A thin-film capacitive humidity sensor was tested in transformer oil for moisture sensing. This sensor was found to respond well to moisture levels from dry to very wet conditions over a wide temperature range, typical for oil in transformers in cold and warm weather. Routine calibration was done in oil for the temperature range 20 to 80 degrees C for each sensor. The moisture content of oil may be obtained as percent saturation or as ppm moisture level from the current output of the sensor. Further testing was done in a 500 kVA test transformer in which the sensor unit was installed. It was possible to follow the moisture changes during load variation. The ppm moisture content changed roughly in the same manner as the temperature, but the saturation curve showed a tendency to maintain a steady value, except during shutdown. Moisture saturation in oil increased abnormally due to the slow return of moisture from oil to the paper insulation. The moisture sensor should therefore be a valuable tool in warning of hazardous conditions from moisture saturation in oil.
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