Abstract:
A power generating system includes an energy source coupled to a DC link through a first power converter and a second power converter to couple the DC link to a power grid. A first controller in the power generating system regulates voltage on the DC link and a second controller regulates a parameter of the energy source. A dynamic parsing controller coupled to the first power converter and the second power converter selectively parses the output signals of the first and second controllers and generates operating commands for the first and second power converters based at least in part on the parsed output signals.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to power generating systems connected to a grid and, more particularly, to control of a wind power generating system. 
         [0002]    Wind turbine generators are regarded as environmentally friendly and relatively inexpensive alternative sources of energy that utilize wind energy to produce electrical power. A wind turbine generator generally includes a wind rotor having turbine blades that transform wind energy into rotational motion of a drive shaft, which in turn is utilized to drive a rotor of an electrical generator to produce electrical power. Modern wind power generation systems typically take the form of a wind-farm having multiple such wind turbine generators that are operable to supply power to a utility system. 
         [0003]    Some wind turbine generators have a variable frequency operation and require a variable frequency power electronic converter to interface the wind turbine generator output with the utility grid. In one common approach, the wind turbine generator output is directly fed to a power electronic converter where the generator output frequency is rectified and inverted into a fixed frequency as needed by the utility system. 
         [0004]    One of the challenges associated with such systems is control of wind turbines in the event of a weak and/or resonant grid connection. For example, to compensate for a weak grid connection use of series compensation, such as a series capacitor bank, is one means of increasing transmission capability in the grid, but this has potential limitation. Series compensation may lead to sub-synchronous resonance modes that can couple to the power electronic converter controllers causing control instability. Resonant mode coupling challenges may also occur at super synchronous frequencies due to the interaction of distributed shunt capacitance and line inductance in the network. 
         [0005]    Therefore, it is desirable to determine a method and a system that will address the foregoing issues. 
       BRIEF DESCRIPTION 
       [0006]    In accordance with an embodiment of the present invention, a power generation system including an energy source coupled to a DC link through a first power converter is provided. The power generation system also includes a second power converter for coupling the DC link to a power grid, a first controller for regulating voltage on the DC link and a second controller for regulating a parameter of the energy source. The system further includes a dynamic parsing controller coupled to the first power converter and the second power converter and configured to selectively parse the output signals of the first and second controllers and generate operating commands for the first and the second power converters based at least in part on the parsed output signals. 
         [0007]    In accordance with another embodiment of the present invention, a control system for a wind power generating system including a wind turbine coupled to a DC link through a first power converter and a second power converter for coupling the DC link to a power grid is provided. The control system includes a DC link controller for regulating voltage on the DC link and a torque controller for regulating torque of the wind turbine. The control system further includes a dynamic parsing controller coupled to the first power converter and the second power converter and configured to selectively parse the output signals of the torque and DC link controllers and generate operating commands for the first and second power converters based at least in part on the parse output signals. 
         [0008]    In accordance with yet another embodiment of the present invention, a method of supplying electrical power to a power grid is provided. The method includes generating the electrical power from an electrical source and coupling the electrical source to the power grid through a first power converter and a second power converter. The method also includes controlling the first and second power converters by interfacing output signals of a first controller configured to regulate voltage on the DC link and a second controller configured to regulate a parameter of the energy source. 
     
    
     
       DRAWINGS 
         [0009]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0010]      FIG. 1  is a diagrammatical representation of a conventional wind power generating system connected to a power grid; 
           [0011]      FIG. 2  is a diagrammatical representation of a grid connected wind power generating system, in accordance with an embodiment of the present invention; 
           [0012]      FIG. 3  is a diagrammatical representation of a detailed block diagram of an unified controller of  FIG. 2 , in accordance with an embodiment of the present invention; 
           [0013]      FIG. 4  is graphical representation of frequency responses of function blocks of  FIG. 3 ; and 
           [0014]      FIG. 5  is graphical representation of an amplitude response plot of function blocks of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    As discussed in detail below, embodiments of the present invention function to provide methods and systems to control grid connected power generating systems. Although the present discussion focuses on control of power electronic converters in a wind power generating system, the present invention is applicable to any power generating system with a dispatchable or intermittent input energy source and a power electronic converter interface. For example, the present invention is also applicable but not limited to solar power generation, marine hydrokinetic power generation, microturbines, and fuel cell systems. 
         [0016]      FIG. 1  shows a conventional grid connected wind power generating system  10 . The system includes a wind generator  12 , a generator side converter  14 , and a grid side converter  16 . The system further includes a grid side controller  18 , a generator side controller  20 , and a power grid  22 . Power grid  22  typically includes conventional synchronous generators  24  and electrical loads  26 . A resonant circuit  42  is also formed in power grid  22 . A direct current (DC) link  28  connects generator side converter  14  and grid side converter  16 . Generator side converter  14  converts alternating current (AC) power generated by wind generator  12  into DC power. Grid side converter  16  then converts the DC power to AC power at a frequency compatible with power grid  22 . 
         [0017]    The combination of grid side controller  18  and grid side converter  16  functions as a current controlled source for grid  22 . In other words, grid side controller  18  controls the phase and amplitude of the output current  30  of grid side converter  16  relative to the measured terminal voltage (Vpcc). In one embodiment, grid side controller  18  and grid side converter  16  may alternatively function as a voltage controlled source. Grid side controller  18  typically includes a phase locked loop (PLL)  32 , a DC voltage regulator  34 , and an AC current regulator  36 . PLL  32  senses three phase voltages of the power grid and generates a frequency and phase reference for grid side converter  16 . DC voltage regulator  34  helps in maintaining the DC link voltage at a desired value. Current regulator  36  generates the output current reference for grid side converter  16  based on the PLL output and the DC voltage regulator output. Generator side controller  20  generates operating or switching signals for generator side converter  14 . In one embodiment, a turbine controller  38  provides a torque reference to generator side controller  20  based on wind velocity or rotor speed of the wind turbine. The turbine controller generates the torque reference to maximize the energy captured from the wind. 
         [0018]    Grid resonance modes can couple to grid side controller  18  through the voltage and current measurement signals. In some embodiments, the grid side control of DC voltage can be susceptible to interaction with weak grids or grid resonant modes. This interaction may occur due to the effect of weak or resonant grid conditions on the forward loop transfer function of the DC link voltage control. In another embodiment (not shown in  FIG. 1 ) to overcome control instability problems caused by weak or resonant grid conditions, generator side converter  14  controls DC link voltage, grid side converter  16  controls wind turbine torque. However, this type of control may result in wind turbine torque instability due to the effect of weak or resonant grid conditions on the forward loop transfer function of the torque control. 
         [0019]      FIG. 2  shows a grid connected wind power generating system  60  in accordance with an embodiment of the present invention. Power generating system  60  includes a unified controller  62  that coordinates the control of signals necessary for the transfer of power from the wind turbine generator  12  to the converter DC link  28  and power grid  22 . The unified controller functions may include, but are not limited to, controlling output power, generator torque, generator speed, and DC link voltage. In one embodiment, unified controller  62  includes a torque controller  80  and a voltage controller  82 . It should be noted that torque controller  80  may be replaced by or supplemented with other controllers to control parameters of generating systems such as active power, generator speed, or output current. 
         [0020]    In one specific embodiment, unified controller  62  includes a coupled or a multivariable controller that receives torque reference command T*, torque feedback signal T, DC link voltage reference command Vdc*, and DC link voltage feedback signal Vdc. The controller  62  then provides control signals or switching commands  66  and  68  to generator side converter  14  and grid side converter  16 , respectively. In one embodiment, the torque control and the DC link voltage control functions are split between generator side and grid side converters in a parsing controller  81  based on various strategies such as parsing the control signals based on frequency response or based on amplitudes, for example. 
         [0021]      FIG. 3  shows a detailed block diagram of the unified controller  62  of  FIG. 2  in accordance with an embodiment of the present invention. Unified controller  62  includes torque controller  80 , DC link voltage controller  82 , and dynamic parsing controller  81 . Both torque and DC link voltage controllers  80 ,  82  may comprise proportional-integral (PI) type controllers. Torque controller  80  receives the torque reference command T* and the torque feedback signal T and generates a torque control signal r 1  based on a difference between T* and T. Similarly, voltage controller  82  generates a voltage control signal r 2  based on a difference between reference DC link voltage command Vdc* and DC link voltage feedback signal Vdc. 
         [0022]    In one embodiment, dynamic parsing controller  81  includes function blocks  84 ,  86  for torque control and function blocks  88 ,  90  for voltage control to parse the torque and voltage control signals r 1  and r 2  between the generator side converter and the grid side converter in response to system dynamics. In this way, the control and disturbance response of the torque and DC link voltage signals will be improved. In one embodiment, function blocks  84 ,  86 ,  88 ,  90  may be dynamic, linear, or nonlinear scalar blocks, or combinations of these. Further, the parameters of the function blocks may be dynamically adjusted based on other control signals such as output power, generator speed, voltage magnitude, or current magnitude. 
         [0023]    Function blocks  84 ,  86 ,  88 ,  90  may parse the torque and voltage control signals r 1  and r 2  based on any of a number of strategies. One strategy is to parse the signals based on frequency response such that high frequency and low frequency signals are separated. For example, the high frequency and low frequency signals may be defined by the operator and, in one embodiment, low frequency signals refer to signals of bandwidth lower than 5 or 10 Hz, and high frequency signals refer to signals with bandwidth higher than 5 or 10 Hz. The operator as used herein refers to an authorized person who controls the operation of a power generating system and has authority to control parameters of the controllers. In one embodiment function blocks  86  and  88  may comprise high pass filters, and function blocks  84  and  90  may comprise low pass filters. The transfer functions F 12 (s) and F 22 (s) of the function blocks  88 ,  90  for the DC-link voltage control may be given as 
         [0000]        F   12 ( s )= k   11   Ts/ 1 +Ts   (1)
 
         [0000]        F   22 ( s )= k   21 1/1+ Ts   (2)
 
         [0000]    where s is a Laplace operator, and T, k 11 , and k 21  are constants determined based on network elements and network conditions such as power level, voltage and current magnitude, faults, or generator speed. In one embodiment, constants T, k 11 , and k 21  are dynamic and may vary depending on system conditions. For example, T may comprise a time constant determined from the parsing frequency bandwidth selected. Thus, this implementation will parse control based on the frequency content of the voltage control signal. The high frequency content of the signal (freq&gt;½πT) will be directed to the generator side converter  14 , and the low frequency (e.g., steady-state) content will be directed to the grid side converter  16 . Similar transfer functions may be used for control blocks  84  and  86  for controlling the torque. The frequency parsed signals r 11  and r 2   h  from the torque control signal and the voltage control signal are then combined to generate an active-current signal Ir_gen for the generator side converter  14 . Further, frequency parsed signals r 1   h  and r 2   l  are combined to generate an active-current signal Ir_grid for the grid side converter  16 . 
         [0024]    In another embodiment, based on system conditions, function blocks  88 ,  90  for dc-link voltage control may be designed as below: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       F 
                       12 
                     
                      
                     
                       ( 
                       
                         r 
                         2 
                       
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               k 
                               11 
                             
                             · 
                             
                               ( 
                               
                                 
                                   r 
                                   2 
                                 
                                 - 
                                 
                                   F 
                                   max 
                                 
                               
                               ) 
                             
                           
                         
                         
                           
                             
                               r 
                               2 
                             
                             ≥ 
                             
                               F 
                               max 
                             
                           
                         
                       
                       
                         
                           0 
                         
                         
                           
                             
                               F 
                               max 
                             
                             &gt; 
                             
                               r 
                               2 
                             
                             &gt; 
                             
                               F 
                               min 
                             
                           
                         
                       
                       
                         
                           
                             
                               k 
                               11 
                             
                             · 
                             
                               ( 
                               
                                 
                                   r 
                                   2 
                                 
                                 - 
                                 
                                   F 
                                   min 
                                 
                               
                               ) 
                             
                           
                         
                         
                           
                             
                               r 
                               2 
                             
                             ≤ 
                             
                               F 
                               min 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       F 
                       22 
                     
                      
                     
                       ( 
                       
                         r 
                         2 
                       
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               k 
                               11 
                             
                             · 
                             
                               F 
                               max 
                             
                           
                         
                         
                           
                             
                               r 
                               2 
                             
                             ≥ 
                             
                               F 
                               max 
                             
                           
                         
                       
                       
                         
                           
                             
                               k 
                               11 
                             
                             · 
                             
                               r 
                               2 
                             
                           
                         
                         
                           
                             
                               F 
                               max 
                             
                             &gt; 
                             
                               r 
                               2 
                             
                             &gt; 
                             
                               F 
                               min 
                             
                           
                         
                       
                       
                         
                           
                             
                               k 
                               11 
                             
                             · 
                             
                               F 
                               min 
                             
                           
                         
                         
                           
                             
                               r 
                               2 
                             
                             ≤ 
                             
                               F 
                               min 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0000]    This implementation will parse control signals based on the amplitude of the torque control signal r 1  and voltage control signal r 2 . For example, signals between F max  and F min  will affect only the grid side converter, while signals outside of F max  and F min  will affect the gen side converter. F max  and F min  are again dynamic constants and are determined based on network elements and network conditions such as power level, voltage and current magnitude, faults, and generator speed. 
         [0025]    It should be noted that the implementations of function blocks  84 ,  86 ,  88 , and  90  given in equations 1, 2, 3, and 4 are just for exemplary purposes and in one embodiment, function block  84  may have unity gain, function block  86  may have zero gain, function block  88  may be a high pass filter and function block  90  may be a low pass filter. Similarly in another embodiment, function blocks  84 ,  86  may parse the signal based on frequency response whereas function blocks  88 ,  90  may parse the signal based on amplitude of the control signal. In yet another embodiment, where a third converter with an energy storage device or a load and a respective controller is used in the system, the control signals may be parsed in three different components. Thus, various different combinations may be used for implementation of function blocks  84 ,  86 ,  88 , and  90 . 
         [0026]      FIG. 4  shows simulated frequency responses  100  and  102  of function blocks  84  and  86  or  88  and  90  in accordance with an embodiment of the present invention. In both responses  100 ,  102 , horizontal axis  104  represents frequency in Hz, and vertical axis  106  represents amplitude gain in decibel (db). It can be seen from response  100  that below a frequency of x Hz the amplitude gain is below 3 db, and beyond that the amplitude gain is 0 db. On the contrary, for response  102 , the amplitude gain is below 3 db for frequencies above x Hz. Thus, the function blocks  84  and  86  or  88  and  90  parse the control signals between two frequencies and control the grid side converter and the generator side converter. It should be noted that the frequency x Hz may be determined by the operator, and in one embodiment it may be 5 Hz or 10 Hz. In another embodiment, the frequency is determined based on the sub synchronous resonance frequency. 
         [0027]      FIG. 5  shows a simulated amplitude response plot  120  of function blocks  88 ,  90  for the implementation described in equations (3) and (4). In plot  120 , horizontal axis  122  represents the amplitude of voltage control signal r 2 , and vertical axis  124  represents the amplitude of response signals r 2   h  and r 2   l . Plot  126  represents response signal r 2   h  of function block  88 , and plot  128  represents response signal r 2   l  of function block  90  respectively. It can be seen from plots  126  and  128  that the voltage control signal r 2  is parsed based on a lower cut off amplitude Fmin and a upper cut off amplitude Fmax as described in equations (3) and (4). 
         [0028]    One of the advantages of the present control system is that it can provide a more stable control behavior for weak or resonant grid networks by dynamically directing the control signals to the more stable converter platform, depending on system conditions. As will be appreciated by those of ordinary skill in the art, even though the above discussion focuses on wind power generating system, the control method can also be used in other uncontrollable power generating systems connected to the power grid such as photovoltaic, microturbines, marine hydrokinetic, and fuel cell systems. In such power generating systems, the grid side and the generator side converter controls may be implemented as a coupled or multivariable control. Similarly, even though the discussion focuses on parsing the signals in two components, in certain embodiments where more than two power converters or two controllers are utilized, the control method may also split the control signal in more than two components. Further, torque control and DC-link voltage control can be split between the generator side and the grid side based on several strategies. 
         [0029]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.