Thorough testing

Thursday, 21 November, 2013 - 14:45

A wind turbine must always be tested as a complete system. Due to the large dimensions, of course this is not possible in the laboratory, but only in the test field. The largest component that can still be tested in the laboratory is the nacelle, where the effects of the rotor and of the electrical network can be modelled as realistically as possible.

The Fraunhofer Institute for Wind Energy and Energy System Technology (Fraunhofer IWES) is going to commission a large-scale test bench at the end of next year in Bremerhaven, Germany, with a planned driving power of 10 MW. The foundations for this were laid in September. The Fraunhofer IWES already has two test benches for testing rotor blades (see OWI 3/2013, p. 28). The nacelle test bench is now to become the centrepiece of the Dynamic Nacelle Testing Laboratory (DyNaLab), which will enable field tests under realistic conditions in the lab and will also house smaller dynamic testing facilities for converters, generators, bearings and main shafts.

An important advantage of the locations is the short transport distance from the heavy duty wharf in the Labrador Harbour to the test bench, which is designed for a maximum nacelle mass of 400 t. The test piece is inserted into the test bench on a self-propelled heavy duty module, set down onto the foundations and then clamped.

Modelling of real-life conditions
The torque generated by wind is reproduced by two synchronous machines arranged in tandem with a driving power of 5 MW each. This enables the introduction of a nominal torque of 8.6 into the test piece. 13 mN.m are also possible in the short term.

However, the thus generated torsion only creates a load in direction of rotation. If gusts of wind, turbulences or angled wind streams hit the rotor, this generates additional bending moments and thrust forces which must also be represented on the test bench. The simulation of these additional wind loads occurs via a hydraulic force transmission.


Six individually controlled hydraulic cylinders transmit different forces to the rotating main shaft of the nacelle via a powerful triangular loading disk (see image).
The hexapod arrangement of the hydraulic cylinders can represent bending moments of approx. 20 mN.m and in addition a shear force of approx. 1.9 mN.

As the rotor is missing as well as the tower, the real conditions must be modelled. The aim is to reproduce the real environment as precisely as possible. For this reason, the Fraunhofer IWES calculates the interaction between nacelle and rotor and uses this information to develop real-time models as well as control algorithms required for this.

Artificial medium voltage grid
The electrical network that feeds into the wind energy plant is also reproduced in the DyNaLab. The artificial medium voltage grid with 44 MVA inverter capacity enables simulation of short-circuits and other short-term events in the network, which impact on the nacelle as complete system. This means the Fraunhofer IWES can represent different load situations under reproducible conditions and test the behaviour of a wind energy plant in case of storm, faulty pitch control, grid outages or emergency stops. The DyNaLab can simulate different operating conditions any number of times and should save time in the end by shortening so far lengthy field tests through usage on the test bench.

But the Fraunhofer IWES is not the only supplier of nacelle test services. There are three additional manufacturer-independent MW test benches, which are roughly equivalent based on the driving power. Some manufacturers also have their own test benches...

You can read the complete article written by Detlef Koenemann
in the latest issue of OFFSHORE WIND INDUSTRY.