TRANSEC - Moisture monitoring & drying

The issue: Moisture is threatening the transformer

Moisture is one of the major cause of failures for power transformers and one of the main degradation factors for the insulation paper. It is therefore increases the risks of operation failures and shortens the life expectancy of the asset.
Unfortunately, moisture can appear in a transformer from several sources which are external or internal and it has a complex dynamic between the oil and paper within the transformer.
The use of silica gel breathers, sealed tanks, or nitrogen blankets can avoid all or at least the most part of moisture from the atmosphere affecting the transformer. However, when the transformer is energized, the production of water inside the power transformer is a natural and inevitable occurrence over time due to the depolymerization of the cellulose paper.

Only continuous filtration is efficient in removing moisture from a transformer

As moisture is created continuously and as it is one of the major concern for the transformer safety and life expectancy, it seems counter intuitive to apply a temporary solution for this continuous problem. Also it is important to note that more than 98% of the water in a transformer is in the paper while a very low amount is dissolved in oil. The diffusion time of water from paper to oil is very slow. This is why punctual filtrations are not solving the moisture issue.

The issue: Assessing the amount of water in the transformer

  • More than 98% of the water present in the transformer is contained in the paper insulation while less than 2% is in the oil. Unfortunately the insulation paper is not easily accessible for water assessment.
  • Water solubility in oil varies depending on oil temperature and therefore PPM varies also. Hence it is not possible to just check the PPM value to define the amount of water in paper.
  • If PPM and oil temperature are known some curves exist in order to link the water PPM in oil and the water content in paper. But these curves are only really valid at equilibrium which is in general never reached on a running transformer.


  • As the water diffusion time is faster from the paper to the oil than the opposite it is possible to reach on a same transformer several very different PPM values for the same oil temperature even at a few days difference
  •  As a result of the previous points, taking an oil sample once or twice per year in order to assess the moisture level in the transformer seems irrelevant
  • Furthermore, oil samples can be contaminated at the sampling time or in the laboratory. As the moisture level is anyway very low in oil, any contamination (simple contact with the ambient air) will affect the sample ppm result. The chart shows PPM analysis results of 7 different laboratories on 3 different oil samples. It is clear that for moisture analysis the error range is very wide.


The solution: TRANSEC online drying system


  1. Frame
  2. Inlet ball valve 
  3. Pump
  4. Inlet moisture and temperature sensor
  5. Oil pre-treatment filter (10 microns)
  6. Connecting pipes 
  7. Adsorption tanks 
  8. Inlet sampling valve
  9. Air bypass valve between cylinders 1 & 2
  10. Fine filter (3 microns)
  11. Outlet moisture and temperature sensor
  12. Deaerator
  13. Deaerator air bypass valve 
  14. Outlet sampling valve 
  15. Flow indicator
  16. Outlet ball valve
  17. Control and monitoring unit 
  18. Flow meter
  19. Leak sensor & leak tray (optional) 

TRANSEC Online Drying System uses molecular sieves in order to extract moisture from the oil. These granules (non chemical) contain many pores of 3 Angstrom diameter which is the exact size to catch water molecules. That way other components like gases molecules with larger or smaller diameter are not filtered by these sieves.

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