Abstract:
A wind turbine testing system is disclosed for testing at least a part of the nacelle components of a wind turbine system when mounted on a load carrying structure of a nacelle, said wind turbine testing system comprising a test bench being arranged to hold said load carrying structure including said nacelle components, a grid simulation system comprising a power converter system and a simulation controller being arranged to be electrically coupled to at least one of said nacelle components and being adapted for providing a simulated utility grid on the basis of a power supply and at least one control signal established by said simulation controller, and a wind simulation system comprising a wind turbine shaft rotating means arranged to be coupled to a rotating part of said generator system or a generator-related system of a nacelle or a part of a nacelle located in said test bench.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of pending International patent application PCT/DK2007/000337 filed on Jul. 3, 2007 which designates the United States and claims priority from Danish patent application PA 2006 00913 filed on Jul. 3, 2006, the content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a wind turbine testing system for testing at least a part of the nacelle components of a wind turbine system. 
       BACKGROUND OF THE INVENTION 
       [0003]    A general challenge related to wind turbines is when wind turbines are coupled to weak utility grids. A coupling to a weak grid may under some circumstances result in transfer of, e.g., transients, noise or voltage drops or peaks between the utility grid and a wind turbine coupled to the grid. 
         [0004]    Testing the individual parts of a wind turbine system for the capability of complying with different grid conditions before the wind turbine system is shipped from the factory—often to very distant parts of the world—is, therefore, essential. 
         [0005]    It is, however, not sufficient to make sure, that the electrical parts of the wind turbine system, such as the generator and the power frequency converter, are able to manage simulated grid conditions corresponding to what could be expected during operation of the wind turbine when they are tested as stand-alone units. This is due to the fact that the varying rotation speed of the drive train axis and the mechanical and electrical interactions between the different parts of the wind turbine nacelle have an important influence on the performance of the wind turbine system. 
         [0006]    It is one of several objects of the invention to establish a system which is able to physically simulate a coupling between a wind turbine system and a grid or parts of a wind turbine system and a grid. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention relates to a wind turbine testing system for testing at least a part of the nacelle components of a wind turbine including an electrical generator system, said nacelle components being mounted on a load carrying structure of a nacelle of said wind turbine system, said wind turbine testing system comprising
       a test bench being arranged to hold said load carrying structure of said wind turbine nacelle including said nacelle components,   a grid simulation system comprising
           a power converter system and   a simulation controller being arranged to be electrically coupled to at least one of said nacelle components and being adapted for providing   a simulated utility grid on the basis of   a power supply and   at least one control signal established by said simulation controller, and   
           a wind simulation system comprising
           a wind turbine shaft rotating means arranged to be coupled to a rotating part of said generator system or a generator-related system of a wind turbine nacelle or a part of a wind turbine nacelle located in said test bench.   
               
 
         [0017]    An advantageous feature of the invention is that the characteristics of a wind turbine system or at least part thereof may be tested in a broad physically simulated environment in the sense that wind resulting inputs and grid resulting inputs may be evaluated in combination, thereby providing a unique opportunity to incorporate feedback to and from the grid in a realistic situation or a certain desired test situation. 
         [0018]    The wind turbine testing system according to the provisions of the invention is in particular advantageous for wind turbine systems, such as nacelle, generator, power frequency converters, pitch control, etc., as such systems must be optimized for the purpose of avoiding decoupling from the grid due to deviations from the ideal outputs from the wind turbine system or the grid, so that such decoupling may in fact be avoided or at least be performed by a smooth decoupling. 
         [0019]    Although the wind turbine system can comprise a complete wind turbine nacelle or only parts thereof, a generator must be comprised within the system being tested in order to test the interaction between the wind turbine system and a simulated grid. 
         [0020]    The use of a test bench is advantageous in the sense that the test setup may be standardised and provide reproducible results. 
         [0021]    The simulation controller of the grid simulation system may be one single unit or form part of a distributed simulation controller network. In a further embodiment the power converter system of the wind turbine testing system may be formed by the power inverter of a wind turbine to be tested. In other words, the power inverter may form a part of the external test setup or alternatively be established by the wind turbines own power inverter if such is present. According to a further embodiment, the power converter system of the grid simulation system may be established rather primitively by means of simple resistor(s) combined with an on/off switch controlled by the simulation controller, e.g., a manual switch. 
         [0022]    The purpose of the wind simulation system is to establish resulting movement of moving parts of the wind turbine system as if the system was activated by wind. 
         [0023]    The wind simulation system may relate to any parameter relevant to wind simulation, such as wind speed, temperature, turbulence, resulting torque of the rotating system, etc. 
         [0024]    This wind simulating may be more or less sophisticated, but it is typically important that the resulting wind simulating relates at least somewhat to real conditions. 
         [0025]    The shaft rotating means of the wind simulation system may, e.g., be torque or speed adapted to provide typical wind-resulting conditions as input to the nacelle by means of the rotating system. 
         [0026]    The shaft rotating means may typically be mechanically coupled to a rotating part of the generator system or generator-related system, such as a hub or a gear box, in order to make a generator-related part rotate as if the wind turbine system was activated by certain wind conditions. 
         [0027]    As for examples of the implementation of different simulated grid conditions, the skilled person is kindly referred to the detailed description of the present invention, particularly the discussion of  FIGS. 7-12 . Other examples of simulations of grid and wind conditions are disclosed in “Development of a unified design, test, and research platform for wind energy systems based on hardware-in-the-loop real time simulation”, M. Steurer et al., Power Electronics Specialists Conference, 2004, PESC 04, 2004 IEEE 35 th  annual, Aachen, Germany. 
         [0028]    In a preferred embodiment of the present invention, said test bench is arranged to hold a complete nacelle of said wind turbine system. 
         [0029]    When used for testing the system before it is send off, it is advantageous to be able to test a complete assembled nacelle. Therefore, preferably a full-scale nacelle test bench is provided for testing and simulating varying mechanical, wind, and/or grid situations, etc. In this case, either a complete nacelle or one or more parts of it mounted on the load carrying structure, such as a generator, may be held or fixated mechanically sufficiently in order to allow the mechanical energy to be transferred from the wind simulation system to a rotatable part or movable part of the parts being tested. 
         [0030]    In yet a preferred embodiment of the invention, said shaft rotating means comprises means for being mechanically connected to a low speed shaft of a wind turbine system. 
         [0031]    In order to be able to simulate different wind conditions in the case where a gear box is included in the parts being tested, comprising means for being connected to a low speed shaft, which is connected to the input of the gear box, is advantageous. 
         [0032]    In another preferred embodiment of the invention, said shaft rotating means comprises means for being mechanically connected to a high speed shaft of a wind turbine system. 
         [0033]    In order to be able to simulate different wind conditions in the case where a gear box is not included in the parts being tested, comprising means for being connected to a high speed shaft, which is connected directly to the rotor of the generator, is advantageous. 
         [0034]    In an embodiment of the invention, said simulated utility grid comprises means for being electrically coupled to a generator output transformer of said nacelle components. 
         [0035]    In some wind turbine systems, the electrical interface between the generator and the (simulated) utility grid comprises a simple transformer instead of or in combination with a power frequency converter. Such a generator output transformer is provided for stepping up the typically less than 1 kV, such as 690 V, output from the generator to a higher voltage compatible with a subsequently coupled converter or grid, such as 3 kV or 30 kV. Therefore, the simulated utility grid of the present invention preferably comprises means for interfacing to such a generator output transformer. 
         [0036]    In a preferred embodiment of the invention, said simulated utility grid comprises means for being electrically coupled to a power frequency converter of said nacelle components. 
         [0037]    Some generator types, such as Doubly-Fed Induction Generators (DFIG) and synchronous multi-pole generators require some sort of frequency conversion to be included in the electrical coupling to a utility grid. Typically, this frequency conversion is performed by means of a power frequency converter. Therefore, the simulated utility grid of the present invention preferably comprises means for interfacing to such a power frequency converter. It should be emphasized that the invention relates as well to generator types, such as synchronous multi-pole generators, where all of the power is transferred between the generator and the utility grid through the power frequency converter, as to generator types, such as DFIG, where some of the power is transferred through the power frequency converter and some is transferred directly between the stator of the generator and the utility grid. 
         [0038]    In a preferred embodiment of the invention, said power converter system is arranged to simulate different grid conditions in response to control signals from said simulation controller. 
         [0039]    The control signals from the simulation controller may address any interesting and relevant grid conditions, such as faults, extremes, sudden or slow changes in frequencies, transients, etc., thereby enabling a robust testing and an advantageous way of optimising the performance of the wind turbine systems on site. Such tests may also result in significant improvement with respect to cost-efficiency as different parts of the wind turbine system may be tested “in vivo”. 
         [0040]    The control signals from the simulation controller may be designed for any desired simulation purpose and the signals may, e.g., relate to specific grid situations known to be of relevance to specific components of the system, thereby allowing relevant tests and optimisation of dedicated components, or the controller may, e.g., establish grid simulations for the purpose of verifying that the complete wind turbine system performs as expected, e.g., according to specific grid codes. 
         [0041]    In a further preferred embodiment of the invention, said grid conditions comprise fault conditions, weak grid conditions and/or asymmetric grid conditions. 
         [0042]    A weak grid may, e.g., be understood as the part of the grid, which under certain conditions may be influenced by coupled systems such as wind turbines. 
         [0043]    Thus, the term weak grid may typically be used with respect to a grid where it is necessary to take voltage level and fluctuations into account because there is a certain risk that the values might exceed the requirements in standards. Weak grids may, e.g., relate to more remote or peripheral locations where the grid is designed for small loads. In connection with a wind turbine system, a weak grid may typically be a system where the amount of wind energy that can be absorbed is limited by the grid capacity and, moreover, where the grid under different conditions may transfer noise or undesired changes in level from one connection point to the grid coupled wind turbine system. 
         [0044]    In yet a preferred embodiment of the invention, said grid conditions comprise voltage variations such as voltage drops, voltage dips or voltage increases, short-circuits such as short-circuits between ground and phases or short-circuits between two or more phases, cos(φ) variations such as increase or decrease of reactive power to and/or from the grid, frequency variations of phases, current variations such as dips or increases, curve form variations of individual, some or all phases and any combination thereof. 
         [0045]    Preferably, the grid simulation system is able to simulate substantially all different grid conditions including faults and extremes, which the wind turbine system might meet during operation, so that the performance of the wind turbine system under such conditions can be tested thoroughly before the wind turbine system is shipped to its site of operation. 
         [0046]    In a preferred embodiment of the invention said grid conditions involve time varying grid conditions. 
         [0047]    Preferably, the above mentioned range of grid conditions also includes grid conditions that vary over time. Time varying conditions or specific curve form emulation may be combined with any of the above-mentioned grid conditions. 
         [0048]    In an embodiment of the invention, said power supply is provided by a utility grid. 
         [0049]    Using a utility grid is one way of supplying the necessary power for the wind turbine testing system or a part of it. Evidently, any suitable power supply may be applied within the scope of the invention as long as the resulting simulations correspond to the intended grid and wind simulations. 
         [0050]    In an embodiment of the invention, said power converter system comprises a switching semiconductor based converter. 
         [0051]    A preferred way of implementing a power converter comprises a suitable number of switching semiconductors. The semiconductors may be controlled or hardware configured according to conventional converter techniques. 
         [0052]    In an embodiment of the invention, said semiconductor based converter is a thyristor based converter. 
         [0053]    The semiconductors may, e.g., comprise transistors or thyristors such as BPT (bipolar transistor), IGBT (insulated gate bipolar transistor), MCT (metal-oxide-semiconductor controlled thyristor), IGCT (insulated gate controlled thyristor) or GTO thyristor (Gate-Turn-Off). A presently preferred semiconductor is a GTO thyristor. 
         [0054]    Evidently, the grid simulating system may be based on any switching component which is able to establish the desired grid conditions. 
         [0055]    In an embodiment of the invention, said grid simulation system further comprises a grid input transformer coupled between said power supply and said power converter system. 
         [0056]    Advantageously, a transformer is provided for establishing compatibility between the power supply, which may, e.g., be a utility grid at 30 kV, and the power converter of the grid simulation system operating at, e.g., 3 kV. 
         [0057]    In an embodiment of the invention, said grid simulation system further comprises a grid output transformer coupled between said power converter system and said simulated utility grid. 
         [0058]    In an embodiment where the grid simulation system provides a simulated utility grid at, e.g., 30 kV, the output of the power converter has to be stepped up from the typical operation level of the power converters around 2 or 3 kV. 
         [0059]    In an embodiment of the invention, said wind turbine testing system further comprises a nacelle stress simulating system, a wind turbine system temperature simulating system, and/or a wind turbine system humidity simulating system. 
         [0060]    Advantageously, different other physical conditions may be established in order to provide an even broader and more realistic physical simulation. 
         [0061]    In another aspect of the invention, it relates to the use of a wind turbine testing system as described above to establish and evaluate feedback from at least a part of a wind turbine system under physically simulated wind conditions into a grid. 
         [0062]    Establishing and evaluating feedback from the wind turbine system enables the skilled personnel performing the test to estimate whether the wind turbine system meets a given set of specified requirements under different wind conditions. 
         [0063]    In a preferred embodiment of the invention, a wind turbine testing system as described above is used to establish and evaluate feedback from at least a part of a given wind turbine system under physically simulated wind conditions into a grid before shipping said wind turbine system from the factory, wherein said simulated wind conditions correspond to wind conditions known to be present at the specific site of operation of said given wind turbine system. 
         [0064]    In order to improve the simulations, the wind conditions simulated by the wind simulation system may be correlated with conditions known to be present at specific sites with respect to, e.g., sudden change of wind speed, wind gusts, fluctuating wind direction, turbulence, wake effects, etc. Particularly, the simulated wind conditions can be correlated to the site of operation of the specific wind turbine system being tested if such site is known at the time of testing. 
         [0065]    In yet a preferred embodiment of the invention, a wind turbine testing system as described above is used to establish and evaluate feedback from at least a part of a given wind turbine system under physically simulated climatic conditions into a grid before shipping said wind turbine system from the factory, wherein said simulated climatic conditions include wind conditions, temperature conditions and/or humidity conditions corresponding to climatic conditions known to be present at the specific site of operation of said given wind turbine system. 
         [0066]    The simulations can be improved even further, if not only the simulated wind conditions but also other simulated climatic conditions are correlated with conditions known to be present at specific sites with respect to, e.g., temperature and humidity, etc. Particularly, the simulated climatic conditions can be correlated to the site of operation of the specific wind turbine system being tested if such site is known at the time of testing. 
         [0067]    In an advantageous embodiment of the invention, a wind turbine testing system as described above is used to establish and evaluate feedback from at least a part of a given wind turbine system under different grid conditions. 
         [0068]    Establishing and evaluating feedback from the wind turbine system enables the skilled personnel performing the test to estimate whether the wind turbine system meets a given set of specified requirements under different grid conditions. 
         [0069]    In a further advantageous embodiment of the invention, a wind turbine testing system as described above is used to establish and evaluate the compliance of at least a part of a given wind turbine system to a set of grid codes before shipping said wind turbine system from the factory, wherein said set of grid codes corresponds to grid codes applying at the specific site of operation of said given wind turbine system. 
         [0070]    Depending on the site of operation of a given wind turbine system, it is normally required to comply to a set of grid codes representing a number of requirements with regard to the response and reaction to certain specified grid conditions of a wind turbine system coupled to a utility grid. 
         [0071]    A wind turbine system may be analysed, tested or verified according to different grid codes in a simulated “in vivo” situation, preferably with respect to both wind and grid conditions. The simulation controller may simulate any relevant grid code in an environment relevant to a specific analysis. In this way, a wind turbine system or a part of a wind turbine system may be tested in order to verify that the wind turbine reacts in compliance with specific grid codes. Particularly, the compliance of a wind turbine system to a set of grid codes applying at the site of operation of the specific wind turbine system can be tested if such site is known at the time of testing. 
         [0072]    In yet a further advantageous embodiment of the invention, a wind turbine testing system as described above is used to establish and evaluate the compliance of at least a part of a given wind turbine system to a set of grid codes comprising requirements of the response of a wind turbine system to different grid conditions including voltage variations such as voltage drops, voltage dips or voltage increases, short-circuits such as short-circuits between ground and phases or short-circuits between two or more phases, cos(φ) variations such as increase or decrease of reactive power to and/or from the grid, frequency variations of phases, current variations such as dips or increases, curve form variations of individual, some or all phases or combinations thereof. 
         [0073]    Preferably, the set of grid codes with regard to which the compliance of a wind turbine system is tested should include substantially all different grid codes, that the wind turbine system might meet during operation, such grid codes covering grid conditions including faults and extremes, so that the compliance of the wind turbine system to all relevant grid codes can be tested thoroughly before the wind turbine system is shipped to its site of operation. 
         [0074]    In yet a further aspect of the invention, it relates to a method of testing a least a part of the nacelle components of a wind turbine system when mounted on a load carrying structure of a nacelle of said wind turbine system by means of a wind turbine testing system comprising the steps of:
       mounting said nacelle components onto a load carrying structure of a wind turbine nacelle,   placing said load carrying structure in a test bench of said wind turbine testing system,   coupling a shaft rotating means of a wind simulation system of said wind turbine testing system to a rotating part of a generator system or a generator-related system of said wind turbine nacelle,   coupling a simulated utility grid of a grid simulation system of said wind turbine testing system electrically to an electrical nacelle component,   rotating said rotating part of a generator system by means of said wind simulation system simulating different wind conditions,   exposing said nacelle component to different simulated grid conditions by means of said grid simulation system, and   establishing and evaluating feedback from said nacelle components under different simultaneously simulated wind and grid conditions.       
 
         [0082]    Performing the steps of the above mentioned method enables the skilled personnel performing the test to estimate in a reliable and reproducible way whether the wind turbine system meets a given set of specified requirements under different wind and grid conditions. 
         [0083]    In a preferred embodiment of the invention, it relates to a method of testing a least a part of the nacelle components of a wind turbine system when mounted on a load carrying structure of a nacelle of said wind turbine system by means of a wind turbine testing system comprising the steps of:
       mounting said nacelle components in a wind turbine nacelle,   placing said wind turbine nacelle in a test bench of said wind turbine testing system,   coupling a shaft rotating means of a wind simulation system of said wind turbine testing system to a rotating part of a generator system or a generator-related system of said wind turbine nacelle,   coupling a simulated utility grid of a grid simulation system of said wind turbine testing system electrically to an electrical nacelle component,   rotating said rotating part of a generator system by means of said wind simulation system simulating different wind conditions,   exposing said nacelle component to different simulated grid conditions by means of said grid simulation system, and   establishing and evaluating feedback from said nacelle components under different simultaneously simulated wind and grid conditions.       
 
         [0091]    Advantageously, a complete assembled nacelle can be placed in the test bench and all the components of the nacelle can be tested simultaneously by following the steps of the above mentioned method. 
         [0092]    In yet a preferred embodiment of the invention, it relates to a method of testing as described above, wherein said wind turbine testing system comprises a wind turbine testing system according to the present invention. 
         [0093]    It is advantageous to use a wind turbine testing system as described above for the test as described in the above mentioned method, because the wind turbine testing system is designed exactly for that purpose. 
         [0094]    In a further preferred embodiment of the invention, it relates to a method of testing as described above, wherein said simulated grid conditions include fault conditions of a utility grid. 
         [0095]    In order to make the test performed according to the above mentioned method as complete as possible, it should include substantially all grid conditions that a wind turbine system could be expected to meet during operation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0096]    Some embodiments of the invention will now be described with reference to the figures, where 
           [0097]      FIG. 1  illustrates a large modern wind turbine known in the art as seen from the front, 
           [0098]      FIG. 2  illustrates a cross section of an embodiment of a simplified nacelle known in the art as seen from the side, 
           [0099]      FIG. 3  illustrates a test bench for a nacelle according to an embodiment of the invention, 
           [0100]      FIG. 4  illustrates a test bench for a generator according to an embodiment of the invention, 
           [0101]      FIG. 5  illustrates a schematically represented grid simulating system test setup, 
           [0102]      FIG. 6  illustrates a principle output of a power converter applied according to an embodiment of the invention, 
           [0103]      FIG. 7  illustrates a grid simulation of a voltage drop up to 60% of nominal voltage at all phases, 
           [0104]      FIG. 8  illustrates a grid simulation of a three-phase short-circuit and a voltage drop to about 35% of nominal voltage, 
           [0105]      FIG. 9  illustrates a grid simulation of a two-phase fault with voltage level 100%, 0°/50%, 180°/50%, 180°, and 
           [0106]      FIG. 10  illustrates a grid simulation of a voltage drop up to about 20% of nominal voltage. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0107]      FIG. 1  illustrates a modern wind turbine  1  comprising a tower  2  placed on a foundation and a wind turbine nacelle  3  positioned on top of the tower  2 . The wind turbine rotor  4 , comprising three wind turbine blades  5 , is connected to the nacelle  3  through the low speed shaft which extends out of the front of the nacelle  3 . 
         [0108]      FIG. 2  illustrates a simplified cross section of a nacelle  3  as seen from the side. Nacelles  3  exist in a multitude of variations and configurations but in most cases the drive train  14  in the nacelle  3  comprises one or more of the following components: a gear  6 , a coupling (not shown), some sort of braking system  7  and a generator  8 . A nacelle  3  of a modern wind turbine  1  can also include a power frequency converter  9  (also called an inverter) and additional peripheral equipment such as further power handling equipment, control cabinets, hydraulic systems, cooling systems and more. 
         [0109]    The weight of the entire nacelle  3  including the nacelle components  6 ,  7 ,  8 ,  9  is carried by a load carrying structure  10 . The components  6 ,  7 ,  8 ,  9  are usually placed on and/or connected to this common load carrying structure  10 . In this simplified embodiment, the load carrying structure  10  only extends along the bottom of the nacelle  3 , e.g., in form of a bed frame to which some or all the components  6 ,  7 ,  8 ,  9  are connected. In another embodiment, the load carrying structure  10  could comprise a gear bell  11  transferring the load of the rotor  4  to the tower  2 , or the load carrying structure  10  could comprise several interconnected parts such as latticework. In this embodiment of the invention, the drive train  14  is established in a normal operation angle NA of 8° in relation to a plane perpendicular to the centre axis through the tower  2 . 
         [0110]      FIG. 3  illustrates a partial cross-section of a test bench  12  forming a part of a wind turbine testing system testing a wind turbine nacelle  3  as seen from the side. Evidently, the illustrated test bench  12  forms only one of several different possible configurations of such a test setup within the scope of the invention. 
         [0111]    In this embodiment of the invention, the test bench  12  comprises drive means  13  in the form of an electric motor  15  and a gear  16  between which a braking system  17  and a flexible coupling  18  are positioned. 
         [0112]    The output shaft of the test bench gear  16  is connected to a flexible coupling  18  of the test bench  12 , which is connected to load applying means of the test bench  12  in the form of radial load means  20 , which will be described more thoroughly under the description of  FIG. 4 . 
         [0113]    The radial load means  20  comprising a shaft adapter  24  are connected to an input shaft  21  of a nacelle  3 , which in this case is the input shaft  21  of a wind turbine gear  6 , which via a brake system  7  and coupling (not shown) is connected to a generator  8  inside the nacelle  3 . In this embodiment, the nacelle  3  further comprises a power frequency converter  9 . The power frequency converter comprises an electrical interface  91  to a grid simulation system  92 . Basically, the wind turbine nacelle  3  may be interfaced to a grid by any suitable power transferring system comprising the relevant components for the specific application. Such components may, e.g., comprise a combination of one or more transformer(s), frequency converter(s), rectifiers, power buffers, power cables, etc. 
         [0114]    In this embodiment, the wind turbine equipment—in form of the drive train components  6 ,  7 ,  8  connected to each other by the high-speed output shaft  27  from the gear box  6  in the nacelle  3 —is positioned in an angle A of 6° in relation to a horizontal plane, in that the tower connection flange  23  of the nacelle  3  is rigidly connected to a substantially horizontal connection flange of the test bench  12 . Since the nacelle  3  in real life would be connected to a substantially horizontal connection flange at the top of a wind turbine tower  2 , this angle A corresponds to the angle NA of these specific drive train components when positioned in an ordinary operating wind turbine  1 . 
         [0115]    In another embodiment of the invention, the connection flange of the test bench  12  could be angled in relation to a horizontal plane, or the entire test bench  12  could be angled or comprise means for controlling the angle of the test bench  12  and/or the tested equipment  22 . 
         [0116]    In another embodiment of the invention, the connection flange of the test bench  12  could also comprise means  19  for providing load to the equipment in order to increase the efficiency and/or realism of the test. Such load applying means  19  could for instance apply loads to a yaw mechanism (not shown) of a wind turbine  1 , the load carrying structure  10  of a nacelle  3  or the input shaft  21 , or it could apply loads to the tested equipment  22  in any other way. 
         [0117]    In this embodiment of the invention, the radial load means  20  are at least in principle the only load applying means  19  of the test bench  12  applying direct load to the input shaft  21  of the tested equipment  22 . In another embodiment, however, the test bench  12  could further comprise load applying means  19  for applying load to the input shaft  21  of the equipment  22  or any other part of the equipment  22  in any feasible direction such as axially, diagonally or from varying directions. 
         [0118]    In a preferred embodiment of the invention, the generator  8  in the nacelle  3  is connected to the grid simulating system  92  enabling the generator  8  to act indirectly as a load applying means  19  of the test bench  12  during the test, in which it is possible to simulate different grid situations such as extreme overload situations, fault situations, short circuits, asymmetric phase amplitude and angle or other either independently or co-operating grid situations. The different situations of the grid will, thereby, indirectly apply different load situations on the tested equipment  22  through the generator  8 . 
         [0119]    In another embodiment, the generator  8  could simply be connected to the utility grid in the same way as it would be in an operating wind turbine  1 . 
         [0120]    In this embodiment of the invention, the test bench  12  comprises noise reducing means  28  in the form of a sound absorbing box  28  substantially enclosing the drive means  13  of the test bench  12 , hereby enabling that the noise produced by particularly the drive means  13  are absorbed by the box  28 , hereby reducing the noise emission from the test bench  12 . 
         [0121]    In another embodiment, the entire test bench  12  could be enclosed by a sound absorbing box  28  or the individual parts of the test bench  12  producing noise could be individually equipped with noise reducing means  28 . In this embodiment, the noise reducing means  28  are passive but, in another embodiment, the means  28  could be active, e.g., by providing noise in anti-phase or in other ways. 
         [0122]    In this embodiment of the invention, the test bench  12  further comprises climate controlling means  29  in form of a climate box  29  substantially enclosing the wind turbine equipment  22  or at least a part of the equipment  22  to be tested on the test bench  12 . 
         [0123]    In this embodiment, the climate box  29  enables that the temperature inside the box  29  can be adjusted and controlled freely between −45° C. and 55° C. when the tested equipment  22  is inactive and not operating, and between −40° C. and 90° C. during operation of the tested equipment  22 . These temperature ranges are sufficient in the present embodiment of the invention to provide an efficient and/or realistic environment for the tested equipment  22 , but in another embodiment, the test bench  12  could comprise means for controlling the ambient temperature of the equipment  22  within different ranges, and the climate controlling means  29  could further comprise means for controlling other climate parameters such as the humidity and/or the air pressure. 
         [0124]      FIG. 4  illustrates a variant of a wind turbine testing system according to a further embodiment of the invention. In this embodiment, the wind turbine testing system is coupled to only a part of a wind turbine system, namely a wind turbine generator  48  as seen in perspective. 
         [0125]    In this embodiment, a test bench  42  is in a substantially horizontal position when the wind turbine generator  48  is lifted onto and rigidly connected to the bench  42 . 
         [0126]    A wind simulation system  45  comprising an electrical motor is driven to establish desired physical conditions. The wind simulation system  45  transfers rotation to the generator  48  by means of a mechanical coupling  49 , and the complete test bench  42  may be tilted to simulate further desired conditions physically. 
         [0127]    The wind turbine testing system further comprises an electrical interface  191  to a grid simulation system (not shown) enabling coupling of the generator to a simulated grid. 
         [0128]      FIG. 5  illustrates a schematically represented electrical diagram of a wind turbine testing system according to one of several embodiments within the scope of the invention. 
         [0129]    The illustrated embodiment comprises a grid simulation system  59  and a wind simulation system  69  mutually coupled via a wind turbine system  74 . In this present embodiment, the wind turbine system comprises a nacelle  72  or nacelle parts. 
         [0130]    The grid simulation system  59  and the wind simulation system  69  are both coupled to a power supply, such as the utility grid  500 . Evidently, the simulating systems may be supplied from two different power supplies. 
         [0131]    The grid simulating system  59  comprises a power converter  50  controlled by a simulation controller  51 . The simulation controller  51  controls the power converter  50  to establish the intended grid simulation at the output of the grid simulating system  59 . The power converter  50  is coupled to the utility grid  500  via a transformer  54  and a switch  56 , and it is furthermore coupled to the wind turbine system  74  via a transformer  55 , a switch  57  and an electrical interface  58 . 
         [0132]    The illustrated power converter  50  may, e.g., comprise an ALSPA VDM 7000 medium voltage inverter, the illustrated switches  56 ,  57  may, e.g., comprise 30 kV/50 Hz switches, the transformer  54  may, e.g., comprise a three-phase 13 MVA 30 kV/3.1 kV transformer, and the transformer  55  may, e.g., comprise a three-phase 13 MVA 3.1 kV/30 kV transformer. 
         [0133]    The illustrated wind turbine system  74  comprises a nacelle  72  comprising a mechanical transmission  62 , such as a gear box, connected to a generator  78  of the nacelle  72 . The generator may, e.g., be electrically coupled to the electrical interface  58  via a generator output transformer  75  such as a simple transformer or a power frequency converter depending on the type of the tested wind turbine system  74 . The coupling must match the effective output of the grid simulating system  59 , here the output of the transformer  55 . In the embodiment of  FIG. 5 , the generator output transformer  75  is shown as a simple transformer, whereas in the embodiments of  FIGS. 2 and 3  are shown a power frequency converter  9 . 
         [0134]    Thus, the grid simulation system  59  may be established to provide a grid simulated output which may be fed directly into a wind turbine system  74  comprising a power converter  9  or, e.g., to a wind turbine system  74  only comprising a generator output transformer  75  as illustrated in the present embodiment. The illustrated generator output transformer  75  transforms the output of the generator  78  of the nacelle  72  of, e.g., about 690 V to 30 kV. 
         [0135]    The wind simulation system  69  is basically established for providing a situation at the mechanical input of the nacelle  72  corresponding to certain wind conditions. The wind simulation system  69  comprises a frequency converter  60  connected to a motor  61  and thereby controlling the same. The motor  61  is mechanically and rotatable coupled to the nacelle  72  via a gear  63  providing a slow high-torque rotation. 
         [0136]    The frequency converter  60  is here coupled to the utility grid  500  via a transformer  64  and a switch  66 . The transformer  64  may, e.g., comprise a 13 MVA 30 kV/3.1 kV transformer. 
         [0137]    The illustrated frequency converter  60  may, e.g., comprise an ALSPA VDM 7000 medium voltage inverter. 
         [0138]    For reference to  FIGS. 3 and 4 , the grid simulation system  59  and the wind simulation system  69  of the wind turbine testing system may preferably be parts of the test benches  12 ,  42  shown in  FIGS. 3 and 4 , where main parts comprise the motor  61 , which may correspond to the motor  15  of  FIG. 3  or the motor  45  of  FIG. 4 , and the gear  63 , which may correspond to the gear  16  of  FIG. 3 . In the small-scale test bench of  FIG. 3 , where, e.g., a generator  8  is tested separately from the rest of the nacelle components, no gear  6  is required, so far as the motor and gear are compatible. Also, in an embodiment, the grid simulating system  59  and electrical interface  58  may correspond to the grid simulating system  92  and electrical interface  91  of  FIG. 3 . 
         [0139]    The illustrated embodiment facilitates testing of the nacelle  72  in a broad simultaneous physical simulation covering both a simulation of the grid and the wind. Evidently, further parameters may be included in the test setup, such as temperature, humidity, mechanical stress, tilting, etc. of an individual component of a wind turbine system or all or most of them in combination. 
         [0140]    The control of the illustrated test setup may be established in several different ways depending on the purpose of the simulation, and the test results may be measured with different test methods at different measure points of the wind turbine testing system. 
         [0141]    It should be noted that the illustrated grid simulating system may also be comprised by more simple or primitive contact systems with or without associated transforming circuitry, such as arrangements of switches and/or resistors or other loads, e.g., for establishing a voltage dividing circuit for simulating voltage drops. Such simulating systems would typically be suitable for testing very specific, typically only a few, grid conditions. 
         [0142]    The power converter(s) may be self-commutated converters as well as externally controlled converters. In a preferred embodiment of the invention, the converter circuit is a thyristor converter circuit, converting the power from AC to DC and again to AC, giving an advantageous possibility of controlling a plurality of parameters. In another embodiment of the invention, the converter system may also be a direct AC converter or other types of converters or units with the functionality of a converter known to a person skilled in the art. The converters may further be manually clocked, self-clocked, grid-clocked, load-clocked, machine-clocked or the like. By controlling the converter circuits, it is possible to control a plurality of parameters, giving the possibility of simulating a plurality of different scenarios, such as grid faults, grid variations, different grid codes, variations of the wind turbine or the like. 
         [0143]    Simulations concerning grid codes, grid stability and the like are powerful tools for optimisation of the wind turbine parameters. It is possible to use the power converters to simulate different kinds of voltage drops or increases on the grid, short circuits between ground and phases, short circuits between two phases, short circuits between three phases, reactive compensation, frequency variations, different phase angles, different amplitude curve forms, and other different relevant simulations. Furthermore, it is possible to simulate and identify different time periods concerning different scenarios. 
         [0144]    By placing different sensors, such as accelerometers, heat sensors, acoustic sensors, heat cameras, voltage and current sensors, and a plurality of different other relevant sensors known to a person skilled in the art, it is possible to monitor the effects of the before mentioned simulations on the wind turbine. 
         [0145]      FIG. 6  illustrates an exemplary output of a power converter, such as the power converter  50 . The illustrated converter, an ALSPA VDM 7000 medium voltage inverter, is a multi-level converter, such as a 3-level neutral point clamped converter, thereby delivering an output having a relatively low harmonic distortion when coupled to a motor or a transformer. The illustrated output illustrates phase to phase voltage as a function of time. 
         [0146]      FIG. 7  illustrates a grid simulation of a voltage drop to 60% of nominal voltage at all phases with a configuration of 18 MVA/18 MVA installed inverter power. The two stated power values refer to the two sides of the power frequency converter, respectively. 
         [0147]    v U [kV] designates the voltages of the different phases of the output of the grid simulating system, measured in kVolt. 
         [0148]    i u [A] designates the currents of the different phases of the output of the grid simulating system, measured in Ampere. 
         [0149]    v abs [pu] and i abs [pu] designate the combined absolute voltage and the combined absolute current, respectively, per unit, i.e. a value of 1 indicates 100% of nominal voltage or current of the phases of the grid simulation system. 
         [0150]    In this simulation, the test period starts at about 0.06 seconds, and it is seen that the voltage level of the system stabilizes after about 2.3 seconds. 
         [0151]      FIG. 8  illustrates a grid simulation of a three-phase short-circuit and a voltage drop to about 35% of nominal voltage with a configuration of 18 MVA/27 MVA installed inverter power. 
         [0152]    v U [kV] designates the voltages of the different phases of the output of the grid simulating system, measured in kVolt. 
         [0153]    i u [A] designates the currents of the different phases of the output of the grid simulating system, measured in Ampere. 
         [0154]    v abs [pu] and i abs [pu] designate the combined absolute voltage and the combined absolute current, respectively, per unit, i.e. a value of 1 indicates 100% of nominal voltage or current of the phases of the grid simulation system. 
         [0155]    In this simulation, the test period starts at about 0.06 seconds, and it is seen that the voltage level of the system stabilizes after about 2.6 seconds. 
         [0156]      FIG. 9  illustrates a grid simulation of a two-phase short-circuit fault with voltage level 100%, 0°/50%, 180°/50%, 180° with a configuration of 18 MVA/27 MVA installed inverter power. The two faulty phases act as return path for the current of the working phase. 
         [0157]    v U [kV] designates the voltages of the different phases of the output of the grid simulating system, measured in kVolt. 
         [0158]    i u [A] designates the currents of the different phases of the output of the grid simulating system, measured in Ampere. 
         [0159]    v abs [pu] and i abs [pu] designate the combined absolute voltage and the combined absolute current, respectively, per unit, i.e. a value of 1 indicates 100% of nominal voltage or current of the phases of the grid simulation system. 
         [0160]    In this simulation, the test period starts at about 0.06 seconds, and it is seen that the voltage level of the system stabilizes after about 2.8 seconds. 
         [0161]      FIG. 10  illustrates a grid simulation of a voltage drop up to about 20% of nominal voltage with a configuration of 18 MVA/36 MVA installed inverter power. 
         [0162]    v U [kV] designates the voltages of the different phases of the output of the grid simulating system, measured in kVolt. 
         [0163]    i u [A] designates the currents of the different phases of the output of the grid simulating system, measured in Ampere. 
         [0164]    v abs [pu] and i abs [pu] designate the combined absolute voltage and the combined absolute current, respectively, per unit, i.e. a value of 1 indicates 100% of nominal voltage or current of the phases of the grid simulation system. 
         [0165]    In this simulation, the test period starts at about 0.06 seconds, and it is seen that the voltage level of the system stabilizes after about 2.8 seconds.