Directional sensitivity of a fibre-optic sensor to acoustic signals in transformer oil

The detection and location of partial discharges (PDs) in power transformers would be greatly facilitated if sensors could be placed inside the tank thus avoiding the multiple paths affecting conventional piezoelectric sensors attached to the outside. This paper reports measurements of the acoustic pressure waves in transformer oil using an optical fibre coil: the pressure induces phase lags in light transmitted through the fibre which can be detected by interferometric means. The coil´s response to plane waves was investigated for various angles of incidence and various frequencies in the range of interest (50 to 300 kHz). The response was modelled and showed that the positions of the nulls were very sensitive to small misalignments but the response to a broad spectrum signal would probably be reasonably flat and allow triangulation of correlated PD pulses