S-Taper Fiber Based Moisture Sensing in Power Transformer Oil

A moisture sensing technique for real-time monitoring of the moisture content in transformer oil based on an S-taper fiber structure, is proposed and experimentally demonstrated, with the advantages of high sensitivity, excellent repeatability, simple fabrication, compact structure, and resistance to ambient temperature variation. By analyzing the physical model of the S-taper fiber, the quantitative relationship between the wavelength change of the transmission dip in the transmission spectrum of the S-taper fiber and the moisture content is established. Then, the S-taper fibers with different structural parameters, such as the waist diameter and the axial offset, were fabricated in the laboratory, and actual measurements in transformer oil samples with different moisture content are carried out. The results show that the transmission dip experiences the red-shifts with decreasing moisture, which could be used to correlate/trace moisture content. It is demonstrated that the S-taper fiber achieves higher detection sensitivity with a decreasing waist diameter or increasing axial offset. For the S-taper fiber with a waist diameter of $50 \mu \text{m}$ and an axial offset of $110 \mu \text{m}$ , the sensitivity and the lower detection limit reach up to 0.48 nm/ppm and 2.19 ppm, respectively. Therefore, the S-taper fiber sensor could effectively in situ monitor the moisture content in the transformer oil in real-time, which helps to detect the insulation damp problem in the early stage of the transformer in time and ensure its long-term safe operation.