Advances in material science have significantly improved the reliability and performance of power cables since their invention over 150 years ago. This is perhaps most evident in medium-voltage (MV) and high-voltage (HV) power cable applications.
Unlike low-voltage cables — which consist of a central current-carrying conductor surrounded by insulation and an earthing medium to carry residual fault currents — MV and HV cables require a more sophisticated construction to avoid electrical irregularities at the interfaces between the conductor and the insulation and between the insulation and the earthing medium. This is where the semi-conductive layers come in.
Semi-Conductive Shielding and Stress Control
Semi-conductive layers form the basis of the inner and outer shields around the conductor and the insulation, serving as the boundary that contains the electric field and ensuring smoother and more efficient power transmission. Without them, the electrical interfaces can become irregular, leading to high-voltage stress points and premature cable failure.
In addition, with the proper grounding, the semi-conductive layers evenly distribute any radial stresses within the insulation. They also eliminate longitudinal stresses by bringing the entire outer surface of the insulation to the ground potential.
The Role of Polymer Selection and Compounding Technology
The inner and outer semi-conductive layers are co-extruded to create smooth, contaminant-free interfaces. This co-extrusion process is critical for maintaining the integrity and reliability of the cables. Semi-conductive materials combine an ethylene-based polymer, a conductive material such as carbon black, and various processing and performance-enhancing additives. The goal is to achieve stable conductivity and smooth, defect-free extrusion. Tighter controls in polymer ingredient selection, combined with improvements in production methods, have enabled semi-conductive compounds to meet extremely tight limits regarding overall surface smoothness and the absence of lumps, pips or agglomerates.
Carbon black is what imparts the semi-conductive material with controlled volume resistivity properties. To be effective, however, the carbon black must have a high surface area and high molecular structure. It’s important to maintain the structure of the carbon black during the production process of the semi-conductive compounds. The carbon black must be thoroughly dispersed in the polymer matrix, creating a homogeneous compound free of bumps, protrusions and other impurities.
The polymer should also not degrade during the compounding process, requiring kneading with low shear to reduce damage to the material’s physical properties, maintain precise temperature control and ensure high dispersion of the carbon black. In addition, the peroxide soaking system must avoid generating fines, ensure good dispersion of peroxide in the compound to avoid processing issues in cables, and maintain consistency in peroxide in each granule.
Meeting the Demanding Requirements of Modern CV Lines
To help cable producers maximize production, compound manufacturers design their products to cross-link at the speed of the production line, withstand process temperatures without scorching and avoid degradation in long runs. Modern equipment can even predict such processing behavior during compounding to maintain competitive performance.
For example, we optimized ESCONTEK® compounds for processing at high line speeds. These products contain high-quality raw materials that we meticulously mix with carbon black to create a clean, smooth surface with minimal residue and ionic impurities. The resulting field-proven formulations have high scorch resistance, allowing long run times without degradation, and cross-link quickly in continuous vulcanization (CV) tubes to meet the demanding requirements of modern CV production lines.
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