Plumbing to new depths - undersea electricity cables
Supplying electricity across water expanses can only be completed through durable cables. Kristin Hessen, Communications and Marketing Director, from Nexans, Halden, Norway, outlines the developments in cross-linked polyethylene conductors for more efficient power delivery.
High voltage (HV) submarine power cables have been used to transfer power beneath the sea for decades. They play a crucial role in reinforcing networks and are set to play an increasingly important role in the development of renewable energy resources.
For over 80 years HV submarine power cables have been manufactured using fluid-filled cables (filled with oil) to insulate conductors. A proven performer since the late 1930s, paper-insulated, fluid-filled cables with extruded insulation have been refined constantly to deliver high voltages in harsh environments, including underground, tunnels, shafts and deep ocean depths.
A tube-like stranded copper or aluminium conductor carries oil under pressure, which permeates the surrounding paper layers to ensure a paper insulation that is pressurised with pores and gaps that are fluid filled, under all operating conditions.
The use of a lead sheath to prevent water penetration has also been dominant for submarine cables, with a semiconducting tape-layer, or semiconducting polyethylene jacket, in place to prevent over-voltages causing puncture of the lead sheath.
Submarine cables also normally have steel wire armour and corrosion protection provided by a bitumen coating and polypropylene yarn outer layers. For application at greater depths of water, the armour consists of two cross-wound layers to provide a torsion balanced design.
In contrast, normal submarine cable designs must be bonded at both ends, and, as a result, experience significant armour losses. For high capacity applications, the chosen armour material may be copper or aluminum where appropriate from a mechanical point of view, and has to be designed for the actual application. Their characteristics include –
- High reliability at high voltages.
- A homogeneous insulation system ensured by fluid- impregnated paper.
- Custom-designed for tough environments.
- Flexible factory joints and field joints are developed for application up to 525kV.
The drawback of fluid-filled cables, however, is that they require an oil feeding system to pump oil through the cable to ensure they work correctly. This comprises a pumping station at each end of the circuit, with the ability to compensate for oil volume changes in the cables that occur because of variations in temperature, caused by load changes and/or environmental conditions.
If the insulation is damaged, fluid-filled cables have been designed to maintain their oil supply for around 21 days.
High voltage cross-linked polyethylene (XLPE) cables have been under development since the 1960s. They comprise a stranded copper or aluminium conductor with an extruded insulation system, and various shields, sheaths and armouring. Just like fluid-filled cables, the normal application has a lead sheath and the same basis for choice of armour materials.
Like fluid-filled cables, XLPE cables can be equipped with optical fibres integrated in the cable design/armour to allow the utilities to carry out thermal monitoring, or even optimise the real carried ampacity.
Cross linked polyethylene technology has several advantages compared to fluid-filled cables, including –
- Flexibility, lightness and strength.
- No need for an auxiliary fluid-pressure system.
- Low maintenance compared to fluid-filled cables (related to hydraulic system).
- Pre-made accessories for land application and inclusion in repair joints in limited water depths.
- Less need for reactors for compensation at cable ends in the grid.
Using XLPE cables in submarine applications also presents challenges. They need to be made water tight and resistant to corrosion and abrasion, caused by sea currents and waves. In deep water they must also withstand high pressures, since they are subjected to a 20bar (200m in depth). In comparison, when a fluid-filled cable is used, the fluid pressure will nearly balance the water pressure. Laying the cable at this depth also subjects it to great mechanical strain due to the long unsupported weight.
Greece’s Public Power Corporation (PPC), the country’s largest electrical power utility, has commissioned the design, manufacture and installation of a new high-voltage AC power link between Evia and the region of Attika. The link will be known as the Nea Makri-Polypotamos HV submarine power link and will facilitate the development of wind power projects totaling around 400MW on Evia, the second largest of the Greek islands, by enabling the power produced to be exported directly into PPC’s national power transmission grid.
The 150kV link will interconnect the Nea Makri substation on the coast of Attika with the Polypotamos substation on Evia. There will be three sub-sea cable circuits that will take a 21km route across the Gulf of Evia in waters reaching a maximum depth of 85m.
The 150kV submarine cables will be manufactured in a specialised submarine cable facility in Halden, Norway. They will feature XLPE insulation and comprise three power cores with a copper cross-section of 630mm2. Three cable circuits will be installed across the Gulf of Evia to provide two circuits in normal use with one spare to ensure continuity of operation. Each circuit will have a nominal capacity of 200 Megavolt Ampere (MVA).
Installation is scheduled to be carried out by ship. The cables will be buried approximately one metre below the seabed. The project is scheduled for completion within 36 months.
The 150kV underground cables will be a single core design for installation in three circuits, each circuit comprising three individual cables, over a 2.75km route on the Attika side and a 330m route on the Evia side.
Kristin Hessen, Communications and Marketing Director, Nexans, Kraft sjøkabel, umbilical, HV PEX-kabel og HV laboratorium Postboks 42, 1751 Halden.Besøksadresse: Knivsøvn.70, Norway. Tel: +47 22 88 61 11. Email: Kristin.firstname.lastname@example.org Website: www.nexans.com