Measurement of ester-based transformer oil aging using tapered single mode-multimode-single mode fiber structure

Nowadays, ester-based transformer oil has been an interest in high voltage transformers as an insulating and cooling medium. In-service transformer oil is subjected to electrical and thermal stresses, leading to degradation process of the oil. In this work, a tapered single mode-multimode-single mode (SMS) optical fiber structure was used to detect transformer oil aging based on the change in oil refractive index (RI). An optical sensor was fabricated by splicing technique and chemical etching. For comparison, five samples of transformer ester oils were examined. The results show that high-level-aging transformer oil possesses a greater RI than fresh transformer oil. A correlation analysis was conducted among sensor output signal, oil absorption spectrum test, and breakdown voltage test. The performance of the sensor demonstrated good sensitivity and linearity of 20.41 nm/RIU and 95.95%, respectively. The single mode-multimode-single mode sensor was easily fabricated, low cost and could be used for real-time high voltage transformer monitoring.