Insulating materials under the long-term action of external electrical stress, leading to a gradual decline in various properties. The main cause of aging is partial discharge.
The phenomenon that insulating materials cause various properties to gradually decline under the long-term action of external electrical stress is called electrical aging of materials. There are many factors that cause aging, including electric current, dielectric loss, electromagnetic force, partial discharge and other factors.
However, it is reflected in the current practical application that the main factor that affects the electrical aging life of the material is partial discharge. The reason is that the manufacturing process contains a part of the air gap or other impurities that are easier to break down than the insulating medium. When the applied voltage increases , Causing partial breakdown of this part of the air or other components to produce partial discharge.
Partial discharge damages insulating materials greatly, but for different materials, due to the different material structures, the process of partial discharge damage to materials is also different.
For example, when polyethylene is used in the manufacture of high-voltage cables, partial discharge accelerates its cracking, and the mica insulation of generator coils is considered to be the main cause of damage due to increased ion bombardment. Generally, the breakdown field strength of the material under discharge conditions is used to evaluate the electrical aging parameters.
In the process of partial discharge destroying insulating materials, irreversible damage marks are always left, which reflect obvious changes in the material's dielectric and mechanical properties.
(1) The low-molecular-weight polar substances or acids generated by the discharge penetrate into the material, so that the volume resistance of the material decreases, and the loss tangent increases. This kind of change is especially obvious in the oil-paper insulation.
(2) Make the material lose elasticity, become brittle or crack.
(3) After the discharge, gas and ions are left behind, so that the material discharges and the voltage gradually decreases, which changes various discharge characteristics.