Patent Publication Number: US-2022231622-A1

Title: Methods of Operating Doubly-Fed Induction Generator Systems

Description:
TECHNICAL FIELD 
     The present invention relates to methods of operating doubly-fed induction generator (DFIG) systems, and in particular when the DFIG system is used to charge an AC power line that forms part of a local AC power network and/or when the local AC power network is operated in an islanded mode before it is connected to a remote AC power network or utility grid by closing a remote circuit breaker, and for a short time thereafter. 
     BACKGROUND ART 
     DFIGs are well known induction electric machines that are often used for renewable power generation. 
     A typical DFIG includes a stator having a three-phase stator winding and a rotor having a three-phase rotor winding. Usually the stator winding is electrically connected to an alternating current (AC) power network or utility grid operating at a fixed frequency (the “grid frequency”), e.g., 50 or 60 Hz. The DFIG is an electric machine whose mechanical speed (i.e., the speed at which the rotor shaft rotates) may be varied by adjusting the frequency of the AC current fed into the rotor winding (the “rotor frequency”). In practice, this means that the rotor frequency and the grid frequency are often different. 
     A DFIG system may include the DFIG (i.e., the induction electric machine) and associated power converter, controller etc. The DFIG system may include output terminals (e.g., three terminals for a three-phase output) that may be electrically connected to the AC power network or utility grid by an external AC circuit. The external AC circuit is typically a three-phase circuit and may include a step-up transformer and circuit breaker. The circuit breaker is normally on the high voltage (HV) side of the step-up transformer and may sometimes be referred to as an HV circuit breaker. 
     The stator winding is electrically connected to output terminals of the DFIG system. 
     The rotor winding is electrically connected to the output terminals of the DFIG system by means of slip rings and a power converter. The power converter includes a first active rectifier/inverter (or “machine-side converter”) with AC terminals electrically connected to the rotor winding, and direct current (DC) terminals. The power converter also includes a second active rectifier/inverter (or “grid-side converter”) with DC terminals electrically connected to the DC terminals of the machine-side converter by a DC link, and AC terminals electrically connected to the output of the DFIG system, optionally by means of a transformer. 
     For wind or hydro power generation, for example, the rotor shaft of the DFIG is mechanically connected to a turbine assembly with turbine blades that may be rotated by the wind or by water flow. The rotor shaft of the DFIG may be mechanically connected to the turbine assembly by means of a drive chain so that rotation of the turbine assembly causes the rotor shaft to rotate. The drive chain may include a gear box. 
     The stator winding is electrically connected to the output terminals of the DFIG system and in normal operation should provide three-phase AC power at the grid frequency. 
     The magnetic field created in the rotor of the DFIG is not static but rotates at a speed that is proportional to the rotor frequency. This means that the rotating magnetic field passing through the stator windings of the DFIG not only rotates in response to the physical rotation of the rotor, but also because of the rotational effect produced by the AC current fed into the rotor winding (the “rotor current”). Therefore, both the rotation speed of the rotor and the frequency of the rotor current determine the speed of the rotating magnetic field passing through the stator winding, and consequently the frequency of the AC current induced in the stator winding (the “stator frequency” and the “stator current”). 
     During normal operation, the stator frequency should be constant and equal to the grid frequency despite changes in the rotor speed—which will be caused by changes in wind speed or water flow speed, for example. This means that the rotor frequency must be continually adjusted to compensate. 
     The rotor winding is electrically connected to the output terminals of the DFIG system by means of the power converter. The machine-side converter may be controlled to set the electrical parameters of the rotor current, including the rotor frequency. To set these electrical parameters, the machine-side converter may use vector control, e.g., two-axis vector control in a rotating reference frame (e.g., a dq-reference frame) as explained in more detail below. 
       FIGS. 1 and 2  show a basic DFIG system  1  including a DFIG  2  whose rotor shaft is mechanically connected to a turbine assembly  4  by means of a drive train  6 . The turbine assembly  4  includes a plurality of blades that may be rotated by the wind or by water flow for hydro power generation. The rotation speed of the turbine assembly  4  may be regulated by a turbine regulator (not shown). In the case of hydro power generation, the turbine regulator may control the rotation speed by opening and closing wicket gates that control the flow of water to the turbine assembly. For wind power generation, the corresponding turbine regulator may control the rotation speed of the wind turbine assembly by pitch control of the blades, for example. 
     The stator winding of the DFIG  2  is electrically connected to output terminals  8  by a three-phase AC circuit  10 . 
     The output terminals  8  of the DFIG  2  are electrically connected to a remote three-phase AC power network or utility grid  20  (hereinafter “remote power network”) by means of an external three-phase AC circuit that includes an AC power line  12 , an HV circuit breaker  14  and a step-up transformer  16 . In  FIG. 1 , the AC power line  12  is shown electrically connected to a remote switchyard  18  which is electrically connected in turn to the remote grid  20  by means of a remote circuit breaker  22 . Both the HV circuit breaker  14  and the remote circuit breaker  22  are closed. The external three-phase AC circuit forms part of a local three-phase AC power network (hereinafter “local power network”) to which the DFIG system  1  is electrically connected. In  FIGS. 1 and 2 , the local power network is shown to be electrically connected to the remote power network and is operating in a “grid-connected mode”. 
     The rotor winding is electrically connected to a machine-side converter  24  by a three-phase AC circuit  26 . A grid-side converter  28  is electrically connected to the machine-side converter  24  by a DC link  30  with one or more capacitors. The grid-side converter  28  is electrically connected to the output terminals  8  by a three-phase AC circuit  32  that includes a transformer  34 . The grid-side converter, the remote switchyard and the remote power network have been omitted in  FIG. 2  for clarity. 
     It will be understood that the DFIG  2  has a transfer ratio between the stator winding and the rotor winding which is typically not 1:1—much like a transformer. In order to consistently model and calculate DFIG behaviour, rotor values are normally referred to the stator and this is normally indicated by adding a dash or index to the value. All rotor values mentioned herein are referred to the stator, but without any special indication or marking. 
     The machine-side converter  24  includes a plurality of controllable semiconductor switches that are controlled to turn on and off for power conversion. Similarly, the grid-side converter  28  includes a plurality of controllable semiconductor switches that are controlled to turn on and off for power conversion. 
     With reference to  FIG. 2 , a controller  36 A for the machine-side converter  24  includes a pulse pattern generator  38  for generating drive pulses for controlling the semiconductor switches of the machine-side converter to turn on and off. The drive pulses are generated using output signals from a direct axis (or “d-axis”) current controller  40  and a quadrature axis (or “q-axis”) current controller  42 . 
     The controller  36 A controls the machine-side converter  24  according to a known control scheme while the local power network is operating in the grid-connected mode. 
     The rotor current I r  may be measured using suitable current transducers or other measuring devices and is converted from the three-phase reference frame to the dq-reference frame based on a transformation angle γ r . The dq-reference frame is a rotating reference frame, typically rotating at the stator frequency of the DFIG  2 . In the dq-reference frame, the measured value of the rotor current has a d-axis component (or “d-axis rotor current I dr ”) and a q-axis component (or “q-axis rotor current I qr ”). 
     The d-axis current controller  40  receives an input signal ΔI d  derived from a difference between a d-axis rotor current reference I dr * and the d-axis rotor current I dr  and provides a d-axis rotor voltage V dr  that is converted from the dq-reference frame to the three-phase reference frame using the transformation angle γ r . The q-axis current controller  42  receives an input signal ΔI qr  derived from a difference between a q-axis rotor current reference I qr * and the q-axis rotor current I qr  and provides a q-axis rotor voltage V qr  that is converted from the dq-reference frame to the three-phase reference frame using the transformation angle γ r . 
     The rotor current reference Id r * may be provided by an active power, torque or speed controller, for example, and may be indicative of a desired active power, torque or speed for the DFIG  2 . The d-axis rotor voltage V dr  provided by the d-axis current controller  40  is used to control the semiconductor switches of the machine-side converter  24  to achieve the desired active power, torque or speed that corresponds to the current reference I dr *. The rotor current reference I qr * may be provided by a reactive power, voltage or power factor controller, for example, and may be indicative of a desired reactive power, voltage or power factor for the DFIG  2 . The q-axis rotor voltage V qr  provided by the q-axis current controller  42  is used to control the semiconductor switches of the machine-side converter  24  to achieve the desired reactive power, voltage or power factor that corresponds to the current reference I qr *. 
     The d-axis current controller  40  and the q-axis current controller  42  can be proportional-integral (PI) controllers, for example. 
     The d-axis and q-axis rotor voltages V dr , V qr  derived by the d- and q-axis current controllers  40 ,  42  are converted from the dq-reference frame to the three-phase reference frame based on the transformation angle γ r  and provided as an input to the pulse pattern generator  38 . 
     In the known control scheme, the transformation angle γ r  (or “rotor angle”) used to convert between the three-phase and dq-reference frames is derived from the angle of the rotor shaft (the “mechanical angle”) γ m  and the angle of the stator voltage (the “stator angle”) γ s  as follows: 
     
       
         
           
             
               γ 
               r 
             
             = 
             
               
                 γ 
                 s 
               
               - 
               
                 γ 
                 m 
               
             
           
         
       
     
     The stator angle γ s  corresponds to the integral of the stator angular frequency ω s  (i.e., γ s (t)=INT{ω s }dt) and can be determined from the measured stator voltage using a phase-locked loop (PLL), for example. 
     The mechanical angle γ m  corresponds to the integral of the rotor shaft speed ω m  (i.e., γ m (t)=INT{ω m }dt) and may be determined using a suitable encoder or other measuring device within the DFIG. 
     The rotor angle γ r  corresponds to the rotor angular frequency ω r  (i.e., γ r (t)=INT{ω r }dt=INT{ω s −ω m )}dt). It can therefore be seen that the rotor angular frequency ω r  is not normally substantially constant but will normally vary in response to changes in the rotor shaft speed, e.g., as a result of changing operation points. 
     The grid-side converter  28  may be controlled to set the electrical parameters of the grid current and may transfer active power between the machine-side converter and the AC power network or utility grid. The grid-side converter  28  may also be controlled to maintain the DC link voltage. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method of operating a doubly-fed induction generator (DFIG) system comprising an induction electric machine including a stator having a stator winding and a rotor having a rotor winding, wherein the stator winding is electrically connected to at least one output terminal and the rotor winding is electrically connected to the at least one output terminal of the DFIG system by means of a power converter that includes:
         a first active rectifier/inverter with alternating current AC terminals electrically connected to the rotor winding, and direct current DC terminals; and   a second active rectifier/inverter with DC terminals electrically connected to the DC terminals of the first active rectifier/inverter by a DC link, and AC terminals electrically connected to the at least one output terminal;       wherein the method comprises the step of controlling the first active rectifier/inverter so that the frequency of the AC current at its AC terminals is kept substantially constant during at least one of a “line charging mode” and an “islanded mode”, i.e., a substantially constant rotor frequency is maintained during one or both of these operating modes as herein defined so that the induction electric machine behaves like a synchronous electric machine.   

     Put another way, a substantially constant rotor frequency is imposed on the DFIG system so that the induction electric machine will behave like a synchronous electric machine, for example in regard to how it reacts in changes in grid frequency, drift etc. This also means that existing control strategies for synchronous electric machines may be used to control the induction electric machine in some circumstances. 
     The DFIG system will normally be electrically connected to, or form part of, a local three-phase AC power network (hereinafter “local power network”) that includes an AC power line. The local power network may also include additional components such as a local circuit breaker, a step-up transformer etc. The local power network may be electrically connected to a remote three-phase AC power network or utility grid (hereinafter “remote power network”) by means of a remote circuit breaker that is electrically connected to the AC power line. 
     When the remote circuit breaker is closed, the local power network is electrically connected to the remote power network and may be considered to be operated in a “grid-connected mode”. 
     When the remote circuit breaker is open, the local power network is not electrically connected to the remote power network and may be considered to be operated in one of two different modes, namely:
         a “line charging mode” where the AC power line of the local power network is electrically connected to the DFIG system and is charged by the DFIG system, and   an “islanded mode” where the AC power line has been charged by the DFIG system and the local power network is not yet electrically connected to the remote power network.       

     As described in more detail below, in the grid-connected mode, frequency and voltage regulation will typically be handled by the remote power network. Although in the islanded mode, other power sources may optionally be electrically connected to the local power network, these power sources will typically have a smaller rating than the DFIG system such that frequency and voltage regulation of the local power network will normally be determined by the DFIG system. Typically, the DFIG system will also have the largest effective inertia, and will therefore determine the frequency of the local power network. 
     As the local power network is transitioned through the various operating modes, the DFIG system will normally be operated in a corresponding sequence of steps, for example, from standstill to normal operation. The first active rectifier/inverter of the DFIG system may be controlled using different control schemes during these different steps. 
     The DFIG system will normally be operated according to the present method (i.e., to maintain a substantially constant rotor frequency) when the local power network is operated in the line charging mode or the islanded mode. The DFIG system may also be operated according to the present method for a short period of time immediately after the remote circuit breaker is closed to connect the local power network to the remote power network (i.e., when the local power network is operated in the grid-connected mode). After the remote circuit breaker is closed, the DFIG system will normally transition to normal operation, i.e., where the active rectifier/inverter is controlled using a known control scheme such as the control scheme described above with reference to  FIG. 2 , for example. More specifically, the first active rectifier/inverter will normally be controlled according to an appropriate control scheme during the operating steps of the DFIG system that correspond to the line charging, islanded and grid-connected modes of the local power network. 
     The frequency of the AC current at the AC terminals of the second active rectifier/inverter (i.e., the stator frequency) may vary during the at least one of the line charging mode and the islanded mode when the rotor frequency is kept substantially constant. The stator frequency may also vary during a grid-connected mode after the remote circuit breaker is closed and the local power network is electrically connected to the remote power network. 
     The step of controlling the first active rectifier/inverter may further comprise using a rotor angle as a transformation angle, e.g., to convert between a three-phase reference frame and a rotating reference frame. 
     The rotor angle may be derived by a rotor angle generator. 
     In one arrangement, the rotor angle generator may derive the rotor angle from a constant or substantially constant rotor frequency reference (e.g., a pre-set value or a value that is derived from a look-up table with reference to a pointer such as stator power, grid power, rotor shaft speed etc. during the operating mode(s)). The rotor angle may correspond to the integral of the rotor frequency reference (i.e., γ r =INT{ω r *}dt) as explained in more detail below, where γ r  is the generated rotor angle and ω r * is the rotor frequency reference. 
     The rotor frequency reference is indicative of a desired rotor frequency that is to be maintained for the DFIG during the relevant operating mode(s). 
     It will be understood that the derived rotor angle will maintain a substantially constant rotor frequency. 
     In one arrangement, the rotor angle generator may derive the rotor angle using an algorithm as a function of stator power, grid power, rotor shaft speed etc. during the operating mode(s). It will be understood that the derived rotor angle will maintain a substantially constant rotor frequency. In one arrangement, the rotor angle generator may derive the rotor angle using a controller (e.g., a proportional-integral (PI) controller or other suitable controller). The controller may receive an input signal that is derived from a difference between a speed reference and a measured rotor shaft speed. The speed reference may be provided by a speed regulator that regulates the rotor shaft speed of the DFIG, e.g., a turbine regulator that may control the flow of water to a hydro turbine by opening or closing wicket gates, or the blade pitch in the case of a wind turbine. The output of the controller may be a dynamic rotor angle that may be added to a base rotor angle that is derived from the stator angle and the mechanical angle. 
     The first active rectifier/inverter may be controlled using vector control, e.g., two-axis vector control. 
     A controller for the first active rectifier/inverter may include a pulse pattern generator for generating drive pulses for controlling the semiconductor switches of the first active rectifier/inverter to turn on and off. The drive pulses may be generated using output signals from a first axis controller and a second axis controller. In one arrangement, one of the first axis controller and the second axis controller may be omitted or replaced with a pre-set value which may be zero. In one arrangement, where the rotating reference frame is a dq-reference frame, the first axis controller is a direct (or “d-axis”) controller and the second axis controller is a quadrature (or “q-axis”) controller. 
     The rotor current may be measured using suitable measuring devices and may be converted from the three-phase reference frame to the rotating reference frame (e.g., the dq-reference frame) based on the rotor angle that is derived by the rotor angle generator. The dq-reference frame is a rotating reference frame, typically rotating at the stator frequency of the DFIG. In the dq-reference frame, the measured rotor current has a d-axis component (the “d-axis rotor current”) and a q-axis component (the “q-axis rotor current”). 
     The first axis controller may receive an input signal derived from a difference between a rotor current reference and a measured rotor current in the rotating reference frame. In one arrangement, the d-axis controller may receive an input signal derived from a difference between a d-axis rotor current reference and the d-axis rotor current. The second axis controller may receive an input signal derived from a difference between a rotor current reference and a measured rotor current in the rotating reference frame. In one arrangement, the q-axis controller may receive an input signal derived from a difference between a q-axis rotor current reference and the q-axis rotor current. 
     The rotor current reference and the measured rotor current for the first axis may be provided to a first summing node that subtracts the measured rotor current from the rotor current reference and provides the difference to the first axis controller. The rotor current reference and the measured rotor current for the second axis may be provided to a second summing node that subtracts the measured rotor current from the rotor current reference and provides the difference to the second axis controller. 
     The rotor current reference for the first axis controller may be indicative of a desired first parameter (e.g., active power, torque or speed) and the output signal from the first axis controller may control the semiconductor switches of the first active rectifier/inverter to achieve the desired level of the first parameter that corresponds to the rotor current reference for the first axis. The rotor current reference for the second axis controller may be indicative of a desired second parameter (e.g., reactive power, voltage or power factor) and the output signal from the second axis controller may be used to control the semiconductor switches of the first active rectifier/inverter to achieve the desired level of the second parameter that corresponds to the rotor current reference for the second axis. 
     The first axis controller may be a PI controller and the second axis controller may be a PI controller, for example. 
     The output signal from the first axis controller may be used to control active power of the DFIG and consequently the rotational speed, torque or stator active current. The output signal from the second axis controller may be used to control reactive power of the DFIG and consequently the stator voltage, stator reactive current, or stator power factor. 
     The rotor angle derived by the rotor angle generator may be used as a transformation angle for the controller when converting between the three-phase reference frame and the dq-reference frame. The output signals from the first axis controller and the second axis controller may be converted from the dq-reference frame to the three-phase reference frame using the rotor angle to derive control signals for the pulse pattern generator, for example. 
     The controller may include a stator angle generator that derives a stator angle from a measured value of the stator voltage using a phase-locked loop (PLL), for example. The stator current may be measured using suitable current transducers or other measuring devices and may be converted from the three-phase reference frame to the rotating reference frame (e.g., the dq-reference frame) based on the stator angle. In the dq-reference frame, the measured stator current has a d-axis component (the “d-axis stator current”) and a q-axis component (the “q-axis stator current”). 
     The input signals to the first axis controller and the second axis controller may be further derived from the measured stator current for the first axis and the second axis, respectively (i.e., the d-axis stator current and the q-axis stator current). In one arrangement, the first axis (i.e., d-axis) stator current may be provided to a first controller or gain function and the second axis (i.e., q-axis) stator current and the measured value of the stator voltage may be provided to a second controller or gain function. The output of the first controller or gain function may be provided to the first summing node and used to derive the input signal to the first axis controller and the output of the second controller or gain function may be provided to the second summing node and used to derive the input signal to the second axis controller. The first and second gain functions may be implemented as a constant gain value, a first order transfer function such as a low pass function, or a PID function, for example. 
     Using the stator current and stator voltage to derive the input signals for the first axis controller and second axis controller allows the controller to correctly align the rotating reference frame with the stator voltage. When using a substantially constant rotor frequency to derive a rotor angle as the transformation angle to convert between the three-phase and dq-reference frames, for example, this alignment might be lost in the case of loading the DFIG with active power. The relationship between stator current and rotor current in the case of correct alignment is explained in more detail below. 
     The present invention further provides a DFIG system comprising:
         an induction electric machine including a stator having a stator winding and a rotor having a rotor winding, wherein the stator winding is electrically connected to at least one output terminal of the DFIG system and the rotor winding is electrically connected to the at least one output terminal by means of a power converter that includes:
           a first active rectifier/inverter with alternating current AC terminals electrically connected to the rotor winding, and direct current DC terminals; and   a second active rectifier/inverter with DC terminals electrically connected to the DC terminals of the first active rectifier/inverter by a DC link, and AC terminals electrically connected to the at least one output terminal; and   
           a controller adapted to control the first active rectifier/inverter so that the frequency of the AC current at its AC terminals is kept substantially constant during at least one of a “line charging mode” and an “islanded mode”.       

     The controller may be adapted to carry out the method described above. 
     The present invention further provides an alternative method of operating a DFIG system comprising an induction electric machine including a stator having a stator winding and a rotor having a rotor winding, wherein the stator winding is electrically connected to at least one output terminal and the rotor winding is electrically connected to the at least one output terminal of the DFIG system by means of a power converter that includes:
         a first active rectifier/inverter with alternating current AC terminals electrically connected to the rotor winding, and direct current DC terminals; and   a second active rectifier/inverter with DC terminals electrically connected to the DC terminals of the first active rectifier/inverter by a DC link, and AC terminals electrically connected to the at least one output terminal;       wherein the method comprises the step of controlling the first active rectifier/inverter so that the frequency of the AC current at its AC terminals is determined with reference to the difference between a speed reference and a measured rotor shaft speed of the DFIG.   

     The present invention further provides an alternative DFIG system comprising:
         an induction electric machine including a stator having a stator winding and a rotor having a rotor winding, wherein the stator winding is electrically connected to at least one output terminal of the DFIG system and the rotor winding is electrically connected to the at least one output terminal by means of a power converter that includes:
           a first active rectifier/inverter with alternating current AC terminals electrically connected to the rotor winding, and direct current DC terminals; and   a second active rectifier/inverter with DC terminals electrically connected to the DC terminals of the first active rectifier/inverter by a DC link, and AC terminals electrically connected to the at least one output terminal; and   
           a controller adapted to control the first active rectifier/inverter so that the frequency of the AC current at its AC terminals is determined with reference to the difference between a speed reference and a measured rotor shaft speed of the DFIG.       

     Further features of the alternative method and the alternative DFIG system are as herein described and with particular regard to the control scheme described with reference to  FIG. 13  that may optionally be used during at least one of a “line charging mode” and a “islanded mode”. In one arrangement, a rotor angle generator may derive a rotor angle that may be used as a transformation angle to convert between a rotating reference frame and a three-phase reference frame. In one arrangement, the rotor angle generator may derive the rotor angle using a controller (e.g., a proportional-integral (PI) controller or other suitable controller). The controller may receive an input signal that is derived from a difference between the speed reference and the measured rotor shaft speed of the DFIG. The speed reference may be provided by a speed regulator that regulates the rotor shaft speed of the DFIG, e.g., a turbine regulator that may control the flow of water to a hydro turbine by opening or closing wicket gates, or the blade pitch in the case of a wind turbine. The output of the controller may be a dynamic rotor angle that may be added to a base rotor angle that is derived from the stator angle and the mechanical angle. 
     A first axis (e.g., a q-axis) controller may receive an input signal derived from a difference between a rotor current reference and a measured rotor current. The measured rotor current may be an absolute value, for example. The output signal from the first axis controller may be converted from the dq-reference frame to the three-phase reference frame using the rotor angle to derive control signals for the pulse pattern generator. 
    
    
     
       DRAWINGS 
         FIG. 1  is a schematic diagram of a basic DFIG system; 
         FIG. 2  is a schematic diagram of part of the basic DFIG system of  FIG. 1  showing a controller for the machine-side converter; 
         FIG. 3  is a schematic diagram of a possible practical implementation of a DFIG system according to the present invention; 
         FIG. 4  is a flow diagram of an operating sequence for the DFIG system of  FIG. 3 ; 
         FIG. 5  is a flow diagram of a blackstart step of the operating sequence; 
         FIG. 6  shows characteristic values of the blackstart step; 
         FIG. 7  is a flow diagram of a line charging step of the operating sequence; 
         FIG. 8  shows characteristic values of the line charging step; 
         FIG. 9  shows characteristic values of a network connection step of the operating sequence; 
         FIG. 10  is a schematic diagram of part of the basic DFIG system of  FIG. 1  showing a first controller for the machine-side converter according to the present invention; 
         FIG. 11A  is a schematic diagram of a first rotor angle generator; 
         FIG. 11B  is a schematic diagram of a second rotor angle generator; 
         FIG. 12  is a schematic diagram of a second controller for the machine-side converter according to the present invention; and 
         FIG. 13  is a schematic diagram of a third controller for the machine-side converter according to the present invention. 
     
    
    
       FIG. 3  shows a possible practical implementation of a DFIG system  100  specifically adapted for hydro power generation. The DFIG system  100  includes a DFIG  102  whose rotor shaft is mechanically connected to a turbine assembly  104  by means of a drive train  106 . The turbine assembly  104  includes a plurality of blades that may be rotated by water flow for hydro power generation. The rotation speed of the turbine assembly  104  is regulated by a turbine regulator (not shown) which controls the rotation speed by opening and closing wicket gates (not shown) that control the flow of water to the turbine assembly. 
     The stator winding of the DFIG  102  is electrically connected to output terminals  108  by a three-phase AC circuit  110 . The AC circuit  110  includes a circuit breaker  112  and a phase reversal switch  114 . 
     The output terminals  108  of the DFIG system  100  are electrically connected to a remote three-phase AC power network or utility grid  128  (hereinafter “remote power network”) by means of an external three-phase AC circuit  116 . The AC circuit  116  includes a first switchyard  118  to which the output terminals  108  are electrically connected by a step-up transformer  120  and a local (or HV) circuit breaker  122 . Additional DFIG systems may be electrically connected to the first switchyard  118  as shown. 
     The first switchyard  118  is electrically connected to a second switchyard  124  by an AC power line  126 . Additional AC power lines may be electrically connected to the second switchyard  124  as shown. 
     The AC circuit  116  forms part of a local three-phase AC power network (hereinafter “local power network”) that is electrically connected to the remote power network  128  through the second switchyard  124 , and in particular by means of a remote circuit breaker  168 —see below. In this arrangement, the local power network would extend as far as the second switchyard  124  and possibly to further parts of the remote power network as long as there is no other significant power source with a higher rated power and/or inertia. Any additional DFIG systems electrically connected to the first switchyard  118  may be operated in a “slave mode” where they follow the operation of the DFIG system  100  shown in  FIG. 3 . Alternatively, any additional DFIG systems may be operated with a known control scheme such as the control scheme described with reference to  FIG. 2 , for example. As such, any additional DFIG systems may be considered to be local power sources that do not have a higher rated power or inertia. 
     The second switchyard  124  is electrically connected to the remote power network  128  operating at a fixed grid frequency, e.g., 50 or 60 Hz, when the remote circuit breaker  168  is closed. 
     The rotor winding of the DFIG  102  is electrically connected to three machine-side converters  130   a - 130   c  arranged in parallel by a three-phase AC circuit  132 . It will be readily understood that the number of machine-side converters is not limited to three and will depend on the overall system requirements. A crowbar  134  is electrically connected to the AC circuit  132 . A grid-side converter  136   a - 136   c  is electrically connected to each machine-side converter  130   a - 130   c  by a DC link  138   a - 138   c  with one or more capacitors. Each DC link  138   a - 138   c  includes a DC chopper  140   a - 140   c.    
     Each grid-side converter  136   a - 136   c  is electrically connected to the output terminals  108  by a three-phase AC circuit  142   a - 142   c  that includes a transformer  144   a - 144   c.  The AC circuits  142   a - 142   c  are electrically connected to a circuit breaker  146  that is electrically connected in turn to the output terminals  108  by a three-phase AC circuit  148 . 
     A pre-charge circuit/auxiliary grid  150  is electrically connected to the AC circuit  148  and includes a circuit breaker  152 . The pre-charge circuit/auxiliary grid  150  is electrically connected to each DC link  138   a - 138   c  by a contactor  154   a - 154   c,  a pre-charge transformer  156   a - 156   c  and a rectifier  158   a - 158   c.  The pre-charge circuit/auxiliary grid  150  includes a transformer  160  and is electrically connected to an electric machine  162  whose rotor shaft is driven by a prime mover, for example a diesel engine  164 . 
     Other electrical loads may be electrically connected to the pre-charge circuit/auxiliary grid  150  as shown. 
     All circuit breakers (CBs), contactors etc. are controlled to open and close by a controller (not shown). 
     From standstill (step  0 ) the DFIG system  100  may be operated in a sequence of steps shown in  FIG. 4 . The operation sequence will also transition the local power network through various operating modes, and in particular a line-charging mode, an islanded mode, and eventually a grid-connected mode. 
     Pre-Charge Blackstart Step (Step  1 ) 
     Summary: The pre-charge circuit/auxiliary grid  150  receives power from the electric machine  162  and the DC links  138   a - 138   c  are charged using the pre-charge circuit/auxiliary grid to an initial DC link voltage. 
     Blackstart Step (Step  2 ) 
     Summary: The excitation of the DFIG  102  is ramped up, until the grid-side converters  136   a - 136   c  may be started. After starting the grid-side converters  136   a - 136   c,  the DC link voltage is increased from the initial DC link voltage. 
     The detailed sequence for the blackstart step is shown in  FIG. 5 . Characteristic values are shown in  FIG. 6 . In particular,  FIG. 6  shows:
         the load on the diesel engine  164  that drives the electric machine  162  supplying power to the pre-charge circuit/auxiliary grid  150  (the “diesel load”),   the load on the turbine assembly  104  (the “turbine load”),   the DC link voltage, and   the stator voltage.       

     Switch state during the blackstart step:
         circuit breaker  112  is closed,   phase reversal disconnector  114  is connected to “turbine” (T),   circuit breaker  146  is closed,   local circuit breaker  122  is open,   pre-charge contactors  154   a - 154   c  are closed, and   circuit breaker  152  is open.       

     Control state during the blackstart step:
         turbine regulator controls the opening/closing of the wicket gates to control rotor shaft speed,   grid-side converters  136   a - 136   c  control DC link voltage, and   machine-side converters  130   a - 130   c  control rotor current amplitude and frequency.       

     After the DC links  138   a - 138   c  have been pre-charged to the initial DC link voltage in step  1 , the blackstart step may be started. 
     The turbine regulator may bring the rotor shaft speed of the DFIG  102  to close to the synchronous speed. For example, the rotation speed of the turbine assembly  104 , and hence the rotation speed of the rotor shaft, may be controlled by operating the wicket gates to control the flow of water to the turbine assembly  104 . 
     The speed setpoint will preferably lead to low slip to minimise the rotor active power. 
     The machine-side converters  130   a - 130   c  are started (step  2   b ) and inject a magnetisation current into the DFIG  102  to excite it. As a result of the excitation, the stator voltage will increase. Any active current flow on the stator, will apply a torque to the rotor shaft. 
     During step  2   c,  the magnetisation current reference is increased by a ramp. When the stator voltage reaches a pre-defined level (e.g., 20% to 32% of the rated stator voltage), the grid-side converters  136   a - 136   c  are started (step  2   d ). The grid-side converter controls the DC link voltage. 
     The DC link voltage reference is increased by a ramp (step  2   e ). With increasing DC link voltage from the initial DC link voltage, the active power flow through the pre-charge rectifiers  158   a - 158   c  will cease, and instead the active power will start to flow from the stator via the transformers  144   a - 144   c  into the DC link. 
     As it is unloaded, the pre-charge circuit/auxiliary grid  150  may be disconnected from the DC links  138   a - 138   c  by opening the pre-charge contactors  154   a - 154   c  (step  2   f ). However, it may be required that the pre-charge contactors  154   a - 154   c  remain closed to supply power to the DC links  138   a - 138   c  in case of a transient during the following steps, which leads to a decrease in DC link voltage. 
     With fully ramped up DC link voltage, optimization of the overall DFIG system  100  (e.g., power converter and/or turbine assembly operation) may be carried out (step  2   g ). 
     The DFIG system  100  now operates in isolated step (step  3 )—see below. 
     Isolated Step (Step  3 ) 
     Summary: The DFIG system  100  is operated in steady state where the rotor shaft speed is controlled by the turbine regulator at de facto no load condition. The machine-side converters  130   a - 130   c  are excited by the DFIG  102  to a level in the range of 20% to 32% of rated stator voltage. The grid-side converters  136   a - 136   c  control the DC link voltage and cover losses within the electrical system. 
     Line Charging Step (Step  4 ) 
     Summary: The AC power line  126 , which was previously not energized, is electrically connected to the DFIG  102  through the step-up transformer  120  by closing the local circuit breaker  122 . The power converter is operated to increase the line voltage and to transition the local power network into a line charging mode of operation. 
     The detailed sequence for the line charging step is shown in  FIG. 7 . Characteristic values are shown in  FIG. 8 . In particular,  FIG. 8  shows:
         the line voltage,   the load on the diesel engine  164  that drives the electric machine  162  supplying power to the pre-charge circuit/auxiliary grid  150  (the “diesel load”),   the load on the turbine assembly  104  (the “turbine load”),   the DC link voltage, and   the stator voltage.       

     Switch state during the line charging step:
         circuit breaker  112  is closed,   phase reversal disconnector  114  is connected to “turbine” (T),   circuit breaker  146  is closed,   local circuit breaker  122  is open and will be closed during line charging (circuit breakers  166 A,  166 B that connect the AC power line  126  to the first and second switchyards  118 ,  124  are closed),   pre-charge contactors  154   a - 154   c  are open or closed, and   circuit breaker  152  is open.       

     Control state during the line charging step:
         turbine regulator controls the opening/closing of the wicket gates to control rotor shaft speed,   grid-side converters  136   a - 136   c  control DC link voltage, and   machine-side converters  130   a - 130   c  control rotor current amplitude and frequency. The first, second or third control schemes described below may be used to control the machine-side converters  130   a - 130   c  during the line charging step and while the local power network is operated in the line charging mode. The second control scheme may be preferred.       

     The speed setpoint should consider the operational characteristic of the machine-side converters  130   a - 130   c  with preference to medium output voltage and of the turbine assembly  104 , which is preferably run at low speed. 
     The line charging step is initiated (e.g., by a control signal) and preparation is made to close the local circuit breaker  122  (step  4   a ). The circuit breakers  166 A,  166 B that connect the AC power line  126  to the first and second switchyards  118 ,  124  are closed. If all necessary internal conditions are fulfilled, the controller (not shown) closes the local circuit breaker  122  to connect the DFIG system to the first switchyard  118  (step  4   b ) and transition the local power network to a line charging mode. 
     With the closing of the local circuit breaker  122 , the AC power line  126  will be electrically connected to the DFIG system  100  and may be energised. Due to the AC power line&#39;s capacity against ground, an inrush current is expected which imposes a transient in the DFIG  102  and the power converter. The stator voltage is expected to partially decrease. 
     After receiving the closed feedback from the local circuit breaker  122 , and after the transient due to line connection is over, the power converter starts to increase the stator voltage and at the same time the AC power line voltage (step  4   c ). 
     With increasing voltage, the losses of the DFIG system will increase and are covered by the turbine assembly  104 . For example, the wicket gates opening will be adjusted. 
     Islanded Step (Step  5 ) 
     Summary: The AC power line  126  is charged to a voltage of &gt;90% of rated voltage. 
     Passive loads may be electrically connected and active power may be consumed up to a pre-defined level. The local power network is operated in an islanded mode. During the islanded mode there is no other significant power source with a higher rated power and/or inertia connected to the local power network. The DFIG system  100  therefore regulates the voltage and frequency of the local power network. Any additional DFIG systems electrically connected to the first switchyard  118  may support the DFIG system  100  and may be operated in a “slave mode”, i.e., where the additional DFIG system follow the operation of the DFIG system and may consequently be considered to be an extension of the DFIG system. 
     The first, second or third control schemes described below may be used to control the machine-side converters  130   a - 130   c  during the islanded step and while the local power network is operated in the islanded mode. The third control scheme may be preferred. 
     Network Connection Step (Step  6 ) 
     Summary: The power converter and the DFIG control prepares the settings for connection of the islanded local power network to the remote power network  128  through the second switchyard  124 . This transitions the local power network from the islanded mode to a grid-connected mode (or normal mode). 
     Characteristic values for the network connection step are shown in  FIG. 9 . In particular,  FIG. 9  shows:
         the line voltage,   the line frequency,   the opening/closing of the wicket gates that control flow of water to the turbine assembly  104  (the “hydraulic power”),   the turbine speed, and   the stator voltage.       

     Switch state during the network connection step:
         circuit breaker  112  is closed,   phase reversal disconnector  114  is connected to “turbine” (T),   circuit breaker  146  is closed,   local circuit breaker  122  is closed (circuit breakers  166 A,  166 B that connect the AC power line  126  to the first and second switchyards  118 ,  124  are closed),   pre-charge contactors  154   a - 154   c  are open,   circuit breaker  152  is open,   remote circuit breaker  168  is open and will be closed during the network connection step.       

     Control state during the network connection step:
         turbine regulator controls the opening/closing of the wicket gates to control rotor shaft speed,   grid-side converters  136   a - 136   c  control DC link voltage, and   machine-side converters  130   a - 130   c  control rotor current amplitude and frequency. The first, second or third control schemes described below may be used to control the machine-side converters  130   a - 130   c  during the network connection step as the local power network is transitioned from the islanded mode to the grid-connected mode. The third control scheme may be preferred.       

     During the network connection step, it is necessary to connect the local power network to the remote power network  128 . 
     It is assumed that:
         the remote circuit breaker  168  which connects the remote power network  128  to the second switchyard  124  is physically remote to the DFIG system,   there is no instant signalling to the controller for the power converter,   the difference between the frequency of the DFIG system (i.e., the “stator frequency”) and the frequency of the remote power network  128  (the “grid frequency”) is very small, and in particular is small enough for a remote synchrocheck to allow the electrical connection to be made by closing the remote circuit breaker  168 , and   the installed power in the remote power network  128  is higher than the installed power in the islanded local power network.       

     This means that the instant of connection by the remote synchrocheck is not known exactly and that, after connection, the stator frequency will be forced to follow the grid frequency (i.e., the frequency of the remote power network). 
     The turbine regulator (not shown) may control the rotation speed of the turbine assembly  104  (e.g., by operating the wicket gates) to a speed which is pre-defined for islanded operation. This rotation speed should take into consideration the operational characteristic of the power converter with preference to medium output voltage and of the turbine assembly, which is preferably run at low speed. 
     When it is intended to connect the AC power line  126  to the interconnected remote power network  126 , a signal may be sent to the power converter controller and the turbine regulator. 
     The controller and turbine regulator will apply limits to the active and reactive power, as well as to the wicket gate opening. Transients will be used to detect the remote connection and the DFIG system  100  switches to normal operation (step  7 ). The local power network is transitioned to a grid-connected mode. 
     The difference between the stator frequency ω s  and the grid frequency ω g  on network connection will result in a change in the rotor shaft speed of the DFIG  102 . If the grid frequency is higher than the stator frequency, the rotor shaft speed will increase and the turbine regulator may adjust the wicket gates to reduce rotor shaft speed accordingly. If the grid frequency is lower than the stator frequency, the rotor shaft speed will decrease and the regulator may adjust the wicket gates to increase rotor shaft speed accordingly. 
     Normal Operation Step (Step  7 ) 
     Summary: The DFIG system  100  is electrically connected to the remote power network  128  and operates within normal parameters for hydro power generation. 
     The local power network operates in a grid-connected mode (or normal mode). 
     The first, second or third control schemes described below may be used to control the machine-side converters  130   a - 130   c  for a short time after the remote circuit breaker  168  is closed and the local power network is transitioned to the grid-connected mode. The third control scheme may be preferred. After a short time (e.g., less than 15 min for a manual transition or less than about 100 ms for an automatic transition) the machine-side converters  130   a - 130   c  may be controlled by a known control scheme as described with reference to  FIG. 2 , for example. 
     Houseload (Step  8 ) 
     Summary: The DFIG system  100  is operated in steady state where the rotor shaft speed is controlled by the turbine regulator at de facto no load condition. The machine-side converters  130   a - 130   c  are excited by the DFIG  102  to a level in the range of about 90% of rated stator voltage. The grid-side converters  136   a - 136   c  control the DC link voltage and covers losses within the electrical system. The high stator voltage allows connection of the auxiliary grid  150  to the stator. 
     Synchronisation HV Side (Step  9 ) 
     Summary: The DFIG system  100 , which was previously operated in the houseload step, is synchronised to the remote power network grid  128 . 
       FIG. 10  shows a first controller  36 B according to the present invention. The controller  36 B is similar to the controller  36 A shown in  FIG. 2  and like parts have been given the same reference sign. The controller  36 B may be used to control the machine-side converters  130   a - 130   c  of the DFIG system  100  shown in  FIG. 3 . 
     To improve clarity, only one of the machine-side converters is shown in  FIGS. 10, 12 and 13 . The grid-side converters, the remote switchyard, the remote power network and other non-essential components have also been omitted. 
     The first controller  36 B controls the machine-side converter  130   a  according to a first control scheme. 
     The first controller  36 B includes a pulse pattern generator  38  for generating drive pulses for controlling the semiconductor switches of the machine-side converter  130   a  to turn on and off. The drive pulses are generated using output signals from a direct axis (or “d-axis”) current controller  40  and a quadrature axis (or “q-axis”) current controller  42 . 
     The rotor current I r  may be measured using suitable current transducers or other measuring devices and is converted from the three-phase reference frame to the dq-reference frame based on a transformation angle γ r . The dq-reference frame is a rotating reference frame, typically rotating at the stator frequency of the DFIG  102 . In the dq-reference frame, the measured value of the rotor current has a d-axis component (or “d-axis rotor current I dr ”) and a q-axis component (or “q-axis rotor current I qr ”). 
     The d-axis current controller  40  receives an input signal ΔI dr  derived from a difference between a d-axis rotor current reference I dr * and the d-axis rotor current I dr . The q-axis current controller  42  receives an input signal ΔI qr  derived from a difference between a q-axis rotor current reference I qr * and the q-axis rotor current I qr . The d-axis rotor current reference I dr * may be provided by an active power, torque or speed controller, for example, and may be indicative of a desired active power, torque or speed for the DFIG  102 . The d-axis current controller  40  uses the input signal ΔI dr  to derive a d-axis rotor voltage V dr  to control the semiconductor switches of the machine-side converter  103   a  to achieve the desired active power, torque or speed that corresponds to the d-axis rotor current reference I dr *. The q-axis rotor current reference I qr * may be provided by a reactive power, voltage or power factor controller, for example, and may be indicative of a desired reactive power, voltage or power factor for the DFIG  102 . The q-axis current controller  42  uses the input signal ΔI qr  to derive a q-axis rotor voltage V qr  control the semiconductor switches of the machine-side converter  130   a  to achieve the desired reactive power, voltage or power factor that corresponds to the q-axis rotor current reference I qr *. 
     The d-axis current controller  40  and the q-axis current controller  42  may be proportional-integral (PT) controllers, for example. 
     The d-axis and q-axis rotor voltages V dr , V qr  derived by the d-axis and q-axis current controllers  40 ,  42  are converted from the dq-reference frame to the three-phase reference frame based on the transformation angle γ r  and provided as an input to the pulse pattern generator  38 . 
     The transformation angle γ r  used to convert between the three-phase and dq-reference frames is a rotor angle and is provided by a rotor angle generator  44 . Unlike the controller  36 A shown in  FIG. 2 , the rotor angle γ r  is not derived from the mechanical angle and the stator angle. Instead, as shown in  FIG. 11A , the rotor angle generator  44  may derive the rotor angle γ r  by integrating a substantially constant (or pre-set) rotor frequency reference ω r * that is indicative of the desired rotor frequency to be maintained at the AC terminals of the machine-side converter  130   a  as represented below: 
     
       
         
           
             
               γ 
               r 
             
             = 
             
               INT 
               ⁢ 
               
                 { 
                 
                   
                     ω 
                     r 
                   
                   * 
                 
                 } 
               
               ⁢ 
               dt 
             
           
         
       
     
     Alternatively, as shown in  FIG. 11B , the rotor angle generator  44  may derive the rotor frequency reference ω r * using a look-up table with reference to one or more system parameters such a stator power, grid power, rotor shaft speed etc., and represented in  FIG. 11B  by “X”. The rotor angle generator  44  may then derive the rotor angle γ r  by integrating the rotor frequency reference ω r * as represented above. 
     The first controller  36 B uses the rotor angle γ r  generated by the rotor angle generator  44  to control the semiconductor switches of the machine-side converter  130   a  to achieve and maintain the desired rotor frequency during the appropriate operating steps of the DFIG system  100 . 
       FIG. 12  shows a second controller  36 C according to the present invention. The second controller  36 C is similar to the first controller  36 B shown in  FIG. 10  and like parts have been given the same reference sign. 
     The second controller  36 B controls the machine-side converter  130   a  according to a second control scheme. 
     The d-axis current corresponds to the active current and the q-axis current corresponds to the reactive current only if the rotating reference frame is correctly aligned with the stator voltage. When using a substantially constant rotor frequency to derive a rotor angle as the transformation angle to convert between the three-phase and dq-reference frames, this alignment might be lost in the case of loading the DFIG  2  with active power. 
     It is known that in the case of correct alignment, the stator current and rotor current have the following relationship: 
     
       
         
           
             
               I 
               
                 d 
                 ⁢ 
                 r 
               
             
             = 
             
               
                 
                   - 
                   
                     
                       L 
                       s 
                     
                     
                       L 
                       h 
                     
                   
                 
                 ⁢ 
                 
                   I 
                   
                     d 
                     ⁢ 
                     s 
                   
                 
               
               + 
               
                 
                   
                     L 
                     s 
                   
                   
                     
                       L 
                       h 
                     
                     ⁢ 
                     
                       R 
                       
                         F 
                         ⁢ 
                         e 
                       
                     
                   
                 
                 ⁢ 
                 
                   V 
                   
                     d 
                     ⁢ 
                     s 
                   
                 
               
               - 
               
                 
                   
                     R 
                     s 
                   
                   
                     
                       L 
                       h 
                     
                     ⁢ 
                     
                       ω 
                       s 
                     
                   
                 
                 ⁢ 
                 
                   I 
                   
                     q 
                     ⁢ 
                     s 
                   
                 
               
             
           
         
       
       
         
           
             
               I 
               
                 q 
                 ⁢ 
                 r 
               
             
             = 
             
               
                 
                   - 
                   
                     
                       L 
                       s 
                     
                     
                       L 
                       h 
                     
                   
                 
                 ⁢ 
                 
                   I 
                   
                     q 
                     ⁢ 
                     s 
                   
                 
               
               - 
               
                 
                   V 
                   
                     d 
                     ⁢ 
                     s 
                   
                 
                 
                   
                     L 
                     h 
                   
                   ⁢ 
                   
                     ω 
                     s 
                   
                 
               
               + 
               
                 
                   
                     R 
                     s 
                   
                   
                     
                       L 
                       h 
                     
                     ⁢ 
                     
                       ω 
                       s 
                     
                   
                 
                 ⁢ 
                 
                   I 
                   
                     d 
                     ⁢ 
                     s 
                   
                 
               
             
           
         
       
     
     where: 
     I dr  is the d-axis rotor current, 
     I qr  is the q-axis rotor current, 
     I ds  is the d-axis stator current, 
     I qs  is the q-axis stator current, 
     L s  is the stator inductance, 
     L b  is the main inductance, 
     R Fe  is the iron resistance, 
     V ds  is the d-axis stator voltage, 
     R s  is the stator resistance, and 
     ω s  is the stator angular frequency. 
     This alignment may be corrected on the basis of the relationship between the stator current I s  and the rotor current I r . 
     As shown in  FIG. 12 , the second controller  36 C includes a stator angle generator  46  that derives a stator angle γ s  from a measured value of the stator voltage V s  using a PLL, for example. The stator current I s  may be measured using suitable current transducers or other measuring devices and is converted from the three-phase reference frame to the dq-reference frame based on the stator angle γ s . In the dq-reference frame, the measured value of the stator current has a d-axis component (or “d-axis stator current I ds ”) and a q-axis component (or “q-axis stator current I qs ”). The d-axis stator current I ds  is provided to a gain function  48  and then to the summing node  52  that provides the input signal ΔI dr  to the d-axis current controller  40 . The q-axis stator current I qs  is provided to a gain function  50  and then to the summing node  54  that provides the input signal ΔI qr  to the q-axis current controller  42 . The gain function  50  also receives the measured stator voltage V s . 
     In case the measured d-axis and q-axis rotor currents I dr  and I qr  do not fulfil the above relationship with the respective d-axis and q-axis stator currents I ds  and I qs , the gain functions,  48 ,  50  are implemented to correct for the d-axis and q-axis currents, respectively. The gain functions  48 ,  50  may be implemented as a constant gain value, a first order transfer function such as a low pass function, or a PID function, for example. 
     In one particular implementation, the gain function  48  may be represented by: 
     
       
         
           
             - 
             
               
                 L 
                 s 
               
               
                 L 
                 h 
               
             
           
         
       
     
     and the gain function  50  may be zero. 
     The second controller  36 C may align the rotor d-axis with the stator d-axis. Conventionally, d-axis control is associated with active power and the q-axis control is associated with reactive power. The stator voltage measurement is needed to distinguish between active and reactive power current components. 
       FIG. 13  shows a third controller  60  according to the present invention. 
     The third controller  60  controls the machine-side converter  130   a  according to a third control scheme. 
     The controller  60  includes a pulse pattern generator  62  for generating drive pulses for controlling the semiconductor switches of the machine-side converter  130   a  to turn on and off. 
     A turbine regulator  64  includes a grid frequency controller  66  that derives power reference P hyd * by comparing a measured grid frequency ω g  (e.g., the frequency of the local power network when operated in an islanded mode before it is electrically connected to the remote power network or immediately after a connection has been made and the local power network is operated in a grid-connected mode) with a grid frequency reference ω g *. The power reference P hyd * is provided to a power controller  68  which may adjust the flow of water to the turbine assembly  4  by controlling the wicket gates. The power controller  68  may also use the power reference P hyd * as the pointer to a look-up table to derive an optimum mechanical speed reference ω m *. 
     The rotor current I r  may be measured using suitable current transducers or other measuring devices and is converted from the three-phase reference frame to an absolute value of the rotor current I absr . In particular, the rotor current I r  may be converted using the following equations: 
     
       
         
           
             
               I 
               
                 α 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 r 
               
             
             = 
             
               I 
               
                 a 
                 ⁢ 
                 r 
               
             
           
         
       
       
         
           
             
               I 
               
                 β 
                 ⁢ 
                 r 
               
             
             = 
             
               
                 ( 
                 
                   
                     I 
                     
                       b 
                       ⁢ 
                       r 
                     
                   
                   - 
                   
                     I 
                     
                       c 
                       ⁢ 
                       r 
                     
                   
                 
                 ) 
               
               
                 3 
               
             
           
         
       
       
         
           
             
               I 
               
                 a 
                 ⁢ 
                 b 
                 ⁢ 
                 s 
                 ⁢ 
                 r 
               
             
             = 
             
               
                 
                   I 
                   
                     α 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     r 
                   
                   2 
                 
                 + 
                 
                   I 
                   
                     β 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     r 
                   
                   2 
                 
               
             
           
         
       
     
     where I ar , I br  and I cr  are the rotor currents for phase “a”, “b” and “c”, respectively. 
     Similarly, the stator voltage V s  may be measured using suitable voltage sensors or other measuring devices and is converted from the three-phase reference frame to an absolute value of the stator voltage V abss . In particular, the stator voltage V s  may be converted using the following equations: 
     
       
         
           
             
               V 
               
                 α 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 s 
               
             
             = 
             
               V 
               
                 a 
                 ⁢ 
                 s 
               
             
           
         
       
       
         
           
             
               V 
               
                 β 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 s 
               
             
             = 
             
               
                 ( 
                 
                   
                     V 
                     bs 
                   
                   - 
                   
                     V 
                     
                       c 
                       ⁢ 
                       s 
                     
                   
                 
                 ) 
               
               
                 3 
               
             
           
         
       
       
         
           
             
               V 
               
                 a 
                 ⁢ 
                 b 
                 ⁢ 
                 s 
                 ⁢ 
                 s 
               
             
             = 
             
               
                 
                   V 
                   
                     α 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     s 
                   
                   2 
                 
                 + 
                 
                   V 
                   
                     β 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     s 
                   
                   2 
                 
               
             
           
         
       
     
     where V as , V bs  and V cs  are the stator voltages for phase “a”, “b” and “c”, respectively. 
     The absolute value of the rotor current I absr  is provided to a summing node  70  where it is subtracted from a rotor current reference I absr *. 
     The rotor current reference I absr * is derived from a stator voltage amplitude controller  72 . The stator voltage amplitude controller  72  subtracts the absolute value of the stator voltage V abss  from a stator voltage reference V abss * using a summing node  74 . The difference between the stator voltage reference V abss * and the absolute value of the stator voltage V abss  is provided to a controller  76  which derives the rotor current reference I absr *. The controller  76  controls the stator voltage amplitude and may be a PI controller or another suitable controller. 
     The difference between the rotor current reference I absr * and the absolute value of the rotor current Iabsr (i.e., ΔI absr  output by the summing node  70 ) is provided to a q-axis current controller  78  which derives a q-axis rotor voltage V qr . The q-axis current controller  78  controls the rotor voltage amplitude and may be a PI controller or another suitable controller. In this arrangement, a d-axis rotor voltage V dr  is zero. 
     The controller  60  includes a rotor angle generator  80  which derives a rotor angle which is used as a transformation angle γ r  to convert the q-axis rotor voltage V qr  into the three-phase reference frame for use by the pulse pattern generator  62 . 
     In the rotor angle generator  80 , a measured value of the mechanical shaft speed ω is subtracted from the optimum mechanical speed reference ω m * provided by the turbine regulator  64  in a summing node  86 . (In an alternative arrangement, the optimum mechanical speed reference ω m * may simply be a constant value as opposed to being provided by the turbine speed controller. In this case, there would be no need for the turbine speed controller to derive the mechanical speed reference.) The difference between the speed reference ω m * and the measured shaft speed ω m  (i.e., Δω) is provided to a speed controller  84  which provides an output to a summing node  90 . The speed controller  84  may be a PI controller or other suitable controller. The output of the speed controller  84  represents a dynamic component of the rotor angle. 
     An initial stator frequency reference ω s0 * is integrated by integrator  86  to derive a stator angle γ s . The mechanical angle γ m  may be derived from a speed encoder fitted to the rotor shaft and is subtracted from the stator angle γ s  in summing node  88  to derive a base component of the rotor angle. A summing node  90  sums the dynamic and base components of the rotor angle (i.e., γ r ,dynamic and γ r ,base) to derive the total rotor angle γ r . The rotor angle γ r  generated by the rotor angle generator  80  is used by the controller  60  to control the semiconductor switches of the machine-side converter  103   a  to maintain a substantially constant rotor frequency. 
     The controller  60  aims to apply the same control scheme for connection to different types of AC power network or utility grid. During a line charging mode of the local power network, the DFIG  102  is effectively creating the islanded network and impregnating the voltage and frequency to the local power network and eventually to other loads that are electrically connected to it. These loads may be passive loads, active loads, or additional generators in the case of a small islanded grid that would typically have small grid inertia (i.e., reaction of grid frequency to active power changes). The dynamic of the remote power network  128  to which the DFIG system  100  is eventually connected is unknown and a robust control structure is needed. At the end of the line charging, the local power network will be electrically connected to the unknown remote power network  128  with an unknown inertia by closing the remote circuit breaker. The control scheme should remain stable during the connection to the remote power network. 
     The rotor angle generator  80  is initialised by the initial stator frequency reference ω s0 *. If the speed reference ω m * is the same as the measured speed ω m , the initial stator frequency reference ω s0 * is integrated to generate the stator angle γ s . The integrator  86  may be initialised by an initialising angle. A deviation between the measured speed ω m  and the speed reference ω m * will lead to a change in the stator frequency. This deviation will be detected and corrected by the turbine regulator  64  in terms of a change in the measured grid frequency ω g , which is the same as the stator frequency. The speed controller  84  will correct for any deviation in the shaft speed. Such speed deviations may happen when a new participant has been electrically connected to the local power network, for example, after the local power network has been electrically connected to the remote power network that has a strong dynamic influence. 
     Whenever there is a transient in the power network that imposes a higher (or lower) torque on the DFIG  102 , the shaft speed will change from the previously steady state. This will cause a reaction in the rotor angle generator  80  which may regulate the stator frequency to a higher or lower value. The turbine regulator  64  will detect the deviation in the stator frequency (which corresponds to the measured grid frequency ω g ) and will use the generated power reference P hyd * to adjust the flow of water to the turbine assembly  104  by controlling the wicket gates to compensate for the higher or lower torque. The rotor frequency also remains substantially constant. The DFIG  102  therefore shows a similar frequency behaviour to a synchronous machine.