The performance characteristics of cable paper primarily encompass physical, mechanical, chemical, and electrical insulation properties, all of which significantly influence its electrical insulation performance.
Physical properties mainly include tightness (density), thickness, and air permeability. Tightness has a substantial impact on the paper's electrical insulation and mechanical properties; generally, as tightness increases, the paper's dielectric constant, dielectric loss tangent, impulse breakdown field strength, tensile strength, and elastic modulus all rise, while the long-term power-frequency breakdown field strength decreases slightly. Thickness significantly affects the dielectric strength of oil-impregnated paper; as the paper becomes thinner, dielectric strength increases while mechanical strength decreases. Consequently, different thickness grades of cable paper are used for cables of varying voltage ratings, and thickness tolerances must be strictly controlled. Air permeability indicates the degree of porosity in the paper structure; typically, lower air permeability correlates with higher dielectric strength in oil-impregnated paper. National standards classify cable paper into three categories-DLZ-I, DLZ-II, and DLZ-III-based on thickness, specifying requirements for thickness deviation, tightness, and air permeability.
Mechanical properties-primarily tensile index, elongation, and tear index-significantly affect the manufacturing quality of cable insulation. Tensile index represents the tension a paper can withstand per unit width and unit mass, with values distinguished by machine direction (longitudinal) and cross direction (transverse). Elongation serves as an indicator of the paper's elasticity. The tear index (transverse) reflects the paper's resistance to transverse tearing during the winding process of electromagnetic wires. National standards specify requirements for longitudinal and transverse tensile indices, elongation, and tear indices for the different DLZ product categories.
Chemical properties include moisture content, ash content, pH value of aqueous extract, and electrical conductivity of aqueous extract. Moisture content significantly influences both mechanical and electrical insulation properties. Ash content represents the total inorganic impurity content within the paper fibers; reducing ash content can improve the paper's dielectric loss characteristics. The pH value of the aqueous extract indicates whether the paper exhibits an acidic or alkaline reaction; excessively high or low pH values impair the paper's thermal stability. The electrical conductivity of the aqueous extract serves as an indicator of the chemical purity of the pulp based on electrical properties. National standards specify clear limits for moisture content, ash content, and the pH and conductivity of the aqueous extract at the time of delivery.
Cable paper must possess excellent electrical insulation properties, primarily characterized by power-frequency breakdown voltage and the dielectric loss tangent. Breakdown voltage refers to the voltage level at which electrical breakdown occurs in the cable paper; it depends largely on physical properties such as density, thickness, and air permeability. The dielectric loss tangent indicates the extent to which the paper generates heat under the influence of an electric field; ash content and density are the primary factors influencing the dielectric loss of wood-fiber paper. National standards stipulate specific technical requirements regarding power-frequency breakdown voltage and the dielectric loss tangent for various categories of cable paper.
Other properties include uniformity and impregnability. Paper uniformity-such as variations in thickness or density-affects mechanical properties and the consistency of breakdown voltage. Cable paper is typically used after being treated with an impregnating agent; the quality of impregnation significantly impacts electrical insulation performance, and impregnability is closely linked to the pulping, beating, and papermaking processes.
