A Capacitive Sensor for Detecting Insulation Degradation by Sensing 2-FAL in Transformer Oil

A capacitive sensor for assessing early degradation of transformer insulation by detecting 2-furfuraldehyde (2-FAL) at the sub-ppm level in transformer oil is presented in this paper. The sensor uses a molecularly imprinted polymer (MIP) in which 2-FAL sensitive receptors are created. The 2-FAL molecules in the oil are adsorbed on the receptor cavity of the MIP which results in a change in the sensor capacitance. The sensing film is prepared by the sol-gel technique using methacrylic acid, divinyl benzene, and azobis. The sensor is successfully tested from sub-ppm to ppm of 2-FAL concentration. The average sensitivity of the sensor is 0.379 pF/ppm, and the average standard deviation is 0.035 pF at 1 kHz in the range of 0-10 ppm. The sensor shows piecewise linear characteristics for sub-ppm and higher ppm ranges. The resolution of the sensor is 0.1 ppm in the sub-ppm range, and hence, it can be used for anticipating early failure as well as provide warning during an advanced stage of deterioration. The average repeatability index of the sensor is +/- 0.033%, which indicates negligible drift and stability of sensing film.