Abstract:
A wind turbine includes a rotor, a permanent magnet generator and a generator converter, whereby an electric or electronic switch, which is arranged between the permanent magnet generator and the generator converter, is provided for selectively connecting the permanent magnet generator to at least one braking resistor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office Application No. 11151237.2 EP filed Jan. 18, 2011. All of the applications are incorporated by reference herein in their entirety. 
       FIELD OF INVENTION 
       [0002]    The present claimed invention relates to a wind turbine, comprising a rotor, a permanent magnet generator and a generator converter. 
       BACKGROUND OF INVENTION 
       [0003]    Wind turbines are provided with a rotor which is connected to a permanent magnet generator producing electricity during movement of a rotor relative to a stator of the permanent magnet generator. The stator comprises a number of coils; the rotor comprises a number of permanent magnets so that an electric voltage is induced when the rotor is turned. 
         [0004]    The permanent magnet generator is connected to a generator converter which transforms the alternating current which is produced by the permanent magnet generator into direct current; the generator converter is preferably connected to a grid converter which transforms the direct current into alternating current with the correct voltage and frequency which is required by a power grid. The power produced by a wind turbine is normally going into the grid. However, this is not possible during a grid drop. In case of a grid drop the turbine will reduce the power production to zero. As a consequence the generator torque drops dramatically. In this case the aerodynamic torque will now be larger than the generator torque and the rotor will accelerate. Hereby the speed of the rotor is no longer controlled and could cause the rotor to go into overspeed. To prevent an uncontrolled rotation it is desired to apply braking to keep control of the wind turbine and to decelerate the rotor. 
         [0005]    In conventional wind turbines air brakes are used, whereby the blades are pitched to reduce the aerodynamic moment. 
         [0006]    Another possibility to apply braking is to use a mechanical brake, e. g. a brake disc so that the reduced generator torque is compensated for. 
         [0007]    In conventional wind turbines braking resistors in a DC-link are used. This means that power resistors are positioned in the DC-link between generator converter and grid converter in order to load the generator and thereby braking the turbine. However, braking resistors in the DC-link require that the generator converter is running. If the generator converter fails it is no longer possible to brake the rotor of the wind turbine by power resistors in the DC-link. 
       SUMMARY OF INVENTION 
       [0008]    It is an object of the present claimed invention to provide a wind turbine, whereby braking of the rotor is possible even when the generator converter is not running. 
         [0009]    The object is achieved in the above defined wind turbine in that an electric or electronic switch, which is arranged between the permanent magnet generator and the generator converter, is provided for selectively connecting the permanent magnet generator to at least one braking resistor. 
         [0010]    The claimed invention is based on the idea that the dependency of the running generator converter can be eliminated when the braking resistors are placed directly in connection with the permanent magnet generator. This is possible because the permanent magnet generator does not require magnetisation current or other active control to produce power. Simply cutting in power resistors directly on the phases of the permanent magnet generator will provide load on the generator and thereby brake the rotor. 
         [0011]    In the wind turbine it is preferred that the electronic switch is controlled by a controller. The controller can e. g. comprise a microprocessor or a similar control device. 
         [0012]    According to a further development of the wind turbine the duty cycle of the electronic switch can be controlled by the controller. The duty cycle is the fraction of time that the electronic switch is in an active state. Accordingly it is intended that the electronic switch is switched on and off according to a certain duty cycle so that braking of the rotor is effected intermittently. 
         [0013]    In the wind turbine it is preferred that a control algorithm is implemented in the controller, monitoring grid status and/or rotor speed and/or temperature of the resistors. 
         [0014]    Accordingly the control algorithm can take into account different status information from the grid and components of the wind turbine. The grid status can best be monitored by monitoring the grid voltage. Rotor speed can be monitored in order to avoid rotor speed above a maximum allowed rotor speed. Temperature of the resistors can be monitored in order to avoid a temperature above a predetermined maximum temperature level. 
         [0015]    According to a preferred embodiment of the wind turbine the electronic switch may comprise a transistor. The transistor can easily be switched electronically; therefore any duty cycle can be chosen when the electronic switch is a transistor. 
         [0016]    Preferably the permanent magnet generator and the generator converter of the inventive wind turbine are connected by preferably three phases, whereby a braking resistor is connected to each phase. Accordingly three switches are needed, one for each phase. 
         [0017]    According to an alternative embodiment of the wind turbine the electric switch can be a relay, which is connected such that the at least one braking resistor is cut-in when a power loss occurs. According to this embodiment the switch is a relay which is connected such that the resistors are cut-in if the relay is inactive. That way the resistors will automatically be cut-in when power is lost so that a braking force is exerted upon the rotor. 
         [0018]    In the wind turbine it may be envisaged that it comprises an electric switch and an electronic switch. This way the advantages of both switches can be achieved. 
         [0019]    In the wind turbine it is preferred that the at least one braking resistor is a power resistor. 
         [0020]    The claimed invention and its underlying principle will be better understood when consideration is given to the following detailed description of preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic view of a wind turbine; 
           [0022]      FIG. 2  is a schematic view of another embodiment of a wind turbine. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0023]      FIG. 1  is a schematic view of a wind turbine  1 , comprising a rotor  2  with three rotor blades connected to a rotor shaft  3  and a permanent magnet generator  4 . The permanent magnet generator  4  comprises a stator with a number of coils and a rotor with a number of permanent magnets, so that an electric voltage is induced when the rotor is turned. 
         [0024]    The permanent magnet generator  4  is connected to a generator converter  5  by three phases  6 ,  7 ,  8 . The generator converter  5  converts the alternating current which is produced by the permanent magnet generator  4  into direct current. A grid converter  9  is connected to the generator converter  5  by two lines  10 ,  11 . The grid converter  9  converts the direct current which is generated by the generator converter  5  into alternating current with the appropriate frequency and voltage as required by a power grid  12 . 
         [0025]    When a grid drop occurs the generator torque, which normally acts upon the rotor shaft  3 , is reduced dramatically so that the aerodynamic torque generated by the wind is larger, consequently the rotor  2  will accelerate. 
         [0026]    A grid drop is monitored by a controller  13 , which monitors e. g. the voltage of the power grid  12 . When a grid drop has been detected the controller  13  switches an electronic switch so that a phase between permanent magnet generator  4  and generator converter  5  is connected to a resistor which brakes the permanent magnet generator  4 . As can be seen in  FIG. 1  three phases  6 ,  7 ,  8  are present between permanent magnet generator  4  and generator converter  5 . Through a switch  14 , which is arranged between the permanent magnet generator and the generator converter, each phase can be connected to a resistor  15 . When a phase of the permanent magnet generator  4  is connected to a resistor  15 , a braking force acting upon the permanent magnet generator  4  is generated. In total three switches  14  are present; one switch  14  and one resistor  15  is allocated to each phase  6 ,  7 ,  8 . Each switch  14  comprises a transistor or similar electronic switching device. The three switches  14  are controlled by a controller  13  which monitors rotor speed, temperature of the resistors  15 , grid status and other operational parameters using a control algorithm. The controller  13  is configured such that the switches  14  are run according to an optimal duty cycle. The controller  13  which comprises a microprocessor calculates the optimal duty cycle with regard to structural loads on the wind turbine  1  and safety requirements. When the switches  14  are switched such that the permanent magnet generator  4  is connected to the resistors  15  a brake torque is generated which acts against the aerodynamic torque so that the effect of the grid drop is compensated for. 
         [0027]      FIG. 2  shows another embodiment of the wind turbine  16 , whereby for like components the same reference signs are used. A rotor  2  with rotor blades is connected via a rotor shaft  3  to a permanent magnet generator  4 . The permanent magnet generator  4  is connected to a generator converter  5  which is further connected to a grid converter  9  which is connected to a power grid  12 . Three phases are present between permanent magnet generator  4  and generator converter  5 , of which only one phase  17  is depicted in  FIG. 2  for clarity reasons. In accordance with the first embodiment a controller  13  controls a switch  14  in the form of a transistor so that the permanent magnet generator  4  can selectively be connected to a resistor  15  in order to generate a braking torque during a grid drop. In addition a relay  18  is present as electric switch which is connected in parallel to the switch  14  such that a braking resistor  19  is cut-in when a power loss occurs. In the case of a total power loss the controller  13  will not work anymore, in this case the relay  18  switches automatically such that the resistor  19  will be automatically cut-in and connected to the phase  17  so that the resistor  19  is connected to the grid converter  4 . Consequently, even when a total power loss occurs, the resistor  19  will generate a braking force which acts upon the permanent magnet generator  4  so that the rotor  2  can be controlled.