Research on Partial Discharge Acoustic Emission Sensing Using Fiber Optic Sagnac Interferometer Based on Shaft–Type Multi–Order Resonant Mode Coupling

In response to the key issues of complex internal structure, significant attenuation of partial discharge (PD) ultrasound signal propagation, and low sensor sensitivity in large oil-immersed power transformers, this paper analyzes the multi-order resonant mode vibration characteristics of the shaft-type fiber optic ultrasound sensor core structure. The displacement distribution patterns of the core structure in both transverse and longitudinal resonant modes are clarified. A strategy using oblique fiber winding rings is proposed to eliminate the problems of strain cancellation and non-accumulation of displacement in transverse and longitudinal resonant modes, which are common in traditional fiber optic ultrasound sensors with parallel fiber windings. Furthermore, design principles are provided to enhance the coverage of the free end and the high-strain regions with semi-high symmetry, as well as the vector-integrated response suitable for multi-order modes. Experimental results show that, in typical PD model detection, the oblique winding sensor exhibits a more prominent response near the high-order resonances of the core, with a detection sensitivity approximately 2.5 times higher than that of the parallel winding structure, and an overall sensitivity at least 7.4 times greater than that of traditional Piezoelectric (PZT) sensors. This demonstrates that the fiber winding method is a key design parameter determining the acoustic-solid coupling efficiency and high sensitivity performance of shaft-type fiber optic interferometric PD sensors, providing a feasible path for high-reliability fiber optic sensing solutions for online monitoring of transformer partial discharges.