Cable Design
Emission Shield
An emission shield is positioned between a semiconducting conductor shield and the insulation that surrounds the conductor shield. The emission shield acts as a barrier that limits or prevents the injection of electrons from the conductor region into the insulation, thereby reducing the likelihood of insulation damage and extending the operational life of the cable.
Preferably, in cables designed for higher voltage applications, an additional emission shield is also provided between the outer surface of the insulation and a semiconducting insulation shield. This second shield further protects the insulation by controlling electric field stresses at the outer boundary of the insulation layer. By stabilising the electrical environment on both the inner and outer sides of the insulation, the cable can operate more reliably under high voltage conditions.
In manufacturing such a cable, the conductor is first provided with a semiconducting conductor shield. The emission shield layer is then applied over this shield, followed by the extrusion of the primary insulation layer before the semiconducting insulation shield is added. These layers are typically applied through controlled multi-layer-extrusion processes to ensure uniform thickness, proper bonding between layers, and consistent electrical performance throughout the length of the cable.
Through this layered construction and shielding arrangement, the cable achieves improved resistance to electrical stress, reduced risk of insulation degradation, and a longer service life in high-voltage power transmission applications.
WTR testing
Even if water-treeing in modern cable is less a concern than during the 70ies. The bad experience during that time should not be repeated. Therefore in certain countries strict requirements are imposed doing a so-called “long term wet ageing test“.
The VDE requirements is the most famous.
Eifelkabel has assisted in several tests passing TB 722 for 66 kV cables and passing the requirements of VDE 0276-620.
He was member of Cigre B1-55, ex-chair of ICC A06D and A13W.
He is also secretary of Cigre B1-92 (lead free cables)
Corrosion
Corrosion is a major concern for sub-sea cable reliability. These cables are normally watertight, however, CIGRE recently recommended to test wet sub-sea array cables for offshore wind-parks. Furthermore, it should be noted that several kilometres of wet medium voltage submarine cables were installed more than twenty years ago and have been operating without any reported problems.
For example, stress introduced electrical degradation (SIED) of the inner semi-conductive screen is influenced by the polymer properties.
Many tapes contain additives such as super-absorbent polymers (SAP) that do not work in salt water and degrade very quickly without any saltwater absorption. A newly developed SAP has been tested for this application.
Detlef Wald was secretary of Cigre working group D1/B1.75 about Corrosion
