The directionality of an optical fiber high-frequency acoustic sensor for partial discharge detection and location

Fiber-optic acoustic sensors are being developed for the detection and loca tion of partial discharges in oil-filled transformers, The partial discharg es can be detected acoustically and located by the triangulation method wit h 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 degrees 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 ma de using a plane wave approximation. The calculation showed that if the dia meter of a fiber coil is less than 20 mm, its directionality is relatively flat within +/-40 degrees for frequencies lower than 150 kHz. The directionality of an optical fiber acoustic sensor for the frequency ra nge of 50-300 kHz was measured experimentally in an oil tank. The effect of reflected waves was avoided by using a gating technique. The experimental results show somewhat different directionality patterns from the theoretica l results but are internally consistent and with maxima and minima at frequ encies close to those predicted. The discrepancy is believed to be due to a resonance effect which is not taken into account in the theoretical model. However, the sensor achieves the directionality performance required for t he application.