Fig. 17. 3D spectrogram of change in the capacitance between the primary and secondary windings.
As observed, STFT makes an apparent deviation between fingerprint and faulty cases. Though 2D spectrogram is am-biguous in some cases, 3D spectrogram provides a good indi-cation of difference. Table IV summarizes the results obtain with trans-admittance transfer function under all cases, where Am and fm are the peak amplitude and frequency of the reso-nance pole, respectively. tm is the time at which the resonance pole occurs.
Also, the resonant frequency location of the fingerprint is different than faulty cases; consequently, a breakdown be-tween the turns or coils of the transformer results in shifting the resonant pole to another frequency or the creation of a new pole.
TABLE 4
SPECTROGRAM SUMMARY OF TRANS-ADMITTANCE TRANSFER FUNCTION
9. DISCUSSION AND CONCLUSION
From the previous results, the following can be concluded:
• There is a small difference in the time domain when comparing voltage or current waveforms from normal to faulty cases.
• As a result of the transfer function in the time and fre-quency domain, deviations are apparent in some faulty cases although it is difficult to make a quantitative evaluation.
• Via STFT, a significant change is clear, particularly the 3D spectrogram gives a clear view. The 2D spectrogram may be confusing sometimes; hence, it can be used as a supplementary factor to 3D.
• STFT shows a good resolution at the resonant peak,