We have 300mw capacity Alternator (Vt = 20kv, rated excitation current = 2075amps) driven by steam turbine. there are 6 bearings. of which 5th & 6th are Gen Brg. After a huge surge accompanied by blackout, when the machine was synchronized thereafter there was heavy shaft vibration (about 180 um, tripping is 250um) on 5th brg shaft. With increasing in excitation current vib increases. So our gen is running with AVR in Manual mode keeping gen o/p voltage at 19kv, i.e we manually control terminal voltage by increasing or decreasing excitation current. But recently there are instances when i noticed at 1400 amps excitation current & var 10mvar sometimes vib is 130um sometimes 160um, load almost remaining constant. no way we can determine whats the problem. All other shaft vib are normal, only 5th is too high & 6th( Gen rear) is around 100um. Normally these used to b around 70-80um. Also there are instances where with increased VAR (during evening time, to keep the voltage we have to increase the excitation current, MVAR increases to 30-35Mvar) vib shoots up. Our Gen is still in load but we are apprehensive. Some RSO Test (at Turbine dead stop) have been done.

 

Santanu... while you may have ruled out a short-circuited rotor-windng, there is always concern about slot-wedge dislodgement.

Have you any noticed measurements of the coolant temperature that could point to rotor damage?

Regards, Phil Corso

 

Can you give us temperatures of brg pad (shoe) and drain oil. and whether you can see the oil stream if there is a sight glass in the tube of drain oil (if this is a sleeve brg) and also for other brds to compare with