Reproduction of dynamic load cases on a 13.2 MW test facility for wind turbine gearboxes F. De Coninck1, W. Desmet1, P. Sas1, J. Peeters2, R. Huijskens2, D. Leimann2 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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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:
2.2
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
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-toback 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
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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:
f
2kt 2 J
Rotor 1
Rotor 2
Figure 2: Two inertia equivalent system
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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)
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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 testrig. 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.
Gearbox Torque sensor
Encoder
Inertia
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.
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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:
Treversal 2
T backlash kt
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:
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T² k t * k * T ² kt ² T ² E kt t 2 2 2 * kt 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
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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 multipole 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 testrig 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,
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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
Non-linear stiffness 0.15
hysteresis 0.1 normalized Torque
linearized stiffness 0.05 0 -0.05 -0.1
backlash
-0.15 -0.2 -0.2
-0.15
-0.1
-0.05 0 normalized angle
0.05
0.1
0.15
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.
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Normalized Torque
overview 0.2 0 -0.2
Torque demand Torque measurement
Normalized Torque
zoom 0.2 0 -0.2
Torque demand Torque measurement time
Figure 8: Comparison of demand and reproduced torque for a specific exemplary batch of StartRun-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
Normalized Torque
overview 0.4 0.3 0.2 0.1 0
Torque demand Torque measurement
Normalized Torque
zoom 0.4 0.35 0.3 0.25
Torque demand Torque measurement
0.2 time
Figure 9: Comparison of demand and reproduced torque for a specific exemplary batch of StartRun-Stop load cases. The Run load case represents a sinusoidal torque variation superposed on a mean torque level.
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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.
Normalized Torque
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
