Advances in cable technology have increased the flexibility of data that can be transmitted down a single line. In today’s high-tech age when fast access to information is of paramount importance, telecommunications and electricity sharing the same distribution network is a huge advantage.

The volume of traffic in data transmission networks is expected to make huge increases year on year, as the world wide web and intranet solutions in business, education and entertainment applications gain in popularity. Fibre optics able to flow down the same cable as electricity almost makes a literal translation of the old adage, information is power.

Utilities can now take advantage of such technology to replace and update ageing cable for the needs of today. Early in 2000, the Finnish Åland Islands, in the Baltic Sea between Finland and Sweden, will see this idea put into practice.

Cables

There are currently three 25-year-old copper cables lying on the sea bed between Åland and Sweden. The age and wear on these 77 kV, 185mm2 submarine cables coupled with their limited transmission capacity (35 MW) necessitates replacement. Åland has suffered from power shortages and cuts due to the damage these cables have received and in addition the risk of failure has increased significantly due to pack ice movement and damage from ship’s anchors. A reliable and uninterrupted power supply will shortly be in place using technology developed by Finnish company NK Energy.

NK Energy, a subsidiary of NK Cables, is supplying approximately 60 kilometres of 110 kV submarine cable and 2.5 km of spare cable to the Åland Islands. The order was placed by the local power company KraftnätÅland AB.

Following feasibility studies in 1995, Kraftnät considered building a new power plant to supply much needed electricity. But the technical viability and flexibility of submarine cable replacement rendered the new build option uneconomical.

The cable to be supplied for this project is one of the world’s first 3-core 110 kV submarine cables. Previously, submarine cables were manufactured and laid in three different phases. For many years, the typical submarine cable projects have consisted of terminal equipment, special cable plus a varying number of submarine repeaters. This concept led to expensive cable and equipment design. When added to cable laying, technology for installation and implementation costs, it is easy to see why Kraftnät Åland considered the new build option. But now all three phases have been combined into a single cable. This solution provides significant savings in cable laying and transmission losses later on during operation.

As well as supplying electricity, the new cable will also transmit data via 36 optical fibres, well above Kraftnnät Åland’s requirements. The contract, valued at FIM60 million (US$10.7m), covers both manufacturing and laying of the submarine and underground cable link from Väddö, Sweden to Tellholmen, Åland.

NK Energy’s contract also includes a 6 km 110 kV underground cable as well as various accessories. Delivery preparations commenced in December last year. A test cable was completed in May 1999 and type testing undertaken during the spring and summer of that year. Manufacture of the actual submarine cable started in August and it will be laid in April-May 2000. NK Energy has previously supplied 45 kV submarine cables to Kraftnät Åland AB for internal electrical transmission within the Åland Islands, so the power company is well acquainted with the cable technology.

Swelling tape

To give the cable its technical name it is HXYLKPJ-W/36F 3x240mm2 110 kV. A solid conductor makes the conductor longitudinally watertight perfectly. Insulation thickness is reduced from the normal level to achieve acceptable cable size. Good semi-conducting swelling tape under the lead sheath makes a good block for longitudinal water penetration in case of cable damage. SZ-stranding of the power cable cores means that they are twisted first clockwise and then anti-clockwise. This system eliminates the need to build a big traditional stranding machine. The 110 kV submarine cable is also unique in terms of dimension. One metre of this 135 mm thick cable weighs nearly 37 kg. The total weight of this single piece cable is approximately 2400 tonnes.

For this submarine cable, the three factory joints have been subjected to mechanical and electrical tests and several water penetration tests. Mechanical tests have been performed according to an international Cigré recommendation. Coiling test, tensile bending test under 95 kN tension over a 4 m wheel, tension test, holding device test, bending fatigue test and two crush tests have been performed. Longitudinal water penetration tests were performed until 27 bar pressure corresponding maximum 270 m water depth along the route. There has also been several test requirements from Kraftnät Åland’s technical specification. All tests have been successfully completed. The optical fibres have been measured during or after each test and no signs of increased attenuation have been found. Tensile and electrical tests for the repair joint still have to be carried during the winter.

Project logistics

The archipelago make-up of the Baltic Sea brings some different logistics to this cable replacement project. A new cable route has been selected to avoid rocky coastal areas, and the new route will be more than 5 km longer because of this. During laying, an ROV camera will be used to check the touch down point in critical areas. The cable will be buried to 10 m water depth in Sweden and 5 m water depth in Åland to be protected from pack ice.

It is due to the shallow or low waters that will allow these cables to be installed without the use of submarine repeaters. Long intercontinental cable routes will continue to use repeaters. The main areas where non-repeater systems have become competitive are country-to-country connections, coastal installations and regions where there are inland lakes and rivers. Cables designed for these conditions will be more economical than those used in deep waters.

The transmission capacity of the link is 80 MW, compared to an unreliable 35 MW previously. Kraftnät Åland needs also new substations in both ends. In Sweden, 77 kV overhead line feeds the Senneby substation. A new transformer changes the voltage level to 110 kV. InÅland, about 15 km of new 110 kV overhead line will be built to Tingsbacka substation. There is a 110/45 kV transformer which feeds Åland’s 45 kV network. In both ends of the link, compensation units are needed to optimise voltage, power losses etc. 110 kV transmission voltage was chosen to optimise the power losses.

From the Finnish mainland there is also a small 45 kV submarine cable connection along islands to Åland archipelago. However, this connection cannot be connected to Swedish network because there is a small phase difference in Swedish and Finnish networks.

Optical unit

NK Cables’ Spiral Space design is used for the optical unit. In this construction the optical fibres are inside a plastic tube in spiral space which is filled with gel. Some swelling tape and mechanical protection is under the lead sheath and plastic jacket of the optical unit. This unit has also been subjected to longitudinal water penetration tests and is between two cable phases under polypropylene fillers.

Spiral Space design was created to make a simple, cost efficient and reliable fibre optic cable to fulfil a large number of different applications starting from duct and directly buried usage to indoor, submarine and optical ground wire (OPGW) cables.

The Spiral Space core (see figure 1) has a number of properties that make it ideal for data transmission and particularly within a dual purpose power cable:

  • Non-metallic core
  • High crush strength
  • Extra sheath
  • Small diameter
  • Easy to install
  • Long manufacturing length

Due to the form of the spiralling channel and the material and dimensions of the core element, the Spiral Space can withstand high crushing forces (ideally suited therefore to underwater pressures). The high crush strength is very important as it brings many advantages, such as easy installation without any protective pipes and by using economical methods such as ploughing. This also makes the Spiral Space design very economical for all direct burial applications. During tests for crush strength, the Spiral Space was far beyond that of stranded loose tube or slotted core designs.

The type of unit used within the Åland cable combines the function of secondary coating and stranding. It has the advantages of loose buffering, but it also has much better mechanical properties such as crush strength, than a stranded loose tube design. This is due to the cross sectional shape of the spiralling channel and the material used. The spiralling channel is usually filled with a filling compound in outdoor cables. The core of this unit is surrounded by protective layers and strength elements.

In Spiral Space, the unit core is basically a single loose tube. It is not, however, a conventional straight tube, but a tube element with a spiralling channel. By using a proper shape and size of the channel cross-section the fibres in the channel will have a well-controlled overlength and strain margin.

The coated fibres are stranded around the central strength member for good stability in varying environmental conditions, as in high and low temperature areas. The stranding of the fibres gives them extra length compared with the length of the submarine power cable. With this feature when the cable is elongated, the fibres just move closer to the centre of the cable. A similar effect can be seen when temperature variations change the physical length of the cable. Although this premise can be seen in loose tube cable, Spiral Space has extra benefits. It is much more rugged due to proprietary core design with a spiralling slot in the core.

Another advantage is the high radial protection of the solid core to the fibres. Overlength of the fibres gives the design good resistance against tensile forces in installation and operation and against temperature changes.

The fibres in the spiralling channel also have well controlled and uniformly distributed fibre overlength. The strain and contraction margins can be adjusted such as required by the applications. During installation and operation the cable can be elongated by a certain amount without any strain in the fibres.

The special core forms an extra plastic sheath around the fibres to prevent water penetration through the cable sheath and also to keep filling compound inside the cable during the cable termination process. Even during an unlikely emergency scenario of the cable sheath and armouring being damaged, the solid core of the Spiral Space protects the optical fibres.

Temperature characteristics in the Åland cable are obviously important. In Spiral Space design the material characteristics of the core and the overlength of fibres are chosen to give an extremely good attenuation stability of fibres.

The number of optical fibres can be up to 192. For the Åland cable, the fibres are grouped into six-fibre units with coloured binders for identification. The whole Spiral Space plastic core has a diameter of just 7.5 mm. Åland cable uses 36 optical fibres allowing data transmission at a rate higher than the requirement of Kraftnät Åland.

NK Energy is confident that the project will present no specific logistical problems. Loading is scheduled to start in the second half of April 2000 when the bay next to the company’s Pikkala factory is free from ice.

With a reliable power supply restored, thus ridding the Åland Islands of shortages and cuts in electricity, this submarine power cable project will prove its economical value when installed in April. With the extra benefits of optical fibre data transmission – each of the 36 fibres able to carry 30 000+ telephone messages – all in the same 135mm diameter cable this project demonstrates that information is indeed power.