In Situ Detection of Transformer Oil-dissolved Trace Hydrogen using a Composite Membrane-modified Fiber-optic Fabry-Pérot Probe

When discharge faults occur inside transformers, hydrogen (H₂) gas is generated and dissolves into the insulating oil. However, conventional dissolved gas analysis (DGA) techniques cannot achieve in situ detection, and existing fiber-optic sensors lack sufficient detection limits. To enable in situ detection of transformer oil-dissolved trace H₂, this work proposes a fiber-optic Fabry-Pérot (F-P) probe based on a composite membrane. The F-P cavity is fabricated by microassembling a single-mode fiber and a ceramic insert, and an ultrathin graphene membrane is transferred onto the cavity end-face as a support layer, followed by deposition of a Pd-Ag alloy as the H₂-sensitive layer and pure Pd as the protective layer. Gas-phase H₂ detection and in situ oil-dissolved H₂ detection experiments are conducted using the fabricated fiber-optic F-P sensor. Results demonstrate that, in gas-phase environments with H₂ concentrations ranging from 0 to 167 ppm, the sensor exhibits a sensitivity of 18.5 pm/ppm and a detection limit of 0.14 ppm, with a response time of 6 minutes. When directly immersed in transformer oil, the sensor achieves a sensitivity of 89.7 pm/ppm and a detection limit of 0.08 ppm for oil-dissolved H₂ concentrations between 0 and 79.1 ppm. Furthermore, excellent recovery and selectivity performance are verified. Therefore, the fiber-optic F-P H₂ sensor proposed in this work enables high-performance in situ detection of oil-dissolved H₂ at sub-100 ppb levels.