1. The current of the insulating material of transformer consists of three parts: instantaneous charging current (generated by displacement polarization of the geometric capacitance of the medium and gradually attenuated with the increase of time, expressed by IC), absorption current (generated by slow polarization, volume charge generated by conductive ions, etc., and also gradually attenuated with the increase of time, expressed by IA) Leakage current (the leakage current is closely related to the ion content of the insulating material itself, expressed by IB).
2. Insulation resistance of transformer insulation material: the quotient of the DC voltage added between two electrodes in contact with the insulator or sample divided by the total current passing through the two electrodes.
3. Volume resistance of transformer insulation material: the quotient of the DC voltage applied between the two electrodes placed on the opposite two surfaces of the sample divided by the steady-state current formed between the two electrodes; that is, the resistance between the insulation material and the two surfaces.
4. Volume resistivity of transformer insulation material: the quotient of the DC electric field strength in the sample divided by the steady-state current density can be seen as the volume resistance in a unit cubic volume. The volume resistivity of insulating materials is usually 109-1021 cm.
5. Surface resistance of transformer insulation material: the quotient of the DC voltage between two electrodes added on the same surface of insulator or sample divided by the current between the two electrodes after a certain electrification time.
6. Surface resistivity of transformer insulation material: the quotient of DC electric field strength on the surface of insulation material divided by current linear density.
7. Factors affecting the resistivity of transformer insulation materials: temperature, humidity, impurities, electric field strength.
(1) Temperature
With the increase of temperature, the resistivity decreases exponentially. The reason - this is because when the temperature increases, the molecular heat movement increases, the average kinetic energy of the molecule increases, the probability of the molecular kinetic energy reaching the activation energy increases, and the ion is easy to migrate.
(2) Humidity
——The insulation resistance decreases with the increase of humidity.
The effect on porous materials (such as paper) is particularly significant, and the resistance of dielectric surface is very sensitive to the influence of surface moisture.
Electrical equipment, especially outdoor equipment, can prevent accidents by regularly checking the change of insulation resistance of equipment.
(3) Impurity
——Increase of impurities, decrease of resistivity, contamination of surface by impurities and absorption of water
Impurities directly increase conductive ions in the dielectric
Impurities are easy to mix with polar materials, which can promote the dissociation of polar molecules.
8. Dielectric - any substance that can establish polarization under the action of an electric field. Insulating material is one of the dielectrics.
9. In the external electric field, induced charge (bound charge) is produced on the surface of the dielectric, which is called polarization of the dielectric.
10. The basic forms of polarization are displacement polarization, relaxation polarization, turning polarization, space charge polarization and spontaneous polarization.
