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Offshore wind energy is one of the big sources of hope for a successful clean energy turnaround in Germany. The existing wind parks in the North Sea and the Baltic Sea, however, have a performance of only 200 megawatts (MW) altogether - low compared to countries like Great Britain, with a current offshore performance of almost 1600 MW, or Denmark, with 850 MW. Germany, however, has very ambitious expansion plans. The German government wants to produce 25 gigawatts (GW) on the sea by 2030. In order to achieve this goal, the speed of expansion has to increase significantly - currently, it is very slow. Besides legal and financial obstacles and problems, technical challenges form other barriers that have to be coped with.

A key role in this process is grid access: how is the electricity to be transported from the wind power stations on the high seas to the mainland ‑ and from there to the consumer? As all of the existing wind parks worldwide are relatively close to the coast, it has been sufficient, so far, to transport the three-phase current from the individual wind power station to a substation platform at sea, and from there via high-voltage alternating current to the mainland. This, however only works with cables no longer than 80 km. If the undersea cables using three-phase current exceed this length, power is lost along the way and practically no energy can be effectively transported. Due to landscape protection and a higher wind yield, most of the German wind parks, planned or under construction, are considerably much farther out on the ocean.

The solution to this problem is High-Voltage Direct Current (HVDC) transmission. In this system, the three-phase current produced by wind turbines is collected and transformed to higher voltage levels at offshore substation platforms, from which it is transported to offshore converting platforms via AC undersea cables, where it is converted into direct current. This direct current is transported to the mainland via HVDC largely lossless, where it is converted back into alternating current and fed into the power grid in a substation platform.

When it concerns overland transport, or undersea transport between two places on the mainland, this technology works well. Worldwide there are HVDC connections, which, altogether, can transmit about 100 gigawatt. At sea, however, things are different: there, only one single HVDC station has been constructed on an offshore platform, so far. The platform BorWin alpha, which was constructed 125 km off the Dutch coast, was built in a shipyard in the Netherlands. The Swedish company ABB provided the electrical equipment for this platform, which can transmit a maximum power of 400 MW.

A little delayed, Siemens, a world market leader in offshore grid connection, is entering this promising market. "We expect big demand in this market - not only in Germany. There is also no alternative for HVDC in offshore projects in British waters, whose future wind parks are planned to be farther out in the sea," says Tim Dawidowsky, CEO of Siemens Business Unit Transmission Solutions since July 2012. In the inverter sector his department has already put six AC offshore grid connections into operation, which have a total power of 1.6 GW. However, the challenges for the first HVDC grid connections are great. "With a weight of 15,000 tons, the HVDC converting platforms are five times as heavy as an offshore AC grid connection. They also have to be mounted in up to 70 m deep water - which is twice as much as the platforms existing so far off the British coast," Dawidowsky explains.

Currently Siemens is processing four orders for offshore HVDC. To build the corresponding offshore converting platforms, Siemens has brought in the shipbuilder Nordic Yards. At a press event at Nordic Yards' shipyard in Wismar, Germany, last week, I had the opportunity to get an impression of the huge dimensions of these grid access components. In a drydock with a length of 370 m rests a steel cuboid, which is 50 m wide, 70 m long, and 35 m high - the so called topside of the converting station HelWin 1. Inside the seven-story construction, employees of the shipyard and specialists from Siemens are currently preparing large halls, in which converters with a total power of 576 MW are soon to be installed. The "life maintaining" systems of the station, like diesel generators for an independent power supply or the seawater cooling, are placed around it.

In front of the drydock lies the already-completed base frame of the converting station BorWin 2, which is also a Siemens project. The carrying system, which is 50 m high and built out of steel pipes up to 80 mm thick, is constructed to lift a converting system ‑ similar to HelWin 1 ‑ out of the North Sea, into a height of about 20 meters above sea level. For this purpose it is anchored with piles in the sea bed.

A long trip to their place of use lies ahead for the topside and base frame. "It will last three weeks to tow HelWin 1's topside from the shipyard at the Baltic Sea around the northern tip of Denmark to a place near the German island Helgoland in the North Sea," Christian Schmitt, Vice President for Offshore Platforms Power Transmission Solutions of Siemens, reported. "Therefore a period of weather, as mild as possible, is essential. As soon as the weather conditions for transportation are determined, we are confident to we will be able to deliver the mentioned data.”

With regard to the original plans, Siemens' HVDC converting platforms are about one year delayed. "We underestimated the manpower requirements, the know-how, the standards that had to be met and the approval process for this project," Tim Dawidowsky says. The four current projects are individual constructions and are regarded as pilot projects by Siemens. The experience gained in this project, as well as more explicit normalization and standardization in the future, are to shorten the delivery time in future.

For an even better handling of the new business field of offshore HVDC grid connection Siemens increased human resources from 180 to around 300 at its location in Hamburg this spring.

HelWin1 topside

The topside of the HVDC converting platform HelWin 1 in the drydock in Wismar, Germany.
 
Image Credit: Eugene Suslo/Shutterstock