Power transformer oil-level measurement using multiple fiber optic sensors

This paper presents two different optical fiber based sensors system for measurement of oil level of a power transformer. One uses a continuously uncladed multimode optical fiber with uncladded portion equals to the measurement range. The other one uses multimode optical fiber with discretized uncladed portions that equals the measurement range. Differential measurement technique has been adopted to eliminate the environmental affect on the measurand using a reference identical multimode optical fiber for both continuous and discrete sensors. A beam splitter is used to split a laser light emitted from He-Ne laser source to produce two identical beams of rays. These two beams are applied to the sensing fiber and to the reference fiber. The sensing principle is based on the evanescent wave interaction. The output intensity of light through the sensing fiber depends on the change in refractive index due to change in relative length of air and transformer oil in proximity of uncladed portion of the sensors. The outputs light from the sensing and reference fibers are applied to two identical potential divider circuits to convert variation of light intensity in to voltages. The voltages of the potential divider circuits are read by a data acquisition card (DAQ) and analysis is carried out using LabView software. The optical sensor has many advantages which includes low cost and simple design, high sensitivity, light weight, immune to electromagnetic interference and radio frequency interference, etc.