Commissioned in May and officially opened in September of this year, NorNed is the world’s longest submarine high-voltage power cable. Linking the power grids of Norway and the Netherlands, it represents an important step in the European Union’s Trans-European Energy Network (TEN-E) initiative, which seeks to create a single electricity market in Europe.

Heather Johnstone, Senior Editor

Although Norway remains outside of the European Union (EU) this has not stopped it enjoying good trading relations with other EU member states, particularly in the export of many of its natural resources, such as crude oil.

In May of this year, Norway began trading another much sought after commodity with the EU, namely electricity, when the NorNed power link that connects the grids of Norway and the Netherlands was commissioned.

At 580 km, the NorNed link is the longest high-voltage submarine power transmission line in the world and is jointly owned and operated by Stanett and TenneT, the Norwegian and Dutch transmission system operators, respectively. It is the first direct power link between Norway and the Netherlands, and is part of the EU’s plan to improve cross-border power infrastructure, reduce blackouts and help create more efficient electricity markets within Europe.

NorNed’s HVDC underwater cable was installed using a specially designed cable-laying ship
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The NorNed interconnection not only provides the opportunity to trade electricity between the two countries, but also to improve the reliability of electricity supply. Norwegian power generation is almost entirely based on hydropower, while power production in the Netherlands is mostly fossil fuel based.

Hydropower is easily regulated and stored in existing dams. This allows the overall system to be optimized by capitalizing on the hydropower regulating capability to cover peak loads during the day in the Netherlands. At night, power can be imported back to Norway to save the electricity energy in the water dams. The result is a more stable output from the fossil fuel fired plants in the Netherlands, reducing carbon dioxide (CO2) emissions. The reduction in CO2 has been estimated at 1.7 million tonnes per year. It is also hoped that the power link will help to promote the greater integration of wind power in the Netherlands because of the greater stabilization of its grid.

Norway’s high dependence on hydropower, however, means that below-normal precipitation can have a profound effect on its entire power system. With the NorNed link operational, Norway now has the option to import power from the Netherlands when it experiences periods of low rainfall.

The NorNed HVDC link has a maximum capacity of 700 MW and an operating current of 824 A. Initially, the NorNed link was intended to provide a transmission capability of 600 MW, corresponding to a maximum conductor temperature of around 40ºC. Subsequent studies demonstrated, however, that there was sufficient margin to allow for an additional 100 MW capacity even during the warmer summer periods, making the effective continuous operational rating 700 MW.

In total, the NorNed project is said to have cost in the region of €600 million ($760 million), and took ten years of planning and a further three years of construction.

ABB of Switzerland was awarded the contract for the construction of the two converter stations – one at Feda in Norway and the other at Eemshaven in the Netherlands, and approximately two-thirds of the cable system. While Nexans of France supplied two 156 km lengths of cable for the deep-water section – laid at depths of up to 410 m – and a shorter length of cable to bring the link ashore in Norway.

Innovative Cable System

The cable system consists of a mass impregnated (MI) paper insulated cable type. Although this cable technology is more than 100 years old and was employed in the first submarine HVDC cable back in 1954, it remains the cable technology employed in over 90 per cent of all HVDC submarine transmission projects.

The cable system is built as a bipolar system consisting of two HVDC cables, each 580 km in length. The system is split into a shallow part supplied by ABB and a deep part delivered by Nexans.

The cable starts at Feda in Norway and runs through a 1.4 km long tunnel down to a jointing chamber. Here the cable is jointed to the submarine cable. From there the cable runs through a 150 m long micro-tunnel down to the seabed approximately 45 m below sea level. The cable then goes 156 km out into the Fedafjord and is jointed to a single-core cable, which marks the beginning of the shallow part of the cable system.

The shallow part is covered by two different cable designs: a single core HVDC cable, which runs approximately 154 km into the North Sea in up to 70 m of water, and a double-core FMI (flat mass impregnated) cable.

The FMI cable has a novel design in that it comprises two independent cable cores made of braided copper stands, each rated 450 kV DC, placed side by side into a common steel wire armour. Each cable core has its own insulation, metallic sheath and plastic sheath. An extruded plastic profile between the cores supports the cable cores. A common counter-helical steel wire armour protects the flat cable. It offers an extremely low outside magnetic field because the conductors with counteracting currents are only 100 mm apart. Also, both cores can be laid in a single operation. The FMI cable run for approximately 270 km

Although the tensional forces during cable laying are moderate in the shallow waters, both the submarine cable types are provided with a strong double-helical steel wire armouring. The wire armour is a part of a cable protection system, which, together with burying of the cable into the sea bottom, protects the cable from external impacts like anchors and fishing gear. Statistics show that the overwhelming majority of submarine cable faults are caused by external impact.

A 28 km portion of the FMI cable is also equipped with a fibre-optical temperature sensor embedded into the armour. It enables a distributed temperature measurement along the cable route closest to the Dutch coast. This route portion is particularly interesting for temperature monitoring because the sea bottom morphology is prone to considerable changes. Temperature measurements can potentially also reveal changes in the cable environment.

Cable System Installation

Nexans’ specialist cable ship called the C/S Nexans Skagerrak installed the complete submarine cable system. It is the first purpose-built vessel to be designed for the transport and installation of submarine high-voltage power cables. The vessel is equipped with a 7000 tonne capacity, 29 m diameter turntable, a computer based laying control system and a state-of-the-art positioning system. The submarine cable was protected on the seabed by burying it at depths of between 1 m and 3 m, and in areas where the nature of the seabed prevented burial, rock dumping protected the cable.

In addition to the submarine cable the shallow part also comprises a pair of 790 mm2 single-core land cables. As the requirements for tensional strength and impact protection are considerably lower than for the submarine cables, the land cables have only one layer of steel wire armouring. The length of the land cables is approximately 1500 m. They were laid in an open trench between the FMI landfall and the converter station in Eemshaven.

Converter Stations

The converter stations are located at the Feda substation in Norway and at the Eemshaven substation in The Netherlands. In Feda, the AC side equipment is connected to the 300 kV AIS substation and placed outdoors. In Eemshaven the AC side equipment is connected to the 380 kV GIS station by cables and the AC filters are located indoors. In addition, the DC side equipment in Eemshaven is installed in a DC hall to avoid problems with flashover due to salt contamination from the sea.

The converter transformers are of a single phase, three winding type. Six double-valves in the valve hall are arranged to provide the +/– 450 kV twelve-pulse converter. The twelve-pulse converter has a voltage rating of 900 kV, while the DC voltage to ground is 450 kV.

Each single valve has 120 thyristors including three redundant devices. The smoothing reactors are 700 mH, oil-insulated type with the bushings protruding into the valve hall (Feda) and DC hall (Eemshaven). The DC cables enter directly into the converter stations so no DC filters are required.

The AC filters and shunt capacitor banks provides a total of 485 Mvar of reactive power in Feda and 432 Mvar in Eemshaven.

The brain of the HVDC transmission is a MACH2-based control and protection system.

Off to a Promising Start

With almost 100 per cent cable availability, the NorNed cable between the Netherlands and Norway has got off to a promising start. According to its operators, in its first two months in operation, the cable generated revenues of close to €50 million, which exceeds all previous estimates. In the business case drawn up for the NorNed cable, annual revenues were estimated at €64 million. Since the NorNed cable came into service on 6 May, it has exceeded the expectations TenneT and Statnett had when embarking into this ambitious project. Low electricity prices in southern Norway combined with high electricity prices in the Netherlands have ensured that the current flow goes from Norway to the Netherlands most of the time – between May and September 1.7 TWh of Norwegian hydropower was imported into the Netherlands, with 0.1 TWh electricity transported in the opposite direction.

The NorNed cable has also increased the resilience of the Dutch electricity market. Analyses indicate that in the first weeks of the go-live the APX index for Dutch next-day electricity would have been over three times higher without the buffering effect of NorNed. As an example, the APX Index for 6 May 2008 reached €180/MWh, without the NorNed cable this price would have spiked to €500/MWh in several occasions – clear example of how the NorNed cable and its imports have added to the stability of the Dutch wholesale electricity market. However, the work does not end there.

Beyond NorNed

There are several other important interconnection projects under construction in Europe and include: Nord E.ON 1, a 400 MW underground/subsea link to integrate Borkum, the world’s largest offshore wind farm, into the German grid; SAPEI, a 1000 MW subsea island connection between Sardinia and the Italian mainland; BritNed, an ambitious 1300 MW submarine cross-border connection between the UK and the Netherlands, due to be commissioned, in 2010, and Skagerrak 4, a 600 MW subsea cable between Southern Norway and Denmark, due to be commissioned in 2014.

Finally, future interconnections rumoured are an interconnection between Norway and Germany, another between Norway and the UK, and even a ‘NorNed 2’.