Tan δ is the tangent of the dielectric loss angle (also known as the dielectric loss factor). By measuring tan δ, a series of insulation defects can be reflected, such as insulation moisture, oil or impregnation dirt or deterioration, and discharge due to air gaps in the insulation. When the insulation is damp or defective, the leakage current will increase, and there will be a large number of bubbles, impurities, and moisture in the insulation, which will intensify the polarization of the interlayer and increase the polarization loss. In this way, the magnitude of the dielectric loss factor is directly related to the quality of the insulation. At the same time, dielectric loss causes internal heating and temperature rise in the insulation, which increases leakage current and polarization. The increase in dielectric loss makes the insulation even hotter, and this cycle may cause breakdown in areas with weak insulation. Therefore, the dielectric loss value reflects both the state of the insulation itself and the process of insulation transitioning from a good condition to a deteriorated condition. Meanwhile, dielectric loss itself is a factor that leads to insulation aging and damage. The purpose of testing the tan δ of the transformer winding and bushing is mainly to check whether the transformer as a whole is damp, whether the insulation oil and paper have deteriorated, whether there is oil sludge attached to the winding, and whether there are serious local defects. It can also measure the tan δ and capacitance of transformer windings as one of the auxiliary means for judging winding deformation. In addition, for voltage induced thermal defects in electrical equipment, it can be mutually verified with the precise temperature measurement test method in infrared thermal imaging testing.
When judging the insulation condition through the tan δ value, it is also necessary to focus on comparing the tan δ values of the equipment over the years and comparing them with similar equipment under the same operating conditions. Even if the tan δ value does not exceed the standard, when the tan δ suddenly increases significantly compared to the past and other equipment under the same operating conditions, it must be dealt with, otherwise accidents often occur during operation.
