THE COMMISSIONING OF BORWIN 1 IN SEPTEMBER 2009 LAUNCHED A NEW DIMENSION in large-scale, long-distance offshore wind power. Located 125 km (78 miles) off the German coast in the North Sea, the world's largest offshore wind park will be linked to the mainland power grid by HVDC Light underground and submarine cable power transmission technology, developed by ABB (Zurich, Switzerland).

When completed, the cable will extend 200 km (124 miles), becoming the world's longest offshore wind power interconnector. Apart from an exceptionally challenging project time frame of just 24 months, the interconnector will cross one of Europe's most-sensitive and heavily protected coastal environments.

German law stipulates it is the responsibility of the transmission system operator (TSO) to provide and bear the cost of connecting offshore wind farms to the mainland power grid. The German Infrastructure Planning Acceleration Act also requires that completion of the cable link coincide with completion of the wind farm. There are four TSOs in Germany, but transpower offshore gmbh (Bayreuth, Germany), formerly E.ON Netz Offshore GmbH, is the utility most involved in meeting the requirements of offshore wind farms.

THE CLUSTER CONCEPT

Responsible for about 40% of the German power grid, transpower is one of the largest TSOs in Europe, operating a 380/220-kV transmission system some 10,700 km (6649 miles) in length and supplying a population of about 20 million with a reliable supply of electrical energy.

The entire German North Sea coastline and parts of the Baltic coastline fall within transpower's control area. Both the North Sea and the Baltic Sea are two of the world's prime offshore wind power locations because of the combination of strong year-round wind resources and relatively shallow water.

In November 2008, more than 55 offshore wind power projects were planned for the North and Baltic seas, the majority of which will require connection to the transpower network. To accommodate the huge surge in offshore wind power, transpower has developed a cluster concept for longer-distance connections, by which wind farms in close proximity to one another will be bundled together to form a cluster. Each wind farm in the cluster will be connected by an underwater ac cable to an offshore cluster converter station. The power will then be converted to dc for transmission through high-voltage direct-current (HVDC) cables to a mainland high-voltage dc-ac converter station and connected to transpower's 380-kV ac transmission system.

With four such clusters planned, transpower's first cluster will be Borkum 2. The first wind farm connection in the Borkum 2 cluster will be BorWin 1 (formerly known as Nord E.ON 1), which will connect the 400-MW Bard Offshore 1 wind farm to the 380-kV mainland system.

LARGEST OFFSHORE WIND FARM

Bard Offshore 1 will consist of 80 wind energy converters (turbines), each with a capacity of 5 MW. This will enter the record books as the world's largest and most-remote offshore wind farm, connected to the mainland by the world's longest offshore wind power link. It will also be the first dc grid connection for offshore wind in Germany. This project has created an array of technical and project management challenges. The sheer scale and remoteness of the project are compounded by the 24-month delivery schedule, coupled with the need to install two HVDC Light cables in one of Europe's most-sensitive and heavily protected coastlines.

Technical reliability is even more critical than usual because of the extreme offshore environmental conditions and the fact the converter station will normally be unmanned and only accessible by helicopter. In addition, an unusually large number of permits were required from various federal, state and county authorities, as well as from national entities, organizations and private individuals. To date, the number of permits obtained for this project exceeds 500.

To meet these and other challenges, transpower required a solution that would provide a transmission system with abnormally high reliability, maximum availability and minimal maintenance. The solution also had to comply with strict grid codes to relieve stress from the wind turbines by isolating electrical transients from the mainland grid. Furthermore, the wind farm had to be designed to withstand the harsh, and often very hostile, weather conditions of the North Sea.

THE SOLUTION

AC transmission is not suitable for connecting long-distance offshore wind farms to the mainland grid. The high levels of capacitance-per-unit length and the large amounts of reactive power produced make it technically infeasible to use ac cables for long-distance transmission applications. Although a classic thyristor-based HVDC system would provide a reliable and efficient alternative, it also would require compensation at the wind farm for the thyristor valves. This would substantially increase the footprint of the platform and the complexity of the solution in such a remote and challenging environment.

However, a technology that offers a technical and economic solution for the challenges of connecting remote wind farms to a mainland grid is ABB's HVDC Light system. This system uses voltage-source converters with state-of-the-art insulated-gate bipolar transistors (IGBTs) to achieve levels of voltage control not otherwise attainable. It is exceptionally compact and offers a number of capabilities that make it ideal for linking remote offshore wind farms to the mainland:

  • Fast and independent control of both active and reactive power

  • Black-start capability to re-energize following mainland blackouts or when there is insufficient wind offshore

  • Full compliance with grid codes

  • MINIMAL ENVIRONMENTAL IMPACT

    Short project implementation time

  • Compact, lightweight and unmanned platform equipped with compact high-voltage equipment housed in modules to protect them from the humid and salt-laden air

  • High-performance, environmentally friendly cables that are mechanically robust and oil free, with a small diameter, neutral electromagnetic fields and no limitations on cable length or distance

  • AMBITIOUS RENEWABLE ENERGY OBJECTIVES

    Freedom to locate the onshore converter station at a key high-voltage substation anywhere on the mainland, rather than on the coast where there is no suitable connection point

  • Decoupling of the onshore and offshore grids to isolate the offshore network from onshore disturbances

  • No synchronous compensator in the offshore network

  • Remote control of the offshore converter platform via ABB's MACH 2 control system, which is currently operating more than 400 HVDC and flexible ac transmission system (FACTS) installations worldwide.

An important factor supporting transpower's decision to select the solution was HVDC Light's unrivaled track record and impressive list of projects using this technology. In particular, a similar HVDC Light interconnection crosses the Baltic Sea between Finland and Estonia. This 105-km (65-mile) power link, which can deliver up to 350 MW of power in either direction through the submarine and underground cables, was commissioned following a 19-month project time frame in 2006. Furthermore, transpower studied two HVDC Light projects completed for StatoilHydro and BP, where power was provided from mainland transmission systems to compact converter stations on platforms in the North Sea.

Another factor in favor of the HVDC Light solution was its ability to meet the strict grid-connection codes. In addition to satisfying the requirements in terms of reactive power and voltage control, HVDC Light also provides frequency support and low-voltage fault ride-through withstand. The result of these inherent design features, together with a so-called chopper solution, is a transmission link that ensures the wind farm is immune to electrical transients from the mainland grid. This means less stress on the equipment connected to the offshore grid.

The part of the German coastline where the HVDC Light cables will be used for the BorWin 1 project is known as the Wadden Sea. It is a coastal marine belt of tidal mudflats that stretches from the Netherlands along the German North Sea coast to Denmark. The area is deemed a wetland of international importance and is currently being considered for inclusion as a World Heritage site. Famous for its flora and wildlife, it is one of the most-important staging areas for migratory birds flying from the Arctic to South Africa.

Crossing such a sensitive and protected area as the Wadden Sea presented a major challenge. The cluster concept significantly reduces the environmental impact by connecting an entire group of wind farms to the mainland with a bundled pair of HVDC Light cables, rather than with multiple cable installations (i.e., one for each wind farm). The cables are very compact, measuring 10 cm (4 inches) in diameter. The copper conductor is surrounded by polymeric insulating material, which is very strong, robust and flexible. As these cables are oil free and there is virtually no electromagnetic field, the cable is regarded as environmentally friendly, a fact that contributed to a major environmental award for an HVDC Light installation in Australia.

Innovative techniques such as hollow ducting for multiple connections and horizontal drilling were used with great success in sensitive areas to avoid surface impact. The cable-laying operation was monitored by conservation experts at all times, and measurements were taken along the route before, during and after installation to ensure minimal impact.

Germany's offshore wind power targets are among the most ambitious in Europe. By 2015, the European Union's largest and most-populous country expects to have 3 GW of offshore wind power. This will rise five-fold within the following five years to 15 GW by 2020 and then double to 30 GW by 2030. By proving that large-scale, long-distance offshore wind farms can be connected to the mainland transmission system, securely, efficiently and cost effectively, the BorWin 1 project is setting the benchmark for future European offshore wind power, demonstrating that technologies are available to enable the ambitious renewable energy targets to be achieved.

Key Technical Data for the BorWin 1 Wind Farm
Number of circuits One
Power rating 400 MW
AC voltage 155 kV (offshore); 380 kV (mainland grid)
DC voltage ± 150 kV
Length of dc underground cable Two 75-km (47-mile) cables
Length of dc submarine cable Two 125-km (78-mile) cables
Project cost US$400 million
Displaced carbon-dioxide emissions Approximately 150,000 tons per annum
Commissioning year 2009

Dr. Constantijn Steinhusen (constantjin.steinhusen@transpower.de) studied electrical engineering at the Institute for Technical Acoustics at RWTH Aachen University and obtained his Ph.D. at the university's Institute of Mining and Metallurgical Machine Design. Steinhusen is an expert in the field of maintenance and operation of modern wind turbines, with his research focused on the monitoring of electronic components in wind turbines. Currently, Steinhusen works for transpower as project manager of the wind park cluster Borkum 2 HVDC link for project BorWin 1.