A study by the IEEE, of 3.787 transformers, with an accumulated 37.692 years of operation, concluded that:
"The insulation is the origin of the majority of the transformer defaults"
Extract from a conclusive study and report, titled Transformer Diagnosis: Part 1.
A Statistical Justification of Preventative Maintenance:
http://www.electricityforum.com/et/june99/trans.html
Presented by Mr. Michael Belanger, of Seidel Inc. in 1999.
The first and most important step of the identification of internal problems of the power transformer is a precise and relevant diagnostic of the actual state of its oil-cellulose system.
Unfortunately it is not commonly recognized that only consequent and complex monitoring only of the content of the aging products in the oil - as water, acids and gases (and of course particles) help avoid big troubles and unnecessary costs.
Therefore universal advice for any user is:
Keep your transformer clean.