Optimization of PS-FBG grating length for ultrasonic signal detection of partial discharge in oil-immersed transformers

• The average strain across the grating region was observed to decrease with increasing PS-FBG grating length. • Under acoustic emission signal excitation, the average response intensity of the PS-FBG with grating length of 30 mm was found to be 172.42 times that of the 5 mm PS-FBG. • A PS-FBG with 30 mm grating can detected pulsed ultrasonic waves induced by tip discharge defects in 10kV Oil-Immersed transformer. Ultrasonic sensors based on phase-shifted fiber Bragg gratings (PS-FBGs) are considered to have strong application potential in transformer partial discharge detection due to their high spectral slope characteristics. However, systematic theoretical guidance for optimizing the grating length of passive fiber Bragg grating (FBG)-based ultrasonic sensors is still lacking, which constrains the further engineering application of PS-FBG sensors. To address this issue, this paper proposed a strain-tuning model of the grating spectrum based on the acoustic wave propagation theory in layered media and the optical transfer matrix theory. On this basis, the response characteristics of PS-FBG sensors with different grating lengths under discharge-induced acoustic fields were investigated through numerical simulations and experimental tests, and the underlying mechanism governing their behavior was revealed. The research results show that PS-FBGs with longer grating lengths are preferable for detecting ultrasonic signals generated by discharges in transformer oil, as they improve the signal response level. Taking into account the limitations of current PS-FBG fabrication techniques and the mechanical strength required during engineering embedding and encapsulation, a grating length of 30 mm was ultimately selected as the sensing element for ultrasonic detection of partial discharge in transformer oil. Theoretically, its average response intensity within the 20–200 kHz frequency range is 172.42 times higher than that of a 5 mm PS-FBG. Experimentally, partial discharge measurements on a full-scale oil-immersed transformer show that the 30 mm PS-FBG yields a pulse amplitude approximately 16 times higher and a signal-to-noise ratio (SNR) improvement of about 22 dB compared with the 5 mm PS-FBG, thereby confirming the effectiveness of the proposed grating-length optimization method for ultrasonic partial discharge detection in oil-immersed transformers.