Investigation of transformer oil aging using no-core optical fiber (NCF) sensor

Transformer oil plays a crucial role in insulation and cooling within high-voltage transformers, but it degrades over time. This research proposes a durable sensor capable of detecting the refractive index (RI) of transformer oil when it exceeds the RI of the sensor structure, known as high refractive index (HRI) sensing. The study utilizes a no-core optical fiber (NCF) to monitor the quality of transformer oil. In this setup, single mode fiber (SMF) is employed as both the input and output of the NCF, forming an SMF-NCF-SMF (SNS) sensor. To date, to the use of an NCF in the SMF-NCF-SMF scheme has not been reported for high RI fiber sensing and transformer oil degradation detection. Additionally, this study provides an analysis of the influence of different diameters and lengths of NCF on the sensor’s sensitivity. The HRI sensing performance of the sensor was evaluated both numerically and experimentally by observing power spectrum changes due to leaky modes interference in response to varying transformer oil RI values from 1.4600 RIU to 1.5500 RIU. The NCF, with a geometry of 1 cm in length and 100 μm in diameter, demonstrated remarkable sensitivity, achieving up to 88.285 dBm/RIU for HRI values within the specified range. The sensor effectively discerned various aging levels of transformer oil in power transformer applications. Additionally, since the NCF structure is entirely composed of silica-based materials, it exhibited significant temperature resistance. These characteristics make the SNS structure well-suited for reliable deployment in challenging thermal environments.