A Novel Application of the Cross-Capacitive Sensor in Real-Time Condition Monitoring of Transformer Oil

This article presents the novel application of a cross-capacitive sensor for monitoring insulation degradation of transformer oil. The condition of insulating oil is determined by measuring the variation of the moisture content and furfural (2-FAL) concentration in the range of 0-60 ppm. The moisture and 2-FAL are two important by-products formed due to the degradation of cellulose paper used for insulating transformer winding. In addition, a small concentration of moisture is always trapped in oil as well as in paper. An accurate and precise capacitive sensor is developed that uses the cross-capacitive structure proposed by Thomson and Lampard. The sensor is developed using bulk brass material and it consists of four identical brass electrodes that are separated by very small gaps. The sensor has been tested at different concentrations of moisture, and 2-FAL in four insulating oil samples that are commonly used in power transformers, and results are compared by a reference meter. The maximum accuracy of the sensor with respect to the commercial meter is 0.77% and the air cross-capacitance value differs calculated absolute capacitance value by 3%. The calibrated response shows a maximum error of 2%. The output of the sensor is highly precise (+/- 0.0026% for moisture, +/- 0.0028% for 2-FAL), fast, drift-free, and the maximum nonlinearity of the response is close to 2%. The maximum standard deviations (SDs) for the measurement of the mineral oil moisture and 2-FAL concentrations are 0.025 and 0.023, respectively. The sensor response is independent of temperature variation.