Analysis of acoustic emissions caused by the partial discharge in the insulation oil

In order to diagnose the deterioration of insulation and evaluate its location in an oil-filled pole transformer, measurements of elastic waves signals by corona discharges in oil were performed based on an acoustic emission (AE) technique. The detected signals in these measurements were analyzed by means of a fast Fourier transform (FFT) and their properties were discussed theoretically. It was found that the AE signals due to the Lamb waves of symmetrical mode caused in a thin steel tank could be easily recognized by the FFT analyses of initial stages of the detected AE signals. This suggests that the accuracy on location of the AE sources in the transformer would be improved. Also, it was shown that a low pass filter was effective to improve the accuracy of location.