Detection of 2-FAL and moisture in transformer oil using an interdigital capacitive sensor for on-site transformer condition monitoring

The insulation system of transformers degrades over time due to harsh environmental and operational conditions, thus reducing their designated useful life. In oil, insulation degradation compounds are released due to transformer insulation degradation. An oil-immersed transformer can be assessed based on the concentration of these compounds in the oil. Transformer paper insulation conditions can be assessed by the level of 2-furfuraldehyde (2-FAL) and moisture in the oil. The dielectric constant of 2-FAL and moisture is very high in comparison to that of fresh oil, making capacitive sensor-based detection of these compounds in oil a promising and economical option. This paper investigates the feasibility of a simple interdigital transducer (IDT) fringing field capacitive sensor in the measurement of the concentration of 2-FAL and humidity in transformer oil. Firstly, an interdigital capacitive (IDC) sensor is simulated using COMSOL Multiphysics software to design and analyse a 1-r-1 IDC sensor where polyvinyl alcohol (PVA) and polydimethylsiloxane (PDMS) are used as sensing materials. Moreover, three different arrangements of electrode structures, which have been designed, are investigated: single-sided (SS), single-sided with shielding (SSWS) and double-sided (DS). Furthermore, the IDC sensors are fabricated and characterised. The fabricated sensor is tested and characterised using oil samples with different concentrations of humidity and 2-FAL (in the 0-60 ppm range). The sensor exhibits the maximum sensitivity for a double-sided 1-1-1 configuration. In the 0-60 ppm range, the sensor exhibits approximately linear response characteristics and very small characteristics drift. The sensor facilitates real-time on-site condition monitoring of in-service transformers and circumvents all the issues of offline monitoring.