Moisture equilibrium and moisture migration within transformer insulation systems

• Uncertainty in effectiveness of methods available to assess a health of an equipment. In [5] e.g. is shown that in spite of low water content in the oil indicated by normal oil sampling procedures a large amount of water can be revealed within a transformer. How to rank the transformer, which needs drying? How to assess a critical level of water contamination to prevent failure or to determine permissible operating conditions, e.g. overloading?