. This unit needs to be refurbished prior to being put back into service. Unit was removed from service due to hydro turbine failure.
I need to send it out to a shop but I would some opinions on the work that I should have done to the unit. I'm looking at testing (before & after) of the stator and poles, cleaning, etc.
I thank you in advance for any responses.

 

[stardelta]

If it was running without problem when it came off there is no reason to suspect any winding issues or other problems. However being stored outside may well have caused the insulation resistance of the winding to have dropped below an acceptable figure due to moisture and condensation, equally the bearings may well have suffered due to the same reason. My Company specify 10Mohm as a minimum. You may even have had what we term as "Vermin entry" which speaks for itself. Its a routine job for a good shop, tell them whats happened, and ask for, before and after test figures relating to the IR and winding resistance figures, dimensional checks relating to bearing housing/journal sizes and condition, and proof that the bearings have been replaced or refurbished. You could even ask for the rotor to be balanced to a higher grade than what was required in the 50`s when it was made which will result in less vibration when running and longer bearing life. Idealy the machine should be load-tested after the repair but not every shop will have facilities for this.

 

[itsmoked]

I suggest you will still need to dry it out after it returns from the shop since it will probably have dropped below the dew point somewhere along the way.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[oftenlost]

I guess that if I were to write a specification outlining tests to be performed, I would include visual inspection with digital photographs, comments concerning the stator wedge integrity and rotor collar appearance. The brush boxes and springs checked for corrosion and the IDs of the boxes for wear, the brush studs for cracked insulation. A megger and PI of the rotor and stator, drop test of the rotor, hipot of the brush rigging. Visual and micrometer test of the bearing and shaft fits, check of the lower flange and rabbet fits. I would specify a solvent or steam clean of components and drying in an oven. I would be tempted to limit oven temperatures as some of the units of this vintage had rag/shellac insulation which does not tolerate temperatures encountered in Class F adjusted ovens. A followup battery of electrical tests including megger, PI, surge comparison of both stator and rotor should be performed. Balancing is possible but costly due to the required mandrels for single bearing units common in this application. The diameter of the rotor is frequently a problem in smaller shops. If plate bearings, I would check to verify appearance visually. You can tell much from the babbit appearance of these units. A topical application of varnish to make it pretty, rings turned and polished would complete the task.

 

[stardelta]

Itsmoked?
Dunno.....depends on what the shop finds and what they think the best repair methods to employ. Stoving is not always 100% effective and the results are often unpredictable. You can stove damp windings off easily enough but when they cool the IR can drop again as you correctly say, given that the insulation material is some 50yrs old its a strong possibility that this may happen. The usual procedure is to stove to about 120degC, if the IR comes up, re-dip while still warm and cure off. Its also possible that the IR may not rise at all, I wasnt around 50yrs ago and wouldnt like to say how the older materials may perform against the newer. There must have been improvements and older insulation is probably not as tolerant to moisture as modern insulation is. If it was in my shop I would say stove it for 12 hours at 120, let it cool to ambient and take a reading, then leave it for a few days to see if it starts drawing in moisture from the atmosphere, take another reading and go from there.

 

[MarktheMarker]

not sure if this will help, but I have searched on the web and have found following site:

http://www.who-sells-it.com/c/industrial-supply-bearings-323.html

with a lot of information to many bearing units and also technical information documents.

kind regards, Mark

 

[CarlPugh]

Back when I was designing transformers, the maximum dryout temperature was something under 100degC. The thinking was that if temperature got over 100degC, any moisture would turn to steam, and the pressure of the steam would damage the insulation.