The value of the dissipation factor tg δ (the factor of dielectric losses) represents the ratio between the capacity and ohmic resistance of the insulation material. Insulation losses its ohmic resistance with age, enlarging the value of tg δ.
Capacity is feature of matter, which is dependent on the distance and the surface area between two plates and on the insulation material between the plates. In a transformer, the plates are represented as a winding against the earthed parts and as the windings against one another.
The measurement is conducted with AC/DC voltage at a given temperature. The measurement is conducted in all the windings, as well as in each individual winding against the earthed parts.