GENERATOR TRIPPING DUE TO VERY HIGH VIBRATION

[appunni]

Sir,
One of our 130MW generators tripped on mechanical lockout relay due to very high vibration of Lower Guide Bearing. The trip is set at 15mils. Normally the vibration was found around 1 to 2 mils. We inspected for any looseness of fasteners, locking system of rotor and stator. Also checked the runner buckets, jets and associated system, inspected for any lost weights inside the rotor portion which were originally present in the rotor at the time of commissioning itself, but nothing found. Then we checked the impedance of all rotor poles. We found that the values were similar for all poles. The appearance of poles were similar, i.e., no shearing and shifting. The interpolar spaces were also checked for any looseness, but nothing found.
After these checks the generator was test started for vibration measurement using another test kit at different loads. Found that the Lower Guide Bearing vibration level gradually increasing while loading and observed 200micro meter at 100MW. Observed 8mils vibration in the panel meter at that time and found increasing while loading further. So we stopped the generator, filtered the bearing oil and analyzed the filtrate sample for checking the presence of any metal particles, nothing observed seriously.
I request your valuable remarks about where should be the fault, which caused vibration. I also request you to give advices for how to proceed further for further checks and for remedial actions.
With regards,
appunni

 

[DougMSOE]

I would ask, does the vibration increase with increase with KW (load) and/or Increase in excitation?

 

[venkat2001]

Certain questions arise from the description. What was the vibration during no load run to full speed? If it was low and comparable to records made earlier, then mechanical problems of loss of weights, etc. can be ruled out. If no load vibration is high, then further investigation of the weights, loss of fasteners, clamps, etc. need to be carried out. If there is sudden rise in vibration on excitation, shorts in rotor or uneven air gap, etc. may be reasons. If there is no jump in vibration during excitation, but it increases during loading, both electrical and hydraulic causes need to be explored.

 

[GusD]

Hello appunni

The LGBearing Vib inreased from a normal 1 or 2 mills to at
least 15 mills.It appears that you have done a lot of checks, including and filtering the lubricating oil.
When conditions are right, the bearings can create an oil
whirl (oil whip)which normally shows itself at < half of the Rpm.A quick test can be made, by changing the Temperature of the bearing oil,if at all possible (either 10 higher or lower)Sometimes it can change the vibration caused from oil whirl.
It might be worthwhile to check for what work was done to the GEN itself or around it,just prior to experience of the hi vibs.Some minor work or simple modification could have caused a condition that excites a LGBearing freq.
If you did vibration analyses spectra,what are the predominant vibration Frequencies in the Spectrum.
That could give you better information as to the nature of the vibration.

GusD

 

[duki]

WHAT TYPE OF VIBRATION DETECTORS WERE DOING THE MONITORING. GRAVITY TYPE(ROBERT SHAWS, ETC) OR PROXIMITY TYPE(BENTLY-NAVADA'S). ON SOME GT'S THERE OCCURS A VIBRATION AT A CERTAIN SPEED IN RUN-UP AND AT A DIFFERENT SPEED ON RUN-BACK. IS YOUR APPLICATION ON A GT?