Abstract:
A method for controlling a frequency converter is provided. The converter includes a rectifier, an inverter which is connected via a DC link to the rectifier, a rectifier controller and an inverter controller. A minimal rectifier DC link voltage for the rectifier controller is determined, a minimal inverter DC link voltage for the inverter controller is determined, a minimal DC link voltage as the maximum of the minimal rectifier DC link voltage and the minimal inverter DC link voltage is determined, and an optimal DC link voltage reference based on the minimum of the minimal DC link voltage and a maximal allowed DC link voltage is determined. The rectifier controller and/or the inverter controller is/are controlled based upon the optimal DC link voltage reference.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European Patent Office Application No. 11173989.2 EP filed Jul. 14, 2011. All of the applications are incorporated by reference herein in their entirety. 
       FIELD OF INVENTION 
       [0002]    A method for controlling a frequency converter, a frequency converter and an electrical energy generating apparatus such as, for example a wind turbine, are provided. Moreover, an electric motor is provided. Particularly, optimisation of a DC link in a conversion of AC input power to AC output power is described. 
       BACKGROUND OF INVENTION 
       [0003]    Wind turbines convert wind energy into electrical energy by using the force of the wind to drive the rotor of a generator. The generator generates AC power having a frequency which depends on the rotational speed of the rotor, i.e. to the wind force. 
         [0004]    Frequency converters or power converters may be used to convert AC power with a variable frequency as provided by the generator of a wind turbine into AC power having the constant frequency of the grid. Usually such a frequency converter has a rectifier for conversion of the AC power to a DC power which is fed to a so called DC link. The DC link connects the rectifier with an inverter which converts the DC power to the AC power with grid frequency. 
         [0005]    For the hardware of a low-voltage frequency converter, the maximum allowed voltage may be higher than 1200V. The DC link voltage (Vdc) of a frequency converter is usually fixed to 1100V at all operating points by controlling the supply network or grid power or/and the generator power. The DC link voltage will determine the losses of the frequency converter and also of the generator. For example, the switching losses of a frequency converter are proportional to the square of the voltage of the DC link. 
         [0006]    As long as the DC link voltage is high enough, all control targets of the generator side and the grid side can be achieved. The necessary or minimal DC link voltage can be determined by the generator voltage, the grid voltage, the generator current and the grid current. 
         [0007]    For variable speed wind turbines the generator voltage, the generator current and the grid current are low at low wind speeds. The necessary DC link voltage at low wind speed conditions is much lower than 1100V. For example, if the grid voltage is 690V, the necessary DC link voltage for a 10% load is less than 1000V. In this case, the fixed DC link voltage increases unnecessary losses at the converter and the generator. 
         [0008]    On the other hand, a fixed 1100V DC link voltage determines the maximum allowed rotor speed and power of the generator in full-load conditions. Therefore, transient over load and over speed of the wind turbine cannot be realised due to the limited fixed DC link voltage. The operating speed and power of the generator can be limited by the fixed DC link voltage as well. 
       SUMMARY OF INVENTION 
       [0009]    An improved conversion of power is provided as claimed in the claims. 
         [0010]    A method for controlling a frequency converter with a rectifier, an inverter which is connected via a DC link to the rectifier, a rectifier controller and an inverter controller, includes the following steps:
   determining a minimal rectifier DC link voltage for the rectifier controller;   determining a minimal inverter DC link voltage for the inverter controller;   determining a minimal DC link voltage as the maximum of the minimal rectifier DC link voltage and the minimal inverter DC link voltage;   determining an optimal DC link voltage reference based on the minimum of the minimal DC link voltage and a maximal allowed DC link voltage; and   controlling the rectifier controller and/or the inverter controller based on the optimal DC link voltage reference.   
 
         [0016]    The optimal DC link voltage reference may, for example, be determined as the minimum of the minimal DC link voltage and a maximal allowed DC link voltage. However, the optimal DC link voltage reference may also be determined in different ways. For example, the optimal DC link voltage reference may be some margin added to minimum of the minimal DC link voltage and a maximal allowed DC link voltage. 
         [0017]    The control method improves efficiency and operating range of the frequency converter and connected generators like in a wind turbine by adjusting the voltage reference of the DC link. This may be done based on operating points of the frequency converter, of the generator and/or of the wind turbine. The DC link voltage may be controlled from the rectifier or generator side and/or from the inverter or grid side. The rectifier controller and/or the inverter controller may be controlled directly or indirectly via another controller or computer. 
         [0018]    A varying DC link voltage is achieved according to the method. The voltage range may for example be between 950 to 1200 V which may depend on the hardware design. The method improves the efficiency of the converter and of a connected generator. The annual energy production of the system like a wind turbine, for example, may be increased while cooling costs may be reduced. The operating range of a wind turbine is increased and overload situations or power boost functions can be realised due to the varying DC link voltage. 
         [0019]    The determination may be based on current operating conditions. This is beneficial in particular for controllers with inner current loops. 
         [0020]    The frequency converter may be connected to a generator or an electric motor with a supply network. This setup, like for example in wind turbines, benefits from the method. The parts of this system may be connected directly or indirectly via, for example filters, detection units and measurement devices or the like. 
         [0021]    The optimal DC link voltage reference may be determined at a rate slower than a bandwidth of the controller. The bandwidth may be the bandwidth of the rectifier controller, of the inverter controller and/or of the DC link control in general. The term controller encompasses all units which take part in the control of the DC link voltage. This slower rate guarantees the stability of the control loop. 
         [0022]    The current of the rectifier controller and/or the inverter controller may be controlled. Alternatively, the voltage or the power may be controlled. Usually a current controller is the inner loop of the controllers so that a control of the current is easy to implement. 
         [0023]    In a second aspect, a frequency converter for conversion of an AC input power to an AC output power is provided. The converter comprises a rectifier, a DC link in communication with the rectifier, an inverter in communication with the DC link, a rectifier controller, an inverter controller and a DC link voltage controller. The DC link voltage controller comprises inputs for the voltages and the currents of the AC input power and of the AC output power and a reference output in communication with the rectifier controller and/or the inverter controller for outputting an optimal DC link voltage reference. The DC link voltage controller produces the optimal DC link voltage reference on the basis of the voltages and the currents of the AC input power and of the AC output power. The same modifications and embodiments as described before apply here. The rectifier may be part of or may be a generator bridge which in normal operation operates as a rectifier. The inverter may be part of or may be a grid or network bridge which in normal operation operates as an inverter. 
         [0024]    The DC link voltage controller may be arranged in the rectifier controller and/or in the inverter controller. It can be chosen if the DC link voltage is controlled from the generator side, the grid side or from both sides. Locating the DC link voltage controller inside these controllers has the benefit of short signal transmission and easy implementation. Of course, it can be located in another controller like for example a wind turbine controller. 
         [0025]    The DC voltage controller may comprise an input for a power of the AC input power, for a torque of the generator, for an angular velocity of the generator and/or for a power of the AC output power. These further inputs can enhance reliability of control. 
         [0026]    The frequency converter may comprise a rectifier current controller and/or an inverter current controller, wherein the reference output of the DC link voltage controller may be in communication with the rectifier current controller and/or the inverter current controller. The communication may be direct or indirect for example via the rectifier controller and/or an inverter controller or another controller. 
         [0027]    In a further aspect, an electrical energy generating apparatus comprising a frequency converter as described above is provided. The same modifications and embodiments as described before apply here. 
         [0028]    The electrical energy generating apparatus may comprise an electrical generator for generating AC power. The converter is beneficial in the combination with a generator. 
         [0029]    Besides for the electrical energy generating apparatus like an electrical generator for generating AC power, the proposed method for controlling dc link voltage may also be used for a frequency converter for driving an electric motor. In a still further aspect, an electric motor comprising a frequency converter as described before is provided. The same modifications and embodiments as described before apply here. 
         [0030]    The electrical energy generating apparatus may be a wind turbine. The described method and apparatuses are beneficial especially for wind turbines which have a generator with varying load depending on the wind force. 
         [0031]    The accompanying drawings are included to provide a further understanding of embodiments. The elements of the drawings do not necessarily scale to each other. Like reference numbers designate corresponding similar parts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  illustrates a schematic view of the electrical equipment of a wind turbine with a frequency converter. 
           [0033]      FIG. 2  illustrates a first implementation of a method for controlling a frequency converter. 
           [0034]      FIG. 3  illustrates a second implementation of a method for controlling a frequency converter. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0035]    In the following detailed description, reference is made to the accompanying drawings which form a part hereof and in which are shown by way of illustration specific embodiments. In this regard, directional terminology, such as “top” or “bottom” etc. is used with reference to the orientation of the figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. 
         [0036]      FIG. 1  shows an overview of the electrical components of an electrical energy generating apparatus like for example a wind turbine  1 . The wind turbine  1  has a rotor  2  with one or more, usually three blades. The rotor  2  is arranged on a rotor shaft  3  which transmits the rotational momentum from the turning rotor  2  to a gearbox  4 . The gearbox  4  transmits the rotation of the rotor shaft  3  to an output shaft  5  with a defined transmission ratio. For direct drive wind turbines, no gearbox is needed. In this case the output shaft and the rotor shaft are the same. 
         [0037]    The output shaft  5  is connected to a generator  6  which transforms the rotational power of the shaft  5  into electrical AC power. The generator  6  may be a synchronous or an asynchronous generator of a single fed or double fed type. The generator  6  is a variable speed generator so that its rotational speed can vary depending on the wind conditions. 
         [0038]    A frequency converter or power converter  7  is in communication with outputs of the generator  6 . The frequency converter  7  provides a fixed frequency to a supply network or (utility) grid  8 . The frequency converter  7  converts parts or the whole of the electrical AC input power having a varying frequency delivered by the generator  6  into an electrical AC output power having a fixed frequency compatible with the grid  8 . Furthermore, the frequency converter  8  can control the output power of the wind turbine  1 . The output of the frequency converter  7  is connected to a filter  9  which can be realised by inductors or the like and further to a transformer  10  which transforms the AC output power of the frequency converter  7  to the level of the grid  8 . 
         [0039]    The converter  7  has a rectifier or generator bridge  11 , inputs of which are connected to the outputs of the generator  6 . The rectifier  11  converts AC power into DC power and puts it out onto a DC link  12 . The DC link  12  connects an inverter or grid bridge  13  with the rectifier  11 . The inverter  13  converts DC power into AC power having a frequency matching to the fixed frequency of the grid  8 . 
         [0040]    A rectifier or generator controller  14  is connected to the rectifier  11 . The controller  14  can also be integrated in the rectifier  11  or in another controller or unit. The connection between the rectifier  11  and its controller  14  can be one or bidirectional. The rectifier controller  14  has inputs for the AC power from the generator  6  and for the DC power from the DC link bridge  12 . Further inputs are for a torque of the AC input power or of the generator  6  and for an angular velocity of the AC input power, the shaft  5  or the generator  6 . 
         [0041]    An inverter or grid controller  15  is connected to the inverter  13 . The controller  15  can also be integrated in the inverter  13  or in another controller or unit. The connection between the inverter  13  and its controller  15  can be one or bidirectional. The inverter controller  15  has inputs for the DC power from the DC link bridge  12 , for the AC power from the inverter  13  and for the AC power from the grid  8 . Further inputs are for a torque of the AC input power or of the generator  6  and for an angular velocity of the AC input power, the shaft  5  or the generator  6 . 
         [0042]    A DC link voltage controller  16  for controlling or adjusting the voltage on the DC link  12  is arranged in the rectifier controller  14  and/or in the inverter controller  15 . The DC link voltage controller  16  can also be located in another controller or it can be a single dedicated controller. Here, two DC link voltage controllers  16  are shown. An implementation with a single DC link voltage controller  16  can be used as well. 
         [0043]    The DC link voltage controller  16  is present in the control system to calculate an optimal DC link voltage reference based on current operating conditions of the converter  7 , the generator  6 , the grid  8  and/or the wind turbine  1 . A necessary DC link voltage is the minimum DC link voltage necessary to control generator  6  and grid  8 . This necessary DC link voltage can be calculated by using some linear equations as below and measured/calculated variables of the wind turbine  1  such as generator voltage/generator speed, generator power/current, grid voltage and grid power/current. 
         [0044]    The minimum or necessary DC link voltage for the rectifier or generator controller  14  can be determined or calculated by the DC link voltage controller  16  according to the following formula: 
         [0000]    
       
         
           
             
               V 
               
                 min 
                  
                 GEN 
               
             
             = 
             
               
                 
                   R 
                   S 
                 
                  
                 
                   I 
                   GEN 
                 
               
               + 
               
                 
                   L 
                   S 
                 
                  
                 
                   
                      
                     
                       I 
                       GEN 
                     
                   
                   
                      
                     t 
                   
                 
               
               + 
               
                 U 
                 GEN 
               
             
           
         
       
     
         [0000]    wherein R s  represents the resistance of the stator winding, L s  represents the inductance of the stator winding, I GEN  represents the generator current and U GEN  represents the generator voltage. 
         [0045]    The minimum or necessary DC link voltage for the inverter or grid controller  15  can be determined or calculated according to the following formula: 
         [0000]    
       
         
           
             
               V 
               
                 min 
                  
                 GRID 
               
             
             = 
             
               
                 
                   R 
                   g 
                 
                  
                 
                   I 
                   GRID 
                 
               
               + 
               
                 
                   L 
                   g 
                 
                  
                 
                   
                      
                     
                       I 
                       GRID 
                     
                   
                   
                      
                     t 
                   
                 
               
               + 
               
                 U 
                 GRID 
               
             
           
         
       
     
         [0000]    wherein R g  represents the resistance of the grid, L g  represents the inductance of the grid, I GRID  represents the grid current and U GRID  represents the grid voltage. 
         [0046]    The minimum or necessary DC link voltage (for the DC link) can be determined or calculated according to the following formula: 
         [0000]        V   min DC =max( V   min GEN   ,V   min GRID ). 
         [0047]    Finally, the optimal DC link voltage reference can be determined or calculated according to the following formula: 
         [0000]        V   DCREF =min( V   min DC   ,V   DCMAX ) 
         [0000]    according to which the reference is set to the minimum of the minimum or necessary DC link voltage and of the maximal allowed DC link voltage. This step enhances the security of operation and can be omitted for example when provisions are made to protect the DC link against too high voltages. 
         [0048]    The DC link voltage reference is then used to set the DC link voltage to a desired point or range. The DC link voltage can be adjusted in a range of approximately 950 to 1200 V depending on the employed hardware and/or the desired or allowed operating conditions. 
         [0049]    The update rate of the DC link voltage reference is normally slower than the bandwidth of the DC link control. 
         [0050]    The DC link voltage reference can be communicated to a current controller of the rectifier controller  14  and/or of the inverter controller  15 . The DC link voltage controller  16  can be integrated in such a current controller. Usually, the current controller forms an inner loop of the rectifier controller  14  and/or of the inverter controller  15 . 
         [0051]      FIGS. 2 and 3  show implementations of the rectifier or generator controller  14  and of the inverter or grid controller  15  in which the DC link voltage controller  16  is integrated. The DC link voltage controller  16  can be implemented in hardware and/or in software. The DC link voltage can be controlled from the generator side or from the grid side. 
         [0052]      FIG. 2  shows a controller in which the DC link voltage control is integrated in the generator controller  14 . The general terms “controller” or “DC link voltage control” encompass all control devices which take part in the control of the DC link voltage. 
         [0053]    The generator controller  14  receives inputs as the AC input power, current and voltage from the generator  6  and an angular velocity of the AC input power or the generator  6 . In a Vdcref calculation unit  17  the reference voltage for the DC link V DCREF  is calculated based on some or all of the inputs and is communicated to a DC link controller  18 . The reference voltage can be calculated according to the above equations. 
         [0054]    The DC link controller  18  additionally receives the actual DC link voltage V DC  and calculates the generator power reference P GenRef . The generator or wind turbine power is controlled by a generator current controller  19  to follow this reference power. The generator current controller  19  is the inner loop of the generator controller  14 . 
         [0055]    The above mentioned DC link voltage controller  16  can consist of the Vdcref calculation unit  17  or of the combination of the Vdcref calculation unit  17  with the DC link controller  18  and/or the generator current controller  19  (either the actual current controller  19   b  itself and/or the reference calculation for the current  19   a ). 
         [0056]    The grid controller  15  receives for example from a wind turbine controller a turbine power reference P WTCRef . Here, only the grid current controller  20  of the grid controller  15  is depicted as only these parts are needed for this example. Further parts of the of the grid controller  15  are not shown for the sake of simplicity. The grid current controller  20  controls the generator power to follow the turbine power reference. 
         [0057]      FIG. 3  shows an implementation where the DC link voltage is controlled from the grid side. Accordingly, the DC link controller  18  is located in the grid controller  15 . 
         [0058]    The grid controller  15  receives inputs as the AC input power, current and voltage from the generator  6  and an angular velocity of the AC input power or the generator  6 . In a Vdcref calculation unit  17  the reference voltage for the DC link VDCREF is calculated based on some or all of the inputs and is communicated to a DC link controller  18 . The reference voltage can be calculated according to the above equations. 
         [0059]    The DC link controller  18  additionally receives the actual DC link voltage VDC and calculates the grid power reference P GridRef . The generator or wind turbine power is controlled by a grid current controller  20  to follow this reference power. The grid current controller  20  is the inner loop of the generator controller  14 . 
         [0060]    The above mentioned DC link voltage controller  16  can consist of the Vdcref calculation unit  17  or of the combination of the Vdcref calculation unit  17  with the DC link controller  18  and/or the grid current controller  20  (either the actual current controller  20   b  itself and/or the reference calculation for the current  20   a ). 
         [0061]    The generator controller  14  receives for example from a wind turbine controller a turbine power reference P WTCRef  and/or a torque reference T WTCRef . Here, only the generator current controller  19  of the generator controller  14  is depicted as only these parts are needed for this example. Further parts of the of the generator controller  14  are not shown for the sake of simplicity. The generator current controller  19  controls the generator power to follow the turbine power and/or torque reference. 
         [0062]    Control of the DC link voltage can be achieved from both sides i.e. from generator and from grid side as well. In this case the generator controller  14  is structured like the one shown in  FIG. 2  while the grid controller  15  is structured like the one shown in  FIG. 3 .