Fiber-Optic Ultrasonic Sensing for Partial-Discharge Detection in Submarine Cable Flexible Joints

Abstract

Traditional active partial-discharge (PD) detection techniques are difficult to deploy for effective monitoring of PD signals in submarine cable flexible joints located far offshore. In addition, owing to the high-frequency damping effect of polymer insulating media, acoustic waves undergo severe attenuation during propagation, and existing fiber-optic ultrasonic detection techniques suffer from insufficient sensitivity. To address these problems, this study integrates a mandrel-type acousto-optic transducer with a fiber-optic interferometric sensing system and establishes a sensitivity–frequency-response model for high-sensitivity narrowband resonant detection. After structural optimization, the mandrel contact surface was curved to match the cylindrical cable surface, enabling conformal installation and stable acoustic coupling between the sensor and the flexible joint. The finite detection range of the sensor is further characterized, and a quasi-distributed array of sensing units is implemented along the sensing arm of a Michelson interferometer to realize effective detection of ultrasonic signals generated by partial discharges in submarine cable flexible joints. Experimental results show that the developed fiber-optic ultrasonic sensor has a resonance frequency of 33 kHz, which is in close agreement with the theoretical value of 33.22 kHz. When the sensor is positioned directly above an artificial PD defect, the minimum detectable apparent charge is 9.4 pC. Taking the sensor sensitivity under this condition as the normalized reference, the sensor is shown to exhibit a circumferential overlap sensing region of 60° between adjacent probes, within which a minimum detectable apparent charge of approximately 18 pC can be maintained. Along the cable axis, a single probe provides an axial physical sensing boundary of ±25 mm, within which a minimum detectable apparent charge of approximately 25 pC can be maintained. These results provide a reliable sensing solution for early fault warning and high-sensitivity detection of partial discharges in submarine cable flexible joints.