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Introduction to HVDC Submarine Power Cables
Released on March 31, 2021 by Leandro Soares da Costa, Director at Tupi Energy
HVDC submarine power cables have been used in offshore transmission lines since 1954, when “Gotland 1”, which was the first commercial HVDC transmission link, came into service in Sweden . These cables are usually buried from 1 to 1.5 metres below the seabed. The cables are also protected with the use of concrete mattress to minimise contact with external agents, or protective collars to reduce friction between cables’ outer layer and the seabed.
Figure 1 shows a submarine power cable’s cross section and its structural layers, which are described as follows:
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Conductor: made of electrical conductive materials through which the current passes;
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Insulation system: made of polyurethane (XLPE) preventing the conductor from coming into contact with the atmosphere;
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Semiconductor: forms a stable dielectric surface between the conductor and the insulation layers;
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Lead and plastic sheaths: prevent the ingress of water protecting the conductor and isolation system;
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Armour layers: made of steel wires to provide tensile resistance to the cable;
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Outer/Protective sheath: protects the structure against harmful external hazards.
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Figure 1 – Cross section of HVDC Submarine Electricity Cable
The most important design inputs for submarine power cables are environmental and sea conditions (temperature, pressure, current, tide and waves), cable route, installation methods, and operating data system (voltage, frequency and minimum current transmission capacity). These factors can drastically affect the cost of projects. For example, adopting an optimised approach to determine the cable length can reduce the number of intermediate connections, therefore, also reducing the project cost.
Detailed design of the installation steps for power cable system and its interfaces is an iterative process which should address scenarios, including contingencies and repairs, such as cable storage, load-out, transport, laying, pull-in at offshore units and landfall, burial, and protection by non-burial methods. In some cases, design factors may imply limitations on installation procedures. For instance, the cable’s minimum bending radius (MBR) can affect the type of vessel and equipment used during installation, as presented in Figure 2.
Figure 2 – Installation of HVDC submarine cables
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The lifetime of submarine cables is driven by the deterioration of the insulation material properties under the influence of a combination of thermal, electrical, chemical and mechanical stresses. These factors progressively decrease the cable’s dielectric strength. Although 70% of cable failure mechanisms are related to external or environmental factors , which cannot be monitored using current monitoring systems, the development of new technologies can improve the life span of submarine power cables.
References:
(1) https://www.hitachiabb-powergrids.com/uk-ie/en/references/hvdc/the-gotland-hvdc-link.
(2) https://europacable.eu/wp-content/uploads/2021/01/Revolve-Europacable-Cable-Power-June-14.pdf.
(3) http://www.emec.org.uk/press-release-study-on-subsea-cable-lifecycle-published/.
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