Dear Hisham4ever ;
I’m not familiar with Siemens GT, I’m more GE HDGT (General Electric Heavy Duty Gas Turbine)…
What’s happened on June 2014? What did you find after the feeder trip? And what was the relation between the FEEDER TRIP and the high vibration on #3 and #4 bearings??
On HDGT when TRIP occurs, automatically the auxiliary lube oil pump starts time to cooldown the machine rotors and theirs bearings. If , for any raison like electrical power cut, the auxiliary pump doesn’t work, the emergency lube oil pump will start to cooldown the rotor and bearings. But if (for any raison) the cooldown process wasn’t correctly performed damages may happen on bearings also rotor bow may happen?!!!..
If on June 2014, the cool-down process wasn’t correctly forwarded, the bearings will be stressed and overheated and this could lead to a partial Babbit melting (bearing white metal) consequently high vibrations will appear!
What about your generator?
Usually 4 poles generators are driven by gas turbines that means their speed is about 1500 or 1800 rpm depending if you are using 50 or 60 Hertz Frequency.
HDGT Rotor exhibits the first RESONANCE between 1100 and 1600 rpm. So if your generator is experiencing any problem leading to a small abnormal level of vibration it will be amplified at the Power Turbine side because it matches with its 1st balance resonance.
Hope this is heplpful