Patent Publication Number: US-8970057-B2

Title: Method of operating a wind turbine, wind turbine, wind turbine controlling system, and processing system

Description:
The present invention relates generally to a method of operating a wind turbine, a wind turbine, a wind turbine controlling system, and a processing system usable for operating a wind turbine. 
     With the rapid increase of large offshore wind farms, a new problem associated with the response of wind turbines to temporary overvoltages has arisen: The majority of wind turbines use voltage source converters with a DC-link. When the grid voltage exceeds a certain voltage limit, the current flow through the line-side converter of the wind turbine may reverse, resulting in a rapidly increasing DC-link voltage. 
     One possibility to handle such situations is to interrupt the connection between the wind turbine and the grid. However, it is desirable to keep the wind turbines connected to the grid even under such circumstances. Thus, different approaches are currently under investigation which enable to keep the wind turbines connected to the grid during temporary overvoltages, while at the same time avoiding rapidly increasing DC-link voltages. 
     SUMMARY OF THE INVENTION 
     According to an embodiment, a method of operating a wind turbine comprising a DC-to-AC voltage converter is provided, the wind turbine being connectable to a grid via the DC-to-AC voltage converter, the method comprising: determining a line voltage of a power line connected between the DC-to-AC voltage converter and the grid; if the determined line voltage exceeds a particular grid voltage threshold value, directly injecting reactive current into the power line, wherein the amount of reactive current injected is chosen such that an output voltage of the DC-to-AC voltage converter is kept within a predetermined voltage range. 
     According to an embodiment, the line voltage is determined using a phase lock loop algorithm. 
     According to an embodiment, a reactive current reference is calculated based on a maximum output voltage of the DC-to-AC voltage converter, the line voltage, and an active current reference. 
     According to an embodiment, the active current reference is derived from a DC link controller output. 
     According to an embodiment, the reactive current reference is calculated according to the following formula:
 
 I   R ≦(√{square root over ( m   2   ·U   max   2 −( U   GD   −ωL·I   A ) 2 )} −U   GQ )/ω L  
 
wherein U GD , U GQ  are the line voltages of the power line along the d/q axis in dq frame, and I A , I R  are the active and reactive currents, m is the maximum allowed modulation index and U max  is the maximum allowed converter output voltage {right arrow over (U)} v . Further, L is the inductance of an inductor (commonly known as “grid inductor” or “grid choke”), and ω is the voltage frequency. I R  calculated in the above equation has a negative value. Thus, the minimum reactive current may be chosen (in terms of amplitude) if I R  is set equal to the right term of the equation. Since a maximum current Imax is fixed, this will leave more space for injection of active current I A . Thus, I R  should preferably be equal to the right term of the equation. According to an embodiment, the reactive current reference is converted into an optimized reactive current reference if a converter output current reference amplitude calculated from the active current reference and the reactive current reference exceeds a predetermined converter output current reference amplitude threshold value.
 
     According to an embodiment, the active current reference is converted into an optimized active current reference if a converter output current reference amplitude calculated from the active current reference and the reactive current reference exceeds a predetermined converter output current reference amplitude threshold value. 
     According to an embodiment, the converter output current reference amplitude is determined according to the following formula:
 
 I   ref =√{square root over ( I   r     —     ref     —     cal   2   +I   a     —     ref     —     cal   2 )}
 
     According to an embodiment, the optimized active current reference/reactive current reference are calculated based on the following formula: 
     
       
         
           
               
             
               { 
               
                 
                   
                     
                         
                       ⁢ 
                       
                         
                           I 
                           R 
                         
                         = 
                         
                           
                             
                               ( 
                               
                                 
                                   
                                     
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                                       2 
                                     
                                     · 
                                     
                                       U 
                                       max 
                                       2 
                                     
                                   
                                   
                                     
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                                       L 
                                       2 
                                     
                                   
                                 
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                           I 
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                               max 
                               2 
                             
                             - 
                             
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                               R 
                               2 
                             
                           
                         
                       
                     
                   
                 
               
             
           
         
       
     
     According to an embodiment, the converter output current is controlled based on the optimized active current reference and the optimized reactive current reference. That is, the amount of reactive current injected is controlled based on the optimized reactive current reference, and the amount of active current injected is controlled based on the optimized active current reference. One effect of this embodiment is that the converter output voltage is kept within Umax, and the converter output current is kept within Imax. A further effect of this embodiment is that, at the same time, the maximum possible active power can be injected into the grid. 
     According to an embodiment, a processing system usable for operating a wind turbine comprising a DC-to-AC voltage converter is provided, the wind turbine being connectable to a grid via the DC-to-AC voltage converter, the processing system comprising: an input unit being configured to receive a line voltage signal indicating the line voltage of a power line connected between the DC-to-AC voltage converter and the grid; a processing unit coupled to the input unit, the processing unit being configured to determine whether the line voltage exceeds a particular line voltage threshold value, and to determine an amount of reactive current which, if directly injected into the power line, keeps an output voltage of the DC-to-AC voltage converter within a predetermined voltage range; and an output unit coupled to the processing unit, the output unit being configured to output a signal indicative of the reactive current to be directly injected into the power line. 
     According to an embodiment, the input unit is further configured to receive an active current reference signal, wherein the processing unit is configured to calculate a reactive current reference indicative of the reactive current to be injected based on a maximum output voltage of the DC-to-AC voltage converter, the line voltage signal, and the active current reference signal, wherein the output signal outputted by the output unit is derived by the processing unit from the reactive current reference. 
     According to an embodiment, the processing unit is further configured to convert the reactive current reference into an optimized reactive current reference if a converter output current reference amplitude calculated from the active current reference and the reactive current reference exceeds a predetermined converter output current reference amplitude threshold value, wherein the output signal outputted by the output unit is derived by the processing unit from the optimized reactive current reference. 
     According to an embodiment, the processing unit is configured to convert the active current reference into an optimized active current reference if a converter output current reference amplitude calculated from the active current reference and the reactive current reference exceeds a predetermined converter output current reference amplitude threshold value, wherein the output signal outputted by the output unit is derived by the processing unit from the optimized active current reference. 
     According to an embodiment, a wind turbine controlling system for controlling a wind turbine comprising a DC-to-AC voltage converter is provided, the wind turbine being connected to a grid via the DC-to-AC voltage converter, the controlling system comprising a processing system according to any one of the embodiments as described above. 
     According to an embodiment, the wind turbine controlling system further comprises a phase lock loop unit being coupled to the input unit of the processing system, the phase lock loop unit being configured to determine the line voltage signal using a phase lock loop algorithm, and to supply it to the input unit. 
     According to an embodiment, the wind turbine controlling system further comprises a DC link controller unit coupled to the input unit of the processing system, wherein the DC link controller unit is configured to control a DC link voltage according to a DC link voltage reference level (the DC link controller unit controls the DC link voltage so that it is always kept at a reference voltage; the DC link controller does this by injecting an appropriate active current), and to output the active current reference signal which is supplied to the input unit. 
     According to an embodiment, the wind turbine controlling system further comprises a converter output current controlling unit coupled to the output unit of the processing system, wherein the converter output current controlling unit is configured to control the converter output current based on the active current reference and the reactive current reference, or based on the optimized active current reference and the optimized reactive current reference. 
     According to an embodiment, a wind turbine comprising a wind turbine controlling system according to any one of the embodiments as described above is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which: 
         FIG. 1  shows a schematic drawing of a wind turbine according to an embodiment of the present invention; 
         FIG. 2   a  shows a schematic drawing of an electrical system having a full scale converter configuration; 
         FIG. 2   b  shows a schematic drawing of an electrical system having a full scale converter configuration according to an embodiment of the present invention; 
         FIG. 3  shows a flow chart diagram of a method of operating a wind turbine according to an embodiment of the present invention; 
         FIG. 4  shows a schematic block diagram of a processing system usable for operating a wind turbine according to an embodiment of the present invention; 
         FIG. 5  shows a schematic block diagram of a controlling system usable for operating a wind turbine according to an embodiment of the present invention; 
         FIG. 6  shows a schematic phasor diagram which may occur when carrying out a method of operating a wind turbine according to an embodiment of the present invention; 
         FIG. 7  shows a schematic phasor diagram which may occur when carrying out a method of operating a wind turbine according to an embodiment of the present invention; 
         FIG. 8  shows a schematic phasor diagram which may occur when carrying out a method of operating a wind turbine according to an embodiment of the present invention; 
         FIG. 9  shows a schematic current diagram which may occur when carrying out a method of operating a wind turbine according to an embodiment of the present invention; 
         FIG. 10  shows a schematic current diagram which may occur when carrying out a method of operating a wind turbine according to an embodiment of the present invention; 
         FIG. 11  shows a schematic current diagram which may occur when carrying out a method of operating a wind turbine according to an embodiment of the present invention; 
         FIG. 12  shows an example of the amount of reactive current to be injected as required by a grid code in case of a low voltage event in the grid. 
     
    
    
     DESCRIPTION 
     The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. 
       FIG. 1  illustrates a common setup of a wind turbine  1000  according to one embodiment of the present invention. The wind turbine  1000  is mounted on a base  1002 . The wind turbine  1000  includes a tower  1004  having a number of towers sections, such as tower rings. A wind turbine nacelle  1006  is placed on top of the tower  1004 . The wind turbine rotor  1008  includes at least one rotor blade  1010 , e.g. three rotor blades  1010 . The rotor blades  1010  are connected to the nacelle  1006  through a low speed shaft which extends out of the front of the nacelle  1006 . In the wind turbine  1000  (e.g. within the nacelle  1006 ), an electrical system may be used which will be described in the following). 
       FIG. 2   a  shows an electrical system  100  of a wind turbine having a converter configuration which may be used in a wind turbine according to embodiments of the present invention. The electrical system  100  is connected to a generator  102  of a wind turbine. The electrical system  100  comprises an AC-to-DC voltage converter  104  (generator-side converter) connected to the generator  102 , a DC-to-AC converter  108  (line-side converter) and a DC link  106  connected between the AC-to-DC converter  104  and the DC-to-AC converter  108 . The DC-to-AC converter  108  is connected via a power line  116  to a transformer  112  which in turn is connected to a power grid  110 . An inductor  114  is located along the power line  116 . The power line  116  is further connected to a capacitor  118 . The electrical system  100 , the generator  102  and the transformer  112  may be part of a wind turbine as shown in  FIG. 1 , and are typically located within the nacelle of the wind turbine. The converter configuration of the electrical system  100  is a full scale converter configuration. “Full scale” in this context means that the full power generated by a generator  102  is converted by the DC-to-AC voltage converter  108  before being supplied to the grid  110 . The DC-to-AC voltage converter  108  has a maximum converter output voltage (U max ) that it can produce based on a fixed DC link voltage and also has a converter output current limit that cannot be exceeded. If the converter current output limit is exceeded, the wind turbine may trip. That is, when there is a HV (high voltage) event on the power line  116  (also referred to as “voltage swell” event), the converter output voltage of the DC-to-AC converter  108  may exceed the maximum converter output voltage U max  of the DC-to-AC converter  108 . As a result, this may cause a converter output current to exceed the converter output current limit of the DC-to-AC converter  108 . This may cause the wind turbine to trip, resulting in the wind turbine being disconnected from the grid  110 . 
     In other words: In case the grid voltage of the power grid  110  exceeds for example beyond 1 p.u. (p.u.=“per unit”, i.e. a ratio of voltage/normal voltage), the DC-to-AC converter  108  (line-side converter) of the wind turbine is required to adjust a line current vector such that the required converter output voltage of the DC-to-AC converter  108  can be kept below the maximum converter output voltage U max  and the converter output current does not exceed the converter output current limit of the DC-to-AC converter  108 . In grid swell condition, since the maximum output voltage U max  of the DC-to-AC converter  108  is limited by the DC-link voltage U DC , the converter output current I G  may become uncontrollable. Also, in a grid swell condition, a significant overshoot of the DC-link voltage U DC  may occur which may lead to an activation of a chopper (not shown) for the dissipation of unknown/uncontrolled power transfer to the grid  110 . In order to avoid such uncontrollable situations, countries like Australia have stipulated High Voltage Ride Through (HVRT) requirements for wind turbines to withstand an over-voltage of 1.3 p.u. for 60 ms without being disconnected. 
     The grid voltage on the grid  110  (high voltage side of the transformer  112 ) is correlated to the line voltage of the power line  116  (low voltage side of the transformer  112 ) by the transformer ratio. Thus, in the present invention, the terms “line voltage” and “grid voltage” may be used interchangeably. 
     However, the situation may be kept under control if reactive power is injected into the power line  116  connected between the DC-to-AC converter  108  and the transformer  112  (and thus into the power grid  110 ) by adjusting the required output voltage of the DC-to-AC converter  108 , as will become apparent in the description below. 
       FIG. 3  shows a flow chart of a method  200  of controlling a wind turbine according to an embodiment of the present invention. At  202 , the method is started. At  204 , the line voltage of the power line is determined. At  206 , it is determined whether the line voltage exceeds a predefined threshold value. If the predefined threshold value is not exceeded, the flow returns to  204 . If the predefined threshold value is exceeded, the flow continues with  208  at which reactive current is directly injected into the power line, wherein the amount of reactive current injected is chosen such that an output voltage of the DC-to-AC converter is kept within a predetermined voltage range. Then, the flow returns to  204 . 
     In the context of  FIG. 2   a , the method shown in  FIG. 3  may be interpreted in a way that in case of HV events on the power line  116 , a calculated amount of reactive current is injected into the power line  116 . This injection is done by adjusting the converter output voltage of the DC-to-AC converter  108  such that the resulting converter output current I G  comprises the calculated amount of reactive current. By controlling the amount of the reactive current injected, the amplitude of the converter output voltage U v  can be kept below the maximum converter output voltage U max , and maximum possible active power can be injected into the power line  116  (and thus into the grid  110 ). As a result, the wind turbine will not trip, and can therefore remain connected to the grid  110  in HV events. HVRT (high voltage ride through) refers to this capability of the wind turbine to remain connected to the grid  110  during such a HV event. 
     In the following description, a theoretical background for embodiments of the present invention will be given. 
     The output voltage of the DC-to-AC converter  108  in  FIG. 2  may be represented by the following voltage equation:
 
 {right arrow over (U)}   v   ={right arrow over (U)}   G   +{right arrow over (U)}   L   ={right arrow over (U)}   G   +jωL{right arrow over (I)}   G  
 
wherein {right arrow over (U)} v  is the converter output voltage of the DC-to-AC converter  108 , {right arrow over (U)} G  is a line voltage at a part of the power line  116  close to the low voltage end of the transformer  112 , {right arrow over (U)} L  is the voltage drop across the inductor  114 , and {right arrow over (I)} G  is the output current of the DC-to-AC converter  108 . Further, L is the inductance of the inductor  114 , and ω is the voltage frequency.
 
     In case of a “healthy” grid (power line) (in a case where there is no grid swell and where the line voltage {right arrow over (U)} G  is within its normal line voltage range), the converter output current {right arrow over (I)} G  is aligned with the line voltage {right arrow over (U)} G  as illustrated in the phasor diagram of  FIG. 6 . That is, only active power is injected into the power line  116 , but no reactive power. 
     Under the grid voltage swell condition, according to an embodiment of the present invention, the converter output current {right arrow over (I)} G  is controlled in such a way that the amplitude of converter output voltage {right arrow over (U)} v  is limited at or below a maximum converter output voltage amplitude U max . The phase of converter output current {right arrow over (I)} G  leads the phase of the line voltage {right arrow over (U)} G  and it can be projected into reactive current I reactive  and active current I active  as shown in the phasor diagram of  FIG. 7 . 
     It should be noted that in the case of grid voltage dip (LV (low voltage) events), reactive (capacitive) current is also normally required by grid operators to be injected into the grid  110  to help in stabilizing the grid  110 . In the case of LV event, the converter output voltage amplitude is always below the limit U max . The phasor diagram as shown in  FIG. 8  illustrates this. 
     The following equations can be derived from a DQ frame model of line current control. 
                   {             L   ⁢       ⅆ     I   R         ⅆ   t         =       U   VD     -     U   GD     +     ω   ⁢           ⁢     LI   A                       L   ⁢       ⅆ     I   A         ⅆ   t         =       U   VQ     -     U   GQ     -     ω   ⁢           ⁢     LI   R                         
wherein U VD , U VQ  are the converter output voltages along the d/q axis in the dq frame, U GD , U GQ  are the line voltages along the d/q axis in dq frame, and I A , I R  are the active and reactive currents. Further, L is the inductance of the inductor  114 , and ω is the voltage frequency.
 
     In the steady state condition, the converter output voltage {right arrow over (U)} v  can be calculated as below 
     
       
         
           
               
             
               { 
               
                 
                   
                     
                       
                         U 
                         VD 
                       
                       = 
                       
                         
                           U 
                           GD 
                         
                         - 
                         
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             LI 
                             A 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         U 
                         VQ 
                       
                       = 
                       
                         
                           U 
                           GQ 
                         
                         + 
                         
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             LI 
                             R 
                           
                         
                       
                     
                   
                 
               
             
           
         
       
     
     The following expression can be used based on the converter voltage limit,
 
 U   VD   2   +U   VQ   2 =( U   GD   −ωLI   A ) 2 +( U   GQ   +ωLI   R ) 2 ≦( m·U   max ) 2   (1)
 
wherein “m” is the maximum allowed modulation index and U max  is the maximum allowed converter output voltage {right arrow over (U)} v .
 
     Using the equations given above, under the steady state condition, the required reactive current for the HVRT condition can be derived as follows
 
 I   R ≦(√{square root over ( m   2   ·U   max   2 −( U   GD   −ωL·I   A ) 2 )}− U   GQ )/ω L   (2)
 
       FIG. 4  shows a processing system  300  usable for operating a wind turbine according to one embodiment of the present invention. The processing system  300  comprises: an input unit  302  being configured to receive a line voltage signal  304  indicating the line voltage of the power line; a processing unit  306  coupled to the input unit  302 , the processing unit  306  being configured to determine whether the line voltage exceeds a predetermined threshold value, and to determine an amount of reactive current which, if directly injected into the power line, keeps an output voltage of the DC-to-AC voltage converter within a predetermined voltage range; and an output unit  308  coupled to the processing unit  306 , the output unit  308  being configured to output a signal  310  indicative of the reactive current to be directly injected into the power line. The output signal  310  may be generated based on formula (2). 
       FIG. 5  shows a possible realization of the processing system  300  shown in  FIG. 4 . In  FIG. 5 , a block diagram of a converter controller system  400  for controlling the DC-to-AC converter  108  is shown. Part  402  of the converter controller system  400  (ride through functionality system) may be regarded as a concrete embodiment of the processing system  300 . Part  402  comprises functionality needed to handle HV events (HVRT functionality) as well as functionality needed to handle LV events (LVRT functionality). 
     The voltage ride through functionality system  402  comprises an input unit  404 , an output unit  406  and a processing unit  408 . 
     The input unit  404  is coupled to a phase lock loop unit  410  being configured to generate a line voltage signal S 1  indicative of the d,q components of the line voltage of the power line  116  using a phase lock loop algorithm, and to supply the line voltage signal S 1  to the input unit  404 . 
     The input unit  404  is further coupled to a DC link controller unit  412  being configured to generate an active current reference signal S 2 , and to supply the active current reference signal S 2  to the input unit  404 . 
     The output unit  406  is coupled to a converter output current controlling unit  414  being configured to control the converter output current based on an active current reference signal S 3  and an reactive current reference signal S 4 . The active current reference signal S 3  may be an optimized active current reference signal, and the reactive current reference signal S 4  may be an optimized reactive current reference signal. 
     The processing unit  408  comprises a first reactive current generator unit  416 , a second reactive current generator unit  418 , a line voltage classifying unit  420 , a switching unit  422 , and a current optimizing unit  424 . 
     Based on a nominal line voltage signal S 5 , the line voltage classifying unit  420  classifies the line voltage into three categories—normal, dip or swell. 
     Depending on the line voltage category determined, a reactive current reference signal S 6  is generated by either the first reactive current generator unit  416  (in case of a dip condition) based on information as shown in  FIG. 12  ( FIG. 12  provides information specifying the amount of reactive current to be injected as required by a grid code in case of a low voltage event in the power line), or from the second reactive current generator unit  418  (in case of a swell condition) based on equation (2). The switching unit  422  is controlled by an output signal S 7  generated by the line voltage classifying unit  420  such that the reactive current reference signal S 6  is calculated from the correct one of the first reactive current generator unit  416  and the second reactive current generator unit  418 , depending on the line voltage. Using the active current reference signal S 2  and the reactive current reference signal S 6 , the line current controlling unit  414  generates a converter output voltage reference signal S 8  based on which the converter output voltage {right arrow over (U)} v  is controlled accordingly. In order to do this, a controller of the DC-to-AC converter  108  may for example generate PWM signals based on the converter output voltage reference signal S 8 , thereby controlling the DC-to-AC converter  108  to adjust the converter output voltage {right arrow over (U)} v  such that the converter output current adopts its target value. This is exemplary shown in  FIG. 2   b : The line voltage signal S 1 , signal S 9  and signal S 12  (and maybe also further signals) are supplied to the converter controller system  400 , based on which the converter controller system  400  determines the converter output voltage reference signal S 8 . The converter output voltage reference signal S 8  is used to generate PWM signals for controlling the DC-to-AC converter  108  such that the converter output current adopts its target value. 
     Signal S 9  is a measured DC link voltage signal (measured at the DC link  106 , signal S 10  is an inductance input parameter (a parameter indicating the inductance of the inductor  114 ), signal S 11  is a DC link voltage target value, and signal S 12  is a measured converter output current signal of the converter  108 . 
     In the case that a voltage dip condition is detected by the line voltage classifying unit  420 , the reactive current reference signal S 6  is calculated by the first reactive current generator unit  416  based on the percentage of the dip according to a grid code as shown in  FIG. 12 . 
     The current optimizing unit  424  (optimal current trajectory control block) is optional and works as follows. A converter output current reference amplitude is computed by the current optimizing unit  424  from the reactive current reference signal S 6  and the active current reference signal S 2 , and is compared with a converter output current reference amplitude limit I max . If the converter output current reference amplitude is less than I max , an active current reference signal S 3  and a reactive current reference signal S 4  are set as following: 
     
       
         
           
             
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     That is, in this case, the active current reference signal S 3  corresponds to the active current reference signal S 2 , and the reactive current reference signal S 4  corresponds to the reactive current reference signal S 6 . 
     However, if the current reference amplitude exceeds the maximum limit I max , an optimization process is applied. The optimization process is carried out as follows: Optimized current references (signals S 3 , S 4 ) are determined using equation (1) as follows: 
     
       
         
           
               
             
               { 
               
                 
                   
                     
                         
                       ⁢ 
                       
                         
                           
                             
                               ( 
                               
                                 
                                   U 
                                   GD 
                                 
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                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     LI 
                                     A 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
                           + 
                           
                             
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                                   U 
                                   GQ 
                                 
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                                     R 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
                         
                         ≤ 
                         
                           
                             m 
                             2 
                           
                           · 
                           
                             U 
                             max 
                             2 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                         
                       ⁢ 
                       
                         
                           
                             I 
                             A 
                             2 
                           
                           + 
                           
                             I 
                             R 
                             2 
                           
                         
                         = 
                         
                           I 
                           max 
                           2 
                         
                       
                     
                   
                 
               
             
           
         
       
     
     Since U GD  is equal to zero in the steady state (this is because the rotation frame, the voltage is aligned with the Q axis in stable condition, and in vertical position to the D axis; U GD  is the voltage that the line voltage is projected to D axis, so it is zero) in the current vector plane, the equations above can be written as following: 
     
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                             
                           ⁢ 
                           
                             
                               
                                 
                                   ( 
                                   
                                     
                                       I 
                                       R 
                                     
                                     + 
                                     
                                       
                                         U 
                                         GQ 
                                       
                                       
                                         ω 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         L 
                                       
                                     
                                   
                                   ) 
                                 
                                 2 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     I 
                                     A 
                                   
                                   ) 
                                 
                                 2 
                               
                             
                             ≤ 
                             
                               
                                 
                                   m 
                                   2 
                                 
                                 · 
                                 
                                   U 
                                   max 
                                   2 
                                 
                               
                               
                                 
                                   ω 
                                   2 
                                 
                                 ⁢ 
                                 
                                   L 
                                   2 
                                 
                               
                             
                           
                         
                       
                     
                     
                       
                         
                             
                           ⁢ 
                           
                             
                               
                                 I 
                                 R 
                                 2 
                               
                               + 
                               
                                 I 
                                 A 
                                 2 
                               
                             
                             = 
                             
                               I 
                               max 
                               2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     By solving the above equations (3), the following optimal current references (signals S 3 , S 4 ) are obtained: 
     
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                             
                           ⁢ 
                           
                             
                               I 
                               R 
                             
                             = 
                             
                               
                                 
                                   ( 
                                   
                                     
                                       
                                         
                                           m 
                                           2 
                                         
                                         · 
                                         
                                           U 
                                           max 
                                           2 
                                         
                                       
                                       
                                         
                                           ω 
                                           2 
                                         
                                         ⁢ 
                                         
                                           L 
                                           2 
                                         
                                       
                                     
                                     - 
                                     
                                       I 
                                       max 
                                       2 
                                     
                                   
                                   ) 
                                 
                                 · 
                                 
                                   
                                     ω 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     L 
                                   
                                   
                                     2 
                                     · 
                                     
                                       U 
                                       GQ 
                                     
                                   
                                 
                               
                               - 
                               
                                 
                                   U 
                                   GQ 
                                 
                                 
                                   
                                     2 
                                     · 
                                     ω 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   L 
                                 
                               
                             
                           
                         
                       
                     
                     
                       
                         
                             
                           ⁢ 
                           
                             
                               I 
                               A 
                             
                             = 
                             
                               
                                 
                                   I 
                                   max 
                                   2 
                                 
                                 - 
                                 
                                   I 
                                   R 
                                   2 
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     That is, if the converter output current reference amplitude exceeds the maximum limit I max , the signals S 3 , S 4  are obtained from signals S 2 , S 6  using equations (4). 
     If the equations (4) are plotted in the current plane, the optimized solutions for active/reactive current references in line voltage swell condition can be visualized. This is shown in  FIG. 9 . Equations (4) to be understood that the currents Ir and Ia are completely recalculated according to equations (4), i.e. in this case the currents Ir and Ia are calculated without using signals S 2  and S 6 . 
       FIG. 9  shows a current trajectory curve obtained after an optimization process based on equation (4). 
     This optimal current trajectory control ensures maximum active current and power output during HVRT while maintaining both the converter output current and converter output voltage within its limits. 
     In the condition of line voltage dip, the situation of  FIG. 10  applies. In this case, since the converter circle contains the current circle, only current limit needs to be satisfied. 
       FIG. 11  applies in the healthy line voltage condition (no reactive current injection needed). Since only active current is required to be injected to the power line, the current trajectory is only at the Y axis. 
     Based on the above description, it has become apparent that, according to an embodiment of the present invention, the following method is applied: 1) First, the active current (Ia_ref) and reactive current (Ir_ref) that need to be injected into the power line are determined. Ir_ref is determined using equation (2) and optimized, if necessary. 2) To cause the DC-to-AC converter  108  to output a converter output current I G  which includes the respective active and reactive current components, the DC-to-AC converter  108  is controlled to output the necessary converter output voltage Uv. The control of the DC-to-AC converter  108  is done using PWM (Pulse Width Modulation) signals generated based on the signal S 8  (i.e. the output signal from the converter output current control unit). 3) The PWM signals are used to control the operation of the DC-to-AC converter  108  such that it outputs the Uv (which is below Umax) together with a corresponding converter output current I G  (comprising the corresponding Ia and Ir components determined) which is below the converter output current limit. Thus, first the reactive current to be injected into the power line  116  is determined such that Uv needed to generate the reactive current is below Umax. Then, the DC-to-AC converter  108  is controlled to have a voltage output Uv which will imply the converter output current I G  with corresponding Ir component as determined earlier. 
     As has become apparent, embodiments of the present invention solve current and power control problems in case of a grid voltage swell. 
     According to embodiments of the present invention, by selecting a proper trajectory of current, the optimal active and reactive current can be injected such that the maximum active power can be transferred to the grid during HVRT. 
     According to embodiments of the present invention, the possibility of turbine tripping and chopper activation during a HVRT condition is minimized. 
     According to embodiments of the present invention, real and reactive power is controlled during voltage ride through condition. 
     According to embodiments of the present invention, by using an optimal trajectory current control, it is possible to provide the optimal active and reactive current injection so that the maximum active power can be transferred to the grid during HVRT. 
     According to embodiments of the present invention, a reactive current reference is computed, thereby minimizing the possibility of chopper activation. In HVRT condition with no activation of a chopper, the power is fed to the grid rather than being dissipated as heat into a chopper resistor. 
     While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.