Views: 0 Author: Site Editor Publish Time: 2026-08-26 Origin: Site
In medium and high voltage cable systems, the termination point represents one of the most electrically stressed regions in the entire network. When the metallic screen and semi-conductive layer are cut back during termination preparation, the equipotential lines become severely compressed at the screen edge, creating an intense electric field concentration. Without proper stress control, this localized field enhancement leads to partial discharge, insulation degradation, and eventual dielectric breakdown.
Heat-shrinkable termination technologies address this challenge through two primary stress control approaches: geometric stress cones and capacitive stress control tubes. Understanding the matching selection between these components and their synergistic application is essential for achieving reliable, long-service-life terminations.
The electric field in a cable termination is governed by the abrupt discontinuity of the cable's conductive screen. In an intact cable, the semi-conductive screen ensures that the electric field is wholly contained within the primary insulation. At the screen cut-back point, however, the equipotential lines converge, producing a stress peak that can exceed the dielectric strength of the insulation material.
This concentration is characterized by both radial and tangential electric field components. The tangential component along the insulation interface is particularly critical, as it drives surface discharge activity. For heat-shrink terminations rated above 6kV, effective measures must be implemented to alleviate this concentration. The stress control strategy fundamentally relies on either geometric field grading (stress cones) or impedance grading (stress control tubes with high permittivity or non-linear resistivity).
A stress cone is a conductive or semi-conductive component with a profiled conical geometry positioned at the screen cut-back point. By extending the conductive surface gradually away from the cable insulation, the stress cone forces equipotential lines to separate and exit the cable insulation more gradually, effectively reducing the field gradient.
The optimal design condition for a stress cone requires that the tangential electric field gradient remains equal along the interface between the stress cone and the cable insulation. This is achieved through precise geometric profiling—typically a curved contour rather than a simple straight taper.
Stress cones for heat-shrink terminations are typically manufactured from silicone rubber or EPDM (Ethylene Propylene Diene Monomer) rubber, loaded with carbon black to achieve semi-conductive properties with volume resistivity in the range of 10³ to 10⁵ ohm·cm. Silicone rubber offers exceptional hydrophobicity, a wide operating temperature range (-50°C to 180°C), and excellent tracking resistance, making it particularly suitable for outdoor applications. EPDM provides outstanding ozone and weathering resistance.
In heat-shrinkable systems, stress cones may be pre-molded and installed as separate components, or integrated into multi-component termination kits.
The stress control tube employs a fundamentally different stress relief mechanism—capacitive grading. These tubes are manufactured from materials with a high relative permittivity (dielectric constant ≥15). When applied over the shield cut-off, they act as distributed capacitors, refracting and distributing the electric field lines evenly along the termination interface.
The stress control function depends on both the resistivity and the high relative permittivity of the material. The result is similar to that of a stress cone: the electric field is graded along the length of the layer, and the field strength at the screen edge is reduced. Some advanced stress control materials also exhibit non-linear resistivity properties, functioning similarly to surge arresters.
Typical heat-shrink stress control tubes are manufactured from cross-linked polyolefin compounds. Key electrical properties include:
· Dielectric Constant: ≥15 (IEC 60250)
· Volume Resistivity: ≥1×10¹⁰ Ω·cm (IEC 60093)
· Continuous Operating Temperature: -40°C to 100°C
· Shrink Temperature: Starts at 90°C, fully recovered at 130°C
These tubes are available in a range of sizes, with shrink ratios typically 3:1, suitable for cable terminations and joints up to 42 kV.
The selection between stress cone and stress tube—or the combination of both—is primarily dictated by the system voltage class.
For low voltage (1kV) applications, electrical stress is relatively benign, and dedicated stress control components are generally not required. The primary function is environmental sealing and physical protection.
For medium voltage (10kV to 24kV) applications, active stress management becomes mandatory. These kits must incorporate stress control tubing or high-permittivity mastic applied directly over the semi-conductive screen cut-back. The stress control tube alone is often sufficient for this voltage range, provided the dielectric constant and positioning are correctly specified.
For 35kV class terminations, the insulation thickness requirements increase substantially. Heat-shrink termination kits for 35kV generally require more complex stress relief geometry—often combining stress control tubes with additional insulation layers. The additional insulation thickness should not be less than 1.5 times the cable's factory insulation thickness.
For higher voltage applications (up to 42kV and beyond), prefabricated rubber stress cones become the dominant stress control element. The stress cone may be supplemented by stress control tubes for enhanced performance.
Proper matching requires careful alignment of component dimensions with cable parameters:
· Cable insulation diameter: The inner diameter of the stress control tube must be selected to ensure proper fit after shrinking.
· Screen cut-back length: The stress control component must fully cover the screen cut-back region.
· Stress cone positioning: The stress cone must be centered properly over the cable lug and screen edge.
Misalignment or improper sizing can result in the stress cone deviating from the intended position, rendering the stress control ineffective and potentially causing breakdown.
The materials of the stress cone and stress control tube must be compatible with the cable's insulation system. For XLPE cables, silicone rubber and EPDM stress cones provide excellent interfacial performance. The stress control tube must also be compatible with any sealing mastics or filling compounds used in the termination assembly.
The most effective approach to electric field concentration suppression combines multiple stress control techniques:
· Primary stress relief is achieved through the geometric stress cone, which provides the fundamental field grading.
· Secondary stress distribution is accomplished through the high-permittivity stress control tube, which smooths the field along the insulation interface.
· Interface management is critical—voids or gaps at the interface between the stress cone and cable insulation can create new stress concentration points.
Finite Element Method (FEM) analysis has become an indispensable tool for optimizing stress control design. FEM enables engineers to:
· Evaluate the effect of different material permittivities on field distribution
· Optimize stress cone geometry (length, curvature, and angle)
· Assess the impact of stress control tube thickness
· Identify potential defect locations (voids, delamination)
The optimal design condition is achieved when the tangential electric field gradient is equalized along the interface between the stress cone and cable insulation.
Recent developments in stress grading materials offer enhanced performance:
· Non-linear conductivity materials with threshold electric fields around 0.8 MV/m and non-linear coefficients of 7.5–11.5 have shown promise for high-voltage applications.
· ZnO microvaristor materials incorporated into stress cones provide field-dependent conductivity that automatically grades the electric field.
· Functionally graded materials (FGM) with continuously varying permittivity can homogenize the tangential electric field.
Even the best-designed stress control system will fail if improperly installed. Critical installation practices include:
· Ensuring interfaces between heat-shrink layers are clean and free from contamination
· Correctly positioning stress control components relative to the screen cut-back
· Applying heat evenly to achieve proper shrinkage without trapping air
· Using appropriate sealing mastics to fill any gaps
The matching selection of stress tubes and stress cones in heat-shrink cable terminations requires a systematic approach that considers voltage class, cable dimensions, material compatibility, and installation conditions. While stress control tubes provide effective capacitive grading for medium voltage applications up to 24kV, stress cones offer superior geometric field control for higher voltage classes. The most robust solutions integrate both approaches, supported by finite element analysis for design optimization and rigorous installation practices.
As cable systems continue to operate at higher voltages and in more demanding environments, the development of advanced stress grading materials—including non-linear conductivity compounds and functionally graded materials—will further enhance the reliability and longevity of heat-shrink terminations. Proper selection, matched with precise installation, remains the cornerstone of electric field concentration suppression in cable termination engineering.
