Generator Electrical Testing

[Melco001]

I have a old generator of a GE7B CTG refurbished 64 MVA, 13.8KV and it has the space heaters powered to preservation, so my question is if it is needed to perform a Stator HiPot test before start up or just a Megger/PI test. I have no any record if somebody did it before.
I will appreciate any help from you.

 

[rmw]

I recommend only the PI test. A hi-pot test can be a destructive test (if it arcs over) and without knowing a lot more about that stator and its present condition, you are taking a mighty big chance with a high pot test. Ask youself if you can live with the results if it arcs over somewhere deep in the core during the high pot......

rmw

 

[wolf39]

Melco001:

How old is the generator and when was the unit refurbished? Which components were refurbished or replaced?

Seems that the stator winding is still the original one. In case refurbishment on the field winding was done, then the rotor was pulled, refurbished and then reassembled. In such case the stator winding may have been mechanically stressed.

You have to bear in mind that at 5 cents per kilowatthour a 50 Megawatt unit can earn as much as US$ 60,000 per day. If the stator winding fails the hi-pot test, you at least are prepared to repair the unit. If the winding fails when in operation and you are committed to deliver kilowatthours, you are in a much more worse situation. Explain this to your superiors.

I would perform a PI test and a hi-pot test with 1.5 times rated voltage (20.7 kV).

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[edison123]

I am with Wolf on this mainly due to the possible collateral damage to the stator core during in-service winding failure, thus prolonging the shutdown.

Known devil is better than an unknown angel.

 

[lansford]

You can research doing a surge comparison test.
JIM

 

[Bekza]

Perhaps its a good idea to find out why the unit was decommissioned, however you mentioned that you do not have historical data! Not bad idea to start new maintanance file:
Option 1: Remove rotor and do proper visual inspection on the stator core and the rotor as well, To see if there's evidence of arcing ,localised overheating or mechical damage to the core lamination, signs of fretting or vibration.Do PI test,how was the machine preserved? Maybe ELCID testing will add value?
Option 2o PI test, do off line Pattial discharge test upto 0.8 rated V. Do you have a plan incase insulation fails Hi-pot test? you might unnecessarily overstress the winding when testing at 20.7kV seing that is the old generator. What class of stator bar insulation?

 

[BSEE1980]

May I suggest you start with a simple 1kv megger? I am concerned about moisture and that will probably show up with a megger. I found a xmfr with moisture problems this way one time that would have certainly failed if it was energized. A few days with heat applied, and it dried up OK.
Then the other posts are right on: hipot is a destructive test if it fails. Rewinding this gen will be very expensive and time consuming.

Steve