Silica rod based optical fibre sensor for high refractive index sensing application in ageing power transformer oil

Abstract

Power transformer is one of the most essential components in power transmission and distribution systems. A thorough inspection of the condition of a power transformer is critical to avert malfunctions. An essential part of this inspection includes degradation control of the transformer oil. In fact, studies have incorporated optical fibre sensors (OFSs) for transformer oil degradation detection owing to the distinct advantages of OFS over conventional methods. Despite the diversity of techniques which have been employed for the developed OFSs, they pose problems of complicated fabrication and cross-sensitivity to temperature. As such, this study reports the original research work on the development of high refractive index (RI) fibre sensors based on silica rod (SR) structure to address the aforementioned problems. This study details the conceptual sensor design, the fabrication, the experimentation, and the application to transformer oil degradation detection. Related mathematical models of the sensor architectures, such as principles of leaky mode interference (LMI) and multimode interference (MMI), were explored to comprehend sensor behaviour. The sensors were numerically analysed using BeamPROP software to determine their functions from field distribution and sensor spectra. Systematic procedures for fabrication and experimentation of the sensor were developed to ensure high repeatability. Notably, four sensor designs are proposed in this study. Design 1 signifies RI sensing based on wavelength shift and spectrum power level change. The use of SR as a sensing element induced the spectrum power level change due to the LMI at the SR section. Meanwhile, spectrum wavelength shift was induced because the input of MMI in MMF was substantially influenced by its surrounding high RI. The sensor responded to the surrounding RI by the changes of dip wavelength and output power level with maximum sensitivity of 38.65 nm/RIU and 63.15 dBm/RIU, respectively. Design 2 is proposed to simultaneously measure high RI and temperature by monitoring the respective output power level and wavelength shift of the single dip transmission spectrum of the sensor. The experimental results revealed that the sensor had RI sensitivity of 108.07 dBm/RIU and temperature sensitivity of 9.31 pm/oC. Design 3 deployed a SR with larger diameter exceeding the MMF core diameter to increase the leakage loss of high-order leaky modes to the surrounding. By monitoring the output power of the interference dip, this sensor achieved 5-fold greater sensitivity than Design 1, which was up to -293.53 dBm/RIU. Design 4 refers to a full intensity-based RI sensor that completely depends on the LMI at the SR section. The measurement of high RI was executed by monitoring the spectrum power level change caused by LMI. The sensitivity of this sensor was 93.82 dBm/RIU. Design 4 sensor was selected and applied in power transformer applications to detect transformer oil degradation due to its compact structure, easy interrogation scheme, and resistance to temperature variations. The findings revealed that the sensor was capable of sensing the variations of oil that belonged to the good and fair regions in accordance to ASTM D1500 colour scale. This scenario highlights the great potential of the sensor for remote in-situ detection of transformer oil degradation.