1. Dielectric coefficient of insulating materials
The relative dielectric coefficient of insulating materials indicates the movement of charges inside insulating materials under the action of electric fields, that is, the degree of polarization. Generally, it decreases step by step with the increase of the frequency of the electric field and increases with the moisture absorption of the material; since temperature affects polarization, a peak value will appear at a certain temperature.
2. Dielectric loss of insulating materials
Under the action of electric fields, insulating materials produce energy loss due to leakage and polarization. Generally, the loss power or loss tangent is used to indicate the magnitude of dielectric loss. Under the action of DC voltage, instantaneous charging current, absorption current and leakage current will pass through. When AC voltage is applied, the instantaneous charging current is reactive current; the leakage current is in phase with the voltage and is active current; the absorption current has both reactive current components and active current components.
3. Breakdown strength of insulating materials
3.1 Thermal breakdown. Under the action of alternating electric fields, heat is generated inside insulating materials due to dielectric loss. If it cannot be dissipated in time, the temperature inside the material will rise, causing the molecular structure to be destroyed and breakdown, which is called thermal breakdown. The thermal breakdown voltage decreases with the increase of the temperature of the surrounding medium. As the material thickness increases, the heat dissipation conditions become worse and the breakdown strength decreases. When the frequency increases, the dielectric loss increases and the breakdown strength also decreases.
3.2 Electric breakdown. Under the action of a strong electric field, the charged particles inside the insulation move violently, collide and ionize, destroy the molecular structure, and finally break down, which is called electric breakdown. The electric breakdown voltage increases linearly with the thickness of the material. In a uniform electric field, unless the impulse voltage is shorter than 10 seconds, the electric breakdown strength is generally unrelated to the voltage action time.
3.3 Discharge breakdown. Under the action of a strong electric field, the bubbles contained in the insulating material discharge due to ionization; impurities are also vaporized by the electric field heating, generating bubbles, which further develops the bubble discharge and leads to the breakdown of the entire material, which is called discharge breakdown.
The breakdown of insulating materials often occurs in the above three forms at the same time, which is difficult to separate. Impregnating insulating materials with insulating paint or glue can not only improve the electric field distribution and increase the electric breakdown strength, but also improve the heat dissipation conditions to increase the thermal breakdown strength.
4. Insulation resistivity
When voltage is applied to an insulating material, there will always be a tiny leakage current flowing through it. Part of this current flows through the interior of the material, and part flows through the surface of the material. Therefore, the insulation resistivity can be divided into volume resistivity and surface resistivity.