29
May
The Insulation System of Submarine Power Cables
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If we would extrude the XLPE insulation layer right away onto the conductor, the grooves, ridges and irregularities of the conductor would generate local stress enhancement and would reduce the dielectric strength of the insulation considerably.
To avoid this, a layer of semi-conductive XLPE is extruded onto the conductor resulting in an extremely smooth dielectric surface towards the insulating XLPE. As the semi-conductive inner layer is completely circular and has a smooth surface, there will be no stress enhancements.
The surface of the inner conductive layer can be made visible using hot-oil bath methods. XLPE becomes transparent at 130◦C and reveals in detail the surface structure of the inner semi-conductive layer.
The triple-extrusion-method also provides a semi-conductive layer outside the insulation layer in order to form a stable dielectric surface not being affected by the outer screen layers. The three layers “conductor screen – insulation wall – insulation screen” make up the cable dielectric system. For high-quality insulation the three layers should always be manufactured simultaneously in a triple-extrusion system.
Semi-conductive XLPE-layers are made of PE-based co-polymers blended up with 40% carbon-black. According to international relevant standards, the volume resistivity shall be lower than 250 Ωm (CENELEC) or lower than 500 Ωm (AEIC, IEC). If the resistivity of the screen materials is too high, impulse voltages in the cable system can induce large stresses in the semi-conductive materials, eventually generating an insulation breakdown. Most modern carbon-black formulations meet the requirements with great margins. The resistivity of carbon-black filled XLPE in practical life is strongly dependent on temperature and mechanical stress, as the contact bridges between the carbon-black particles can close or open under the influence of strain in the polymeric matrix .
There are different varieties of carbon-black, such as furnace-black and acetylene-black with different particle size distribution and purity. Extruding parameters are strongly affected by the carbon-black content and character. Peelable screen systems with reduced adhesion between the insulation wall and the outer XLPE semi-conductive screen are sometimes used for medium-voltage cables. During the preparation of joints or terminations, the outer screen can be peeled off like a banana skin saving working hours.
The semi-conductive layers are applied with 1–2 mm nominal thickness. For long cables, the cost for a thick semi-conductive layer can add up to a considerable sum, as the raw material is fairly expensive.
To avoid this, a layer of semi-conductive XLPE is extruded onto the conductor resulting in an extremely smooth dielectric surface towards the insulating XLPE. As the semi-conductive inner layer is completely circular and has a smooth surface, there will be no stress enhancements.
The surface of the inner conductive layer can be made visible using hot-oil bath methods. XLPE becomes transparent at 130◦C and reveals in detail the surface structure of the inner semi-conductive layer.
The triple-extrusion-method also provides a semi-conductive layer outside the insulation layer in order to form a stable dielectric surface not being affected by the outer screen layers. The three layers “conductor screen – insulation wall – insulation screen” make up the cable dielectric system. For high-quality insulation the three layers should always be manufactured simultaneously in a triple-extrusion system.
Semi-conductive XLPE-layers are made of PE-based co-polymers blended up with 40% carbon-black. According to international relevant standards, the volume resistivity shall be lower than 250 Ωm (CENELEC) or lower than 500 Ωm (AEIC, IEC). If the resistivity of the screen materials is too high, impulse voltages in the cable system can induce large stresses in the semi-conductive materials, eventually generating an insulation breakdown. Most modern carbon-black formulations meet the requirements with great margins. The resistivity of carbon-black filled XLPE in practical life is strongly dependent on temperature and mechanical stress, as the contact bridges between the carbon-black particles can close or open under the influence of strain in the polymeric matrix .
There are different varieties of carbon-black, such as furnace-black and acetylene-black with different particle size distribution and purity. Extruding parameters are strongly affected by the carbon-black content and character. Peelable screen systems with reduced adhesion between the insulation wall and the outer XLPE semi-conductive screen are sometimes used for medium-voltage cables. During the preparation of joints or terminations, the outer screen can be peeled off like a banana skin saving working hours.
The semi-conductive layers are applied with 1–2 mm nominal thickness. For long cables, the cost for a thick semi-conductive layer can add up to a considerable sum, as the raw material is fairly expensive.
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