Highly sensitive and chemically resistant moisture sensor based on polyether sulfone and polyimide composite membrane for power transformer oil

Abstract Moisture is the primary factor leading to the deterioration of transformer oil. Real-time detection of transformer oil moisture content holds significant importance to help power companies monitor the transformer’s operational status, track the change curve of the oil moisture content, identify water ingress causes, and implement preventive measures. However, transformer oil has the characteristics of low moisture content and complex chemical composition, which pose a substantial challenge for sensor monitoring, and require the prepared sensor to be chemically resistant and highly sensitive to moisture. To address these challenges, in this paper, a parallel plate oil moisture sensor with high sensitivity and high chemical resistance based on PES and PI was developed using the MEMS process. The sensor incorporates PES as the chemical resistance layer and PI as the moisture sensitivity layer. The parallel plate design not only optimizes sensitivity by effectively utilizing electric field lines but also allows the passage of water while isolating oil molecules, further enhancing the chemical resistance of the sensor. Experimental results in air demonstrate that the prototype sensor exhibits high sensitivity (~1.26 pF/% RH) across the full humidity range of 0-100% RH. Moreover, the sensor can withstand exposure to a wide range of chemical corrosive gases, including acetone, ammonia, etc. Tests in transformer oil reveal that the sensor has a resolution better than 200 ppm and can effectively measure moisture in the range of 0-1400 ppm, demonstrating very high sensitivity (4 pF/100 ppm) and rapid response (10/23 s).