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Siemens has installed the BorWin2 offshore platform in the North Sea northwest of the island of Borkum – another crucial milestone in the German grid connection projects under contract with the German-Dutch network operator TenneT. Siemens had already erected its first converter platform, HelWin1, off Heligoland in August 2013.
The transmission capacity of BorWin2 is 800 megawatts – enough to meet the power demand of some 800,000 households. On the platforms, the alternating current generated by the wind turbines is transformed into direct current for low-loss transmission to the mainland, where the transmitted energy is converted back into alternating current and fed into the German power grid. Thanks to Siemens direct-current technology, transmission losses are less than four percent.
The platform was towed into its position in the North Sea directly above the support substructure that Siemens had already installed. After the platform was aligned above the substructure, the two parts were combined. This is the most critical part of such an installation sequence, as it calls for a very calm sea. Then the platform was raised using a hydraulic jack-up system. The platform is installed 20 meters above sea level to protect it against giant waves and designed for decades of operation in the rough North Sea.
The BorWin2 platform, which was built by Nordic Yards at the shipyard in Warnemünde, Germany, has a length of 72 meters and a width of 54 meters. Its surface area is more than half the size of a soccer field. It weighs 12,000 tons – more than 20 loaded and fully tanked Airbus A380 super airliners. The platform, which is also equipped with a helipad, has seven decks spanning a total height of 25 meters. These decks house the technology and equipment required for high-voltage direct current (HVDC) transmission as well as living quarters for maintenance and servicing staff.
BorWin2 carries two large converter-transformers and several smaller transformers. The converter transformers weigh 440 tons each and were manufactured at the Siemens transformer factory in Nuremberg, Germany. After careful factory testing the transformers were shipped to the shipyard in Warnemünde, where they were mounted on the so-called topside, the part of the platform that stands above water level. The opening through which the transformers had been lowered by huge cranes was later welded closed, but it can be opened again should a transformer ever need to be replaced.
High-voltage direct-current (HVDC) power transmission technology from Siemens is ideally suited to connect huge offshore wind power plants to the mainland grid. The alternating-current power generated by the wind turbines is transformed into direct current on the offshore platform, transmitted to the land-based converter station at very low loss, and finally reconverted to AC and fed into the grid. The converter technology used by Siemens is called HVDC PLUS, and is essentially a modular multi-level voltage-sourced converter (VSC MMC), a technology that offers considerable technical and operational advantages. Siemens is a leading pioneer in this field.
The power generated by the Veja Mate wind farm and Stadtwerke München’s Global Tech 1 wind farm in the North Sea will be transmitted to the mainland through two subsea 300-kV HVDC cables supplied, bundled, and laid by the well-established cable specialist and Siemens consortium partner Prysmian. The cables, which are 11 centimeters in diameter, equipped with a protective-steel armoring wire layer, and with a 35-millimeter-thick copper conductor, weigh about 30 kilograms per meter.
The cable route covers 125 kilometers at sea between the converter platform and the island of Norderney. It crosses the island and the Wadden Sea in subterranean bores and continues onshore for about 75 kilometers, finally terminating at the converter station at Diele near Papenburg.
The BorWin2 offshore platform was installed on a supporting substructure that had been previously anchored in the North Sea, which is 39 meters deep at that point. This substructure consists of six steel pilings of up to 2.5 meters in diameter and a wall thickness of eight centimeters and is anchored around 50 meters deep in the seabed. With a length of up to 83.5 meters, these pilings are only ten meters shorter than the Statue of Liberty in New York, including its base.
After construction work had been completed in the shipyard in Wismar, three tugboats towed the topside of the converter platform to the North Sea. The steel giant had to be towed around the northern tip of Denmark, because it was too large for the passage through the Kiel Canal, and reached its final destination after covering about 1,000 kilometers.
Thanks to the use of Siemens’ efficient high-voltage direct-current technology HVDC PLUS, the total transmission loss of the BorWin2 grid connection will be less than four percent. The system is based on modular multilevel voltage-sourced converters. Other than conventional HVDC systems, which can only be used in networks with sufficient short-circuit capacity, systems using HVDC PLUS enable island networks to start-up from scratch – an important prerequisite for the operation of an offshore network. The modular VSC technology reduces complexity and the footprint of the installations, which proves a great advantage when space is limited, as is the case on offshore platforms. Moreover, HVDC PLUS ensures a nearly ideally sinusoidal AC voltage curve and a smooth voltage in the DC circuit, which eliminates the need for harmonic filters that would require additional space.