Reinforcement of Generator Stator End Winding Structure by Modal Analysis Approach

Stator end winding of turbine generator experiences high vibrations at resonance frequency in operation which result into basket loosening, insulation failure and ultimately end with no power from machine. Modal analysis is a powerful tool of vibration test to visualize vibration characteristics viz. natural frequencies, amplitudes and mode shapes of dynamic structures. This article elucidates the vibration behavior of stator end winding structure in a 150 MVA, 16 kV, 50 Hz turbine generator of decennial period. Natural frequency test (NFT) was conducted on it in off-line condition. An impact hammer and an accelerometer were jointly used for excitation force and respond vibration respectively. Modal parameters were extracted from the frequency response function through FFT analyzer. Natural frequencies and vibration magnitudes had been obtained. Overhang was reinforced based on the test results. Efficacy of the rectification work was endorsed by post modal analysis. Vibration amplitudes had been brought down to low values in the resonance band width of 95 to 110 Hz.