In this work we present the development and characterization of highly-sensitive fibre optic coils hydrophone for the detection of ultrasonic signals. In particular, fibre coils are a good improvement, in front of electronic sensors, for the application proposed in this work, the detection of ultrasonic signals within power transformers. Several fibre optic coils are tested in a laboratory controlled environment within a suitable ultrasonic frequency range, typically associated to ultrasonic emission in power transformers. We develop and characterize several fibre optic hydrophone coils using a Mach-Zehnder interferometer. A detailed study is carried out on the ideal shape of the hydrophone fibre probe, for parameters such as sensitivity and frequency range. Also, a comparative analysis has been carried out on the sensitivity of commercial PZT to the sensitivity of the fibre optic system under the same test conditions.
Laser optoacoustics is based on the interaction of light with materials producing the thermoelastic effect forming acoustic waves which are characteristic of the medium in which they traverse. This technique is currently on trial for use in biomedical imaging applications and is achieving great success. The work presented here develops an innovative technique for wideband acoustic detection using a fibre optic sensor in a high sensitive multi-coil Mach-Zehnder interferometric configuration. A comparative analysis is performed using both electrical and optical detection techniques on gels which are commonly used to mimic human soft tissue. Indications of future work in this area will be presented throughout this paper.
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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