Major Defects and Diagnostics of Solid Insulation of Power Transformers

The main defects and diagnostics methods of paper-, wood-, and polymer-based solid insulation used in power transformers are presented. Specific examples of pollution, sludging, moisturizing, and mechanical defects and their development during partial and arc discharges are discussed. The distribution of moisture content of cellulose insulation along the height of transformers with natural and forced oil circulation is analyzed. The danger of local wetting of solid insulation is noted. Examples of transformer failures as a result of moistening of wooden strips for attaching taps, as well as defects in epoxy insulation of low-voltage transposed conductor windings, are given.