Solid insulation of power transformer, like in other electrical equipment, consists of paper wrapped around the conductors in the transformer coils plus mineral oil and pressboard to insulate the coils from ground. From the moment a transformer is placed in maintenance, both the solid and liquid insulation begin a slow but irreversible process of degradation. Paper and pressboard are composed primarily of cellulose. As the cellulose molecule degrades, the polymer chain ruptures and the average number of repeating units in each cellulose molecule decrease. With this reduction in the degree of polymerization of cellulose, there is a decrease in the mechanical strength of the cellulose as well as a change in brittleness and color. For field maintenance specialists, it is a big burden trying to sort out the facts, existing measuring results and sometimes the recommendations may be inconsistent with each other. Specialists are still trying to accumulate more and more experience, knowledge, making the application more complex, therefore comprehensive diagnostic is required.