Windtech: Combined component testing in 8MW rig

GERMANY: A full-scale test bench for drivetrain components and complete nacelles up to 8.xMW was inaugurated in October at the Fraunhofer IWES research institute.

Scale… The Dynalab test rig

The dynamic nacelle testing laboratory, or Dynalab, in Bremerhaven, northern Germany, cost €35 million and caters for both geared and direct-drive models. It stands out over other test rigs by incorporating the world's largest and most advanced grid simulator.

The test rig focuses on full system tests, unlike other more traditional rigs that use highly accelerated life testing (HALT) for individual drivetrain components, explained Jan Wenske, wind turbine and system technology head at Fraunhofer IWES.

The HALT methodology is widely used in the wind industry, especially for gearbox life testing, where 20-25 year design life is typically reduced to only three to five months by applying 200-300% overload.

Actual conditions

Dynalab full-system tests aim to get closer to the true conditions faced by all components in a drive system and provides three key benefits, said Wenske.

"The first benefit is in substantially reducing the prototype field-testing period, the second in minimising product development risk, and the third and final benefit is in achieving better reliability and more efficient final products." The total package could help turbine suppliers with early stage detecting and solving technical issues, he said, speeding up type certification from around two years in the field to three to six months.

Dynalab's characteristic tapering steel mounting foot (left) was prominent at the inauguration of the rig, with a large-diameter direct-drive generator attached, the first full-scale drive component to be officially tested. By pushing the big red button at the rig, the modular generator developed by German company Jacobs Powertec started turning. This marked the official start of the test rig, and the electrical certification testing of a first full-scale drive component.

Development of this new generator design is still at an early stage said Wenske: "In a first system analysis and characterising step the experts check all essential elements of the generator and main bearing. The value of such concept analysis is that potential weak points come to the surface, and we can determine the most suitable test profiles and scenarios."

Industry interest

Adwen will be the second client, with drivetrain testing of its 8MW medium-speed offshore turbine, which has a record 180-metre rotor diameter. The comprehensive programme involves simulation of fatigue enhancing conditions and operation in extreme-weather conditions offshore. The verification-test programme covers all the main components of the drivetrain, including rotor bearings, main shaft, gearbox, intermediate coupling, generator and power converter. Maite Basurto, chief technical officer at Adwen, believes that the Dynalab testing will contribute to speeding up the type-certification process of the AD 8-180, as well as produce a more reliable turbine once serial production starts in 2018.

The third product on the Dynalab test bench will be a novel high-temperature superconductor generator being developed by German firm ECO 5.

Because all these components and systems require dedicated Dynalab mounting supports, various modular adaptations will be made available to accommodate nacelles, complete drivetrains and drive components.

Dynalab took six years to develop and build. With no standard guidelines for advanced test benches of this nature, Fraunhofer experts began to develop the main specifications in-house, said Wenske, and then teamed up with Spanish test-rig specialist Idom and two main contractors, power engineering giant ABB and German electric drives specialist LDW for the product design and implementation phases.

Lab conditions

The Dynalab set-up always has to deal with compromises, Wenske admitted. Actual field testing allows a real full-scale offshore turbine to be placed at hub heights of 100 metres and more. This inherently means highly dynamic conditions from the continuous exposure to the combined forces and impacts of wind and waves.

"These real operating conditions are impossible to simulate completely and with sufficient accuracy in a lab," said Wenske. "On the other hand, in a lab, clients never have to wait for sufficient wind and there is no need to ask permission from the utility to conduct combined wind and electrical system behaviour field tests. Perhaps the most important benefits of lab testing are the strong scientific basis of the testing plus the fact that nobody in the wind industry likes to field test prototypes for a 20-year design life period."

For conducting drivetrain and full nacelle testing, Dynalab experts can either apply simulation modes derived from measuring real wind and electric grid conditions or use in-house created artificial test cycle modes. Both simulation mode options aim to test and verify specific system and operating characteristics. The artificial test modes specifically aim to find out whether the assumptions made are indeed right and if and where there could be a need or further room for concept optimising.

Grid simulator

With a rated capacity of 44MW and three external transformers to provide internal medium voltage power supply, Dynalab's grid simulator is considered top of the bill, Wenske said. The system can emulate many situations like controlling each of the three grid phases individually, or allowing the grid voltage to drop from 33kV to 0 volt in a few thousands of a second, remaining at that level for 50-500 milliseconds and then rise up again. The system is also capable of emulating grids up to 440MW capacity by virtual impedance control, can simulate high-voltage ride-through scenarios and multiple grid dips as found in storm conditions.

Grid short circuits and system impacts resulting from pitch-system faults can be introduced, with all possible impacts studied and tested experimentally. Most important, said Wenske, is that all thinkable grid events and wind-load scenarios can be emulated realistically and reproduced in all possible combinations.

Wenske has observed that smaller OEMs in particular still think largely on a component level, believing that models and simulations are possible for everything. "Some even see model simulation as a reliable technology validation tool," he said. While such faith in the validity of computer simulations leads them to believe that advanced simulation models could become a realistic alternative to real testing, the component level testing misses one key test area.

"These experts should consider all the uncertainties that can accumulate between a specific wind volume approaching a rotor and its complete path through a wind turbine system before it actually becomes a load," Wenske said. "In other words, how do we know exactly what goes in and out a system, and what happens in all interfaces and transfer functions between individual components?"

With the Adwen AD 8-180 tests, Dynalab will come close to its nominal maximum capacity. The input torque of this giant is in the 8,500kNm range, whereas Dynalab has 13,000kNm peak and up to 9,700kNm nominal capacity, depending slightly on cooling conditions. Today's largest test benches for HALT testing such drivetrains would, by comparison, require at least double the input torque capacity.

Conducting complete system testing at Dynalab is not cheap, with a full-service package costing about EUR16,000 per day. "The industry will need time to realise the full benefits of Dynalab bench testing. Several industry players have already expressed fears that such tests could become an accepted certification demand similar to what already happened for rotor blades," Wenske concluded. At present, suppliers must submit detailed calculation and modelling results as part of turbine type certification. If Dynalab testing were to become compulsory, this would involve substantial extra effort and costs, which could raise market-entry barriers, especially for smaller OEMs.

HOW IT WORKS: Dynamic nacelle testing laboratory (Dynalab)

The 30-metre high Dynalab hall covers 1,000 square metres, and the overall dimensions of the rig are sufficient to accommodate even the largest direct-drive ring generators up to a 12-metre outer diameter, said Wenske. About 3,500 cubic metres of reinforced concrete was processed for the heavy floor and support/mounting structures for Dynalab's drive components, additional drivetrain/nacelle testing structures and the Stewart platform.

A Stewart platform is essentially a non-rotating load-frame with hydraulic cylinders like a flight simulator that simultaneously allows multiple dynamic movements with six degrees of freedom. Within Dynalab's system configuration it is combined with a rotating output shaft and the assembly can simulate all "normal" wind-turbine-rotor-induced loads transmitted to the drivetrain. These include the axial wind thrust, bending moments, rotating input torque, rotor gravity loads, rotor impact due to yaw actions, vertical and horizontal wind shear due to rotor-plane wind-speed differences. The Stewart platform output shaft is similar to "normal" rotor hubs rigidly attached to a turbine drive system: direct drive, geared and with/without nacelle.

Dynalab drive power is provided by two rigidly coupled 5MW electric motors (total 10MW nominal/15MW peak), as an assembly "flexibly" connected to the Stewart platform via a specially developed homokinetic link-coupling between two shafts, which prevents bending moments and other dynamic loads introduced by the Stewart platform toward the specimen from being passed back into the electric motor drives, potentially reducing their operating life and impacting testing accuracy performance.