Moisture sensor to characterize transformer solid insulations in transformers under non-equilibrium conditions

Moisture is an important variable that must be kept under control to guarantee a safe operation of power transformers. Because of the hydrophilic character of cellulose, water mainly remains in the solid insulation while just a few parts per million are dissolved in oil. The distribution of moisture between paper and oil is not static but varies depending on the insulation temperature and thus, water migration processes take place continuously during transformers operation [1]. In this work, a sensor is presented to determine the moisture content in the solid insulation of transformers during moisture migration processes. The main objective of the design is that the electrodes of the sensor should not obstruct the movement of water from the solid insulation to the oil, so the proposed prototype uses a metallic mesh-electrode. The measurement scheme is based on the characterization of the insulation dielectric response by means of the Frequency Dielectric Spectroscopy (FDS) method.