Dielectric loss is the sum of polarization loss and conductance loss caused by transformer oil under the action of alternating electric field. Dielectric loss factor can reflect the transformer insulation characteristics, reflect the transformer oil in the electric field, oxidation and high temperature under the action of aging degree, reflect the degree of polarity impurities and charged colloid pollution in the oil. In the process of long-term use of transformer, the operation condition of transformer oil can be reflected by medium loss factor test. 1. Analysis of causes for excessive dielectric loss 1. Influence of impurities. Dust and other impurities exist in the oil or solid insulation materials of the transformer during installation. After running for a period of time, colloidal impurities gradually precipitate out. Colloidal particles are small in diameter and diffuse slowly, but have a certain amount of active energy. Particles can automatically coalesce, from small to large,