I agree with Joe Stewart's comment and can offer the following. Dry type transformer insulation is primarily composed of Nomex a hygroscopic material, This material attracts and retains moisture. Steam cleaning is good for new equipment but the force generated by the steam may enter old and brittle insulation causing it to delaminate. Grease and dirt may not be as prevalent as in a generator or motor due to the lack of moving parts and bearings. Typically dust, wet or dry are the most common contaminants. Vacuuming the windings is a good idea as opposed to blowing compressed air that may force the contaminants deeper into the insulation.

I have been successful in drying out electrical apparatus on site using DC welders. A three phase transformer and or generator/motor can have all six leads brought out, (line and neutral) OF BOTH PRIMARY AND SECONDARY WINDINGS and connected in series and have a DC current run through them for the I squared losses to gradually generate heat and dry the winding from the inside out. The temperature should not increase by more than 5°C per hour until the final temperature of 100-125°C is attained. A rapid rise in temperature could cause steam bubbles to damage the insulation. There will be no transformation of voltage as there is no time varying flux.

IEEE 95 and IEEE56 show the trend experienced when drying out windings. Typically Megger values will decrease with an increase in temperature until all of the moisture is removed. Then megger readings will rise.

If an oven is available use it. Typical dry out temperatures were in excess of 250°F, but in no case should they exceed the temperature class of the insulation. Dips and bakes are a good way to seal old and cracked insulation.

Thanks to all, And I hope this helps,