Resolution of high vibration on a Generator

Abstract

This is a synchronous generator driven by a GT through a speed reduction gearbox with a rated power of 25 MW(3600 rpm). Generator bearing high vibration levels resulted in several machine trips during the startup attempt followed by a unit shutdown caused by high lube oil temperature. Generator bearings were replaced, exciter misalignment was rectified and insitu balancing of generator rotor was attempted by plant maintenance personnel but did not yield desirable results preventing machine from attaining FSNL speed with acceptable vibration levels.Data indicated dominant 1X amplitudes up to 90 mm/sec Pk at generator inboard bearing confirming generator rotor unbalance coupled with high degree of anisotropic stiffness between horizontal and vertical planes. This case intends to detail how the high vibration issue was successfully diagnosed using various plots from different machine states and resolved by correct insitu balancing approach coupled with bump test technique to identify and eliminate structural resonance.