reproduction of dynamic load cases on a 13.2 mw test ... · capacity of 16.8 mw. the complexity of...

Reproduction of dynamic load cases on a 13.2 MW test facility for wind turbine gearboxes

F. De Coninck1, W. Desmet

1, P. Sas

1, J. Peeters

2, R. Huijskens

2, D. Leimann

2

1 K.U.Leuven, Department Mechanical Engineering

Celestijnenlaan 300 B, B-3001, Heverlee, Belgium

email: [email protected]

2 Hansen Transmissions International,

De Villermontstraat 9, B-2550 Kontich, Belgium

Abstract In the period 2008-2010, a highly dynamic test facility for multi-megawatt wind turbine gearboxes has

been installed and validated at Hansen Transmissions, Lommel, Belgium. The purpose of the test-rig is to

introduce realistic loading conditions on a test gearbox. The rig is constructed as a back-to-back gearbox

set-up and is one of the largest in the world. It has a nominal power of 13.2 MW and a peak power

capacity of 16.8 MW. The complexity of applying dynamics was tackled by the concept of load cases.

Each load case represents a specific part or phenomenon in the wind turbine behavior. The development

process was split in four phases in order to ensure functionality and robustness in a safe way. Therefore a

1/600 scaled set-up was built on which the control architecture was developed that handles the complex

interaction between the mechanical dynamics and the electrical controller of the test rig. The results on the

scaled set-up were satisfying and led to the validation of the control concepts on the 13.2 MW test-rig with

reproduction of the dynamic load cases for a test configuration featuring two different multi-MW test

gearboxes. The test rig fits in R&D activities such as the experimental validation of dynamic load

simulation models and an extension of the limits of standard prototype validation tests, including new

durability approaches.

1 Introduction

The wind energy industry is quickly growing and the output of new wind turbines increases rapidly. The

need for high product reliability challenges the development of new concepts and forces the manufacturers

to invest in fundamental and elaborate experimental validation of the turbines and subsystems. However,

testing the total system reliability of a wind turbine as a whole in a short term is not straightforward. The

location where the wind turbine is installed is often difficult to access. Wind loads cannot be controlled

while acceleration of reliability tests requires accurately controlled (laboratory) testing. The development

of the 13.2 MW test-rig at Hansen Transmissions started in 2004 with a large investment to develop and

build a highly dynamic test facility for the new generation of wind turbines. Its main purpose is to prove

and improve future concepts of gearboxes for wind turbines. This paper first introduces the functionality

of the test rig and some technical characteristics in order to benchmark the dynamic capabilities of the

Hansen test rig against other installations. Based on this background, the reproduction results for two

highly dynamic load cases are presented. In the last paragraph the main conclusions are presented.

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2 Functionality of the test facility

2.1 A challenging development

The development of a multi-megawatt facility is never straightforward. In this specific case and

considering the dynamic aspects of wind and electricity grid behavior, it was a complex task and a real

challenge. The main aspects to be considered are:

wind turbines are highly dynamic applications and this has to be considered for reproducing

real dynamic operational conditions in a controlled environment, where accuracy and

repeatability are highly important

the size and capacity of the test rig should match the continuously increasing wind turbine

power in the market with consequent increasing gearbox size

a synergy between a mechanical design and a complex electrical controller system is required

for which no straightforward solution is commercially available

2.2 The concept behind the test-rig

The rotor in a wind turbine transfers wind energy into rotational energy, which is consequently transferred

via the drive train to the generator to finally produce electric energy. The majority of currently installed

wind turbines are gear driven [1]. This means that a gearbox increases the slow rotational speed of the

rotor to the high speed of the generator and reduces torque with the same ratio. The main advantage of

gear driven concepts is an important saving in generator size and, thus, in total weight and cost of a wind

turbine. From the wind turbine loads at the rotor hub, mainly drive train torque is determining for the

design of a typical wind turbine gearbox. This torque will vary with wind speed and thus results in various

possible dynamic effects. In addition, the torque on the gearbox at the generator side can also be

influenced by events occurring in the electricity grid. Furthermore, loads change in combination with

rotational speed fluctuations. These simultaneous variations define the highly dynamic loading of a

gearbox in a wind turbine. Knowing and understanding this loading is important to guarantee a reliable

drive train design. Wind turbine manufacturers predict these loads typically with dedicated aero-elastic

simulation codes for so-called design load cases as specified by standards and guidelines, e.g. [2,3]. Much

research is also spent on improving these simulation methods [4] as well as on learning what actually

happens via measurements in wind turbines [5].

2.2.1 A vision on wind turbine behavior

The functionality of the test facility is based on the vision that it is possible to transform wind turbine

behavior into test rig conditions. Therefore, the starting point is the current knowledge of the typical

gearbox loading in a wind turbine, both from simulation of design load cases and experience from

measurements. Figure 1 shows how the vision is implemented, going from a real wind turbine to the

configuration of the new test facility, being a controlled back-to-back set-up. The left side of the back-to-

back gearbox test rig represents the ‘wind & rotor’ and is composed of an electrical machine (motor 1), an

optional speed reducer (3:1 gearbox) and a wind turbine gearbox (gearbox 1). The right side represents the

‘grid & generator’ and is composed of an electrical machine (motor 2) and an optional speed reducer. The

optional speed reducers enable full power testing of wind turbine gearboxes in future so-called hybrid

concepts where a low-speed generator is combined with a gearbox with a lower ratio, e.g. for a nominal

generator speed at 500 RPM rather than the typical speed of a wind turbine generator at 1500 RPM.

Analogous to the operation of a wind turbine, the ‘wind’ side of the test rig is speed controlled, whereas

the ‘generator’ side is torque controlled. In between, the test gearbox (gearbox 2) is driven at a certain

2790 PROCEEDINGS OF ISMA2010 INCLUDING USD2010

time varying speed by the ‘wind’ and loaded with a certain time varying torque by the ‘generator’ and thus

experiences test conditions very similar to wind turbine behavior.

Figure 1: Concept behind the gearbox test-rig

2.2.2 Mechanical design

Figure 3 shows the mechanical design of the test facility on the left and a picture of the actual setup on the

right (note that both figures do not include gearbox 1 or gearbox 2). The location of this facility is the

production plant of Hansen Transmissions in Lommel, Belgium. Its foundation is 35 m by 10 m and a total

of 1000 tons of steel and over 1000 m³ of concrete are used for the rig. Both motor and generator

platforms are moveable in 3 directions for accurately aligning the complete test setup. The total facility

represents an investment of over 10 M€.

From a dynamic point of view, the test rig can be approximated in its most simple form as a system with

two inertia’s J, mainly from the motor rotors, and one torsion spring kt from the gearboxes, shafts and

couplings in between. This resonant frequency is the fundamental frequency that limits the control

performance. The schematic is shown in figure 2 and the frequency given by:

22

J

ktf

Figure 2: Two inertia equivalent system

Rotor 1

Rotor 2

MONITORING AND DIAGNOSTICS OF ROTATING MACHINERY 2791

Figure 3: Mechanical design of the test-rig (left) and picture of the actual test-rig (right)

2.2.3 Performance envelope

The general performance specification is expressed in terms of power, speed and torque limits. The

nominal speed is 1500 RPM and the maximum speed 2600 RPM. At nominal speed, the nominal torque is

84 kNm resulting in 13.2 MW of nominal power. Peak torque at nominal speed is 107 kNm, resulting in

16.8 MW of peak power. This peak performance is limited by the current density in the power electronics

and the capacity of their cooling system, which puts limits on the duration of working in peak conditions.

The torque limit beyond 1500 RPM is determined by the power limit, implying a torque decrease

according to a constant power hyperbolic curve up to 2600 RPM. The chosen power levels put high

demands on the mechanical parts where reliability and safety is crucial. For example the low-speed

coupling in between the gearboxes is designed for a nominal torque of 50 MNm.

2.3 Transforming wind turbine conditions in to characteristic ‘load-cases’

The performance of the test facility enables the engineers to reproduce many effects of wind and grid

dynamic loads. From the available data, typical phenomena have been identified and translated in 11 fully

parameterized ‘load cases’. These load cases are divided in four groups, based on their nature:

1. Start cases (3): represent possible ways a wind turbine can start; each start case is followed by a run

case.

2. Run cases (4): represent what can happen when a wind turbine is in operation; each combination of a

start case and one or various run cases always ends with a stop case.

3. Stop cases (3): represent possible ways a wind turbine can shut down

4. Specials (1): represent special cases which include starting and stopping in one; only one special case

is currently implemented which represents an overspeed test of a wind turbine

All load cases are specified by the required time behaviour of speed and mechanical torque at the high

speed shaft of the tested gearbox. The load cases are modular and can be sequenced in batches.

2.4 Benchmarking the 13.2 MW back-to-back test-rig vs. other approaches

It is important to notice that every test-rig is built with a specific purpose, and that by the choices made

during the design it is only able to generate a limited set of the real-world effects. This is also true for

field testing of wind turbine prototypes, as the weather conditions are not-controllable and it is difficult to

install equipment to accurately measure all the operational parameters. Laboratory testing provides a

well-controlled environment and easy access for measurement equipment in a less harsh (indoor)


environment. The following section benchmarks the 13.2 MW electrical back-to-back approach vs. other

approaches and highlights the most important technical choices that determine the dynamic potential of

the test rig.

2.4.1 Scaled test-rigs

In the development process of the 13.2 MW test facility, it was decided to master the complexity of

dynamic testing by using a 1/600th scale model boasting two 22 kW electrical machines. This approach

takes full advantage of the scalable properties of such a system. The identical control software runs on

both test-rigs, only differing by a scaling factor on input and output. This allows the engineers to safely

test and iterate during the controller implementation. This process was split into four phases [6] for which

this paper presents the results of the last phase: reproduction of dynamic load cases on the 13.2 MW test-

rig. The analogy for the 1/600th scale model is given in Figure 4. The scaled model features perfectly

scaled motor characteristics (power, torque, nominal rpm) and inertia. This required the design of

additional inertia elements, which are introduced before the torque sensor. The gearboxes that are used on

the scaled test rig are three stage planetary units and have a nominal power of 60 kW. Two gear ratio

configurations are used, a symmetrical configuration with a gear ratio of 68 vs. 68, and an unsymmetrical

configuration with a gear ratio of 68 vs. 46. The developed controller functionality is fully able to cope

with any combination of non-identical gearboxes in terms of gear ratio and (non-)reversing behavior. An

important issue during the development was how to cope with

the control deals with gearbox non-linear stiffness.

Figure 4: Scaled test setup for the development of the controller functionality

The scaled set-up also differs from the full scale test-rig in some aspects.

The power rating of the grid power converter is three times larger than the scaled value. This

allows the engineers to very dynamically drive the scaled test-rig, while observing the effects on

the grid power flow, but without suffering from emergency shut-down when exceeding the scaled

value.

Encoder Inertia

Torque sensor

Gearbox


Due to the higher power rating on the gearboxes, the mechanical losses percentage is higher. In

other words, more damping is present in the scaled set-up.

The available motor model in the motor controller is of lower complexity, which introduces some

non-linearity in the torque-response, which the test-rig control software has to compensate for.

The scaled test rig is therefore very suited for the development of torque and speed control functionality,

develop safety functionality, test robustness, evaluate the performance of the ‘load-cases’ functionality in

terms of reproduction accuracy, evaluate electrical interactions between test rig and electricity grid … It

is however not suited to evaluate any gearbox-related effects, as the gearbox concepts are radically

different and therefore not scalable. This fully justifies the use of a full-size facility.

2.4.2 Mechanical loop test-rigs

A new test bench for the multi-megawatt range was introduced by Winergy [7,8] in 2007. This test rig has

a power rating of 14 MW and is based on a mechanically looped back-to-back arrangement of two

gearboxes . It is not known if this test rig has dynamic capacity and if so, what the bandwidth is.

Figure 5: 14 MW mechanical loop test rig

Two aspects with regard to possible dynamic testing need to be discussed for this test rig:

1. High-speed shaft referenced backlash (backlash) and torsional stiffness(kt): The torque loading

in this type of test rig is applied by introducing a phase shift in the mechanical power loop. This

phase shift (or angle shift) is possible due to the finite torsional stiffness of the mechanical loop.

In the case of a torque reversal, which is one of the most demanding dynamic events, the total

amount of backlash also needs to be crossed. Considering a torque reversal dynamic event from

+T to –T, the total angle shift is equal to:

backlash

t

Treversalk

T 2

and needs to be traversed in a time equal to half of the cycle time. The actuation system, which

can be hydraulic (lever-arm) or electromechanical (spindle, gear-based,…) has:

a maximum actuation speed referenced in angle phaseshift (rad/s) or in torque

torqueshift=phaseshift/kt (Nm/s), leading to a maximum frequency freversal= phaseshift / (2*Treversal)

a maximum actuation force, which is function of the amplification factor used in the

actuation system (gear, lever-arm,…), and which should exceed the peak gearbox torque

induced on the actuation system. In an idealized form, the required torque loading energy

is equal to the preloading of the torsional stiffness and takes the following form:


t

t

t

Ttkt

k

Tk

Tk

kE

*2

²

2

²

²*

2

²*

therefore, the power rating of the actuation system for the mechanical loop test rig

amounts to 2*Ekt*freversal. In comparison with the Hansen test rig, the Winergy rig would

require at least a 250 kW actuator system.

2. Dynamic event reflections: a more important issue in the mechanical loop system is that there is

no controller influence on the energy flow for dynamic events. For the fundamental frequency

and higher harmonics, there is no possibility to break-up the energy accumulation or introduce

additional damping. This limits the dynamic potential of the mechanical loop test rig.

The mechanical loop test rig is therefore only suited for quasi static (over-)loading of multi-MW

gearboxes. The investigation of dynamic events and the use of HALT methodologies with excitation

above the fundamental frequency is therefore out of scope for this type of test rig.

2.4.3 Nacelle test-rigs

A very recent evolution is the concept of nacelle test-rigs [9-11]. In this concept, a full wind turbine

nacelle is mounted on the test-rig. The wind loading vector is introduced in a dual approach:

1. Wind torque from a direct drive electrical machine (multi-pole) or from a combined unit of an

electrical machine and a gearbox (very similar to the wind-side part of the Hansen test rig).

2. Wind loads and off-axis torques from a 5-DOF hydraulic actuation system

Two systems are being developed. The MTS system uses a multi-pole electrical machine as the prime

mover. The second system is a 50M€ project to be installed at NAREC in 2011 and code-named ‘Fujin’.

It uses an electrical machine and a reduction gearbox to achieve the turbine torque, although direct drive is

not excluded in the specification yet.

Figure 6: MTS nacelle test rig (left) / NAREC project 'Fujin' (right)

Nacelle test-rigs clearly have a focus on system-level testing. By introducing the 5 additional non-torque

nacelle loads and testing the integrated nacelle system it is believed that reliability issues that otherwise

only occur in the field can be detected.

The non-torque loading system can be made very dynamic (>2Hz) as the nacelle main shaft stiffness is

very large and the displacements are limited. For the torque system, the setup is very similar to the

Hansen test-rig. The dynamic performance will therefore mainly depend upon the control system


performance. Except for cost, direct drive or gearbox based systems are fairly equivalent for dynamic

testing with the exception of two aspects:

1. Angular resolution: For measurement and control of the backlash angle, the angular sensor on

the direct drive needs to have comparable generator side resolution, which means that the required

direct drive encoder resolution is equal to the generator side resolution multiplied with the

gearbox ratio. Typically, this is 6-7 bit more accurate than a gearbox based system. The multi-

pole machine drive system however also needs this accurate information, so this is foreseen.

2. Backlash & flexibility: In gearbox based system, the additional gearbox introduces, next to some

proper dynamics, additional backlash and flexibility in the control loop. This will reduce the

dynamic range of the test rig as compared to the direct drive system.

In general the main source of flexibility and backlash is to be found at the rotor side shafts, couplings and

gears. Here the nacelle test rigs have a drawback for torque based dynamic events, because of the

increased low speed shaft length due to the introduction of the non-torque loading system. The ‘Fujin’

system also boasts a smaller (3 MW vs. 4 MW for Hansen) grid connection with presumably more system

inertia, which will limit the possible speed ramp dynamics.

3 Load case reproduction

From the previous discussion, it is clear that the true dynamic performance of a multi-MW gearbox test-

rig is a complex engineering question. Therefore the performance is estimated from simulation, but

validated trough testing. In this section one identification and three highly dynamic load cases from the

dynamic testing of two fundamentally different multi-MW gearboxes in back-to-back arrangement on the

Hansen test-rig are discussed. First the aspect of the test configuration is introduced.

3.1 Configuration of the test rig

The first step, after mounting the gearboxes on the test-rig, is the definition of the test configuration.

When considering the layout of figure 1, the choice needs to be made which gearbox is gearbox 1, the

sense of rotation and the sign of the applied torque. This amounts to 8 test configurations. When

respecting the vision on wind and grid-side, the sign of the applied torque follows the sense of rotation and

only 4 options, or quadrants, remain. All dynamic testing discussed in this section was performed in all

four quadrants, with the differences (gear ratio, nominal torque,..) automatically handled by the control

system.

Next to the gearbox speed and torque configuration, all necessary safety limits for speed, torque and

electrical power are defined in this step. The control actions that are required upon exceeding a safety limit

have been developed on the scaled test rig.

3.2 Stiffness and backlash measurement

A first load case, which introduces a sinusoidal slowly reversing torque, is used for the measurement of

the non-linear stiffness and the backlash of the complete test-rig system. The test can be performed at any

RPM and peak torque level. More detailed backlash and stiffness information can be obtained by further

instrumenting the individual shafts and couplings in the drivetrain. Figure 7 shows a typical result for

which the torque is normalized to the nominal torque of the smallest gearbox and the angle is normalized

to the angular safety limit. The curve is centered in the Y-axis to compensate for the rotation losses as the

system is running at low speed (200 RPM). The main form of the curve is composed of a loading zone

(left&right) and a backlash travel trough zone (middle). In the backlash zone (-2 to 2 % of nominal

torque) one can identify three steps which correspond to the three gear stages. Between the loading and

unloading curve a hysteresis zone exists, which is due to energy losses. In the loading curve on the left,


two tangents are shown to indicate the non-linear (stiffening) behavior of the drivetrain torsional stiffness.

The linearized stiffness (black dash-dot line) is also given to show the symmetry in the gearbox behavior

and to justify the use of a linearized stiffness in simulation models.

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

normalized angle

norm

alize

d T

orq

ue

Figure 7: Backlash and non-linear stiffness measurement result

3.3 Torque reversals

As mentioned in 2.4.2, one of the most severe load cases is a torque reversal. This load case can be used

to introduce (two-sided) cyclic loading for fatigue/durability testing, or to investigate the behavior of

backlash and non-linear torsional stiffness under different conditions. The most important limitations for

this load case in terms of cycle time are the test-rig fundamental eigenfrequency and the backlash angle.

Figure 8 shows a measurement of the generator side gearbox HSS (high-speed-shaft) input torque for such

a load-case. The top graph gives the time overview, the bottom curve shows a zoom on the torque

reversing load-case. The torque level alternates between 28% and -28% of the nominal torque rating.

About 30% of the cycle time is spent traversing the backlash zone (at almost zero torque). When

subtracting this time from the cycle time, the torque loaded parts of the loadcase have a cycle frequency

that is at 64% of the fundamental frequency. A well controlled amplification of 32% is observed. The

amplification and bandwidth for this test are function of the system and controller tunable damping. More

damping results in less amplification, but also a smaller bandwidth. It is important to notice that the

response peak level is consistent in time, and that it is very feasible to slightly modify the demand level in

order to exactly achieve the desired response measurement.

backlash

hysteresis

Non-linear stiffness

linearized stiffness


-0.2

0

0.2

Norm

alize

d T

orq

ue

overview

Torque demand

Torque measurement

-0.2

0

0.2

zoom

time

Norm

alize

d T

orq

ue

Torque demand

Torque measurement

Figure 8: Comparison of demand and reproduced torque for a specific exemplary batch of Start-

Run-Stop load cases. The Run load case represents the occurrence of torque reversals at the

generator side.

3.4 High-frequency preloaded cycling

A second dynamic load-case is the high frequency preloaded cycling. This load case has a double

purpose. Because a certain preload or mean torque is present, the backlash is not solicited and the system

responds according to the pre-loaded dynamics. By superposing a sinusoidal torque variation, for which

the frequency is not limited by the fundamental resonance, the response of the global system and of local

subsystems can be investigated. For durability testing this load-case can be used for accelerated lifetime

tests or for investigation of subsystem resonances

0

0.1

0.2

0.3

0.4

Norm

alize

d T

orq

ue

overview

Torque demand

Torque measurement

0.2

0.25

0.3

0.35

0.4

zoom

time

Norm

alize

d T

orq

ue

Torque demand

Torque measurement

Figure 9: Comparison of demand and reproduced torque for a specific exemplary batch of Start-

Run-Stop load cases. The Run load case represents a sinusoidal torque variation superposed on a

mean torque level.


Figure 9 shows a measurement of the generator side gearbox HSS (high-speed-shaft) input torque for such

a load-case. The top graph gives the time overview, the bottom curve shows a zoom on the preload torque

with superposed sinusoidal variation. The mean torque level is 30% of the nominal torque rating with a

+/-10% oscillation. The phase angle between demand and measurement is about 90° which indicates that

this test was performed near the fundamental resonance. Instead of yielding a destructive amplification,

the control system stabilizes the test rig and the achieved torque level is about 75% of the demand. Again

the response peak level is consistent in time, so that by modifying the demand level an exact response

measurement can be achieved.

3.5 Grid induced torque peaks

The last load case example, which was already presented for the scaled test setup in [6], concerns short –

time (typically <1s), grid induced torque peaks at the generator side gearbox. These torque peaks

originate from phenomena on the electrical side (generator, drive, grid) and induce high dynamic loads in

the mechanical drivetrain. In order to investigate the influence on the gearbox (gear contact, bearing

condition,…) these short-time high load events are very useful. .

Norm

alize

d T

orq

ue

overview Torque measurement

time

Norm

alize

d T

orq

ue

zoom Torque measurement

0

0.5

1

Norm

alize

d T

orq

ue

overview

Torque measurement

0

0.5

1

time

Norm

alize

d T

orq

ue

zoom

Torque measurement

Figure 10: Measured torque at the HSS of the generator side gearbox for four different types of grid

induced torque peaks

Figure 10 shows a series of four different torque peaks, each of which is repeated five times. The mean

torque level is 20% of the nominal torque and the added overload peaks vary between 65-90% of the

nominal torque. This test was performed at nominal RPM which shows that the test rig is able to induce

and remove almost the full load of a multi-MW gearbox in less than a second.


4 Conclusions

This paper presents the final step, taken by Hansen Transmissions, in collaboration with KULeuven, to

develop and build a highly dynamic test facility for testing wind turbine gearboxes under realistic

conditions. In this paper reproduction results for dynamic load cases are presented. It is shown that the

functionality to perform these load-cases and the necessary safety and robustness is achieved. It is shown

as examples that the non-linear stiffness, backlash and hysteretic losses can be quantified. Three highly

dynamic load cases: torque reversals, high-frequency preloaded cycling and grid induced torque peaks can

be reproduced.

It was shown in a benchmarking exercise that amongst the few test-rig alternatives, the Hansen 13.2 MW

test rig is a unique facility with a clear focus on gearbox dynamic behavior. The 13.2 MW test rig will

facilitate R&D activities within the design, simulation, test and validation departments. With its test rig,,

Hansen Transmissions has made an important step in extending today’s testing methods for further

proving and improving future concepts for gearboxes in the next generation of wind turbines.

Acknowledgements

The R&D activities on the 13.2 MW test rig are performed in the framework of a research project

supported by the Institute for the Promotion of Innovation by Science and Technology in Flanders

(IWT Vlaanderen).

References

[1] BTM Consult ApS. International Wind Energy Development, World Market Update 2008. March

2009.

[2] ISO-IEC 61400-1 Third Edition. Wind turbines – Part 1: Design requirements. August 2005.

[3] Germanischer Lloyd Windenergie. Guideline for the Certification of Wind Turbines. Edition 2003

with Supplement 2004.

[4] Peeters J. Simulation of dynamic drive train loads in a wind turbine PhD dissertation, K.U.Leuven,

Department of Mechanical Engineering, Division PMA, Leuven (Heverlee), Belgium, 2006.

available online: http://hdl.handle.net/1979/344

[5] Söker H, Klosse R, Schwarte J, Winkelmann J, Peeters J, Ongena C, Hauptmann S. PROTEST –

Procedures for Testing and Measuring Wind Energy Systems, Drive Train Case Study. Proceedings

of German Wind Energy Conference DEWEK 2008.

[6] J. Peeters, D. Leimann, R. Huijskens, F. De Coninck, First Results of Hansen’s 13MW test facility for

wind turbine gearboxes, Stokholm, Sweden, September, EOW 2009

[7] Hanspeter Dinner , Trends in wind turbine drive trains, www.EES-KISSsoft.ch

[8] Winergy Latest (newsletter), www.winergy-ag.com

[9] MTS Wind Power Solutions brochure, www.mts.com

[10] ‘Fujin’ datasheet, www.narec.co.uk

[11] Stuart Herbert, Full Scale Testing Facilities – Ensuring Reliability for Offshore Wind, European

Wind Energy Conference EWEC, Warsaw, Poland, 2010 April 20-23


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