Abstract:
A power converter configured to improve power capture in a wind turbine during low wind speed operation is disclosed. The power converter converts the power generated by the alternator of the wind turbine into a suitable AC current for delivery to a utility grid or to an electric load independent of the utility grid. The power converter is configured to operate in multiple operating modes, utilizing both synchronous and non-synchronous control methods, to extend the operating range of the power converter. During non-synchronous operation, the power converter utilizes a modulation routine that may either vary the dead-time compensation period during a constant modulation period or vary the modulation period with a constant on-time. A seamless transfer between non-synchronous and synchronous control methods with low total harmonic distortion (THD) improves the range of power generation for wind generators.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional application Ser. No. 61/577,447, filed Dec. 19, 2011, the entire contents of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to power converters and, more specifically, to improved control of and/or power conversion from polyphase alternating current (AC) machines during low speed operation. 
         [0003]    In recent years, increased demands for energy and increased concerns about supplies of fossil fuels and their corresponding pollution have led to an increased interest in renewable energy sources. Two of the most common and best developed renewable energy sources are photovoltaic energy and wind energy. Other renewable energy sources may include fuel cells, hydroelectric energy, tidal energy, and biofuel or biomass generators. However, using renewable energy sources to generate electrical energy presents a new set of challenges. 
         [0004]    Many renewable energy sources provide a variable supply of energy. The supply may vary, for example, according to the amount of wind, cloud cover, or time of day. Further, different energy sources provide different types of electrical energy. A wind turbine, for example, is better suited to provide Alternating Current (AC) energy while a photovoltaic cell is better suited to provide Direct Current (DC) energy. Due to the variable nature of the energy supplied as well as the varying type of energy generated, power converters are commonly inserted between the renewable energy source and the utility gird or an electrical load, if operating independently of the utility grid. 
         [0005]    It is known that power converters have inherent losses which prevent all of the power generated by the renewable energy source from being converted to usable electrical energy. At low levels of power generation, the energy losses may be greater than the power being generated by the renewable energy source. The power converter is typically switched off to avoid an operating condition in which the power generation system is actually using more energy than it is generating. 
         [0006]    Thus, in order to maximize the efficiency of the power generation system, it is desirable to capture energy generated at low power generation levels and to provide a converter able to efficiently operate at those low power generation levels. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    The subject matter disclosed herein describes a system and method for controlling polyphase machines during low speed operation and, more specifically, a system and method for controlling power transfer from an alternator while the alternator is being driven at low speeds. 
         [0008]    According to one aspect of the present invention, improved power capture in a wind turbine during low wind speed operation is disclosed. A power converter is provided to convert the power generated by the alternator of the wind turbine into a suitable AC current for delivery to a utility grid or to an electric load independent of the utility grid. The power converter is configured to operate in multiple operating modes, utilizing both synchronous and non-synchronous control methods, to extend the operating range of the power converter. A seamless transfer between non-synchronous and synchronous control methods with low total harmonic distortion (THD) improves the range of power generation for wind generators. 
         [0009]    The non-synchronous control method extends the low speed power transfer capability of a wind turbine. To efficiently capture power during low wind speed operation a variable frequency pulse width modulation (PWM) including dead time control approach is used. Because conventional switching methods are highly inefficient at low power levels, resulting in switching losses that exceed power production, the power converter is typically not operated during periods of low power production. The variable PWM frequency significantly reduces the losses associated with the switching of the solid state power devices during power conversion. Thus, the variable PWM frequency allows the power conversion system to capture power generated during low wind speed operation. Utilizing this PWM switching method, the usable operating range of a wind turbine is extended downward to capture this untapped power under present converter designs. 
         [0010]    According to one embodiment of the invention, the power converter is configured to operate with wind turbines and to operate in multiple power transfer modes. During periods in which the wind is blowing above traditional cut-in speeds, a first synchronous control method transfers power from the alternator to the utility grid or electrical load. As the wind speed is reduced, the power and, consequently, the output voltage and frequency generated by the alternator are reduced. The synchronous control method reduces the modulated voltage. During periods in which the wind speed is reduced, the modulation frequency may similarly be reduced to reduce switching losses in the power converter. 
         [0011]    As the power levels continue to drop beyond the PWM continuous switching efficiency range, additional steps may be taken to reduce power consumption in the power converter and to continue transferring power generated by the wind turbine over an increased operating range. According to one embodiment of the invention, the dead time period is increased and the maximum on time for modulation of the converter is reduced. Optionally, blanking times may be introduced at periodic intervals into the modulation method. During periods in which the modulation is disabled, the back-emf at the input of the converter may be read to obtain an electrical angle of the voltage being generated. Obtaining the back-emf during these periods extends the operating range of synchronous control of the converter. As a result, low power levels are captured and converter losses are minimized in this area of very low power utilizing the dead time compensation due to the reduction in switching losses by removing the diode recovery losses. As the power from the alternator continues to drop, control of the power switches is modified to allow for discontinuous current from the alternator. Each of the phases from the alternator are alternately connected to either the positive or the negative rail of the DC bus at a minimum on time. The current will remain somewhat sinusoidal resulting in lower torque ripple on the alternators. As a result of the multiple operating modes, the operating range of the converter is extended without excessive current spiking while not adding any detrimental effects to the wind generator. 
         [0012]    According to one embodiment of the invention, a power converter includes an input configured to receive power from a multi-phase AC source, a DC bus having a positive and a negative rail, a plurality of positive switching devices, and a plurality of negative switching devices. Each positive switching device selectively connects one phase of the AC source to the positive rail of the DC bus, and each negative switching device selectively connecting one phase of the AC source to the negative rail of the DC bus. A memory device stores a series of instructions, and a controller is configured to execute the series of instructions. The controller executes the instructions to determine a magnitude of power generated by the AC source, and execute a modulation module to generate a positive control signal for each positive switching device and a negative control signal for each negative switching device. The control signals are generated in a first operating mode when the AC source is generating a magnitude of power greater than a first threshold, and the control signals are generated in a second operating mode when the AC source is generating a magnitude of power less than the first threshold. During the second operating mode, each of the positive switching devices are controlled to connect each phase of the AC source to the positive rail in tandem and each of the negative switching devices are controlled to connect each phase of the AC source to the negative rail in tandem. During the first operating mode the controller executes the modulation module with a fixed modulation frequency and a fixed dead time, and during the second operating mode the controller executes the modulation module with a fixed on time and a varying modulation frequency. During the second operating mode, the controller may access a lookup table stored in the memory device defining a rate of change of the modulation frequency as a function of the current modulation frequency, where the modulation frequency may vary from about 10 kHz to about 50 Hz. 
         [0013]    According to another aspect of the invention, the control signals are generated in a intermediate operating mode when the AC source is generating a magnitude of power less than the first threshold and greater than a second threshold, and the second threshold is less than the first threshold. With the intermediate operating mode, the second operating mode executes below the first and the second thresholds. During the intermediate operating mode the controller executes the modulation module with a blanking time periodically disabling the control signals. 
         [0014]    According to one embodiment of the invention, a power converter includes an input configured to receive power from a multi-phase AC source, a DC bus having a positive and a negative rail, a plurality of positive switching devices, and a plurality of negative switching devices. Each positive switching device selectively connects one phase of the AC source to the positive rail of the DC bus, and each negative switching device selectively connecting one phase of the AC source to the negative rail of the DC bus. A memory device stores a series of instructions, and a controller is configured to execute the series of instructions. The controller executes the instructions to determine a magnitude of power generated by the AC source, and execute a modulation module to generate a positive control signal for each positive switching device and a negative control signal for each negative switching device. The control signals are generated in a first operating mode when the AC source is generating a magnitude of power greater than a first threshold, and the control signals are generated in a second operating mode when the AC source is generating a magnitude of power less than the first threshold. During the second operating mode, the controller periodically disables the control signals for a blanking time. During the first operating mode the controller executes the modulation module with a fixed modulation frequency and a fixed dead time. 
         [0015]    According to another aspect of the invention, the control signals are generated in a third operating mode when the AC source is generating a magnitude of power less than a second threshold, where the second threshold is less than the first threshold. During the third operating mode, each of the positive switching devices are controlled to connect each phase of the AC source to the positive rail in tandem and each of the negative switching devices are controlled to connect each phase of the AC source to the negative rail in tandem. 
         [0016]    According to another embodiment of the invention, a method of converting power from a renewable energy source having variable power generation capability is disclosed. The method includes the steps of monitoring a level of power generated by the renewable energy source, controlling a power converter in a first operating mode via pulse width modulation having a fixed modulation frequency and a fixed dead time compensation when the level of power generated is above a first predetermined threshold, and controlling the power converter in a second operating mode via pulse width modulation module having a periodic blanking time, wherein the blanking time is repeated at a periodic interval during each cycle of a fundamental frequency of a voltage generated by the renewable energy source and wherein during the blanking time the pulse width modulation is disabled. 
         [0017]    According to another aspect of the invention, the method includes the step of controlling the power converter in a third operating mode when the level of power generated is below a second predetermined threshold via pulse width modulation having a variable modulation frequency and a fixed on time, where the second predetermined threshold is less than the first predetermined threshold. 
         [0018]    According to another aspect of the invention, the renewable energy source generates a multi-phase AC input voltage and controlling the power converter in the third operating mode further comprises the steps of connecting each of the phases from the AC input voltage to a positive rail of a DC bus in the power converter in tandem, and connecting each of the phases from the AC input voltage to a negative rail of a DC bus in the power converter in tandem, where each of the phases are alternately connected to the positive and negative rails. 
         [0019]    According to yet another embodiment of the invention, a power converter includes an input configured to receive power from an AC source, a DC bus having a positive rail and a negative rail, at least one positive switching device selectively connecting the input to the positive rail of the DC bus as a function of a corresponding positive gating signal, at least one negative switching device selectively connecting the input to the negative rail of the DC bus as a function of a corresponding negative gating signal, a memory device storing a series of instructions, and a controller. The controller is configured to execute the series of instructions to execute a modulation routine to generate each of the positive and negative gating signals, determine a magnitude of power generated by the AC source, generate the positive and negative gating signals for each of the positive and negative switching devices in a first operating mode when the magnitude of power generated by the AC source exceeds a first predefined threshold, and generate the positive and negative gating signals for each of the positive and negative switching devices in a second operating mode when the magnitude of power generated by the AC source is less than the first predefined threshold. During the first operating mode, the controller periodically inserts a blanking time in the modulation routine, disabling the positive and negative gating signals during the blanking time. During the second operating mode, each of the positive switching devices connects the input to the positive rail in tandem and each of the negative switching devices connects the input to the negative rail in tandem. 
         [0020]    According to still another aspect of the invention, during the second operating mode the controller may vary the dead time via a current controller that varies the dead-time as a function of the current transferred from the AC source to the DC bus. The controller also executes the modulation routine with a varying modulation period and a fixed on time. 
         [0021]    These and other objects, advantages, and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         [0022]    Various exemplary embodiments of the subject matter disclosed herein are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
           [0023]      FIG. 1  is a schematic representation of a converter according to one embodiment of the invention; 
           [0024]      FIG. 2  is a schematic representation of an inverter according to one embodiment of the invention; 
           [0025]      FIG. 3  is a graphical representation of the power generated by a wind turbine as functions of rotor speed and wind speed; 
           [0026]      FIG. 4  is a block diagram representation of one embodiment of the invention; 
           [0027]      FIG. 5  is a graphical representation of a portion of one modulation period according to one embodiment of the invention; 
           [0028]      FIG. 6  is a graphical representation of dead time compensation; 
           [0029]      FIG. 7  is a graphical representation of dead time control with a fixed modulation period; 
           [0030]      FIG. 8  is a graphical representation of a variable modulation period; 
           [0031]      FIG. 9  is a graphical representation of a three phase alternating current of the converter of  FIG. 1  operating with dead time control at a first dead time; 
           [0032]      FIG. 10  is a graphical representation of a three phase alternating current of the converter of  FIG. 1  operating with dead time control at a second dead time, the second dead time greater than the first dead time. 
           [0033]      FIG. 11  is a graphical representation of a three phase voltage present at the terminals of the converter of  Fig.1  during operation under continuous pulse width modulation; 
           [0034]      FIG. 12  is a graphical representation of a three phase voltage present at the terminals of the converter of  Fig.1  during operation under pulse width modulation with a periodic blanking time; and 
           [0035]      FIG. 13  is a graphical representation of one phase of the three phase voltage of 
           [0036]      FIG. 12  over one period of the voltage. 
       
    
    
       [0037]    In describing the preferred embodiments of the invention which are illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word “connected,” “attached,” or terms similar thereto are often used. They are not limited to direct connection but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    The various features and advantageous details of the subject matter disclosed herein are explained more fully with reference to the non-limiting embodiments described in detail in the following description. 
         [0039]    Turning initially to  FIG. 1 , an exemplary converter  10  incorporating one embodiment of the present invention is illustrated. The converter  10  includes three input terminals, T 1 -T 3 , configured to receive input voltages. Each of the input terminals, T 1 -T 3 , of the illustrated embodiment is configured to receive one phase of a multi-phase voltage, V 1 -V 3 , generated by an alternator  6 . The alternator  6  may generate, for example, three phase alternating current (AC) power. An input filter  28  is connected in series with each of the terminals, T 1 -T 3 . 
         [0040]    The converter  10  receives the multiphase AC input voltage, V 1 -V 3 , at the terminals, T 1 -T 3 , and outputs a desired DC voltage, Vdc, present on a DC bus  12  using switching devices,  20  and  21 . The DC bus  12  includes a positive rail  14  and a negative rail  16  which are made available at outputs, +Vdc and −Vdc. As is understood in the art, the positive rail  14  and the negative rail  16  may conduct any suitable DC voltage potential with respect to a common or neutral voltage and are not limited to a positive or a negative DC voltage potential. Further, either of the positive rail  14  or the negative rail  16  may be connected to a neutral voltage potential. The positive rail  14  typically conducts a DC voltage having a greater potential than the negative rail  16 . 
         [0041]    The switching devices,  20  and  21 , are typically solid-state power devices.  FIG. 1  shows the switching devices,  20  and  21 , as bipolar junction transistors (BJTs); however, it is contemplated that any suitable switching device according to the application requirements may be used, including, but not limited to, insulated gate bipolar transistors (IGBT), field effect transistors (FET), silicon controlled rectifiers (SCR), thyristors such as integrated gate-commutated thyristors (IGCT) or gate turn-off thyristors (GTO), or other controlled devices. A diode  22  is connected in parallel to each of the switching devices,  20  and  21 , for reverse conduction across the switching device,  20  and  21 , as required when the switching device,  20  and  21 , is turned off This diode  22  may also be a part of the semiconductor switch. For each phase of the input, a positive switch,  20 , is connected between the input terminal, T 1 -T 3 , and the positive rail  14  of the DC bus  12 , and a negative switch,  21 , is connected between the input terminal, T 1 -T 3 , and the negative rail  16  of the DC bus  12 . Each of the positive switching devices  20  are controlled by a positive gate signal  24  and each of the negative switching devices  21  are controlled by a negative gate signal  25 . Each of the positive and negative gate signals,  24  or  25 , is enabled or disabled to selectively permit conduction through the positive or negative switching devices,  20  or  21  respectively. A capacitance  50  is connected between the positive rail  14  and the negative rail  16  of the DC bus  12 . The capacitance  50  may be a single capacitor or any number of capacitors connected in series or parallel according to the system requirements. The capacitance  50  is configured to reduce the magnitude of ripple voltage resulting from the voltage conversion between the input voltage and the DC bus  12 . 
         [0042]    A controller  40  executes a series of stored instructions to generate the gate signals,  24  and  25 . The controller  40  receives feedback signals from sensors corresponding to the amplitude of the voltage and/or current at various points throughout the converter  10 . The locations are dependent on the specific control routines being executed within the controller  40 . For example, input sensors,  26   a - 26   c , may provide an amplitude of the voltage present at each input terminal, T 1 -T 3 . Optionally, an input sensor,  26   a - 26   c , may be operatively connected to provide an amplitude of the current conducted at each input terminal, T 1 -T 3 . Similarly a current and/or a voltage sensor,  28  and  30 , may be operatively connected to the positive rail  14  and the negative rail  16 , respectively, of the DC bus  12 . The controller  40  interfaces with a memory device  42  to retrieve the stored instructions and with a communication port  44  to communicate with external devices. The controller  40  is configured to execute the stored instructions to control the converter  10  as described herein. 
         [0043]    Referring next to  FIG. 4 , an exemplary power conversion system includes a first power converter  10  and a second power converter  60 , operating as an inverter, connected by a DC bus  12 . Optionally, an energy storage device  18  may be connected between the positive rail  14  and the negative rail  16  of the DC bus  12 . The alternator  6 , such as the generator of a wind turbine, supplies power to the converter  10 , which is converted to a DC voltage on the DC bus  12 , and the inverter  60 , in turn, supplies power to an electrical load  4  or to a utility grid (not shown) from the DC bus  12 . The storage device  18  may also include a DC to DC converter to convert the DC voltage present on the DC bus  12  to a suitable DC voltage level according to requirements of the storage device. The storage device may be, for example, a lead-acid battery, a lithium ion battery, a zinc-bromide battery, a flow battery, or any other suitable energy storage device. The DC to DC converter operates to transfer energy between the DC bus  12  and the storage device  18  according to the application requirements. 
         [0044]    Referring now to  FIG. 2 , an exemplary inverter  60  is connected to the DC bus  12 . The inverter  60  converts the DC voltage from the DC bus  12  to an AC voltage suitable to be supplied, for example, to the utility grid or an electrical load, such as a motor. The conversion is performed using switching devices  70  which selectively connect either the positive rail  14  or the negative rail  16  to one of the phases of the output voltage. The switching devices  70  are typically solid-state power devices.  FIG. 2  illustrates the switching devices  70  as bipolar junction transistors (BJTs); however, it is contemplated that any suitable switching device according to the application requirements may be used, including, but not limited to, insulated gate bipolar transistors (IGBT), field effect transistors (FET), silicon controlled rectifiers (SCR), thyristors such as integrated gate-commutated thyristors (IGCT) or gate turn-off thyristors (GTO), or other controlled devices. A diode  72  is connected in parallel to each of the switching devices  70  for reverse conduction across the switching device as required when the switching device  70  is turned off. This diode  72  may also be a part of the semiconductor switch. Each switching device  70  is controlled by a gate signal  74 . The gate signal  74  is enabled or disabled to selectively permit conduction through the switching device  70 . 
         [0045]    A controller  90  executes a series of stored instructions to generate the gate signals  74 . The controller  90  receives feedback signals from sensors corresponding to the amplitude of the voltage and/or current at various points throughout the inverter  60 . The locations are dependent on the specific control routines being executed within the controller  90 . For example, sensors,  76   a - 76   c , may provide an amplitude of the voltage present at each phase of the output terminal  62 . Optionally, the output sensor,  76   a - 76   c  may be operatively connected to provide an amplitude of the current conducted at each phase of the output terminal  62 . Similarly a current and/or a voltage sensor,  78  and  80 , may be operatively connected to the positive rail  12  and the negative rail  16 , respectively, of the DC bus  12 . The controller  90  interfaces with a memory device  92  to retrieve the stored instructions and with a communication port  94  to communicate with external devices. According to one embodiment of the invention, the first converter  10  and the second converter  60  are separate modules having separate controllers  40 ,  90  and memory devices  42 ,  92  configured to control operation of the respective power converter. Optionally, a single controller and memory device may be configured to control operation of both power converters. 
         [0046]    In operation, the converter  10  converts the power supplied from a variable power energy source to power available on the DC bus  12  of the converter. Subsequent energy storage devices  18  or inverter modules  60  may be connected to the DC bus  12  either to store the power generated by the energy source or to deliver stored power to an electrical load  4  (see also  FIG. 4 ). The first power converter  10  is configured to transfer power from the source  6  to the DC bus  12  and the second power converter  60  is configured to transfer power from the DC bus  12  to the load  4 . The controller  40 ,  90  of each power converter  10 ,  60  executes one or more control modules which generate gating signals  24 ,  25 , or  74  to selectively connect the switches  20 ,  21 , or  70 , respectively, between the DC bus  12  and either the input terminals, T 1 -T 3 , or the output  62  according to the desired form of power conversion. According to one embodiment of the invention, a wind turbine may include blades that rotate a low speed drive shaft as a function of the speed of the wind. The low speed drive shaft is input to a gearbox, which, in turn, rotates a high speed drive shaft output as a function of its gearing. The high speed drive shaft rotates the rotor portion of the alternator  6 , generating AC voltages, V 1 -V 3 , on the stator. 
         [0047]    Referring next to  FIG. 3 , a graph  100  illustrates the relationship between power generated by the alternator  6  as a function of the rotor speed for an exemplary wind turbine operating under varying wind speeds. The speed of the turbine blades may be controlled, for example, by varying the pitch of the blades. Thus, for a constant wind speed, the speed of rotation of the low speed drive shaft and, consequently, the speed of rotation of the rotor in the alternator  6  can be varied. However, the potential exists that the pitch of the blades may not be adjustable at a fast enough rate to respond to varying wind conditions. In addition to, or in lieu of, pitch control, the converter  10  may help regulate the speed of the alternator  6  by regulating current drawn from the alternator  6  such that a variable braking force is applied to the alternator  6 . The electronic control of the current may, therefore, compensate for variations in the wind speed to maintain operation at the maximum power point. 
         [0048]    As further illustrated in  FIG. 3  by the dashed line  101 , operation of an alternator  6  may follow a squared power rule, where the power produced by the turbine increases as the square of the wind speed. For each wind speed, the controller  40  is configured to operate at a maximum power point (MPP), such that the maximum power that may be generated by the alternator at that wind speed is transferred to the DC bus  12 . Tracking these maximum power points at the various wind speeds results in the exponential, squared power curve  101  until rated power production occurs. At that point, the controller  40  is configured to limit power production to the rated value to prevent damage to the alternator  6  or to the components of the converter  10 . The controller  40  may be configured to execute control routines both to control the pitch of the blades and to control the current conducted between the alternator  6  and the DC bus  12 . Optionally, separate controllers  40  may be used, each executing one of the control modules. 
         [0049]    In order to regulate the current drawn from the alternator  6  during normal operating conditions, the controller  40  may implement a first current regulator configured for synchronous control of the current from the alternator  6  to the DC bus  12 , as is known in the art. A synchronous current regulator receives a current reference and using measured current signals determines a current error value. The synchronous current regulator then determines a desired controlled current to compensate for the current error value. The controller  40  then determines appropriate gating signals,  24  and  25 , to selectively connect each phase of the input terminals, T 1 -T 3 , to the DC bus  12  to produce the desired controlled current between the alternator  6  and the DC bus  12 . 
         [0050]    Because the alternator  6  generates AC power, the controller  40  also requires knowledge of the electrical angle of the AC voltages present at the input terminals, T 1 -T 3 . When operating above a minimum speed, the controller  40  may determine the electrical angle by detecting the back-emf present at the alternator  6 . As the speed of rotation of the alternator increases, the amplitude of the back-emf similarly increases. However, the back-emf is a function of the alternator parameters as well as a function of the rotor speed. Thus, the minimum speed at which the back-emf may be detected is a function of the application. However, the amplitude of the back-emf may typically be reliably detected between about 10% and about 20% of the rated speed of the alternator  6 . 
         [0051]    Referring next to  FIG. 5 , the synchronous current regulator uses the desired controlled current value and the detected electrical angle of the alternator  6  to generate a voltage reference signal  154  to generate gating signals  24 ,  25 . In  FIG. 5 , generation of gating signals  24 ,  25  for a segment of one cycle for one phase of the AC voltage according to an exemplary sine-triangle PWM modulation technique  150  is illustrated. In the sine-triangle PWM modulation technique  150 , a triangular waveform  152  is compared to the voltage reference  154  to generate the gating signals,  24  and  25 . One period of the triangular waveform  152  is defined by the switching period  156  of the PWM routine. During the switching period  156 , if the voltage reference  154  is greater than the triangular waveform  152 , the positive gating signal  24  is set high while the negative gating signal  25  is set low. If the voltage reference  154  is less than the triangular waveform  152 , the positive gating signal  24  is set low while the negative gating signal  25  is set high. It is contemplated that other modulation techniques, as would be known to one skilled in the art, may also be used to generate the output voltage, such as space-vector or multi-level switching. Further, the modulation techniques may be implemented by comparing analog signals, as shown in  FIG. 4 , digital signals, such as a register being incremented up and down, or a combination thereof. 
         [0052]      FIG. 5  illustrates ideal switching conditions under which the positive gating signal  24  and the negative gating signal  25  simultaneously invert states such that the positive switch  20  and the negative switch  21  are not simultaneous conducting. In practice, however, the switches,  20  and  21 , are not ideal and are not switched as indicated in  FIG. 5 . Referring also to  FIG. 6 , each of the switches,  20  and  21 , requires a finite time to turn off, t off , or to turn on, t on . In order to prevent simultaneous conduction of the positive switch  20  and the negative switch  21 , a dead time compensation may be used. The dead time, t d , is typically set longer than the turn off time, t off , of the switches,  20  or  21 . When either the positive gating signal  24  or the negative gating signal  25  is commanded to turn off, as illustrated at the switching instant, t sw , the controller  40  delays setting the other of the positive gating signal  24  or the negative gating signal  25  to on for the duration of the dead time, t d , preventing simultaneous conduction of both a positive and a negative switch,  20  and  21 , on the same phase, which creates a short between the positive rail  14  and the negative rail  16  of the DC bus  12 . The delay in a switch,  20  or  21 , turning off, t off , results in a short period  27  of unwanted conduction and the delay in a switch,  20  or  21 , turning on, t on , results in a short period  29  of unwanted non-conduction. 
         [0053]    As previously indicated, knowledge of the electrical angle of the AC power produced by the AC alternator  6  is required for the synchronous current regulator to control power transfer from the alternator  6  to the DC bus  12 . The angular position of the alternator  6  is typically obtained from the electrical waveform generated. Using, for example, measurements of the back-emf voltage, a phase-locked loop can extract the angular position of the alternator  6 . As the speed of the rotor slows, the magnitude of the back-emf decreases until the amplitude becomes too low to accurately detect. Previously, converters  10  would need to shut down to prevent instability, an inability to transfer power, and/or potential damage to the inverter resulting from generating gating signals,  24  and  25 , without accurate knowledge of the electrical angle. This minimum speed at which the converter  10  could operate is also known as the cut-in speed. Although the converter  10  ceases operation, the alternator  6  is still capable of generating power below the cut-in speed. 
         [0054]    In order to improve efficiency of the alternator  6  and to continue receiving the power generated by the alternator  6  during low-speed operation, the converter  10 , as disclosed herein, executes in multiple operating modes to expand its operating range. As discussed above, the converter  10  executes a synchronous control method in a first operating mode at or above a first threshold. This first threshold corresponds to the operating speed of the alternator  6  at which the back-emf of the voltage generated by the alternator  6  may be reliably detected, which is typically about 10-20% of rated speed. During operation in the first operating mode, the modulation routine executes with a fixed period, T 1 , and a fixed dead-time compensation, t d . Optionally, the modulation frequency and, consequently, the period may vary during the first operating mode as a function of the frequency of the voltage being generated by the alternator  6 . The range of switching frequency may be, for example between 5-10 kHz. 
         [0055]    Modulation techniques control the positive switches  20  and the negative switches  21  to alternately connect each of the terminals, T 1 -T 3 , between either the positive or negative rail,  14  and  16 , of the DC bus  12 . Referring next to  FIG. 11 , the resulting modulated voltage waveforms from alternately connecting each of the terminals, T 1 -T 3 , between either the positive or negative rail,  14  and  16 , of the DC bus  12  is illustrated. As the speed of the alternator  6  decreases, the frequency and amplitude of the back-emf in the alternator similarly decrease. However, because the inverter  60  connected to the power converter  10  is generating an AC voltage for connection to a utility grid or to an electrical load  4 , the power conversion system maintains a generally constant level of DC voltage on the DC bus  12 . Consequently, as the amplitude of the back-emf decreases, the peak amplitude of the modulated waveforms remains the same and becomes much greater than the amplitude of the back-emf generated by the alternator  6 , introducing significant noise or uncertainty in attempting to read the value of the back-emf. Referring also to  FIGS. 12 and 13 , the difference in magnitudes of the modulated voltage  121  compared to the magnitude of the back-emf voltage  123  during low frequency operation of the alternator  6  is illustrated. 
         [0056]    To improve the range over which the controller may reliably measure the back-emf, the controller  40  may enter a blanking control operating mode. As the operating frequency of the alternator  6  decrease, the blanking control operating mode is configured to introduce a short interval, or blanking time  120 , during which the modulation is stopped. During the blanking time  120 , the controller  40  may read the back-emf voltage without interference from the modulated voltage. The blanking time  120  is short enough such that the inertia of the alternator  6  and the blades of the wind turbine keep the alternator  6  rotating with little or no change in speed of the alternator  6 . The blanking time  120  is introduced at a periodic interval throughout one cycle of the fundamental frequency of the voltage produced by the alternator  6 . During periods of modulation, the power generated by the alternator  6  is transferred to the DC bus  12 . Introduction of a blanking time, as described above, allows the power converter  10  to temporarily discontinue modulation and read the back-emf. The electrical angle of the back-emf is determined and corresponding adjustments made to the angle used by the controller  40  to perform modulation. Modulation of the switches,  20  and  21 , is resumed at the modified angle to transfer power from the alternator  6  to the DC bus  12 . Thus, the operating range at which the back-emf control is performed may be extended to about 5% of the amplitude of the rated speed of the alternator  6 . 
         [0057]    Referring next to  FIG. 7 , as the speed and, therefore, the corresponding power generated of the alternator  6  decreases, the converter may also be configured to operate in another operating mode having a fixed period, T 1 , and varying dead-time control. The duration of the fixed period, T 1 , is selected to be the same as the period  156  used by the controller  40  during operation in the prior operating mode. Similarly, the initial dead-time, t dx , for dead-time control is selected to be the same as the dead time, t d , used during operation in the prior operating mode. As a result, the transition from operation with the synchronous current regulator or with the current blanking control to operation with the varying dead-time has no step change in either of these operating parameters. 
         [0058]    Although there is no step change in operating parameters, there is a change in modulation technique between operating modes. As discussed above with respect to  FIG. 5 , pulse width modulation generates gating signals,  24  and  25 , as a function of the electrical angle of the input voltage at each terminal, T 1 -T 3 . As a result, the positive gate signals  24  and the negative gate signals  25  are different for each phase of the input terminals, T 1 -T 3 . In contrast, during dead time control, the converter  10  generates substantially identical positive gate signals  24  and negative gate signals  25  for each of the terminals, T 1 -T 3 . The resulting effect is that each of the positive switches  20  are turned on in tandem and each of the negative switches  21  are turned on in tandem. The controller  40  generates the gating signals,  24  and  25 , such that the positive switches  20  and negative switches  21  are alternately pulsed on and off for short durations as controlled by the dead time, t d . 
         [0059]    The multi-phase inductor  28  connected in series between each phase of the input terminals, T 1 -T 3 , and each of the switches,  20  or  21 , limits the rate of change of the current. In addition, the amplitude of the voltage produced by the alternator  6  is lower at low speeds also reducing the rate of change of current through the inductor  28 . Thus, although simultaneously switching each of the positive switches  20  or negative switches  21  would otherwise establish a short circuit across the alternator  6 , the resulting current waveforms during this operating mode are generally sinusoidal, as illustrated in  FIGS. 9 and 10 . 
         [0060]    The magnitude of the current is a function of the duration of the gating signal,  24  or  25 , to each of the switches,  20  or  21  respectively. The dead-time, t d , and the on time, t on , are inversely related, meaning that as the on time, t on , decreases, the dead-time, t d , increases. The controller  40  is configured to execute a second current regulator, for example, a proportional-integral (PI) regulator used to control the dead-time, t d , as a function of the current produced by the alternator  6 . A progression from the longest on time, t on1 , to the minimum on time, t on3 , for operation under dead-time control is illustrated in  FIG. 7 , plots (a)-(c) respectively. Upon initially switching into dead time control, the alternator  6  is generating the greatest amount of energy and the greatest amount of power may be transferred between the alternator  6  and the DC bus  12 . The initial on time, t on , is, therefore, at its greatest duration. As the wind speed continues to decline, the power levels that the alternator  6  is capable of producing continues to decline, requiring a decrease in the on time, t on , for each switch,  20  or  21 . At some point, the converter reaches a minimum on time, t on , which corresponds to a point at which the losses generated by the switches,  20  and  21 , exceed the power transferred during the on time, t on . At this point, the controller  40  begins to vary the modulation frequency. 
         [0061]    The transition described above, allows the controller  40  to transfer into dead-time control with no step change in operating parameters. Although there is no step change in operating parameters, there is a change in modulation technique. As discussed above with respect to  FIG. 5 , pulse width modulation generates gating signals,  24  and  25 , as a function of the electrical angle of the input voltage at each terminal, T 1 -T 3 . During synchronous current control, the positive gate signals  24  and the negative gate signals  25  are different for each phase of the input terminals, T 1 -T 3 . In contrast, during dead-time control, the converter  10  generates substantially identical positive gate signals  24  and negative gate signals  25  for each of the terminals, T 1 -T 3 . The resulting effect is that each of the positive switches  20  are turned on in tandem and each of the negative switches  21  are turned on in tandem. The controller  40  generates the gating signals,  24  and  25 , such that the positive switches  20  and negative switches  21  are alternately pulsed on and off for short durations as controlled by the dead time, t d . 
         [0062]    The multi-phase inductor  28  connected in series between each phase of the input terminals, T 1 -T 3 , and each of the switches,  20  or  21 , limits the rate of change of the current. In addition, the amplitude of the voltage produced by the alternator  6  is lower at low speeds also reducing the rate of change of current through the inductor  28 . Thus, although simultaneously switching each of the positive switches  20  or negative switches  21  would otherwise establish a short circuit across the alternator  6 , the resulting current waveforms during this operating mode are generally sinusoidal, as illustrated in  FIGS. 9 and 10 . 
         [0063]    Referring next to  FIG. 8 , in order to continue transferring energy from the alternator  6  to the DC bus  12  after the minimum on time, t on , has been reached, the controller  40  executes a modulation routing in which the on time, t on , remains constant and the modulation period, T, varies. For example, plot (a) of  FIG. 8  may represent the initial operating point in this operating mode. The initial period, T 1 , is equal to the period, T 1 , used during the transition illustrated in  FIG. 7  and the on time, t on , corresponds to the minimum on time, t on3 . As a result, the transition between operating modes again has no step changes with respect to the modulation period, T, or the on time, t on . 
         [0064]    As illustrated in  FIG. 8 , the converter  10  holds the on time, t on , constant and controls the modulation period. The initial modulation period, T 1 , may be, for example, 100 μsec which corresponds to a 10 kHz switching frequency. As the current provided by the alternator  6  continues to decrease, the modulation period may be extended, for example, to T 2  and subsequently to T 3 . It is contemplated that the modulation period may be extended to at least 20 msec, which corresponds to a 50 Hz switching frequency. Thus, as the wind speed and the corresponding rotor speed decreases, the converter  10  continues operation across a broader operating range to increase the amount of energy obtained from the wind turbine. 
         [0065]    During operation at variable modulation frequency, the controller  40  may access a look up table stored in memory  42  to facilitate operation because the relationship between changes in the amplitude of the current and the duration of the modulation period is nonlinear. For example, a 10 μsec change in the modulation period when operating at a 10 kHz switching frequency (i.e. a 100 μsec period) represents a greater percentage increment than when operating at a 50 Hz switching frequency (i.e. a 20 msec period). In order to improve the response time of the controller  40  to variations in the amplitude of the current during low power operation, the modulation period is changed at larger increments when the converter  10  is operating at lower switching frequencies than when the converter is operating at higher switching frequencies. The lookup table may store the desired incremental changes in the modulation frequency at varying operating points. 
         [0066]    As the wind speed and the corresponding power produced by the alternator  6  begins to increase, the controller  40  reverses the steps through the operating modes. Initially, the controller  40  operates with a fixed on time, t on , and reduces the modulation period, T, until it again reaches the desired duration for operation in the first and second operating modes. The transition to operation with a fixed modulation period, T, and variable on time, t on , from operation with a variable modulation period, T, and a fixed on time, t on , is again seamless because both operating modes encompass the common operating point. Similarly, as the wind speed and the corresponding power produced by the alternator  6  continue to increase, the dead time, t d , is reduced until it reaches the dead time, t d , for operation in the first operating mode. At this point, the alternator  6  is producing power at a sufficient level that the controller  40  may accurately determine the back-emf of the alternator  6 . The controller begins monitoring the back-emf and determines the corresponding electrical angle, for example, using a phase-locked loop and may then switch back to operation in the first operating mode with the synchronous current regulator. Again the transition between modes is seamless because the period, T, and dead time, t d , are the same for each mode at the transition point. 
         [0067]    It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention