Insulating Paper And Its Generation Requirements

Insulating paper is an insulating material widely used in the electrical field, mainly used in various electrical equipment to insulate and protect. Insulating paper is usually made from cellulose materials, such as wood, cardboard, or kraft paper, and has undergone a special process to improve its electrical insulating properties.

Insulating paper is a general term for electrical insulating paper, which not only has good insulation properties and mechanical strength, but also has the characteristics of high temperature resistance, oil resistance, and chemical corrosion resistance. Insulating paper can be processed according to different needs and made into various shapes and specifications to adapt to different applications.

Application of insulating paper

At present, insulating paper has been widely used in transformers, cables, motors, batteries, etc. In power transformers, insulating paper is used as a solid insulating material in oil-immersed transformers, for separators and barriers for windings, and for wrapping around current-carrying conductors. In high-voltage cables, insulating paper is used as the insulating layer of the cable, protecting the conductors inside the cable and preventing current leakage. In motors, insulating paper is used as slot insulation, phase insulation, end laminates, etc., to improve the insulation performance and mechanical stability of the motor. Insulating paper is also used in batteries, especially lithium battery power banks, as an isolation layer to prevent short circuits. Due to its excellent comprehensive properties, insulating paper has a wide range of applications in electric power, electronics, communications, aerospace and other fields. With the development of the power grid to high voltage, large capacity and compact power grid in the future, the electrical performance control of insulating paper is particularly important for the reliable and stable operation of transformers.

The important properties of insulating paper

The dielectric constant, dielectric loss and some physical properties of insulating paper have an important impact on its insulation performance. Improve the dielectric properties of insulating paper, improve the insulation reliability and efficiency of electrical equipment such as power transformers.

1. Dielectric constant requirements: The dielectric constant of the insulating paper should be as low as possible to reduce the concentration of electric field in the insulating paper, thereby improving the breakdown strength and the energy storage density of the capacitor. The relative dielectric constant of cellulose insulating paper is usually in the range of 3.5~4.4, and its dielectric constant can be reduced by modification, such as using BTCA (bis(trichloromethyl)carbonate) modification, so as to optimize the electric field distribution in the oil and paper in the transformer and reduce the occurrence of insulation accidents.

2. Dielectric loss requirements: The dielectric loss of insulating paper should be as small as possible to reduce the energy loss in the AC electric field and improve the efficiency of the capacitor. Materials with high dielectric losses will generate more heat in the alternating electric field, causing the device to overheat, affecting the stability and life of the device.

3. Breakdown strength: The insulating paper should have high breakdown strength, that is, it can withstand high electric field without breakdown. The breakdown strength is closely related to the dielectric constant and dielectric loss, and the optimization of the two helps to improve the electrical strength of the insulating paper.

4. Physical properties: Tightness is one of the important indicators of insulating paper. The higher the cellulose content, the stronger the polarity and the higher the breakdown strength. Paper thickness has a great influence on breakdown strength. The thinner the cable paper, the thinner the tape gap oil film and the higher the breakdown strength. The air permeability also reflects the porosity of the paper, the smaller the air permeability, the smaller the void, the more impermeable the gas, and the higher the electrical strength.

Requirements for the production of insulating paper

Factors affecting the dielectric properties of insulating paper include paper composition, structure and production process environment:

1. Paper composition structure

Differences in fiber raw materials: Different kinds of fibers (such as cellulose, aramid, etc.) have different chemical structures and physical properties, which determine the electrical properties such as dielectric constant, conductivity and breakdown strength of insulating paper. Aramid insulating paper is widely used in specific equipment due to its excellent high-temperature stability and electrical properties. The number and distribution of polar groups such as hydroxyl groups on the surface of the fiber affect the hydrophilicity and dielectric constant of the insulating paper. The morphology (e.g., length) and structure (e.g., inter-fiber tightness and internal refinement) of a fiber also affect its dielectric properties. Through chemical modification, such as esterification or etherification, the polarity of the fiber surface can be adjusted to adjust the electrical properties of the insulating paper. Factors such as crystallinity and orientation can also affect the electrical properties of insulating paper, and fibers with high crystallinity generally have better insulation results. Interactions between fiber feedstocks, as well as the combination of additives, such as nanoparticle doping, can alter carrier transport characteristics, which can affect the overall performance of conductivity and breakdown strength.

Purity: High-purity fiber raw materials are an important prerequisite for the production of high-quality insulating paper, and the ash content in the pulp has a significant impact on the electrical properties of insulating paper. For example, the ash content requirement for the 500 kV transformer inter-turn paper is less than 0.21%, and the ash content requirement for the UHV paper is even lower. The doping ratio and type of nanoparticles can affect the trap characteristics in the insulating paper system, which in turn can affect the electrical properties.

2. Production process:

The production process of insulating paper has a significant impact on its insulating properties. The degree of pulping is an important factor affecting the performance of insulating paper. During the pulping process, the degree of fiber cutting, swelling, filament broomization and fine fibrosis will affect the bonding between the base paper fibers of the insulating paper and the paper tightness. The relationship between paper tightness and insulation performance is shown in the figure. The high degree of pulping, good fiber dispersion and tight connection improve the mechanical properties and electrical strength of the insulating paper. Drying removes moisture from wet paper, enhances the bond between fibers, improves paper strength and smoothness, and thus increases electrical strength. Calendering improves the smoothness, tightness and airtightness of the paper, all of which have a direct effect on the electrical properties of the paper. Hot pressing promotes the formation of hydrogen bonds between fibers, and the cellulose molecular chains are more firmly bonded, thus improving electrical performance. The control of the heat treatment temperature is crucial to the electrical properties, which affects the activity and collision probability of the free hydroxyl groups on the fiber surface, which in turn affects the formation of hydrogen bonds.