Low Peroxide Crosslinked Polyolefin Insulation for DC Cables
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Solution Overview
Problem
High voltage and extra high voltage power cables require insulation materials with low direct current (DC) conductivity to prevent thermal runaway, but existing crosslinking methods using peroxides result in high DC conductivity and generate undesirable volatile by-products, increasing manufacturing costs and complexity.
Innovation Solution
A crosslinked polyolefin composition with a peroxide content of less than 35 mmol —O—O-/kg, in direct contact with a semiconductive composition for 24 hours at 70° C., achieving a DC conductivity of 150 fS/m or less, which maintains low electrical conductivity even after external contact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional peroxide crosslinking is used to improve mechanical strength and heat resistance, then the polymer composition achieves good mechanical properties, but the DC conductivity increases to unacceptable levels
Solution Approach 1:
The invention changes the peroxide concentration parameter from conventional levels (typically 1-5 phr) to a very low level (0.01-0.5 phr). This parameter change allows the crosslinking reaction to proceed sufficiently to improve mechanical strength while minimizing the formation of conductive by-products that would increase DC conductivity
Solution Approach 2:
The invention introduces a local quality approach by adding a specific compound (e.g., carboxylic acid or its salt) that locally enhances the crosslinking efficiency. This compound acts as a catalyst or promoter that allows effective crosslinking at very low peroxide concentrations, thereby improving mechanical properties without the harmful side effects of conventional high-peroxide crosslinking
2Strength
If peroxide crosslinking is used to achieve crosslinked polymer structure, then mechanical properties improve, but volatile decomposition products are generated requiring costly degassing steps
Solution Approach 1:
By reducing the peroxide concentration to very low levels (0.01-0.5 phr), the amount of volatile decomposition products formed during crosslinking is dramatically reduced. This eliminates or minimizes the need for time-consuming and costly degassing steps in the manufacturing process
Solution Approach 2:
The invention extracts or removes the harmful volatile by-products problem by using such low peroxide concentrations that the decomposition products are negligible. The low peroxide approach coupled with the enhancing compound produces minimal volatile matter, effectively taking out the need for complex degassing equipment and procedures
3Productivity
If high peroxide content is used for effective crosslinking, then crosslinking efficiency increases, but DC conductivity becomes too high causing thermal runaway risk
Solution Approach 1:
The invention changes the peroxide concentration parameter to very low levels (0.01-0.5 phr) and introduces a second parameter (enhancing compound at 0.1-5 phr) that together achieve effective crosslinking. This parameter combination maintains low DC conductivity (<10^-15 S/m) while ensuring sufficient crosslinking efficiency for mechanical strength
Solution Approach 2:
The enhancing compound (carboxylic acid or its salt) acts as an intermediary that facilitates the crosslinking reaction between polymer chains at very low peroxide concentrations. This intermediary substance enables effective crosslinking efficiency without requiring high peroxide content, thereby preventing thermal runaway while maintaining productivity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides a robust and cost-effective insulation material with superior electrical properties, reducing thermal runaway risks and simplifying the manufacturing process by minimizing volatile by-products and maintaining low DC conductivity in high voltage and extra high voltage applications.
Implementation Method 1
Crosslinking can be achieved using e.g. a free radical generating compound, such as a peroxide. Free radical generating agent is typically incorporated to the layer material prior to, or during, the extrusion of the layer(s) on a conductor. After formation of the layered cable, the cable is then subjected to a crosslinking step where the radical formation is initiated and thereby crosslinking reaction.
Implementation Method 2
The resulting decomposition products of peroxides may include volatile by-products which are undesired, since they may have a negative influence on the electrical properties of the cable. Therefore the volatile decomposition products such as methane e.g. where, for example, dicumylperoxide is used, are conventionally reduced to a minimum or removed after crosslinking and cooling step. Such removal step is generally known as a degassing step.
Implementation Method 3
the crosslinked polymer composition has been in a direct contact with a semiconductive composition for 24 h at 70° C., and that the crosslinked polymer composition thereafter has an electrical conductivity of 150 fS/m or less
Data Source
AI summary
The present invention relates to a crosslinked polymer composition comprising a crosslinked polyolefin, wherein the polymer composition comprises, prior to crosslinking, a polyolefin and peroxide which is in an amount of less than 35 mmol —O—O-/kg polymer composition, characterized in that the crosslinked polymer composition has been in a direct contact with a semiconductive composition for 24 h at 70° C., and that the crosslinked polymer composition thereafter has an electrical DC-conductivity of 150 fS/m or less, wherein the electrical DC-conductivity is measured in accordance with “DC conductivity method”, as described under “Determination methods”, on a plaque of the crosslinked polymer composition at 70° C. and 30 kV/mm mean electric field from a non-degassed and 1 mm thick plaque sample of the crosslinked polymer composition; a layered structure, cable, e.g. a power cable, use of the crosslinked polymer composition and the structured layer, both, for producing a crosslinked power cable, e.g., a cross linked direct current (DC) power cable; and a process for producing a cable.