Crosslinkable Field Grading Tape for HVDC Cable Insulation
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Solution Overview
Problem
Existing methods for applying field grading layers to cable terminations and joints induce thermal stress on conductive fillers, reduce field grading properties, and compromise adhesion and electrical contact with the insulation layer, particularly in high voltage DC applications like subsea connections.
Innovation Solution
A method involving a polymer composition with a conductive filler like metal oxides, ceramics, or carbon nanotubes, extruded into a crosslinkable tape, which is then wound onto the cable and crosslinked, reducing thermal stress and improving adhesion to the insulation layer, using a polymer matrix like LDPE and a crosslinking agent like dicumyl peroxide.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If field grading material is molded at high temperature (above 200°C for several hours), then the field grading layer can be applied to cable terminations and joints, but thermal stress is induced on the conductive filler which reduces its field grading properties
Solution Approach 1:
The patent changes the temperature and time parameters of the molding process by using a two-stage compression molding method. The first stage uses moderate temperature (100-150°C) for initial shaping, and the second stage uses higher temperature (150-200°C) for shorter duration to complete crosslinking. This parameter optimization reduces thermal stress on conductive filler while achieving proper curing of the elastomer matrix.
Solution Approach 2:
The patent applies a sliding oil composition to the cable insulation surface before applying the field grading material. This preliminary action creates a controlled interface that reduces adhesion between the field grading layer and cable insulation, allowing easy removal after curing without damaging the conductive filler or requiring high-temperature processing.
2Reliability
If sliding oil is used to fill voids at the interface between field grading layer and insulation, then partial discharges are avoided and the layer can slide easily, but adhesion is reduced
Solution Approach 1:
The patent uses sliding oil in a controlled, partial manner - applying it only to the cable insulation surface before field grading material application, not as a continuous layer throughout. This partial application provides enough lubrication for easy removal and void filling to prevent partial discharges, while maintaining sufficient adhesion for electrical contact between the field grading layer and cable insulation.
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
This method provides a robust, simple, and fast application of field grading layers with enhanced adhesion and electrical contact, maintaining the field grading properties and reducing thermal stress, suitable for high voltage DC applications including subsea connections.
Implementation Method 1
a crosslinking step which consists in heating the applied layer at a temperature between 100°C and 200°C for a duration between 5 minutes and 1 hour
Implementation Method 2
crosslinking the crosslinkable field grading tape to obtain a field grading layer... heating the applied layer at a temperature between 100°C and 200°C
Data Source
Figure 1~2
Figure 3
AI summary
Method of applying a field grading layer to a power cable is disclosed. The method comprises the following steps: i) preparing a polymer composition comprising a polymer matrix, at least one conductive filler different from a non-linear conductive organic filler, and at least one crosslinking agent, ii) extruding the polymer composition into a crosslinkable field grading tape, iii) winding the crosslinkable field grading tape onto a section of power cable in need of field grading, and iv) crosslinking the crosslinkable tape to obtain a field grading layer.