HVDC Transmission for Offshore Wind Farms:Cable Technologies

Cable Technologies

Nowadays, there are three types of DC cables available in the market:

• Self-contained fluid filled cables

• Mass-impregnated cables

• Extruded insulation cables

Self-contained fluid filled (SCFF) cables have as insulation system consisting of a paper impregnated with a low-viscosity fluid. The cable has a central duct where the fluid is maintained under pressure allowing it to reach to the insulator. Since fluid is in constant circulation, hydraulic and pumping stations are essential. However, the possibility of occurrence of oil leakage is a potential risk for the SCFF cables [58].

Mass-impregnated (MI) cables have as insulator a high-density paper impreg- nated in a high-viscosity fluid. This type does not require fluid pressure supplying. So, MI cables may be installed in a very long length without the large oil duct system.

Extruded cables use cross-linked polyethylene (XLPE) as insulator. This option has attractive features such as: (i) XPLE cables are lighter than MI or SCFF since having better insulation properties so less insulation is needed. (ii) XLPE cables have smaller banding radius than MI and SCFF so to install and to transport is relatively easy. (iii) As MI cables, XPLE cables do not need an oil duct system, so there is not risk of oil leakage [58].

AC cables, XLPE type, up to 200 MW to 150 kV have been successfully installed for submarine applications [5]. However, as was mentioned above the cable distance is limited by the reactive power demand, which is the product of the capacitive characteristic of the cable. This limitation can be overcome with the assistance of reactive compensation on both sides of the cable, but the cost of installing more platforms to locate such compensation makes it unaffordable. LCC HVDC uses MI-insulated cables, which are made from copper with high-density

oil-impregnated paper as an insulator. It is also possible to use SCFF cables. XLPE cables have been only applied for HVDC with VSC technology. XLPE are made using a solid dielectric instead of pressurized liquid or oil-impregnated paper. XLPE cable has insulation degradation to voltage polarity reversal, therefore it is not recommended for HVDC with LCC technology [59].

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