Dielectric dissipation factor

OELCHECK test instrument: Baur DTL-C
Sample quantity: approx. 200 ml
Unit: dimensionless
Test result: dissipation factor tan δ, relative dielectric constant εr
Analysis for: transformer oils, oils for electric drives
Brief description: In order to define the tan δ, the measuring cell of the device is filled with 40 ml of the oil and heated to a standard 90 °C. A test voltage of 2,000 V at a frequency of 50 Hz is applied across the two electrodes of the measuring cell, which are arranged at a distance of 2 mm.

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.

Underlying test standard: DIN EN 60247