Submarine Baltic exchange

Talk of creating a power link between the Baltic States and the Nordic region began back in the 1990s, but little movement followed. However, it has finally become a reality thanks to the Estlink coming online.

Bernt Abrahamsson, ABB Power Systems. Finland

In early December 2006 the official inauguration ceremony for the ‘Estlink’ – the first power link between the Baltic States and the Nordic countries – took place simultaneously in Estonia and Finland. The link between these two key grids is intended to improve grid reliability and help to avoid future black-outs, as well as contributing to industrial competition by facilitating energy trading and improving supply to customers.

Plans for a submarine power cable that could facilitate the sharing of electricity between the Baltic and the Nordic regions had been in existence since the 1990s. But it was the harsh winter of 2002/2003, resulting in an increased demand for power, which created energy shortages across the regions that highlighted the pressing need for such a project.

The Estlink submarine cable connects the Harku 330 kV converter station outside Tallinn and the Espoo 440 kV converter station near Helsinki. It is owned and operated by a special purpose company, Nordic Energy Link AS, formed in 2004 as a partnership between the three Baltic power utilities – Eesti Energia, Latvenergo and Lietuvos Energija – as well as Pohjolan Voima and Helsinkin Energia, both of Finland.

The Estlink submarine cable is one of the priority projects in the European Union’s (EU’s) plan to ensure continuity of supply in the power system, improve cross-border power infrastructure and help create more efficient power markets in Europe. It is the first common project between Baltic and Finish energy companies and cost a total of E110 million ($144 million).

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The 350 MW cable was built primarily to provide the Nordic countries with electricity produced in the Baltic States, and is expected to transmit an estimated 2 TWh of power. The link will also be used to support the Baltic Sea region during network emergencies.

For example, ‘Black Start’, can be implemented to aid restoration of the Estonian grid in the unlikely event of a complete network collapse.

Estlink construction

In 2004, ABB was awarded the contract to construct the Estlink, with a 20-month time frame. In fact, the project was completed in 19 months thanks to a modular approach, with much of the equipment being factory assembled. This compares with the 30 months often required for similar installations.

For the Estlink, ABB has supplied the 350 MW HVDC (high-voltage direct current) Light cable link, as well as converter stations at both ends of the cable link. It consists of two parallel cables along its entire length. In total 210 km of HVDC Light oil-free cable has been supplied from ABB’s cable factory in Karlskrona, Sweden, which specializes in the production of long, powerful, submarine high voltage cables.

HVDC Light System

The technology chosen for this project was ABB’s HVDC Light system based on VSCs (voltage source converters), employing state-of-the-art turn-on/turn-off IGBT power semiconductors that operate with high frequency pulse width modulation

This is a mature, non-conventional technology that offers maximum security of supply, minimal environmental impact, negligible electromagnetic fields, secure power control and quick power restoration in the event of a black-out. The system is based on compact bipolar converter stations and extruded polymeric DC cables.

HVDC Light converters provide high speed control of both active and reactive power in both networks. The HVDC Light technology allows power flows to be directly controlled in a manner not possible over a conventional, free flowing alternating current (AC) transmission line.

The system will also help to integrate the electrical systems in the Baltic region, and stabilize electricity grids on both sides of the interconnection.

The Estlink operates at à‚±150 V DC, and is rated at 350 MW of active power in either direction, with a low ambient temperature overload capacity of 365 MW. The two converter stations, at Harku and Espoo, are capable of generating/consuming up to 125 Mvars of reactive power independently of each other, and independently of the active power transfer.

HVDC cables

The lower losses of the HVDC Light cable provide an ideal solution for transmitting power over long distances. This type of cable has a polymeric insulation specifically adapted for direct current.

The conductor screen, insulation and insulation screen are extruded in a triple extrusion process. Radial water sealing is created by applying a corrosion resistant lead sheath. Longitudinal water sealing is achieved by using a water swelling material applied under the lead sheath.

The mechanical strength of the cable is provided by steel wire armour. An outer serving of bitumen bonded polypropylene yarn protects the armour. The strength and flexibility make the HVDC Light cables well suited for severe installation conditions both underground and at sea.

Laying the two bundled, parallel submarine cables that run for 74 km across the Gulf of Finland
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The land cable has a diameter of 93 mm with a 2000 mm2 aluminium conductor, and weighs 11 kg per metre. The submarine cable has a diameter of 96 mm and a 1000 mm2 copper conductor with steel armour, and weighs 26 kg per metre.

Compared with traditional paper insulated cables and overhead lines the polymeric cable has greater mechanical flexibility and strength. It is also environmentally friendly as it is oil-free, and electromagnetic fields surrounding the cable are minimal, far lower than current EU limits.

Cable route

The cable route starts with a 9 km land cable from Harku converter station in Estonia. A submarine cable then runs 74 km across the Gulf of Finland to the Finnish shore, where a land cable of 22 km continues to the converter station at Espoo.

The Estonian land cable part was delivered on 24 cable drums and the weight of the shipment was about 200 tonnes. The Finnish land cable was delivered on 60 cable drums and that shipment weighed approximately 500 tonnes. The land cables were sent in 22 shipments across the Baltic Sea to the installation sites.

Cable installation

ABB appointed Global Marine Systems Limited as the specialist contractor for the submarine cable installation project. The first stage of the project was for Global Marine to equip the Sovereign, its advanced offshore cable laying vessel, with two 17 m diameter cable tanks and loading arms to accommodate the size of the Estlink cables.

The Estlink submarine cable was loaded directly from the factory in Sweden onto the Sovereign. The cable was produced in long lengths, which minimized the need for jointing operations, and was tested during offshore trials prior to sailing for Estonia.

The cable weighs some 3850 tonnes and was laid in one section, with the two power cables bundled together to minimize the electromagnetic field and to enable the cables to be buried by water jetting in a single run. The burial depth requirements to give protection for the cable against fishing gear and ship anchors were precise.

The daily flow of power between Estonia and Finland
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In general this was maintained at no greater than 1 m, and no more than 0.6 m in areas where the soils had high thermal conductivity.

Throughout the laying process ‘touchdown’ monitoring, using the Super Mohawk underwater inspection ROV, was carried out to monitor the touch down position of the cable on the seafloor, to ensure that it was laid at the correct tension and avoiding seafloor obstacles. In fact, a number of ordnances were discovered extremely close to the installed route.

Estlink operation

The Estlink began commercial operation in January this year. The utilization can be viewed live at Shown here is the graph for week seven, indicating the daily flow of power between Finland and Estonia – the positive figure indicates that Finland is importing power from Estonia, as expected. The also graph shows that on a number of occasions the link was being utilized up to its 350 MW capacity.

The graph also gives an indications of the increased opportunities for power trading made possible by the capability of HVDC systems to control the exact amount of power transferred between individual grids. For example, on the days of Saturday, Sunday, Tuesday and Wednesday, Estonia was actually transferring to Finland the 350 MW of power agreed in the base contract. However, Finland had also contracted on those days to sell 70 MW of power on to one of the other Baltic States. This power was transferred directly from Estonia via the Baltic Ring, thus on those days the Estlink appears to show a utilization of 280 MW (350 MW capacity less the 70 MW sold on by Finland).

Finally, although it is early days for the Estlink, all the indications are that this grid interconnection will enable the successful sharing of electricity between these two regions.

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