Publish Time: 2026-05-21 Origin: Site
Pre‑fabricated cable terminations have gained widespread acceptance in high‑voltage power transmission due to their installation simplicity. However, field failures of silicone rubber pre‑fabricated cable accessories have been increasingly reported. Investigations reveal that the interface between the stress cone and cable insulation constitutes the most critical region, where inadequate interface pressure or improper installation can precipitate breakdown. Field statistics indicate that approximately 70% of early‑stage cable faults originate from accessory interfaces.
The electrical performance of this interface fundamentally depends on two interrelated parameters: interface pressure and dielectric breakdown strength. As interface pressure increases, the real contact area between insulation surfaces expands, micro‑voids diminish, and interfacial electrical strength consequently improves. When pressure falls below design requirements, breakdown voltage substantially decreases, representing a primary cause of termination failure.
The correlation between interface pressure and dielectric breakdown strength has been established through systematic laboratory investigations. Studies on XLPE/silicone rubber interfaces demonstrate that as interface pressure increases from 0.1 MPa to 0.5 MPa, the ramped DC breakdown voltage rises from 22.34 kV to 46.84 kV. This monotonic relationship is attributed to the morphology of the contacting surfaces: under higher compressive stress, contact asperity density increases, creating more obstacles that impede breakdown propagation, while micro‑voids—which serve as the primary pathways for interfacial breakdown—are substantially reduced.
For AC applications, a consistent pattern emerges: breakdown voltage decreases with diminishing interface pressure. Consequently, adequate interfacial pressure is essential to prolong the service life of cable accessories.
Installation defects in cable terminations can be categorized into three primary types based on their physical manifestation and impact on interface performance.
During on‑site assembly, the stress cone may be positioned either short of or beyond its optimal location relative to the insulation shielding layer cut‑off. Finite element simulations employing electro‑thermal‑mechanical multi‑physics coupling have revealed distinct failure patterns for each misalignment type.
Under insufficient installation (axial displacement negative), an electric field elevation zone emerges between the shielding layer cut‑off and the stress cone root. The highest interface temperature occurs at a displacement of ‑7.5 mm, while maximum interface pressure appears at ‑2.5 mm. Under over‑installation (positive displacement), electric field distortion develops at the shielding layer cut‑off, with the magnitude of field inhomogeneity increasing as displacement grows. Peak interface pressure occurs at +5.0 mm, accompanied by significant pressure mutation at the cut‑off location.
The interface pressure between cable body and termination is governed by interference fit, material properties, and geometric parameters. The maximum interface pressure is situated at the maximum radius region of the stress cone. Interface pressure scales proportionally with spring compression before reaching 90 mm, after which further compression to 85 mm yields minimal variation. Notably, interface pressure is strongly influenced by the stress cone’s elastic modulus, whereas the elastic modulus of XLPE insulation exerts comparatively minor effect.
The nonlinear deformation of rubber stress cones during interference fit installation can adversely affect interface pressure distribution. Analysis incorporating the Yeoh hyperelastic material model has demonstrated that interface pressure typically exhibits a "high‑at‑ends, low‑in‑middle" distribution pattern.
Surface defects—including scratches from insulation peeling, inadequate polishing, and residual contamination—can produce micro‑air‑gaps at the XLPE/stress cone interface. Field investigation of a 110 kV termination failure revealed that inadequate polishing of the insulation surface created air gaps, which sustained partial discharges under operating voltage, accelerating insulation aging and eventually culminating in dielectric breakdown. Similarly, a 220 kV case demonstrated that poor surface conformity between stress cone and cable, combined with minute interfacial voids, permitted long‑term internal discharge activity, ultimately causing main insulation failure.
Silicone grease is commonly applied during termination assembly to reduce friction and facilitate stress cone insertion. However, grease application introduces both beneficial and detrimental effects on interface electrical performance.
Under both AC and impulse voltage stress, breakdown voltage increases with rising interface pressure regardless of whether conventional silicone grease or fluorinated silicone grease is applied. Higher interface smoothness correlates with increased breakdown voltage values, while the presence of scratches substantially reduces breakdown strength—though grease application can significantly mitigate this reduction.
Conversely, silicone grease diffusion into silicone rubber can degrade material properties. When subjected to moisture ingress, grease‑treated samples exhibit markedly greater reduction in breakdown voltage compared to fluorinated grease‑treated counterparts. Additionally, insufficient interference combined with semi‑conductive layer decarburization has been implicated in interface tracking failures, with interference as small as 1 mm leading to premature breakdown.
Modern finite element methods enable quantitative prediction of interface pressure distribution and electric field behavior in terminated configurations.
The electric field surrounding cable terminations depends critically on stress cone positioning relative to the semiconducting layer termination. Finite element computations of 110 kV XLPE terminations reveal that displacement of the stress cone from its designated position produces significant field intensity redistribution . Interface pressure has been shown to correlate with operating voltage magnitude, with both pressure and fluctuation amplitude increasing under elevated voltage conditions.
Concurrently, high‑temperature service conditions alter interface pressure through three competing mechanisms: thermal expansion reduces pressure, the Gough–Joule effect of rubber materials increases pressure with rising temperature, and stress relaxation accelerates pressure decline. Comprehensive analysis indicates that pressure increase dominates under rising temperature conditions, confirming that the rubber Gough–Joule effect governs interface pressure variation.
Field experience consistently demonstrates that installation quality directly governs termination reliability. Stress cone misalignment has been directly implicated in numerous field failures: in one 110 kV integral prefabricated dry‑type terminal incident, inadequate mechanical fixation permitted terminal oscillation and bending, leading to stress cone displacement from its intended position and subsequent excessive local field strength at the insulation shielding cut‑off, culminating in breakdown . Similarly, a 110 kV GIS cable termination failure was attributed to insufficient interface pressure between the epoxy sleeve inner surface and stress cone, producing discharge propagation from cable conductor to grounding flange and inducing complete termination failure .
To mitigate these risks, several measures are recommended:
· Pre‑installation verification: Confirm stress cone positioning and interference parameters against design specifications before final assembly
· Surface preparation: Ensure uniform polishing of XLPE insulation surface to minimize air gaps and scratch defects
· Grease selection: Employ appropriate grease formulations that balance lubrication requirements with long‑term dielectric stability, particularly when moisture exposure is anticipated
· Quality assurance testing: Conduct withstand voltage and partial discharge tests on assembled terminations to verify adequate interface pressure, as inadequately pressured assemblies consistently fail such tests
Installation defects in high‑voltage cable terminations—specifically stress cone misalignment, inadequate interference, and surface imperfections—exert profound influence on both interface pressure distribution and dielectric breakdown strength. The fundamental relationship is clear: increased interface pressure produces greater real contact area, reduced micro‑void density, and consequently higher breakdown voltage. Conversely, installation deficiencies that diminish interface pressure or introduce localized field distortions precipitate premature failure, often within the first years of service.
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