Cable Kraft Paper Guide: Properties & High-Voltage Applications

Discover the key dielectric properties of cable kraft paper. Learn why it remains irreplaceable in HVDC submarine cables and transformer insulation.

Overview and Applications of Cable Kraft Paper

Cable Kraft Paper (commonly referred to as “insulation kraft paper” or “High-Voltage Insulating Paper“) is a high-grade specialized technical paper used in wires, cables, and electrical equipment. It is a type of pure wood pulp insulating paper. Primarily applied via precision helical wrapping (lapping) and combined with insulating oil to form an “oil-paper insulation system,” it performs critical tasks of electrical insulation, electric field shielding, and mechanical protection under extreme high-voltage conditions.

I. Introduction to Cable Kraft Paper

1. Core Properties

High Purity (No Conductive Impurities): Made from 100% unbleached high-quality sulfate coniferous (softwood) wood pulp. The production process involves multiple deionized water washings to strictly control ash content and pH value. It must be entirely free of microscopic conductive impurities, such as iron filings, to prevent high-voltage dielectric breakdown.

High Mechanical Strength (High Tensile Strength): It possesses excellent longitudinal tensile strength and folding endurance, allowing it to withstand the immense mechanical stress and pulling force exerted by high-speed cable wrapping machines.

Excellent Oil Absorption & Homogeneity: The paper features highly uniform density (tightness). Its fiber pores are designed at a sub-micron/nano level to quickly and fully absorb insulating oil after vacuum drying, forming a dense “oil-paper composite insulation.”

Low Dielectric Loss (Low tanδ): Under high-voltage AC or DC electric fields, it exhibits minimal electrical energy loss and heat generation, which significantly extends the service life of the cable.

2. Key Physical Parameters (Typical Ranges)

Standard : Q/JLLS 20471-2020,GB 7969-2003,IEC 60554-3-1
Thickness: 50μm, 70μm,  80μm, 130μm, 180μm, 200μm
Width: 625mm/980mm/1920mm (Ribbon is available)
Roll diameter: 650~1000mm
Weight of each roll: 220-350kg

II. Primary Applications

Cable Kraft Paper plays an irreplaceable role in power grids, telecommunications, and high-voltage substation equipment. Its core application scenarios include:

1. Extra-High Voltage (EHV) & Ultra-High Voltage (UHV) Power Cables (Main Insulation)

Oil-Filled (OF) Cables: In high-voltage AC transmission cables ranging from 110kV to 500kV, cable kraft paper serves as the primary insulation medium. It works in tandem with low-viscosity insulating oil circulated through the hollow conductor for heat dissipation and insulation.
High Voltage Direct Current (HVDC) Submarine Cables: Under DC electric fields, oil-paper insulation does not experience space charge accumulation (a major vulnerability of plastic XLPE insulation that leads to breakdown). Therefore, cable kraft paper remains the absolute, irreplaceable core material for long-distance, large-capacity cross-sea DC power transmissions.

2. Medium & Low Voltage Oil-Impregnated Paper-Insulated Cables

Phase Insulation & Belt Insulation: In traditional three-core power cables of 35kV and below, the paper is wrapped around each individual phase conductor (Phase Insulation) and then wrapped in multiple layers over the stranded cores (Belt Insulation) to provide insulation against the ground.

3. Telecommunication & Signal Cables (Low-Voltage Insulation)Telephone Paper: Extremely thin (0.03 mm–0.05 mm), it is used for core insulation in traditional urban telephone cables. It features high electrical resistivity and a low dielectric constant, effectively preventing signal crosstalk and reducing attenuation.

4. Electrical Equipment (Transformers & Instrument Transformers)
Paper-Covered Wire: Cable kraft paper is precisely wrapped around the surface of rectangular or round copper/aluminum wires to create “paper-covered wires,” which serve as inter-turn (turn-to-turn) insulation for high-power transformer and reactor coils.
Capacitor Bushings: In high-voltage transformer outlet bushings, kraft paper and aluminum foils are wrapped alternately to create a capacitive voltage-dividing structure, which safely controls the internal electric field gradient.