The so-called insulation refers to the use of insulating materials to seal the charged body, thereby isolating the charged body or conductors of different potentials, so that the current can flow according to a certain path. Good insulation is a necessary condition to ensure the normal operation of equipment and lines, and it is also an important measure to prevent electric shock accidents. Insulating materials often play other roles: heat dissipation and cooling, mechanical support and fixation, energy storage, arc extinguishing, moisture proof, mildew proof, and conductor protection.
1. Classification and characteristics of insulating materials.
Under the action of DC voltage, only a very small current passes through, and their resistivity is greater than 109Ω cm. Insulating materials are divided into three categories: gas, liquid and solid. Commonly used gas insulating materials include air, nitrogen, hydrogen, carbon dioxide and sulfur hexafluoride; commonly used liquid insulating materials include mineral oil (such as transformer oil, switch oil, capacitor oil and cable oil, etc.), silicon oil, and castor oil; Commonly used solid insulating materials include insulating fiber products (such as paper, cardboard), insulating impregnated fiber products (such as varnished cloth, paint pipes and cable ties, etc.), insulating paint, glue and welding powder, insulating mica products, electrical films, composites Products and adhesive tapes, as well as plastics and rubbers for electrical engineering. The insulation of electrical equipment should comply with its corresponding voltage level, environmental conditions and use conditions. Should be able to withstand electrical, mechanical, chemical, thermal, biological and other harmful factors for a long time without failure. The quality and service life of electrical products depend to a large extent on the electrical, thermal, mechanical, and physical and chemical properties of the insulating material. Under the action of an external electric field, the insulating material will undergo processes such as polarization, loss, and breakdown. It will also age.
2. Destruction of insulated shoes
Breakdown The phenomenon that the insulation completely loses its insulation performance under the action of a strong electric field and other factors is called insulation breakdown. Breakdown is divided into three types: gas dielectric breakdown, liquid dielectric breakdown and solid dielectric breakdown.
①The breakdown characteristics of gaseous dielectrics can use high vacuum and high pressure methods to improve the breakdown strength of the gas. The gas contains impurities (conductive vapor, conductive impurities), which can reduce the breakdown voltage. After the gas breakdown, when the external applied voltage is removed, the gas insulation performance is quickly restored. After the gas breaks down, a current path is formed in the gap, and the current increases sharply, such as arcs, lightning, fluorescent lights, neon lights, etc. in daily life, forming gas conduction.
②The breakdown characteristics of liquid dielectrics. It is generally believed that the breakdown of pure liquids is similar to that of gases. The breakdown is caused by electron impact ionization. However, the density of the liquid is high, the electron free travel is short, and the accumulated energy is small, so the breakdown is The intensity is higher than that of gas. The breakdown of the liquid dielectric is related to its purity. To ensure the insulation quality, the liquid dielectric must be purified, dehydrated, and degassed before it is used. After the liquid breakdown, when the applied voltage is removed, the liquid insulation performance can be restored to a certain extent.
③Characteristics of breakdown of solid dielectrics The breakdown of solid dielectrics includes electrical breakdown, thermal breakdown, and chemical breakdown. Electrical breakdown is characterized by short voltage application time and high breakdown voltage; the breakdown field strength is closely related to the uniformity of the electric field, and has almost nothing to do with the surrounding temperature and voltage application time. The characteristic of thermal breakdown is that the voltage is longer than the electric breakdown, the breakdown voltage is lower, and the insulation temperature is increased. The breakdown voltage decreases as the surrounding temperature rises, but it has little to do with the uniformity of the electric field. Electrochemical breakdown is the breakdown caused by the combined effect of factors such as freeing, heating and chemical reaction. Electrochemical breakdown is formed under the long-term effect of voltage, and its breakdown voltage is often very low. It is closely related to the free-resistance performance of the insulating material itself, the manufacturing process, and the working conditions. Breakdown has an accumulative effect, that is, only partial damage or incomplete breakdown is caused by one impulse voltage action, and multiple impulse voltage actions result in complete breakdown. The solid dielectric cannot be recovered after breakdown and will lose its insulating properties.
(2) During the operation of the insulation material of the insulation aging equipment, due to the long-term effects of various factors, a series of chemical and physical changes will occur, resulting in the gradual deterioration of its electrical and mechanical properties. This phenomenon is called insulation aging. . In general low-voltage electrical equipment, insulation aging is mainly thermal aging. Every kind of insulating material has a limit heat resistance temperature. When the temperature exceeds this limit temperature when the equipment is running, the aging of the insulating material will be aggravated and the service life of the electrical equipment will be shortened. In high-voltage electrical equipment, insulation aging is mainly electrical aging. It is caused by the partial discharge of the insulating material.