Ctr. for Mater. Technol., Univ. of Technol., P.O. Box 123, Broadway, NSW 2007, Australia
Partial discharge (PD) erosion of insulation is one of the deteriorative mechanisms in transformers and high-voltage plants leading to early failure. Ultrasonic detection techniques are used regularly for location of PDs when electrical method [IEC Pub. 76/IEC 270] used in routine tests or DGA results indicate the presence of PD. The technique uses either externally or internally mounted ultrasonic transducers (J. Unsworth, J. Kurusingal, and R. E. James, Proceedings of ICPADM 94, IEEE). Unfortunately ultrasonic technique requires expert supervision to distinguish PD signal patterns from some typical transformer noise patterns. Moreover, transducers are selected based on commercial availability or for their ability to reject radio interference. A study was conducted to determine the optimum frequency range for PD detection to give maximum range sensitivity and acoustic interference rejection. This paper presents transformer noise recorded in the field as well as in the laboratory with externally and internally mounted transducers. The data records are Fourier analyzed and transducer response corrected to obtain the acoustic noise spectra. It is found that there is an upper cutoff frequency for transformer acoustic noise which is around 75 kHz and signals originating from PD have frequency components much beyond this range. Factors which influence range sensitivity are also discussed.