What is the quantitative impact of the cross-linking degree of xlpe cable material on its electrical properties?
Release Time : 2025-09-25
As a core insulation material for power transmission, the degree of crosslinking in XLPE cable material influences its electrical performance throughout its entire lifecycle, from material design and production to actual operation. Crosslinking essentially refers to the tightness of the three-dimensional network formed by chemical bonds or physical interactions between polyethylene molecular chains. This microstructural difference directly determines the macroscopic electrical performance. From a materials science perspective, increasing the degree of crosslinking significantly alters the crystal morphology and molecular chain mobility of XLPE cable material, thereby affecting key electrical parameters such as dielectric response, breakdown strength, and space charge distribution.
In terms of dielectric properties, increasing the degree of crosslinking generally causes the dielectric loss tangent of XLPE cable material to initially decrease and then increase. When crosslinking is moderate, the crosslinks between molecular chains suppress dipole rotation polarization, reducing dielectric loss. However, when crosslinking is excessive, the rigidity of the crosslinks can induce localized stress concentrations, which in turn increases polarization losses. This nonlinear relationship is particularly pronounced in high-frequency or high-temperature environments. For example, in DC transmission scenarios, the conductivity of highly cross-linked materials may decrease due to the cross-linked network's hindrance to carrier migration. However, excessive cross-linking can alter the distribution of trap energy levels, leading to space charge accumulation and ultimately offsetting the benefits of reduced conductivity.
Breakdown strength, a key metric for measuring the electrical performance of XLPE cable materials, exhibits a typical "bimodal" relationship with cross-linking. At low cross-linking levels, the intermolecular forces are weak, making the material susceptible to thermal breakdown under electric field conditions. As the cross-linking level increases, the three-dimensional network structure becomes more refined, effectively dissipating electric field stress and inhibiting the initiation and growth of electrical dendrites, thereby improving breakdown strength. However, when the cross-linking level exceeds a critical value, defects such as micropores and impurities can form within the material due to residual cross-linking agent or localized overheating. These defects act as electric field concentration points, ultimately reducing breakdown strength. Research has shown that the optimal cross-linking level for XLPE cable materials suitable for high-voltage cables is typically between 70% and 85%, achieving both high breakdown strength and good mechanical properties.
Space charge distribution is a key factor affecting the long-term stability of XLPE cable materials. Changes in crosslinking degree significantly alter the density distribution of deep and shallow traps within the material. Moderate crosslinking increases the number of shallow traps, promoting rapid dissipation of space charge and preventing local electric field distortion. However, in highly crosslinked materials, the proportion of deep traps increases, making space charge more susceptible to capture and long-term accumulation. This can cause localized electric field strengths exceeding the material's intrinsic breakdown field, ultimately leading to insulation failure. This characteristic is particularly pronounced in DC cables, where the mobility of space charge under DC fields is much lower than under AC fields, making it more sensitive to crosslinking degree.
The matching of crosslinking process parameters with crosslinking degree directly impacts the electrical performance of XLPE cable materials. For example, in peroxide crosslinking systems, coordinated control of crosslinker concentration, curing tube temperature, and crosslinking time allows for precise adjustment of crosslinking distribution. In silane crosslinking processes, the temperature and humidity conditions of hydrolysis crosslinking determine the uniformity of the crosslinked network. If process parameters are mismatched with the material formulation, even if the theoretical crosslinking degree meets the specified standard, the actual material may still exhibit localized electrical performance defects due to uneven crosslinking distribution.
The crosslinking degree of XLPE cable material has a multi-dimensional, nonlinear impact on its electrical performance. By optimizing the crosslinking degree, key parameters such as dielectric loss, breakdown strength, and space charge distribution can be synergistically improved, but this requires balancing material cost and process feasibility. In the future, with the advancement of nanocomposite technology and intelligent crosslinking processes, crosslinking degree control in XLPE cable material will evolve towards more precise and dynamic control, providing more reliable insulation solutions for extreme scenarios such as high-voltage direct current transmission and deep-sea cables.