Acoustic detection of defects and aging for stator bar insulation of large generator

A set of acoustic detection facility developed by our research group is used to investigate defects and aging in stator bar insulation, which have been subjected to electrical, thermal and mechanical stresses for 1000 hours, respectively. The acoustic signal analysis in time domain, frequency domain and wavelet transform at four different scales has been conducted, respectively. It is observed that frequency components concentrate mainly about 1 kHz for specimens unaged, while shift to 2 kHz for specimens subjected to electrical stress or mechanical stresses. A new high frequency component at 6 kHz is characteristic of the specimens subjected to thermal stress. It is suggested that electrical or mechanical stress aging would lead to decrease in block density of insulation with epoxy/mica and thermal aging should be contributed to delamination of the insulation. This acoustic detection system is effective for finding defects in stator winding insulation of generator.