Moisture Content Measurement in Transformer Oil Using Micro-nano Fiber

An optical detection method based on micro-nano fiber (MNF) technique is proposed in this paper through a simple single-mode optical fiber. Theoretically, the evanescent field produced by MNF is calculated and its relationship with refractive index and moisture-in-oil is established. Afterwards, MNF probes with a diameter of
$800-125\ \mu \mathrm{m}$
are fabricated and prepared through a self-made melting taper platform. A variety of oil samples with different moisture contents are tested by the MNF sensors with the specific consideration of fiber diameter. The experiment results indicate that the MNF-based optical sensor can achieve real-time measurement of moisture in transformer oil with the sensitivity of 1.8 ppm when the MNF diameter is at 800 nm. Besides, the situation of temperature variation is taken into account during the measurement of moisture-in-oil. The proposed sensing technique shows tremendous advantages, such as little oil sample consumption, high sensitivity, intrinsically safety, flexible installation and real-time response. The MNF based method provides a potentially effective approach for the online monitoring of moisture content in oil-immersed power transformers.