Why are semi-conductive layers important for MV and HV power cables?

Semi-conductive layers are important for several reasons. By forming the basis of the inner and outer shields around the cable conductor and insulation, they act as the boundary layers that contain the electric field, evenly distributing the electrical flow and ensuring smoother, more efficient power transmission. Whereas conductors create higher stress points on top of the strands, semi-conductive layers distribute the voltage stresses evenly, extending the life of cables.

 

Without these semi-conductive layers, electrical interfaces within the cable — particularly, the interfaces between the conductor and insulation, and the insulation and earthing medium — can become irregular, leading to high-voltage stress points and potential cable failure.

 

Outer semi-conductive layers also provide a grounding surface. With the proper grounding, 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.

 

How do low-voltage (LV) power cables differ from MV/HV cables?

LV power cables consist of a central current-carrying conductor surrounded by insulation and an earthing medium to carry residual fault currents. MV and HV applications require a more sophisticated construction to avoid irregularities at the interfaces between the conductor and insulation and between the insulation and earthing medium.

 

What are the material ingredients in a semi-conductive layer?

Semi-conductive layers contain an ethylene-based polymer, a conductive material like carbon black, and various processing and performance-enhancing additives. Because the inner and outer semi-conductive layers are co-extruded, the goal during the compounding process is to achieve stable conductivity and smooth, defect-free extrusion to maintain cable integrity and reliability. Fortunately, tighter controls in polymer ingredient selection, along with various production improvements, have enabled semi-conductive compounds to meet extremely tight limits for overall surface smoothness and eliminate lumps, pips or agglomerates.

 

Why is carbon black so important?

Semi-conductive compounds rely on the careful addition of conductive carbon black to the polymer matrix to impart the material with controlled volume resistivity properties. To be effective, the carbon black must have a high surface area and high molecular structure. It’s also important to maintain its structure during the production process of the semi-conductive compound. The carbon black must be highly dispersed in the polymer matrix, creating a homogeneous compound free of surface bumps, protrusions and other impurities. Any protrusion or void that forms at the interfaces between the shields and the insulation layer can initiate an electrical tree, which serves as a potential failure point.

 

What role does the extrusion setup play in the manufacturing process?

The quality of the extrusion setup plays an important role in the quality of the semi-conducting compounds, necessitating proper settings for material handling processes, screw, tooling and crosshead designs, temperature profiles and CV tube settings. To ensure all these parameters and equipment are set up correctly, our engineers work closely with your process engineers.

 

What quality management processes do you have in place?

We maintain very tight control on all processes, tracking key parameters and communicating with our customers on a regular, consistent basis.  Our engineers also travel frequently to provide hands-on technical support.

 

Are your materials qualified?

Our semi-conductive compounds have been qualification tested at a number of MV cable manufacturers. They have also successfully completed the following long-term and short-term tests:

Long-term qualification per CENELEC HD 620/605.

Accelerated Water Absorption Test:  EM-60

Qualification tests per CENELEC HD 620 and IEC 60502.

Raw material tests for physical properties and electrical properties (aged and unaged).

 

Do you formulate your compounds sustainably?

Yes! We offer compounds formulated with a reduced carbon footprint. Visit our sustainability page to learn more.