Moisture Assessment in Transformer Insulation Liquid using Optical Sensors

Online monitoring of power transformers' insulation systems is essential to detect incipient failures. Moisture, known to be enemy number one of power transformer insulation, is widely determined by capacitive water activity probes or offline by Karl Fisher titration in the laboratory. This contribution presents online moisture monitoring in transformer oil using an optical fiber sensor. Compared to capacitive sensors, optical sensors are insensitive to electromagnetic interference. Two different Fiber Bragg Gratings (FBGs) separated by a gap are coated, one with acrylate and the other with a hygroscopic polyimide film. Mineral oil and synthetic ester Midel 7131 are used for the calibration. The oil sample was exposed to humid air to obtain four different moisture content values. The designed sensor is immersed into a sealed vessel via a surgical needle into the oil. The two FBGs provide two peaks that are used to assess absolute moisture. The distance between peaks is calibrated to ppm of moisture as assessed by Karl Fisher titration. The obtained results indicate the feasibility of moisture measurements in two different insulated liquids.