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IN DEPTH: A bigger test

Gargantuan as the coming generation of ultra-large offshore wind turbines will be, they are more than matched in size and technological complexity by the giant test rigs being built to trial these machines for deployment in the gale-force gusts off Northern Europe.

The 10MW “function tester” charging up this month at the Lindø Offshore Renewables Centre (Lorc) in Munkebo, Denmark, joins Britain’s National Renewable Energy Centre (Narec) and Germany’s Fraunhofer IWES DyNaLab as frontline testing facilities in the region’s biggest offshore construction campaign since the North Sea oil adventure began 40 years ago.

But the Danish rig will be the first to cast a light on the complex interplay of external forces — including wind loads and network connectivity — and internal conditions, such as generator speed on the electrical dynamics on the complete nacelle, from the rotor hub to grid link.

Lorc’s importance to the offshore wind sector can be seen in its heavyweight backers — turbine makers Vestas and Siemens, developers Dong and Vattenfall, and Danish R&D centre DTU — and the first supersize nacelle slated to be hoisted up onto the new test dock, Vestas’ 8MW monster.

Though the flagship V164-8.0MW has been up and turning at the country’s national testing centre in Østerild since spring following in-house drivetrain trials on Vestas’ own test rig at Aarhus, the DKr116m ($20.9m) Lorc rig will offer something different for the second nacelle: an integrated programme checking transmission system, power converter, medium-voltage switchgear, transformers, inverter, “human-machine interface” and control system, all wired into an offshore-specification 33kV grid-link.

“One of the main things that sets our rig apart from others around Europe and the rest of the world is that ours includes the rotor hub,” states Lorc project manager Lars Stylsvig Rasmussen.

“This is important because it enables us to tie in the pitch system and this gives us a very realistic behaviour on both the hardware and software in the turbine because it means we can feed back a torque reference to the rig motor.

“So you have an exact simulation of doing these same tests on site [with a fully assembled turbine] with the advantages of doing so in a closed-loop, controlled environment.

“This rig will be able to perform the most realistic trials in the world.”

The 3,000-tonne GE-built function tester, which measures 29 metres long, 12 metres high and nine metres across, is designed around a 400-tonne permanent-magnet-motor-driven torque system with misalignment coupling that can wrench a drivetrain with up to 12.5MNm — dwarfing the power of the largest industrial rock crushers.

When free of load, the dock’s highly dynamic motor can gear up in milliseconds to generate full operating torque for “optimum simulation of real-world wind conditions”.

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The Lorc test stand also distinguishes itself for its networked electrical functionality. A nacelle on the rig can either be run with real-life 33kV grid connection linked in to a turbine’s high-voltage transformers and switchgears, or a fully controlled “artificial” set-up fed by frequency converters.

“The direct connection gives you a more realistic view when you want to measure the harmonics of a turbine onto the grid, whereas when you want to ‘manipulate’ the grid to see how the turbine might behave in different situations you can use the ‘artificial’,” says Rasmussen. “It’s good to have both options to really know how a turbine will perform once offshore.”

First sketched out in 2009 on the site of Maersk’s recently vacated Odense Steel Shipyard in Munkebo, Lorc navigated its way forward in developing the nacelle test rig by juggling the somewhat conflicting demands of its major shareholders — OEMs and developer-utilities.

“There is a difference in their various philosophies in testing and overall needs,” says Rasmussen. “Turbine makers wanted to prove mechanical reliability, robustness and output, and the utilities of course wanted to test for grid compliance and electrical performance of the products they are going to buy.

“It comes down broadly to a functionality test versus an overload test and it was seen early on that it was not efficient to combine the two tests in one rig, because if a company was interested in one side or the other, they would occupy the whole bench.”

Lorc opted for a twin-dock set-up with the function tester being brought on line to start, and a Halt (highly accelerated lifetime test) rig now in the final design stage with a view to start-up in 2016.

With only Narec and Fraunhofer IWES offering similar-scale nacelle labs to Lorc in Europe, there has understandably been “very big interest” in the Danish facility from offshore wind players.

While Vestas has the function tester locked up for the next 30 months to test its second V164 nacelle, in the longer run, Lorc foresees the dock being used for end-of-production-line tests — quick functionality double-checks of nacelles coming out of final assembly for deployment offshore.

“We had a lot of different concepts in the beginning on how to do testing of full-scale nacelles and I think we came up with a very good solution in dividing it into two stations,” notes Rasmussen. “In the final analysis, with two rigs you get more components and nacelles moving through the facility.”

The function tester is being readied to be run-in with its maiden DUV (device under test), a 3MW Vestas V112 nacelle, before the V164-8.0MW unit arrives at Lorc for installation early in 2015.

“This [function testing programme] will doubtless be very useful for Vestas but like most things in the offshore wind industry it will be a learning experience for everyone,” states Rasmussen.

“We also want to gain some knowledge in how to efficiently verify these turbines. We need to build up expertise in testing and this is a good way to start.”

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