Near-infrared direct optical techniques for the measurement of water content in oil-paper insulation to avoid oommen's moisture equilibrium curves

Measurement and monitoring of moisture is essential to predict life and operation condition of power transformers. Paper-oil insulation in power transformers is degraded and gradually damaged due to electrical, chemical, mechanical and moisture factors. It has been well established from several studies that moisture is a major source of insulation failure in high voltage power transformers. This paper presents direct optical measurements of water content at paper insulation immersed in transformer oil inside a test cell, which contains a water source, and a capillary paper bridge to transfer water to the paper inside an oil reservoir. Optical measurement of water content was carried out in the near infrared region from 900 to 1500 nm band. Experimental studies of light transmission in transformer oil and water are discussed. The criteria to establish the best optical bands for maximum sensitivities are given. The measurement limitations, calibration procedures and an error analysis are presented. The proposed technique can be used for on-line measurements in electrical apparatus that use oil-paper insulation under large electrical field gradients. The presented method has advantages, since it is a direct and fast technique, which measures the water transfer to paper, immersed in oil, and it could be applied in compact, portable equipment at a low cost.