Loss angle is an important property of transformer oil. In ideal dielectric material, the phase anlge between the voltage and current is 90. However, due to impurities, certain leakage current flows through the dielectric, the actual phase anlge is slightly less than 90. Becasue an angle is very low, the tangent is also low.

Dielectric dissipation factor is the sine of the loww anlge. It is an important parameter to find the insulation quality of the transformer oil. Dielectric dissipation value should be low for good quality transformer oil. Higer dielectric dissipation factor indicates the presence of moisture and contaminating agents.