Views: 0 Author: Site Editor Publish Time: 2026-09-17 Origin: Site
In an unscreened separable connector, the cable’s outer semi-conductive screen terminates abruptly at the connector entrance, creating a region of extreme field intensification. Finite element simulations of 12 kV separable connector–bushing assemblies demonstrate that the electric field strength at the screen cutback point can exceed the dielectric withstand capability of the surrounding insulation, with the discharge probability coefficient α confirming this location as the first to initiate discharge. The situation is further aggravated by microscopic air voids at the silicone rubber–XLPE interface, where the permittivity mismatch between air (εᵣ ≈ 1) and silicone rubber (εᵣ ≈ 3) amplifies the local field by a factor of three relative to the nominal design stress.
The triple point—the convergence of the conductor, solid insulation, and ambient medium—represents the most vulnerable location for PD inception. Under AC stress, field enhancement at this point arises from the geometric discontinuity of the electrode contour and the capacitive division of voltage across the multi-dielectric system. Without effective field control, PD activity at the triple junction initiates electrical treeing within the silicone rubber, eventually leading to insulation breakdown.
Geometric methods modify the physical contour at regions of voltage concentration to reduce local field intensity. In separable connectors, the stress cone geometry—specifically the contour of the embedded semi-conductive deflector—plays a decisive role in shaping the equipotential distribution. Simulation studies have shown that embedding a metal mesh within the epoxy resin as a shield, while reducing the distance between the shielding net end and the bushing taper end, effectively homogenizes the electric field across the connector assembly. Additionally, increasing the step size of the separable connector matched with the bushing and adopting a shielding configuration in which the shielding layer is flush with the insulation layer further improves field uniformity.
Parametric field control using nonlinear resistive field grading (NLRFG) materials offers a complementary approach. These materials, typically fabricated by dispersing zinc oxide (ZnO) or silicon carbide (SiC) fillers in silicone rubber or epoxy matrices, exhibit a field-dependent conductivity that sharply increases above a critical threshold, thereby redistributing stress from high-field regions to adjacent low-field areas. Recent fabrication and simulation studies confirm that NLRFG materials effectively homogenize the electric field in cable joints and terminations, reducing peak stress at triple points by up to 40% under both steady-state and transient conditions. The nonlinearity threshold can be tuned through filler loading and particle size distribution, enabling application-specific optimization of the field grading response.
The dielectric strength of a separable interface is not solely determined by material properties; it depends critically on the interfacial pressure that eliminates air gaps and maintains intimate contact between the silicone rubber stress cone and the mating insulation surface. Experimental studies establishing the relationship between interfacial pressure and electric field strength of epoxy/silicone rubber interfaces demonstrate that increased pressure directly correlates with higher breakdown strength. Typical interface pressure values in medium-voltage connectors range from 0.3 to 0.8 MPa, with insertion forces of 500–2,500 N depending on voltage class.
A critical challenge in interface pressure optimization is the long-term retention of contact pressure. Silicone rubber undergoes compression set—permanent deformation under sustained compressive stress—which reduces interfacial pressure over time. Accelerated aging studies show that compression set increases by approximately 1% in hard silicone rubber and 3% in soft and semi-conductive silicone rubber after prolonged service. This degradation is exacerbated by thermal cycling, as the differential thermal expansion between the silicone rubber and the XLPE insulation causes cyclic interfacial stress that accelerates stress relaxation and creep.
To mitigate pressure decay, designers should select silicone rubber formulations with low compression set, optimize the interference fit between the stress cone and the cable insulation, and incorporate elastic elements that maintain pressure through the operating temperature range. The use of lubricants at the interface—while facilitating installation—must be carefully evaluated, as lubricant type and quantity can influence both PD inception voltage and long-term pressure stability.
IEC 60502-4 specifies the type test requirements for separable connectors, including partial discharge measurement at 2U₀ with a threshold of ≤10 pC, AC withstand voltage, and lightning impulse tests. For unshielded connectors specifically, the test sequence also includes mechanical operations and fault current triggering verification, with axial insertion force limits of ≤1,300 N and torque ≤14 N·m. The PD measurement is particularly critical for validating field grading effectiveness, as any residual void or insufficient interface pressure will manifest as elevated PD activity during the factory test.
The suppression of partial discharge in unshielded separable connectors requires a dual-optimization approach that addresses both electric field concentration and interface pressure management. Geometric field control through stress cone contour optimization and shield placement, combined with nonlinear resistive field grading materials, provides effective suppression of field enhancement at the triple junction. Simultaneously, interface pressure optimization—considering material compression set, thermal cycling effects, and interference fit design—ensures that the dielectric interface maintains its withstand capability throughout the connector’s service life. Designers and engineers should treat these two aspects as interdependent rather than independent optimization objectives, as the dielectric performance of the separable interface ultimately depends on the synergy between field grading effectiveness and mechanical contact stability.
