Electric Field Distribution And Interface Pressure Optimization for 35 KV Pluggable High-Voltage Cable Separable Connectors

1. Introduction

Pluggable high-voltage cable connectors are critical components in modern medium-voltage power distribution networks rated up to 35 kV. These separable connectors enable flexible network reconfiguration, facilitate maintenance operations, and accelerate fault restoration, making them indispensable for urban distribution systems and renewable energy installations. However, the electrical and mechanical reliability of such connectors presents two fundamental technical challenges: managing the electric field distribution across the connector assembly and maintaining adequate interface pressure at the silicone rubber–XLPE insulation interfaces throughout their operational lifespan. This article examines the optimization strategies for both aspects, with a focus on design methodologies and engineering practices for 35 kV pluggable systems.

2. Structural Overview and Design Challenges of 35 kV Pluggable Connectors

A typical 35 kV pluggable cable connector comprises a central conductor termination, a pre-molded stress cone made of silicone elastomer, an epoxy insulator interface, and a metallic housing. The stress cone serves a dual function: it controls the electric field gradient along the connector’s insulation boundary and provides the mechanical compression necessary to eliminate air gaps at the critical interfaces. Unlike permanently installed cable joints, pluggable connectors must tolerate repeated mating and unmating operations while maintaining stable interface performance. This operational requirement imposes stringent constraints on both the geometry of the field control elements and the mechanical design of the compression system.

From an electrical perspective, the most vulnerable regions are the interfaces between the silicone rubber stress cone and the cable XLPE insulation, as well as the interface between the stress cone and the epoxy insulator. Any microscopic void or insufficient contact pressure at these boundaries can lead to partial discharge activity, eventually progressing to insulation breakdown. From a mechanical standpoint, the silicone rubber stress cone undergoes substantial expansion during installation to generate the required interface pressure, and must retain sufficient elastic recovery to compensate for creep and stress relaxation effects over decades of operation under variable temperature conditions.

3. Electric Field Distribution Optimization

The electric field distribution inside a 35 kV pluggable connector is primarily shaped by the contour of the stress cone’s embedded semi-conductive deflector. This conductive layer redirects equipotential lines, preventing field concentration at the termination of the cable’s outer conductor screen. Without effective field control, the electric field strength at this point can exceed the dielectric withstand capability of the materials, leading to premature failure.

3.1 Geometry-Based Field Control

Two principal approaches exist for electric field control in medium-voltage cable accessories: geometric methods and parametric methods. Geometric methods modify the physical contour at regions of voltage concentration—such as stress cones, pencil-cuttings, and flared horns—to reduce local field intensity. In contrast, parametric methods apply stress-grading materials (typically semi-conductive or nonlinear resistive coatings) to the insulation surface, altering the potential distribution along the boundary.

For 35 kV pluggable connectors, the stress cone remains the preferred geometric solution. Conventional designs often employ a straight-line cone profile, which yields a non-uniform field distribution along the interface. Recent research has demonstrated that adopting a quadratic cone curve—designed using a number–shape combination method—produces substantially more uniform field distribution. Compared to straight-line stress cones, the quadratic cone design reduces the maximum internal electric field strength by up to 47.43% in cable intermediate joints. With optimized parameters—winding insulation thickness of 10.94 mm, cone length of 50.48 mm, cone curvature radius of 2.3 mm, and stress cone thickness of 3.3 mm—the maximum internal field strength is reduced to 3.32 kV/mm, and the maximum field on the cone surface to 1.67 kV/mm. This parametric design approach enables a quantifiable reduction in partial discharge risk while improving operational reliability.

3.2 Adaptive Geometric Optimization with Genetic Algorithms

Beyond purely empirical geometry tuning, contemporary design practices integrate finite element simulation with genetic algorithms to achieve the optimal stress cone contour. An adaptive geometric optimization method developed for 35 kV European cable connectors integrates electric field simulation with genetic algorithms to adjust structural parameters, mitigating electric field concentration and breakdown risks. This method incorporates two primary objectives into a custom fitness function: electric field uniformity and maximum field intensity. Dielectric constants, temperature effects, and field concentration phenomena are integrated to refine the critical breakdown field threshold. Simulation results show that, compared to conventional approaches, the optimized design reduces the maximum electric field intensity by approximately 10%, from 15.55 kV/mm to 14.60 kV/mm, while experimental validation demonstrates a 5% improvement in pass rate, meeting the requirements of GB 50168—2018. This optimization framework provides a computationally efficient pathway to achieving both electrical reliability and manufacturability.

4. Interface Pressure Optimization

The interface pressure between the silicone rubber stress cone and the XLPE cable insulation is arguably the most critical mechanical parameter affecting long-term electrical performance. Sufficient compression eliminates air gaps and ensures that tangential electric stress is borne by the solid–solid interface rather than by a low-strength air gap. However, excessive pressure can induce creep deformation in the XLPE insulation, leading to gradual pressure loss and eventual interface failure. Interface pressure optimization thus requires a balanced design that respects both the lower bound (to avoid partial discharge) and the upper bound (to prevent material creep).

4.1 Pressure Range Requirements

In dry-type GIS/transformer terminations using silicone stress cones pressurized by springs, the minimum interface pressure at the lowest operating temperature (−15 °C) must guarantee complete interface contact. Conversely, at maximum conductor temperature (90 °C, with emergency operation up to 105 °C), the pressure must not exceed 5 bar to avoid XLPE insulation deformation. Based on established experience, the minimum required pressure is defined as 1 bar at rated conditions. Maintaining this operational window requires not only appropriate spring characteristics—linear compression springs with a favorable spring constant—but also matched mechanical properties between the insulating and semiconductive materials constituting the stress cone.

4.2 Spring-Loaded Compression Systems

In modern pluggable connectors, interface pressure is typically maintained by a combination of the stress cone’s elastic expansion and a dedicated spring-loading mechanism. The compression springs provide a baseline contact pressure that compensates for manufacturing tolerances and thermal expansion mismatches. Adjustable pre-load of the compression springs extends the permissible installation temperature range (0 °C to 40 °C) and ensures optimal contact pressure at both the cable–stress cone interface and the stress cone–epoxy insulator interface regardless of operating temperature. This design flexibility is particularly valuable for field installations where environmental conditions cannot be strictly controlled.

4.3 Material Aging and Stress Relaxation

Silicone rubber, despite its excellent dielectric properties and mechanical resilience, undergoes stress relaxation and elastic modulus changes during long-term thermal aging. Finite element simulations of 220 kV cable joints considering these effects reveal that interface pressure decreases from an initial 0.156 MPa to 0.129 MPa after thermo-mechanical aging at 100 °C for 2040 h, and to 0.107 MPa after aging at 150 °C for 1200 h. The effect of stress relaxation on interface pressure is more significant than modulus changes, and pressure attenuation accelerates with increasing temperature. For 35 kV connectors operating in outdoor or industrial environments with diurnal temperature cycling, these aging characteristics must be incorporated into the initial design margins. Selecting silicone rubber formulations with lower stress relaxation rates and incorporating spring pre-load adjustments that compensate for long-term creep are effective mitigation strategies.

4.4 Hyperelastic Modeling and Simulation

Accurate prediction of interface pressure distribution requires abandoning linear structural mechanics in favor of hyperelastic material models that capture the large-deformation behavior of rubber stress cones. A sophisticated finite element method employing hyperelastic models—such as the Mooney–Rivlin or Ogden formulations—accurately computes stress distributions in rubber bodies with complex geometries under large deformations. This method enables sensitivity analysis of key parameters including interference fit, cone geometry, and material hardness. When combined with electric field simulation in a coupled multi-physics framework, the hyperelastic approach provides a comprehensive design tool that simultaneously optimizes electrical and mechanical performance.

5. Conclusion

The reliable operation of 35 kV pluggable high-voltage cable connectors depends fundamentally on the synergistic optimization of electric field distribution and interface contact pressure. In the electrical domain, adopting quadratic cone stress cone curves and implementing adaptive geometric optimization with genetic algorithms reduces maximum field intensities by up to 47% and accelerates design convergence toward optimal geometry. In the mechanical domain, maintaining interface pressures within the defined 1 bar to 5 bar range through spring-loaded compression systems, combined with hyperelastic finite element modeling, ensures both adequate dielectric sealing and protection against XLPE creep. Accounting for silicone rubber stress relaxation and thermal aging effects extends design validity over the intended service life. With continued advances in multi-physics simulation and the incorporation of field-grading materials, the next generation of pluggable connectors will achieve even higher levels of compactness, reliability, and ease of installation.