Oil-impregnated paper (OIP) bushings are widely used in the power system. In order to detect the weak signal generated by the partial discharge (PD) in bushing online, a fiber-optic ultrasonic sensing system based on Mach-Zehnder interference (MZI) was studied in this paper. The average sensitivity of the OFS is 67.3 dB (0 dB=1 V(m·s^-1)). The performance test of the fiber-optic ultrasonic sensor (OFS) was carried out on a real 110 kV transformer bushing. PD signal was detected by the proposed MZI fiber-optic ultrasonic sensor, when the average discharge volume is 10.4 pC. The experimental results show that the interferometric fiber-optic ultrasonic sensing system can be used for transformer bushing PD ultrasonic signal detection.  It is of great significance to improve the maintenance of the transformer bushing

Partial discharge; ultrasonic; optical-fiber sensor; Mach-Zehnder interference; transformer bushing