Submarine power cables are not new and have been around for more than a century. In early times, they were used to supply lighthouses and ships.
Recent improvements in cable manufacturing processes, d.c. power conversion technology and cable installation methods has seen a spike in their usage for power transmission projects. Modern applications include the power supply to islands, connection of autonomous grids, offshore wind farms, the supply of marine platforms and in short-haul crossings to transport power across rivers, channels or bays.
The calculation of current ratings for modern submarine cables is similar to that of land cables, with only a few nuances. Some important differences which affect the current rating lie in the areas the cables construction and installation conditions.
The final current rating for any cable route must be based on the most onerous conditions. Therefore, several calculations are often required for a project.
The construction of submarine cables is like extruded land cables with some important differences to consider when performing cables rating calculations.
Submarine cable conductors are normally stranded copper as it has a better conductance and corrosion resistance than aluminium. Note however that some submarine cable projects consist of both copper and some aluminium conductor sections and multiple cable rating calculations may be required.
Conductor areas may be non-standard sizes. Submarine cables are not normally bought off the shelf and so conductor areas, rather than being built to standard values (i.e. those in Standard IEC 60228), are made to meet the needs of a project. For example, non-standard conductor sizes of 790 and 1410 mm2 have been used and up to 3000 mm2 are manufactured.
Swelling agents are used in between the conductor layers to provide water tightness and avoid water ingress during a fault.
A conductor screen is made from a layer of semi-conductive XLPE which is extruded over the conductor and used to smooth the electric field intensity during operation between the grooves, ridges and valleys of the conductor and insulation.
For determining current ratings, the thickness of the conductor screen layer can be added to that of the insulation.
Insulation for submarine cables is no different to that of land cables.
XLPE insulation is the first choice for submarine cables.
Water blocking sheath
Sheaths are commonly used in submarine cables.
A sheath is used to protect against the ingress of water to the insulation. The sheath may be made using a variety of materials including aluminium, lead or copper in a variety of shapes. Copper sheaths are used where continuous bending or cable flexing is endured. In medium voltage submarine cables a polymeric sheath in combination with a moisture absorbing agent may be used instead to save on costs.
Expect high circulating losses in submarine cables due to the necessary armouring.
Armour is very prominent in submarine cable construction. The presence of armour will result in significant circulating current losses inside the cable which can greatly reduce the current rating.
There are many possible types of armour materials and constructions. Stainless steel wire armour although expensive is regarded as ideal as it is low loss non-magnetic, provides good mechanical protection due to its high strength and has good seawater corrosion resistance.
It is important to have a detailed knowledge of the installation conditions and the thermal surroundings of a cable. A precise knowledge of these parameters along the cable route can save on initial investment, improve reliability and increase the lifespan of the cable.
Depth of burial
Submarine power cables are usually buried 1-3 m below the seafloor level. The cables are normally installed using a cable plough which is towed from a ship.
Thermal resistivity of subsea soils
Subsea soils usually have a relatively low thermal resistivity of 1 C.w/W or lower.
The thermal resistivity of soils depends on the soil base material, the dry density, distribution of grain size, the compaction, the moisture content and the content of organic materials.
The thermal resistivity of subsea soils does not vary much and due to being water saturated the thermal resistivity is quite low.
The Table below provides assumed thermal resistivity for common subsea soil types.
Table 1. Thermal resistivity for common subsea soil types.
|Soil Type||Thermal resistivity (C.m/W)
IEC 60287-3-1 provides the following guidance based on standard practice by country outlined by the following table.
Table 2. Thermal resistivity of subsea soils standard values by country per IEC 60287-3-1.
|Country||Description||Thermal resistivity (C.m/W)
|Finland||For submarine cables where the soil is completely saturated with water||0.4
|Netherlands||Sub-soil water level near to cables||0.5
|Sweden||For submarine cables where bottom is covered with sand|
Where nothing is known about the seafloor
Note it is important to have actual and sufficient samples of subsea soils for a project along the route of installation. This is because there may be sections of soil containing high organic content and these have much higher thermal resistivity.
The ambient temperature for a cable is defined as the temperature at the locus of the cable if the cable was not there. Ambient temperature is a critical value in all current rating calculations, no matter the installation conditions used or load cases considered.
For unburied submarine cables this is simply the temperature of the seafloor water, which is generally lower and more constant than the temperature at shallower water levels but in fact it varies considerably over the year or over several years. Normally the highest seafloor temperature over a 10 year or more period is used which must be agreed upon.
For buried submarine cables the ambient temperature at the actual burial depth can be considered, which is lower than the seafloor temperature. This value can be calculated using software.
The calculation of current ratings for submarine power cables is straight-forward. The construction and hence the modelling of submarine cables is similar that of land cables.
Additional layers for water-blocking and high circulating current losses due to the presence of armouring both reduce the current-carrying capacity.
The thermal resistivity of buried submarine power cables is 1 C.m/W or less.
The ambient temperature is the maximum seafloor temperature averaged over a period of years or less due and based upon depth of burial.
Don’t forget it’s the most onerous installation condition along the cable route including at or around the ends of the cables that determines the current rating.