Breakdown of solid media

    Under the action of an electric field, the breakdown of solid media may be caused by electrical processes (electric breakdown), thermal processes (thermal breakdown), and electrochemical processes (electrochemical breakdown). After the solid dielectric is broken down, it will leave discharge traces in the breakdown path, such as burned or melted channels and cracks, which will permanently lose its insulation performance, so it is non-self-healing insulation.

    The breakdown process of solid medium in actual electrical equipment is intricate and complicated. It not only depends on the characteristics of the medium itself, but also depends on the type of insulation structure, the uniformity of the electric field, the applied voltage waveform and the voltage time, and the working environment (the temperature and heat dissipation of the surrounding medium). Conditions) and many other factors are related, so it is often necessary to use a variety of theories to explain the breakdown process.

    Commonly used organic insulating materials, such as fiber materials (paper, cloth and fiberboard) and polyethylene plastics, have high short-term electrical strength, but under the long-term action of working voltage, processes such as ionization and aging will occur, which will make it The electrical strength is drastically reduced. Therefore, for such insulating materials or insulating structures, not only should pay attention to their short-term electrical withstand characteristics, but also their electrical withstand performance under long-term working voltage.

    The electrical breakdown of a solid medium refers to a phenomenon in which the medium is directly destroyed and its insulation performance is lost only due to the action of an electric field. There are a small amount of electrons (conduction electrons) in the conduction band energy level in the solid medium. They are accelerated under the action of a strong electric field and continuously collide with atoms (or ions) on the lattice nodes. When the energy gained by the conduction electron from the electric field per unit time is greater than the energy lost in the collision, when the energy of the electron reaches the level that enables the ionization of the lattice atoms (or ions), the number of conduction electrons will increase rapidly, causing the electron avalanche , Destroys the character structure of the solid medium, greatly increases the conductance and leads to breakdown.

    When the conductivity (or dielectric loss) of the medium is small, there is good heat dissipation condition, and there is no partial discharge inside the medium, the breakdown of the solid medium is usually electrical breakdown, and the breakdown field strength is generally up to 10⁵~10⁶kV/m, which is much higher than the breakdown field strength during thermal breakdown, which is only 103~10⁴kV/m.

    The main characteristics of electrical breakdown are: the breakdown voltage is almost independent of the ambient temperature; except for the case of a short time, the breakdown voltage has little relationship with the voltage action time; the heating of the medium is not significant; the uniformity of the electric field affects the breakdown Voltage has a significant effect.

    Thermal breakdown is caused by thermally unstable processes in solid media. When the solid medium bears the effect of voltage for a long time, it will generate heat due to dielectric loss, and at the same time, it will also dissipate heat to the surroundings. If the ambient temperature is low and the heat dissipation conditions are good, the heat generation and heat dissipation will reach a balance under certain conditions. The solid medium is in a thermally stable state, and the temperature of the medium will not continue to rise to cause damage to the insulation. However, if the heat is greater than the heat dissipation, the temperature of the medium will continue to rise, causing the medium to decompose, melt, carbonize or scorch, thereby causing thermal breakdown.

    Under the action of long-term working voltage, the phenomenon that the insulation deteriorates, the electrical strength gradually decreases and causes breakdown due to partial discharges inside the medium is called electrochemical breakdown. Near the final breakdown stage, it may be completed in the form of thermal breakdown due to the excessively high temperature of the deterioration, or it may be completed in the form of electrical breakdown due to the decrease of the electric strength after the dielectric deterioration. Partial discharge is a local discharge caused by defects in the medium (such as air gaps or bubbles). The main reasons for the deterioration, damage, and electrical strength of the medium due to partial discharge are: ①The active gases O₃、NO、NO₂, etc. produced during the discharge process will oxidize and corrode the medium; ②The discharge process has charged particles hitting the medium, causing local temperature Rising, accelerating the oxidation of the medium and increasing the local conductance and dielectric loss; ③The impact of charged particles may also cut off the molecular structure, leading to the destruction of the medium. The influence of these aspects of partial discharge is particularly obvious for organic insulating materials (such as paper, cloth, paint and polyethylene materials, etc.).

    The size of the electrochemical breakdown voltage is very closely related to the voltage application time, but it also varies with the type of medium. When high-field-strength regions appear in insulating materials due to small curvature radius electrodes, tiny air gaps, impurities and other factors, local dendritic discharges often occur first, and slender groove-shaped discharge channels are left on the organic solid medium. This is the degradation of tree branch discharge.

    Under AC voltage, dendritic discharge degradation is the result of electrochemical degradation caused by charged particles produced by partial discharges colliding with the solid medium. Under the impulse voltage, it may be the result that the local electric field strength exceeds the electric breakdown field strength of the material.

(Huiputech)