Fiber-optic acoustic sensors are being developed for the detection and location of partial discharges in oil-filled transformers. The partial discharges can be detected acoustically and located by the triangulation method with the sensors placed inside the transformer tank. In this application, the directionality of the sensor should be as flat as possible within at least ±40° from the sensor axis and for a frequency range up to 150 kHz. A calculation of the directionality of a fiber coil acoustic sensor was made using a plane wave approximation. The calculation showed that if the diameter of a fiber coil is less than 20 mm, its directionality is relatively flat within ±40° for frequencies lower than 150 kHz.
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