Advanced Online Moisture Measurements in Transformer Insulation Using Optical Sensors

The reliable moisture determination in a power transformer is still a challenge for utilities and large power transformer operators. The presence of high moisture can cause serious problems, such as severely reducing the remaining useful life and the operational capability of a transformer. The large insulation structure and slow dynamic moisture migration between cellulose and insulating fluid make it difficult to determine the true moisture level. Fiber optics based sensors offer superior sensing characteristics as compared with other techniques. A key advantage of using fiber optic sensors is their ability to be inserted into the winding structure. However, this is a relatively new technology and so extensive research and testing is required to understand how their measurement is affected by the operation of a transformer, and in particular by any sudden changes during a stepped load. In this work, the application of optical fiber Bragg grating based sensors is investigated under the influence of step cyclic thermal loading condition. The test is similar to the test performed by CIGRE WG D1.52 for evaluating different capacitive water activity probes. The tested optical sensors, coated with polyimide material for moisture sensing, are found to be suitable in comparison with commercial sensors and can be effectively used for moisture measurements in different insulated transformers.