The main insulation test items in power test transformers include ① measurement of insulation resistance; ② detection of leakage current; ③ absorption ratio; ④ tangent of dielectric loss angle; ⑤ insulation oil and AC withstand voltage test. For transformers with a capacity of 3150KVA and above, during major repairs or when necessary, it is essential to measure the tangent of the dielectric loss angle (tgδ) of the winding together with the bushing. This measurement is mainly to check whether the transformer is damp, insulation aging, oil quality deterioration, or contamination on the insulation.
1. Dielectric loss
What is dielectric loss: Under the action of an electric field, the energy loss caused inside the insulating material due to the hysteresis effect of dielectric conductance and dielectric polarization. Also called dielectric loss, referred to as dielectric loss.
2. Dielectric loss angle δ
Under the action of an alternating electric field, the complementary angle (δ) of the angle (power factor angle Φ) between the current phasor and voltage phasor flowing through the dielectric. Referred to as the dielectric loss angle.
3. Dielectric loss tangent value tgδ
Also known as the dielectric loss factor, it refers to the tangent value of the dielectric loss angle, abbreviated as the tangent of the dielectric loss angle. The definition of the dielectric loss factor is as follows:
If the current phasor and voltage phasor of the test sample are obtained, the following phasor diagram can be obtained:
The total current can be decomposed into the capacitive current Ic and the resistive current IR, therefore:
This is exactly the tangent value of the loss angle δ=(90°-Φ). Therefore, modern digital instruments essentially measure δ or Φ to obtain the dielectric loss factor.
Measuring dielectric loss is a traditional and very effective method for judging the insulation condition of electrical equipment. The decline of insulation ability is directly reflected in the increase of dielectric loss. Further, the reasons for the decline of insulation can be analyzed, such as moisture in insulation, contamination of insulation oil, aging and deterioration, etc.
While measuring dielectric loss, the capacitance of the test sample can also be obtained. If one or several of the multiple capacitor screens are short-circuited or open-circuited, the capacitance will change significantly, so capacitance is also an important parameter.
4. Power factor cosΦ
Power factor is the cosine value of the power factor angle Φ, meaning the proportion of active power P in the total apparent power S of the test sample. The definition of power factor is as follows:
Some dielectric loss testers habitually display the power factor (PF:cosΦ) instead of the dielectric loss factor (DF:tgδ). Generally, cosΦ<tgδ, and when the loss is very small, these two values are very close.
