Breakdown strength of insulating materials

The thickness of the breakdown area of insulating material is used to divide the breakdown voltage and is expressed in kilovolt/millimeter. Breakdown of insulating materials can be roughly divided into three forms: electrical breakdown, thermal breakdown and discharge breakdown.

Electrical breakdown. Under the action of strong electric field, charged particles in insulation move violently, collide and dissociate, destroy the molecular structure, and finally break down, which is called electrical breakdown. The breakdown voltage increases linearly with the thickness of the material. In a uniform electric field, unless the time of the impulse voltage is less than 10 seconds, the breakdown strength is generally independent of the time of the voltage action.

Thermal breakdown. Under the action of alternating electric field, the heat generated by dielectric loss in insulating materials will increase the internal temperature of materials if it can not be released in time, leading to the breakdown of molecular structure, which is called thermal breakdown. The thermal breakdown voltage decreases with the increase of the temperature of the surrounding medium, the thickness of the material increases, the heat dissipation condition becomes worse, and the breakdown strength decreases. With the increase of frequency, the dielectric loss increases and the breakdown strength decreases.

Discharge breakdown. Under the action of strong electric field, the bubbles contained in insulating materials discharge due to ionization; impurities are also heated and gasified by electric field, resulting in bubbles, which further develops the discharge of bubbles, leading to the breakdown of the whole material, called discharge breakdown.

Breakdown of insulating materials often occurs simultaneously in the above three forms, which makes it difficult to be completely separated. Impregnating insulating materials with insulating paint or glue can not only improve electric field distribution and electric breakdown strength, but also improve heat dissipation conditions to improve thermal breakdown strength.