Electric Field And Stress Control In Cold-shrink Cable Accessories

Cold Shrink Cable Accessories Electrical stress control refers to the control of the electric field distribution and electric field strength.
This involves using appropriate measures to ensure that the electric field distribution and electric field lines and equipotential lines are in the optimal state, thereby improving the overall reliability and service life.  Due to the distortion of the electric field at the cable shield break and the end insulation break, the uneven and disordered electric field exists simultaneously in both axial and radial distributions, with 50% to 60% distributed at the radial shield break. To eliminate the influence of the uneven electric field, electrical stress control is necessary. Without electrical stress control, the lifespan of the terminal depends on the electrical stress at the end of the shielding layer and the discharge resistance of the main dielectric, and its lifespan generally does not exceed one year. To improve the electrical stress distribution at the cut end of the cable insulation shielding layer, the following methods are generally used: (1) Geometric shape method – using a stress cone to alleviate electric field stress concentration (i.e., increasing the radius of curvature); (2) Comprehensive control method – using a capacitive cone to alleviate electric field stress concentration; (3) Parameter control method – ① using high dielectric constant materials and resistive materials to alleviate electric field stress concentration; ② using nonlinear resistive materials to alleviate electric field stress concentration. The stress control of cold-shrinkable cable accessories generally uses parametric or geometric shape methods, but the parametric method requires adding a large amount of high-dielectric material to the silicone rubber, which reduces mechanical strength and makes it prone to cracking or delamination during expansion, resulting in unstable quality. Currently, the geometric shape method is more commonly used.
The conductive mechanism of liquid semiconducting silicone rubber is different from that of conductors and insulators. It is produced by electrons hopping from one carbon black aggregate to another through the polymer. The overall structure of cross-linked cables includes a conductor shield (inner semiconducting layer) and an insulation shield (outer semiconducting layer). Therefore, cold-shrinkable cable accessories require semiconducting silicone rubber.  Based on the theoretical basis of the resistivity limits of the semiconducting shield, the voltage distributed across the semiconducting shield layer is one-thousandth of the insulation layer voltage, which is quite safe and will not cause insulation breakdown. Therefore, to ensure the safe operation of the cable, the resistivity of the semiconducting layer is generally required to be p ≤ 10 Ω·cm [7]. However, the semiconducting shield layer of cold-shrinkable cable accessories undergoes a limited contraction of 20%–30% after expansion on the cable body. Due to this 20%–30% expansion (stretching), the distance between the carbon particles in the semiconducting shield increases, inevitably leading to an increase in the volume resistivity of the semiconducting shield. Therefore, a resistivity of pv ≤ 10 Ω·cm must be selected to meet the standard requirements.