The main problem in power transformers is the degradation of the isolation, and partial discharges are a major contribution to this shortcoming. Partial discharges inside medium-and high-power transformers generate ultrasonic pressure waves which are usually detected by using acoustic piezoelectric sensors placed outside the transformers. In this paper we describe an optical fibre interferometric sensor we are developing for the detection and location of partial discharges by measuring inside the power transformers. This non-intrusive optical fibre sensor allows a high level of sensitivity to be achieved for the typical small pressure variations created by partial discharges, specially if compared with the results obtained by acoustic piezoelectric external sensors. We present interferometrically detected levels of partial discharge. These results indicate that the developed optical fibre sensor is able to detect a partial discharge in the range of 10 3 pC that produces an optical phase shift of about 2 rad with a resolution of 30 pC in charge means. Finally, the wavelet analysis is used to identify the acoustic patterns instead of the more commonly used Fourier analysis. When transforming the measured signal to the frequency domain, the temporal information disappears. But the acoustic signals really contain numerous transitory pieces of information to be considered. By looking at a Fourier-transformed signal it is impossible to state when a particular event took place. The wavelet analysis helps to solve this problem, and some results are presented here to show this.
Optical fiber technology is able to answer the demanded development of EMI immune and harsh withstanding sensors for the Electric Power Industry, particularly applied to Power Transformers, and specifically for measurements within them. Apart from temperature, the other main magnitudes of interest are vibrations and partial discharges, which are studied here on the base of the interferometric sensors that we are developing for these purposes. This work is centered on the new concept of interferometric sensing by detecting multi-fringe outputs, which is applied for the measurement of each magnitude independently or both at the same time from only one probe. A high resolution read-out of the optical phase without ambiguity is demonstrated with this approach of demodulation and with the probes designed for monitoring inside power transformers. Results of in-field trials are presented for synchronous vibrations (100 Hz -2 kHz) of two power transformers. First results of calibration of acoustic sensing with these probes on the base of the same interferometric concept are also presented in order to apply them for the measurement of partial discharges by detecting ultrasounds (20 kHz -200 kHz). A multi-purpose exploitation of the sensor heads installed at the core and the windings of a power transformer is explored for detecting at the same time both, vibrations and partial discharge induced ultrasonic pulses. The multi-fringe interferometric output due to vibrations is proposed as a repetitive reference for the demodulation of the higher frequency asynchronous signals.
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