Leaner offshore transformer platforms

Thursday, 29 October, 2015 - 11:15
Siemens is looking to revolutionise the connections for offshore wind farms using lighter, less expensive modules. (Illustration: Siemens AG)
Siemens is looking to revolutionise the connections for offshore wind farms using lighter, less expensive modules. (Illustration: Siemens AG)

Siemens is working on making high-voltage direct current (HVDC) power transmission from offshore wind farms more efficient. Starting in 2016, 200-MW modules will become available to make HVDC converters lighter and less expensive. However, regulatory authorities have not given their approval yet, so it may take a while before the technology can be implemented.

After building four large HVDC converters for the transmission system operator TenneT TSO, Siemens took stock of its technology and channelled the experience it had gained into optimisations. The offshore converter stations currently in use weigh approximately 25,000 tonnes an cover an area the size of a football field. Construction takes up to four years, and there are transport risks as well as enormous costs to deal with.

This technology is nevertheless indispensable for offshore wind farms far from the coast because it keeps line losses at 4%, even over long distances. If conventional alternating current were to be used, almost no power at all would survive the trip to shore. "We have had a number of painful experiences, and now we know how to do it right. We are now looking to reduce costs by introducing optimisations," said Jan Mrosik, head of the division Energy Management at Siemens AG.

The technology used in Germany up till now has a standard capacity of 800 MW, but Siemens intends to offer individual modules with 200 MW each. These modules are designed to reduce the current size of installations, making them more compact and requiring significantly less steel. Siemens is also doing away with air-insulated transistor modules. In the area of high-performance electronics, the company will be implementing robust, passive systems that do not require electrical stimulation and also take up less space.

The heart of the system for converting alternating current into direct current is diode rectifier technology. The rectifier, smoothing reactor, cooling system and transformer are all contained in a sealed tank that is filled with a special oil. "Basically, we took standard components and put them together in a new type of system," Mrosik said. The volume is reduced by four-fifths and the weight by two-thirds, resulting in estimated savings of 30%. This only pertains to the offshore connection of the link, which costs between 1 and 1.5 billion euros. "We are also reducing installation time by 20%, line losses to 3% and increasing the maximum transmission capacity to 1.2 GW," Mrosik explained.

Obstacles in the way

Nevertheless, there are still two obstacles that need to be overcome. One of these is regulatory. The concept eliminates the conventional transformer platform previously used by the operator. Instead, power is fed from the power plants directly into the grid operator's modules via Siemens' interface. This makes it necessary to have a converter platform integrated into the wind farm. Due to liberalisation in the EU, production and transmission of electricity need to be clearly separated. "For this reason, we need time to clarify the regulatory framework and ensure non-discriminatory grid access. In addition, all customers and system manufacturers have to agree that Siemens will provide the interface," said Wilfried Breuer, member of management at TenneT. He would like to see a pilot project and does not expect the technology to go into widespread use before 2024 because current contracts have already been awarded. And that is also the second obstacle. When it comes to tenders, two offers are always better than one.

Torsten Thomas

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