The dielectric dissipation factor tan δ is measured with a Baur DTL-C. It provides information on the extent of dielectric losses in the oil occurring during operation. It is defined as the ratio between the active and reactive currents flowing in a measurement circuit. In determining the dielectric dissipation factor, it is assumed that current and voltage in an alternating current circuit are subject to a phase shift. This phase change is due to the fact that molecules in the insulating oil can no longer align with the alternating electric field.
When the oil ages, polar components occur in the oil, leading to a phase displacement and in turn to dielectric losses in the insulating oil. Impurities such as water, dissolved insulating resin and paper or other particles can also have polar effects and thereby influence the dielectric dissipation factor. The tan δ is also significantly influenced by the temperature-dependent viscosity, and with it, the size of the molecules, in addition to the ageing of the oil and the impurities. The sum of these changes is given as tan δ.
At the same time, the dielectric constant εr required for the design of most on-line oil sensors is also defined during measurement. A strong negative development in the loss factor also indicates an unwelcome thermal development within the fluid, which can ultimately lead to thermal breakdown. For this reason, this test is highly significant for assessment of the condition of the complex arrangement of a transformer or oils for electric drives.