In high-voltage power cables, do the dielectric properties of swelling water-blocking yarn affect the uniformity of the electric field distribution in the insulation layer?
Publish Time: 2025-09-16
In the complex structure of high-voltage power cables, each material layer performs specific electrical and mechanical functions. Any slight difference in dielectric properties can cause electric field distortion, thereby affecting the long-term reliability of the insulation system. Swelling water-blocking yarn, as a functional filler material, is typically placed between cable cores, beneath the metal sheath, or around the insulation shield. Its primary function is to rapidly swell upon water intrusion, forming a dense gel that blocks the propagation path of water trees and protects the primary insulation layer. However, when placed in a high electric field environment, whether its dielectric properties will interfere with the surrounding electric field distribution becomes a key issue that designers must carefully assess.Uniform electric field distribution is crucial for the safe operation of high-voltage cables. In an ideal coaxial structure, the electric field strength decreases hyperbolically from the conductor surface outward, and the electrical stress experienced at each point in the insulation layer is controllable and predictable. Introducing materials with significantly different dielectric constants into the insulation system can disrupt this symmetry, leading to localized electric field enhancement or the formation of electric field concentration points, inducing local discharge and accelerating insulation aging. Therefore, the dielectric behavior of water-blocking yarns must be tailored to the electric field environment in which they are located.Swelling water-blocking yarns are composed of a polyester filament base coated with a highly absorbent resin. In their dry state, their overall dielectric constant is primarily determined by the polyester fibers and the resin coating. Polyester inherently has relatively stable dielectric properties, similar to those of polyethylene or cross-linked polyethylene insulation materials, and does not cause significant electric field perturbations. When unhydrated, the resin coating forms a solid, uniformly distributed film, with limited impact on the overall dielectric properties. In this case, the water-blocking yarn, as a cable core filler, primarily resides in low-electric-field regions, where the dielectric difference is insufficient to cause significant distortion.However, the real challenge lies in its "activated" state—the gel formed after swelling upon contact with water. The gel is rich in bound water, and water has a dielectric constant much higher than that of polymer insulation. If the gel forms a continuous conductive path or a localized high-dielectric region under the influence of an electric field, it could alter the local capacitance distribution and lead to electric field redistribution. However, in practical designs, this risk is mitigated by multiple mechanisms. First, water-blocking yarns typically do not directly contact high-electric-field areas, such as the conductor shield or the insulation itself. Instead, they are placed in the cable core gap or inside the outer sheath, where the electric field strength is lower. Second, the expanding gel is not free water but rather a bounded state firmly locked within a polymer network. It lacks ion mobility and does not form a conductive path. Its dielectric response is more similar to that of a polar polymer than liquid water.Furthermore, the water-blocking yarns are distributed discretely within the cable, often in the form of filaments or bundles. Even if they expand locally, they are unlikely to form large-scale dielectric abrupt interfaces. Gel formation occurs through radial expansion, filling voids and ultimately helping to compact the cable core structure, reducing micropores and air gaps, and indirectly improving electric field uniformity. Modern cable designs also optimize the placement and dosage of water-blocking yarns to ensure they perform their water-blocking function without interfering with the electric field gradient of the primary insulation.At the material level, the resin formula is specifically tuned to control its polarity and dielectric loss to avoid excessive heat generation under alternating electric fields. Furthermore, the coating forms a tight interface with the polyester substrate, preventing rapid water penetration along the fiber axis and ensuring a controlled and localized expansion process.Ultimately, the design philosophy of the swelling water-blocking yarn is "invisible protection"—it remains electrically inert under normal conditions and does not participate in the electric field. Under abnormal conditions, it activates to physically isolate water without interfering with the system through electrical properties. With appropriate layout and material matching, its presence does not disrupt electric field uniformity; instead, by eliminating the threat of moisture, it fundamentally maintains the integrity of the insulation system. Throughout the long service life of a high-voltage cable, it acts as a silent guardian, resisting corrosion in the darkness without disrupting the path of light.
