Patent Publication Number: US-10760547-B2

Title: System and method for controlling voltage of a DC link of a power converter of an electrical power system

Description:
FIELD 
     The present disclosure relates generally to electrical power systems and, more particularly, to a system and method for controlling voltage of a DC link of a power converter for an electrical power system, such as a wind turbine power system. 
     BACKGROUND 
     Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to the generator for producing electricity. 
     More specifically, the gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist. 
     Some wind turbine configurations include doubly-fed asynchronous generators (DFAGs). Such configurations may also include power converters that are used to convert a frequency of generated electric power to a frequency substantially similar to a utility grid frequency. Moreover, such converters, in conjunction with the DFAG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid. Alternatively, some wind turbine configurations include, without limitation, alternative types of induction generators, permanent magnet (PM) synchronous generators, electrically-excited synchronous generators, and switched reluctance generators. These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator. 
     At least some known electric utility grids include one or more series-compensated transmission lines. Sub-synchronous control interactions (SSCI) is a phenomenon that occurs when power-electronic converter controls interact with such series-compensated transmission lines. These interactions can sometimes lead to control instabilities if control systems are not tuned properly or if the control margin of the power converter in properly-tuned control systems is not maintained. 
     Accordingly, the present disclosure is directed to systems and methods for optimizing the utilization of DC voltage for electrical power systems, such as wind turbine power systems, so as to address the aforementioned issues. 
     BRIEF DESCRIPTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     In one aspect, the present disclosure is directed to a method for controlling voltage of a DC link of a power converter of an electrical power system connected to a power grid, such as a wind turbine power system. The method includes operating the DC link to an optimum voltage set point that achieves steady state operation of the power converter. Further, the method includes monitoring the power grid for one or more transient events. More specifically, the transient event(s) may be an indicator of one or more sub-synchronous control interaction (SSCI) conditions occurring in the electrical power system. Upon detection of the transient event(s) occurring in the power grid, the method also includes immediately increasing the optimum voltage set point to a higher voltage set point of the DC link. Moreover, the method includes operating the DC link at the higher voltage set point until the sub-synchronous control interaction(s) is damped, thereby optimizing voltage control of the DC link. Accordingly, the method of the present disclosure is also configured to increase an available voltage control margin of the power converter during the SSCI condition(s). 
     In one embodiment, immediately increasing the optimum voltage set point to the higher voltage set point of the DC link may include determining a voltage command for the optimum voltage set point of the DC link and applying the voltage command to the optimum voltage set point to allow the optimum voltage set point to increase towards the higher voltage set point. In such embodiments, the voltage command may include a voltage rate of change and/or a voltage value. 
     In another embodiment, determining the voltage command for the optimum voltage set point of the DC link may include receiving one or more rotor DC current regulator outputs and determining if the one or more rotor DC current regulator outputs are above a predetermined threshold. In further embodiments, the method may include immediately increasing the optimum voltage set point to the higher voltage set point of the DC link if the rotor DC current regulator output(s) are above a predetermined threshold. 
     In additional embodiments, receiving the one or more rotor DC current regulator outputs may include receiving real and imaginary components of the one or more rotor DC current regulator outputs. In another embodiment, determining the voltage command for the optimum voltage set point of the DC link may also include applying a root sum squared method to the real and imaginary components of the rotor DC current regulator output(s) to determine a magnitude of the real and imaginary components of rotor DC current regulator output(s). 
     In several embodiments, determining the voltage command for the optimum voltage set point of the DC link may include filtering the magnitude of the real and imaginary components of the rotor DC current regulator output(s) using a low-pass filter. 
     In another embodiment, determining the voltage command for the optimum voltage set point of the DC link may include applying a hysteresis function to the filtered magnitude of the real and imaginary components of the one or more rotor DC current regulator outputs. 
     In still further embodiments, the method may include, upon detection of the one or more transient events occurring in the power grid, immediately increasing a modulation index limit of the power converter. 
     In another aspect, the present disclosure is directed to a method for controlling voltage of a DC link of a power converter of an electrical power system connected to a power grid. The method includes operating the DC link to an optimum voltage set point that achieves steady state operation of the power converter. Further, the method includes monitoring the power grid for one or more transient events. More specifically, the transient event(s) may be an indicator of one or more sub-synchronous control interaction (SSCI) conditions occurring in the electrical power system. Upon detection of the one or more transient events occurring in the power grid, the method includes limiting a contribution of non-SSCI-related frequency components to a voltage control margin of the power converter until the sub-synchronous control interaction(s) is damped, thereby optimizing an available control margin during the one or more SSCI conditions. 
     In yet another aspect, the present disclosure is directed to an electrical power system connected to a power grid. The electrical power system includes a doubly-fed asynchronous generator (DFAG) and a power converter coupled to the DFAG. The power converter is configured to convert a frequency of generated electric power from the DFAG to a frequency substantially similar to a frequency of the power grid. Further, the power converter includes a rotor-side converter, a line-side converter, a DC link, and a control module having a current damping device. The control module is configured to perform one or more operations, including but not limited to operating the DC link to an optimum voltage set point that achieves steady state operation of the power converter, upon detection of one or more transient events occurring in the power grid, immediately increasing a modulation index limit of the power converter, the one or more transient events being an indicator of one or more sub-synchronous control interaction (SSCI) conditions occurring in the electrical power system, and operating the power converter at the increased modulation index limit until the one or more SSCI conditions is damped, thereby optimizing control of sub-synchronous oscillations. It should be understood that the wind turbine power system may further include any of the additional features as described herein. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  illustrates a perspective view of one embodiment of a portion of a wind turbine according to the present disclosure; 
         FIG. 2  illustrates a schematic view of one embodiment of an electrical and control system according to the present disclosure that may be used with the wind turbine shown in  FIG. 1 ; 
         FIG. 3  illustrates a block diagram of one embodiment of a power converter system according to the present disclosure that may be used with the electrical and control system shown in  FIG. 2 ; 
         FIG. 4  illustrates a block diagram of one embodiment of a rotor converter control module according to the present disclosure that may be used with the power converter system shown in  FIG. 3 ; 
         FIG. 5  illustrates a block diagram of one embodiment of a current damping device according to the present disclosure that may be used with the rotor converter control module shown in  FIG. 4 ; 
         FIG. 6  illustrates a flow chart of one embodiment of a method for controlling voltage of a DC link of a power converter of an electrical power system connected to a power grid according to the present disclosure; 
         FIG. 7  illustrates a schematic view of one embodiment of an SSCI detection module according to the present disclosure that may be used with the rotor converter control module shown in  FIG. 4 ; and 
         FIG. 8  illustrates a flow chart of another embodiment of a method for controlling voltage of a DC link of a power converter of an electrical power system connected to a power grid according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     Referring now to  FIG. 1 , a perspective view of a portion of an exemplary wind turbine  100  is illustrated. As shown, the wind turbine  100  includes a nacelle  102  housing a generator (not shown in  FIG. 1 ). Further, as shown, the nacelle  102  is mounted on a tower  104  (a portion of the tower  104  being shown in  FIG. 1 ). The tower  104  may have any suitable height that facilitates operation of wind turbine  100  as described herein. The wind turbine  100  also includes a rotor  106  that includes three rotor blades  108  attached to a rotating hub  110 . Alternatively, the wind turbine  100  may include any number of rotor blades  108  that facilitate operation of the wind turbine  100  as described herein. In one embodiment, the wind turbine  100  may also include a gearbox (not shown in  FIG. 1 ) operatively coupled to the rotor  106  and a generator (not shown in  FIG. 1 ). 
     Referring now to  FIG. 2 , a schematic view of one embodiment of an electrical and control system  200  that may be used with the wind turbine  100  is illustrated. As shown, the rotor  106  includes the rotor blades  108  coupled to the hub  110 . The rotor  106  also includes a low-speed shaft  112  rotatably coupled to the hub  110 . The low-speed shaft  112  is coupled to a step-up gearbox  114  that is configured to step up the rotational speed of low-speed shaft  112  and transfer that speed to a high-speed shaft  116 . In alternative embodiments, the wind turbine  100  may include a direct-drive generator that is rotatably coupled to the rotor  106  without any intervening gearbox. Further, as shown, the high-speed shaft  116  is rotatably coupled to the generator  118 . In another embodiment, the generator  118  may be a wound rotor, three-phase, double-fed induction (asynchronous) generator (DFAG) that includes a generator stator  120  magnetically coupled to a generator rotor  122 . In an alternative embodiment, the generator rotor  122  may include a plurality of permanent magnets in place of rotor windings. 
     Still referring to  FIG. 2 , the electrical and control system  200  may also include a turbine controller  202 . The turbine controller  202  may include at least one processor and a memory, at least one processor input channel, at least one processor output channel, and may include at least one computer (none shown in  FIG. 2 ). As used herein, the term computer is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a processor, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits (none shown in  FIG. 2 ), and these terms are used interchangeably herein. In one embodiment, memory may include, but is not limited to, a computer-readable medium, such as a random access memory (RAM) (none shown in  FIG. 2 ). Alternatively, one or more storage devices, such as a floppy disk, a compact disc read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) (none shown in  FIG. 2 ) may also be used. Also, in one embodiment, additional input channels (not shown in  FIG. 2 ) may be, but are not limited to, computer peripherals associated with an operator interface such as a mouse and a keyboard (neither shown in  FIG. 2 ). Further, in one embodiment, additional output channels may include, but are not limited to, an operator interface monitor (not shown in  FIG. 2 ). 
     Processors for the turbine controller  202  are configured to process information transmitted from a plurality of electrical and electronic devices that may include, but are not limited to, voltage and current transducers. RAM and/or storage devices store and transfer information and instructions to be executed by the processor. RAM and/or storage devices can also be used to store and provide temporary variables, static (i.e., non-changing) information and instructions, or other intermediate information to the processors during execution of instructions by the processors. Instructions that are executed include, but are not limited to, resident conversion and/or comparator algorithms. The execution of sequences of instructions is not limited to any specific combination of hardware circuitry and software instructions. 
     Referring still to  FIG. 2 , the generator stator  120  is electrically coupled to a stator synchronizing switch  206  via a stator bus  208 . In one embodiment, to facilitate the DFIG configuration, the generator rotor  122  may be electrically coupled to a bi-directional power conversion assembly  210  via a rotor bus  212 . Alternatively, the generator rotor  122  may be electrically coupled to the rotor bus  212  via any other device that facilitates operation of the system  200  as described herein. As a further alternative, the system  200  may be configured as a full power conversion system (not shown) known in the art, wherein a full power conversion assembly (not shown in  FIG. 2 ), that is similar in design and operation to power conversion assembly  210 , is electrically coupled to the generator stator  120 , and such full power conversion assembly facilitates channeling electric power between the generator stator  120  and an electric power transmission and distribution grid  213 . In certain embodiments, the stator bus  208  transmits three-phase power from the generator stator  120  to the stator synchronizing switch  206 . The rotor bus  212  transmits three-phase power from the generator rotor  122  to the power conversion assembly  210 . In another embodiment, the stator synchronizing switch  206  may be electrically coupled to a main transformer circuit breaker  214  via a system bus  216 . In an alternative embodiment, one or more fuses (not shown) are used instead of the main transformer circuit breaker  214 . In another embodiment, neither fuses nor the main transformer circuit breaker  214  are used. 
     In addition, as shown, the power conversion assembly  210  includes a rotor filter  218  that is electrically coupled to the generator rotor  122  via the rotor bus  212 . A rotor filter bus  219  electrically couples the rotor filter  218  to a rotor-side power converter  220 , and rotor-side power converter  220  is electrically coupled to a line-side power converter  222 . The rotor-side power converter  220  and the line-side power converter  222  are power converter bridges including power semiconductors (not shown). In the illustrated embodiment, the rotor-side power converter  220  and the line-side power converter  222  are configured in a three-phase, pulse width modulation (PWM) configuration including insulated gate bipolar transistor (IGBT) switching devices (not shown in  FIG. 2 ) that operate as known in the art. Alternatively, the rotor-side power converter  220  and the line-side power converter  222  may have any configuration using any switching devices that facilitate operation of the system  200  as described herein. Further, the power conversion assembly  210  may be coupled in electronic data communication with the turbine controller  202  to control the operation of the rotor-side power converter  220  and the line-side power converter  222 . 
     In further embodiments, a line-side power converter bus  223  may electrically couple the line-side power converter  222  to a line filter  224 . Also, as shown, a line bus  225  may electrically couple the line filter  224  to a line contactor  226 . Moreover, as shown, the line contactor  226  may be electrically coupled to a conversion circuit breaker  228  via a conversion circuit breaker bus  230 . In addition, the conversion circuit breaker  228  may be electrically coupled to main transformer circuit breaker  214  via system bus  216  and a connection bus  232 . Alternatively, the line filter  224  may be electrically coupled to the system bus  216  directly via the connection bus  232  wherein any protection scheme (not shown) is configured to account for removal of the line contactor  226  and the conversion circuit breaker  228  from the system  200 . The main transformer circuit breaker  214  may be electrically coupled to an electric power main transformer  234  via a generator-side bus  236 . Further, the main transformer  234  may be electrically coupled to a grid circuit breaker  238  via a breaker-side bus  240 . The grid circuit breaker  238  may be connected to electric power transmission and distribution grid  213  via a grid bus  242 . In an alternative embodiment, the main transformer  234  may be electrically coupled to one or more fuses (not shown), rather than to the grid circuit breaker  238 , via the breaker-side bus  240 . In another embodiment, neither fuses nor the grid circuit breaker  238  is used, but rather the main transformer  234  may be coupled to the electric power transmission and distribution grid  213  via the breaker-side bus  240  and the grid bus  242 . 
     In another embodiment, the rotor-side power converter  220  may be coupled in electrical communication with the line-side power converter  222  via a single direct current (DC) link  244 . Alternatively, the rotor-side power converter  220  and the line-side power converter  222  may be electrically coupled via individual and separate DC links (not shown in  FIG. 2 ). Further, as shown, the DC link  244  may include a positive rail  246 , a negative rail  248 , and at least one capacitor  250  coupled between the positive rail  246  and the negative rail  248 . Alternatively, the capacitor  250  may include one or more capacitors configured in series or in parallel between the positive rail  246  and the negative rail  248 . 
     The turbine controller  202  may also be configured to receive a plurality of voltage and electric current measurement signals from a first set of voltage and electric current sensors  252 . Moreover, the turbine controller  202  may be configured to monitor and control at least some of the operational variables associated with the wind turbine  100 . In particular embodiments, each of three voltage and electric current sensors  252  may be electrically coupled to each one of the three phases of grid bus  242 . Alternatively, the voltage and electric current sensors  252  are electrically coupled to the system bus  216 . As a further alternative, the voltage and electric current sensors  252  may be electrically coupled to any portion of the system  200  that facilitates operation of the system  200  as described herein. As a still further alternative, the turbine controller  202  is configured to receive any number of voltage and electric current measurement signals from any number of the voltage and electric current sensors  252 , including, but not limited to, one voltage and electric current measurement signal from one transducer. 
     Referring still to  FIG. 2 , the system  200  also includes a converter controller  262  that is configured to receive a plurality of voltage and electric current measurement signals from a second set of voltage and electric current sensors  254  coupled in electronic data communication with the stator bus  208 , a third set of voltage and electric current measurement signals from a third set of voltage and electric current sensors  256  coupled in electronic data communication with the rotor bus  212 , and a fourth set of voltage and electric current measurement signals from a fourth set of voltage and electric current sensors  264  coupled in electronic data communication with the conversion circuit breaker bus  230 . The second set of voltage and electric current sensors  254  may be substantially similar to the first set of voltage and electric current sensors  252 , and the fourth set of voltage and electric current sensors  264  may be substantially similar to the third set of voltage and electric current sensors  256 . Further, the converter controller  262  may be substantially similar to the turbine controller  202  and may be coupled in electric data communication with the turbine controller  202 . Moreover, the converter controller  262  may be physically integrated within the power conversion assembly  210 . Alternatively, the converter controller  262  may have any configuration that facilitates operation of the system  200  as described herein. 
     In another embodiment, the electric power transmission and distribution grid  213  may include one or more transmission lines  270  (only one shown for clarity) that are coupled to the grid bus  242  via a grid coupling  272 . The transmission lines  270  and/or the electric power transmission and distribution grid  213  may include one or more series compensation elements  274 , such as one or more capacitors, to facilitate reducing reactive power losses within the transmission lines  270 . As described herein, the series compensation elements  274  may create one or more sub-synchronous resonances within electric power transmission and distribution grid  213 . Further, the transmission lines  270  and/or the electric power transmission and distribution grid  213  may also include one or more switches  276  coupled to each series compensation element  274 . The switches  276  couple and decouple the series compensation elements  274  to and from the electric power transmission and distribution grid  213 , respectively, as desired. More specifically, the switches  276  may be opened to couple the series compensation elements  274  to the electric power transmission and distribution grid  213 , and the switches  276  may also be closed to decouple the series compensation elements  274  from the electric power transmission and distribution grid  213 . The electric power transmission and distribution grid  213  may also be operatively coupled to one or more loads  278  for providing power to loads  278 . 
     During operation, wind impacts the rotor blades  108  and the blades  108  transform wind energy into a mechanical rotational torque that rotatably drives the low-speed shaft  112  via the hub  110 . The low-speed shaft  112  drives the gearbox  114  that subsequently steps up the low rotational speed of the low-speed shaft  112  to drive the high-speed shaft  116  at an increased rotational speed. The high-speed shaft  116  rotatably drives the generator rotor  122 . A rotating magnetic field is induced by the generator rotor  122  and a voltage is induced within the generator stator  120  that is magnetically coupled to the generator rotor  122 . The generator  118  converts the rotational mechanical energy to a sinusoidal, three-phase alternating current (AC) electrical energy signal in the generator stator  120 . The associated electrical power is transmitted to the main transformer  234  via the stator bus  208 , the stator synchronizing switch  206 , the system bus  216 , the main transformer circuit breaker  214  and the generator-side bus  236 . The main transformer  234  steps up the voltage amplitude of the electrical power and the transformed electrical power is further transmitted to the electric power transmission and distribution grid  213  via the breaker-side bus  240 , the grid circuit breaker  238 , and the grid bus  242 . 
     In certain embodiments, a second electrical power transmission path is provided. Electrical, three-phase, sinusoidal, AC power is generated within the generator rotor  122  and is transmitted to the power conversion assembly  210  via the rotor bus  212 . Within the power conversion assembly  210 , the electrical power is transmitted to the rotor filter  218  wherein the electrical power is modified for the rate of change of the output voltage associated with the rotor-side power converter  220 . The rotor-side power converter  220  acts as a rectifier and rectifies the sinusoidal, three-phase AC power to DC power. The DC power is transmitted into the DC link  244 . The capacitor  250  facilitates mitigating DC link voltage amplitude variations by facilitating mitigation of a DC ripple associated with AC rectification. 
     The DC power is subsequently transmitted from the DC link  244  to the line-side power converter  222  wherein the line-side power converter  222  acts as an inverter configured to convert the DC electrical power from the DC link  244  to three-phase, sinusoidal AC electrical power with pre-determined voltages, currents, and frequencies. This conversion is monitored and controlled via the converter controller  262 . The converted AC power is transmitted from the line-side power converter  222  to the system bus  216  via the line-side power converter bus  223  and the line bus  225 , the line contactor  226 , the conversion circuit breaker bus  230 , the conversion circuit breaker  228 , and the connection bus  232 . The line filter  224  compensates or adjusts for harmonic currents in the electric power transmitted from the line-side power converter  222 . The stator synchronizing switch  206  is configured to close to facilitate connecting the three-phase power from the generator stator  120  with the three-phase power from the power conversion assembly  210 . 
     The conversion circuit breaker  228 , the main transformer circuit breaker  214 , and the grid circuit breaker  238  are configured to disconnect corresponding buses, for example, when current flow is excessive and can damage the components of the system  200 . Additional protection components may also be provided, including the line contactor  226 , which may be controlled to form a disconnect by opening a switch (not shown in  FIG. 2 ) corresponding to each of the lines of the line bus  225 . 
     The power conversion assembly  210  compensates or adjusts the frequency of the three-phase power from the generator rotor  122  for changes, for example, in the wind speed at the hub  110  and the rotor blades  108 . Therefore, in this manner, mechanical and electrical rotor frequencies are decoupled from stator frequency. 
     Under some conditions, the bi-directional characteristics of the power conversion assembly  210 , and specifically, the bi-directional characteristics of the rotor-side power converter  220  and the line-side power converter  222 , facilitate feeding back at least some of the generated electrical power into the generator rotor  122 . More specifically, electrical power is transmitted from the system bus  216  to the connection bus  232  and subsequently through the conversion circuit breaker  228  and the conversion circuit breaker bus  230  into the power conversion assembly  210 . Within the power conversion assembly  210 , the electrical power is transmitted through the line contactor  226 , the line bus  225 , and the line-side power converter bus  223  into the line-side power converter  222 . The line-side power converter  222  acts as a rectifier and rectifies the sinusoidal, three-phase AC power to DC power. The DC power is transmitted into the DC link  244 . The capacitor  250  facilitates mitigating the DC link  244  voltage amplitude variations by facilitating mitigation of a DC ripple sometimes associated with three-phase AC rectification. 
     The DC power is subsequently transmitted from the DC link  244  to the rotor-side power converter  220  wherein the rotor-side power converter  220  acts as an inverter configured to convert the DC electrical power transmitted from the DC link  244  to a three-phase, sinusoidal AC electrical power with pre-determined voltages, currents, and frequencies. This conversion is monitored and controlled via the converter controller  262 . The converted AC power is transmitted from the rotor-side power converter  220  to the rotor filter  218  via the rotor filter bus  219  and is subsequently transmitted to the generator rotor  122  via the rotor bus  212 , thereby facilitating sub-synchronous operation. 
     The power conversion assembly  210  is configured to receive control signals from the turbine controller  202 . The control signals are based on sensed conditions or operating characteristics of the wind turbine  100  and the electrical and control system  200 , received by the turbine controller  202  and used to control operation of the power conversion assembly  210 . Feedback from sensors may be used by the system  200  to control the power conversion assembly  210  via the converter controller  262  including, for example, the conversion circuit breaker bus  230 , stator bus and rotor bus voltages or current feedbacks via the second set of voltage and electric current sensors  254 , the third set of voltage and electric current sensors  256 , and the fourth set of voltage and electric current sensors  264 . Using this feedback information, and for example, switching control signals, stator synchronizing switch control signals and system circuit breaker control (trip) signals may be generated in any known manner. For example, for a grid voltage transient with predetermined characteristics, the converter controller  262  can at least temporarily substantially suspend the IGBTs from conducting within the line-side power converter  222 . Such suspension of operation of the line-side power converter  222  can substantially mitigate electric power being channeled through the power conversion assembly  210  to approximately zero. 
     Referring now to  FIG. 3 , a schematic diagram of one embodiment of a power converter system  300  that may be used with the electrical and control system  200  (shown in  FIG. 2 ) is illustrated. As shown, the power converter system  300  includes the rotor-side power converter  220  and the line-side power converter  222 . Further, as shown, the power converter system  300  also includes a torque regulator  302 , a reactive power regulator  304 , a synchronizing phase-locked loop (PLL)  306 , and a DC voltage regulator  308 . 
     As such, the torque regulator  302  is configured to transmit a first rotor current command signal  312  to the rotor-side power converter  220 , and more specifically, to a rotor converter control module  314  thereof. In such embodiments, the first rotor current command signal  312  can be used to adjust a rotor current based on a desired generator torque command signal  316  received from the turbine controller  202  (shown in  FIG. 2 ). Further, as shown, the reactive power regulator  304  is configured to receive a stator voltage and reactive power command signal  318  from the turbine controller  202  and transmit a second rotor current command signal  320  to the rotor converter control module  314 . As such, the second rotor current command signal  320  can be used to control a power factor of the generator  118  by adjusting a ratio of real power to reactive power of the generator  118 . In certain embodiments, the torque regulator  302  and the reactive power regulator  304  may be housed within the converter controller  262 . In an alternative embodiment, the torque regulator  302  and/or the reactive power regulator  304  may be housed within any other suitable controller, such as the turbine controller  202 . 
     The synchronizing PLL  306  is configured to receive a rotor position feedback signal  322  from a rotor position sensor (not shown) and a stator voltage feedback signal  324  from the second set of voltage and electric current sensors  254  (shown in  FIG. 2 ). As such, the synchronizing PLL  306  is configured to determine a transformation angle signal  326  and a reference angle signal  328  that can be used to transform rotor voltages and rotor currents between two or more signal reference frames, such as a time-based reference frame and a phasor-based reference frame. In one embodiment, the transformation angle signal  326  and the reference angle signal  328  can be used to transform rotor voltages and rotor currents to one or more phasors that include X and Y components of the rotor voltages and/or rotor currents. As used herein, an X component refers to a real component of a phasor, and a Y component refers to an imaginary component of a phasor. The transformation angle signal  326  and the reference angle signal  328  can be transmitted to the rotor converter control module  314  and to a line converter control module  330  that is positioned within line-side power converter  222 . The DC voltage regulator  308  receives a DC voltage reference signal  332  that is set, for example, during wind turbine commissioning, and transmits a line current command signal  334  to the line converter control module  330 . The line current command signal  334  is used to adjust a DC voltage of the DC link  244  (shown in  FIG. 2 ). 
     The rotor converter control module  314  is coupled to a rotor converter switching array  336 , and the line converter control module  330  is coupled to a line converter switching array  338 . In one embodiment, the rotor converter switching array  336  and the line converter switching array  338  each includes a plurality of IGBT switching devices (not shown). Alternatively, the rotor converter switching array  336  and/or the line converter switching array  338  may include any suitable switching devices that enable the rotor-side power converter  220  and the line-side power converter  222  to operate as described herein. In one embodiment, the rotor converter control module  314  and the line converter control module  330  may use pulse-width modulation to control a duty cycle of a rotor converter switch control signal  340  and of a line converter switch control signal  342 , respectively. The rotor converter switch control signal  340  controls a switching behavior of the rotor converter switching array  336 , and the line converter switch control signal  342  controls a switching behavior of the line converter switching array  338 . As such, the rotor converter switching array  336  and the line converter switching array  338  may be controlled to produce one or more desired rotor and/or stator voltage and/or current characteristics. 
     Although not shown in  FIG. 3 , one or more control components of the power converter system  300  may receive one or more feedback signals to facilitate maintaining proper operation of the power converter system  300 . Such feedback signals include, without limitation, a DC voltage signal, a three-phase rotor current signal (such as from the third set of voltage and electric current sensors  256 ), a three-phase stator current signal (such as from the second set of voltage and electric current sensors  254 ), a three-phase line current signal (such as from the fourth set of voltage and electric current sensors  264 ), a three-phase stator voltage signal (such as from the second set of voltage and electric current sensors  254 ), and/or a rotor position signal. 
     Referring now to  FIG. 4 , a schematic diagram of one embodiment of a rotor converter control module  314  that may be used with power converter system  300  (shown in  FIG. 3 ) is illustrated. As shown, the rotor converter control module  314  includes a current transform module  402 , an impedance feedforward module  404 , a regulator module  406 , a voltage transform module  408 , and a current damping device  410 . 
     More specifically, as shown, the current transform module  402  receives a current feedback signal  412  that includes current measurements from the third set of electric current sensors  256  (shown in  FIG. 2 ) of each phase of the rotor bus  212 . In one embodiment, the current feedback signal  412  may include one or more current components from the electric power transmission and distribution grid  213  via the power converter system  300  and/or via the generator  118 . In one embodiment, one or more current components may include, for example, one or more sub-synchronous current frequency components and/or one or more grid frequency components that substantially conforms to a frequency of electric power transmission and distribution grid  213 . The current transform module  402  receives the transformation angle signal  326  and transforms the three-phase instantaneous currents of the current feedback signal  412  into a phasor-based reference frame. Thus, as shown, the current transform module  402  transmits a current feedback phasor  414  to a current feedback comparator  416 . The current feedback comparator  416  receives a current command phasor  418 , which includes the first rotor current command signal  312  and the second rotor current command signal  320  (both shown in  FIG. 3 ), and calculates a difference between the current feedback phasor  414  and the current command phasor  418 . The current feedback comparator  416  transmits the resulting difference as a current error phasor  420  to the regulator module  406  and to the current damping device  410 . 
     Still referring to  FIG. 4 , the regulator module  406  receives the current error phasor  420  and performs proportional plus integral feedback regulation to adjust an output of the regulator module  406  to facilitate reducing an error of the current error phasor  420  to substantially 0. The regulator module  406  then transmits a resulting regulator output phasor  422 , which is a voltage phasor signal, to a regulator adder  424 . 
     In addition, as shown, the impedance feedforward module  404  receives the current command phasor  418  and a slip frequency signal  426  and computes an amplitude of a feedforward command phasor  428  as a feedforward voltage phasor signal to supplement a closed-loop current regulation of regulator module  406 . 
     Further, in one embodiment, the current damping device  410  receives the current error phasor  420  and facilitates reducing an amplitude of one or more current frequency components represented by current error phasor  420 . In certain embodiments, the one or more current frequency components are sub-synchronous to a current frequency of the electric power transmission and distribution grid  213  (shown in  FIG. 2 ). As used herein, the term “sub-synchronous” refers to a frequency that is less than a reference frequency, and in certain embodiments, a frequency that is less than the frequency of the electric power transmission and distribution grid  213 . Moreover, as shown, the current damping device  410  transmits a resulting damping control phasor signal  434 , which is a voltage phasor signal, to the regulator adder  424  and a sub-synchronous control interactions (SSCI) detection module  440 , which is described in more detail with reference to  FIG. 7 . The output  728  from the SSCI detection module  440  (e.g. a voltage command  716 ) can then be sent to the DC voltage regulator  308 , e.g. in response to detecting an SSCI. 
     The regulator adder  424  combines the regulator output phasor  422 , the feedforward command phasor  428 , and the output from the SSCI detection module  440 , and transmits a resulting voltage command phasor  430  to a modulation index module  438 . In certain instances, the modulation index module  438  is configured to increase a modulation index limit of the power converter  210 , e.g. in response to detecting an SSCI. Further, as shown, the resulting voltage command phasor  430  is also transmitted to the voltage transform module  408 , which transforms the voltage command phasor  430  to a time-based reference frame using the transformation angle signal  326 . In addition, as shown, the voltage transform module  408  outputs a resulting three-phase sinusoidal voltage command signal  432 . The voltage command signal  432  is modulated by a pulse-width modulation (PWM) module  436  to generate the rotor converter switch control signal  340 . As such, the control module  314  transmits the rotor converter switch control signal  340  to the rotor converter switching array  336  (shown in  FIG. 3 ) to control a switching operation, such as a duty cycle, of the switching devices within the rotor converter switching array  336 . 
     Referring now to  FIG. 5 , a schematic diagram of a portion of the current damping device  410  that may be used with the rotor converter control module  314  (shown in  FIG. 3 ) is illustrated. As shown, the current damping device  410  may include an integrator module  502 , an input transform module  504 , one or more sub-synchronous damping control (SSDC) regulator modules  506 , and an output transform module  508 . The integrator module  502  receives a predetermined sub-synchronous frequency signal  510  that, in one embodiment, represents one or more predetermined sub-synchronous current frequencies to be damped. The sub-synchronous frequency signal  510  is selected as a frequency of a reference frame upon which the sub-synchronous frequency of the grid resonance is acted upon by the SSDC regulator module  506 . In one embodiment, the reference frame may have a substantially zero frequency, such that a frequency of one or more signals entering the SSDC regulator module  506  will be equal to a frequency of signals seen from a stationary reference frame. In another embodiment, the reference frame may be selected to rotate near an anticipated frequency of the sub-synchronous grid resonance. Selection of the appropriate sub-synchronous frequency signal  510  is dependent upon the remainder of the system in which the current damping device  410  is embedded, and is done during design studies for tuning the sub-synchronous damping feature of the system. 
     Thus, as shown, the integrator module  502  integrates the sub-synchronous frequency signal  510  and transmits a resulting sub-synchronous angle signal  512  to a reference angle comparator  514 . The reference angle comparator  514  calculates a difference between sub-synchronous angle signal  512  and reference angle signal  328 , and outputs a resulting sub-synchronous reference angle signal  516  to input into the transform module  504  and into a sub-synchronous orientation adder  518 . The input transform module  504  receives the current error phasor  420 , and performs a transformation of the current error phasor  420  using the sub-synchronous reference angle signal  516 . More specifically, in certain embodiments, the input transform module  504  transforms the current error phasor  420  into a rotating reference frame that includes two components, a and ( 3 , using the following equations:
 
α= x *cos θ+ y *sin θ  (Equation 1)
 
β= x *=sin θ+ y *cos θ  (Equation 2)
 
where x is a real component of the current error phasor  420 ,
 
y is an imaginary component of the current error phasor  420 , and
 
θ is the sub-synchronous reference angle signal  516 .
 
     The rotating reference frame that includes α and β rotates substantially at the frequency of the sub-synchronous current frequency. Thus, as shown, the input transform module  504  transmits a current error transform signal  520  that includes α and β to the SSDC regulator module  506 . The current error transform signal  520  includes a frequency component that is substantially equal to the sub-synchronous current frequency. In one embodiment, the SSDC regulator module  506  includes, and/or is configured to perform, a proportional-plus-integral transfer function. Alternatively, the SSDC regulator module  506  includes any suitable transfer function or other algorithm that enables the current damping device  410  to operate as described herein. The SSDC regulator module  506  integrates and adds a gain to current error transform signal  520 . The SSDC regulator module  506  then transmits a resulting current sub-synchronous damping transform signal  522  to the output transform module  508 . In certain embodiments, the sub-synchronous damping transform signal  522  includes a frequency component that is substantially equal to the sub-synchronous current frequency. 
     Still referring to  FIG. 5 , the sub-synchronous orientation adder  518  combines the sub-synchronous reference angle signal  516  with an orientation adjustment reference signal  524 , and transmits a resulting output orientation signal  526  to the output transform module  508 . Selection of the orientation adjustment reference signal  524  is dependent upon the remainder of the system in which the current damping device  410  is embedded, and is done during design studies for tuning the sub-synchronous damping feature of the system. The output orientation signal  526  is used to adjust an orientation of an output phasor generated by the output transform module  508 . The output transform module  508  transforms the current sub-synchronous damping transform signal  522  to a phasor-based reference frame, in a substantially inverse manner as is performed by the input transform module  504 . As such, an inverse of Equation 1 may be performed on an a component of the sub-synchronous damping transform signal  522 , and an inverse of Equation 2 may be performed on β component of the sub-synchronous damping transform signal  522 . The output transform module  508  outputs a resulting damping control phasor  434  as shown in  FIG. 4 . The damping control phasor  434  includes a frequency component that is substantially equal to a difference between the frequency of the electric power transmission and distribution grid  213  and the sub-synchronous current frequency. The damping control phasor signal  434  may also have real and imaginary components, which are further described herein with reference to  FIG. 7 . 
     Referring now to  FIG. 6 , a flow diagram of one embodiment of a method for controlling voltage of a DC link of a power converter of an electrical power system connected to a power grid, such as a wind turbine power system, is illustrated. In general, the method  600  will be described herein with reference to the wind turbine  100  and control system  200  shown in  FIGS. 1-5 and 7 . However, it should be appreciated that the disclosed method  600  may be implemented with rotor blades having any other suitable configurations. In addition, although  FIG. 6  depicts steps performed in a particular order for purposes of illustration and discussion, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods disclosed herein can be omitted, rearranged, combined, and/or adapted in various ways without deviating from the scope of the present disclosure. 
     As shown at ( 602 ), the method  600  may include operating the DC link  244  to an optimum voltage set point that achieves steady state operation of the power converter  210 . As shown at ( 604 ), the method  600  may include monitoring the power grid for one or more transient events, e.g. a short-lived burst of energy such as a low-voltage-ride-through event. More specifically, the transient event(s) may be an indicator of one or more sub-synchronous control interaction (SSCI) conditions occurring in the electrical power system. SSCIs are a phenomenon that occurs when power-electronic converter controls interact with series-compensated transmission lines. Thus, as shown at ( 606 ), the method  600  may include determining whether one or more transient events are detected. If so, as shown at ( 608 ), the method  600  includes immediately increasing the optimum voltage set point to a higher voltage set point of the DC link  244 . In one embodiment, the control module  314  may be configured to immediately increasing the optimum voltage set point to the higher voltage set point of the DC link  244  by determining a voltage command  716  for the optimum voltage set point of the DC link  244  and applying the voltage command  716  to the optimum voltage set point to allow the optimum voltage set point to increase towards the higher voltage set point. As shown at ( 610 ), the method  600  may include operating the DC link  244  at the higher voltage set point until the sub-synchronous control interaction(s) is damped, thereby optimizing voltage control of the DC link  244 . 
     The method  600  of  FIG. 6  can be better understood with reference to  FIG. 7 . As shown, a schematic diagram  700  of one embodiment of the SSCI detection module  440  according to the present disclosure is illustrated. More specifically, as shown, the SSCI detection module  440  may determine the voltage command  716  for the optimum voltage set point of the DC link  244  by receiving one or more rotor DC current regulator outputs  702 ,  704 . For example, as shown, the control module  314  may receive real and imaginary components of the rotor DC current regulator outputs  702 ,  704  (e.g. real and imaginary components of the damping control phasor signal  434 ). As such, the control module  314  can then determine if the received rotor DC current regulator outputs  702 ,  704  are above a predetermined threshold. 
     More specifically, the rotor DC current regulator outputs  702 ,  704  are a good indicator of an SSCI condition since the rotor DC current regulator attempts to regular current in the specific frequency range related to sub-synchronous resonant frequencies. Thus, when the outputs of this regulator are oscillating (i.e. non-zero), there is likely some currents in the system oscillating in the sub-synchronous frequency range. In such embodiments, the method  600  may include immediately increasing/boosting the optimum voltage set point to the higher voltage set point of the DC link  244  if the rotor DC current regulator outputs are above the predetermined threshold. Thus, by boosting the DC voltage if a transient event occurs, the control behavior following such an event is improved. 
     Still referring to  FIG. 7 , as shown at  706 , the control module  314  may determine the voltage command  716  for the optimum voltage set point of the DC link  244  by processing the rotor DC current regulator outputs  702 ,  704 , e.g. using any suitable statistical analysis. For example, in one embodiment, the processor  706  may use the square root of the sum of the squares of the real and imaginary components of the rotor DC current regulator output(s)  702 ,  704  to determine a magnitude  708  thereof. This magnitude  708  can then be filtered via filter  710 . For example, in certain embodiments, the filter  710  may be a low-pass filter. In addition, as shown, a hysteresis function  714  may be applied to the filtered value  712 . For example, in certain embodiments, the hysteresis function  714  may include comparing the filtered value  712  to a predetermined threshold. If the value is below the threshold, the control module  314  assumes no SSCI condition is occurring in the power grid. Alternatively, if the value is at or above the threshold, the control module  314  may be configured to toggle between the optimum voltage set point for the DC link  244  and the maximum optimum voltage set point for the DC link  244 . Thus, as shown, the output of the hysteresis function  714  may correspond to the voltage command  716 . Accordingly, the voltage command  716  may be a desired rate of change, e.g. in volts/second of the DC voltage regulator reference, or a voltage value, e.g. in volts. Thus, as shown in  FIG. 4 , this signal can be used by the DC voltage regulator. 
     The above-described embodiments facilitate providing an efficient and cost-effective power converter. The power converter damps, or reduces oscillations of, sub-synchronous currents that may be present within the power converter and/or an electric utility grid. As such, the wind turbine power system described herein may be coupled to the electric utility grid while minimizing damage to the system and/or to one or more electric utility grid components that may result from otherwise undamped sub-synchronous current resonances. More specifically, the embodiments described herein enable a wind turbine power system with a doubly-fed asynchronous generator to be coupled to an electric utility grid that includes one or more series-compensated transmission lines. 
     Referring now to  FIG. 8 , a flow chart of another embodiment of a method for controlling voltage of a DC link of a power converter of an electrical power system connected to a power grid is illustrated. In general, the method  800  will be described herein with reference to the wind turbine  100  and control system  200  shown in  FIGS. 1-5 and 7 . However, it should be appreciated that the disclosed method  800  may be implemented with rotor blades having any other suitable configurations. In addition, although  FIG. 8  depicts steps performed in a particular order for purposes of illustration and discussion, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods disclosed herein can be omitted, rearranged, combined, and/or adapted in various ways without deviating from the scope of the present disclosure. 
     As shown at ( 802 ), the method  800  includes operating the DC link  244  to an optimum voltage set point that achieves steady state operation of the power converter. As shown at ( 804 ), the method  800  includes monitoring the power grid for one or more transient events. More specifically, the transient event(s) may be an indicator of one or more sub-synchronous control interaction (SSCI) conditions occurring in the electrical power system. As shown at ( 806 ), the method  800  includes determining if a transient event is detected. If so, as shown at ( 808 ), the method  800  includes limiting a contribution of non-SSCI-related frequency components to a voltage control margin of the power converter  210  until the sub-synchronous control interaction(s) is damped, thereby optimizing an available control margin during the one or more SSCI conditions. 
     Exemplary embodiments of a wind turbine, power converter, and methods of converting power are described above in detail. The methods, wind turbine, and power converter are not limited to the specific embodiments described herein, but rather, components of the wind turbine, components of the power converter, and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the power converter and methods may also be used in combination with other wind turbine power systems and methods, and are not limited to practice with only the power system as described herein. Rather, one embodiment can be implemented and utilized in connection with many other wind turbine or power system applications. 
     Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.