Patent Publication Number: US-10326352-B2

Title: Control of AC source inverter to reduce total harmonic distortation and output voltage unbalance

Description:
FIELD OF THE INVENTION 
     The present subject matter relates generally to power converters and more particularly to systems and methods for controlling a power converter. 
     BACKGROUND OF THE INVENTION 
     DC to AC inverters can be used to generate a multiphase output from a DC power source. For instance, DC to AC inverters can be used in aviation applications to generate a three-phase AC output at a fundamental frequency of about 400 Hz from a DC power source. DC to AC inverters can include a plurality of bridge circuits that include semiconductor switching elements (e.g. IGBTs, SiC transistors, etc.) that are controlled using pulse width modulation techniques to convert a DC input to, for instance, a three-phase output. The inverter can additionally provide a neutral output by adding an additional bridge circuit to form a neutral leg of the inverter. The DC to AC inverter can be used to power single phase loads by coupling loads to one of the output phases and to the neutral output of the inverter. 
     Powering single phase loads from a multiphase DC to AC inverter can result in unbalanced loading of the inverter. Unbalanced loading can lead to creation of negative sequence and zero sequence currents by the inverter. These currents can create the same negative sequence and zero sequence components in the output voltage of the inverter. In addition, six-pulse inverters can create harmonics at, for instance, negative fifth harmonic and seventh harmonic of the fundamental frequency. Twelve-pulse inverters can create harmonics at, for instance, the negative eleventh harmonic and thirteenth harmonic of the fundamental frequency. 
     DC to AC inverters can be controlled using various control schemes, including natural frame (abc) control schemes, stationary reference frame control schemes, or synchronous reference frame control schemes. In a natural frame control scheme, identical control structures are used for each phase of the inverter. The control structure for each phase can include a voltage controller and a current controller. The voltage controller can be an outer control loop relative to the current controller and can generate a current reference for the current controller. The current controller can generate a voltage command, which can be used to determine gate timing signals for driving the switching elements of the bridge circuit. If the inverter includes a neutral leg, the voltage command for the neutral leg bridge circuit can be an average value of the voltage command for each of the phases of the inverter. The neutral leg typically does not have its own controller. 
     The controllers in a natural frame control scheme essentially operate at DC. As a result the controllers have infinite gain at DC and finite gain at the fundamental frequency, the negative sequence, the negative fifth harmonic, the positive seventh harmonic, and other harmonics of interest.  FIG. 1  depicts a graphical representation  50  of the gain of the natural frame controller.  FIG. 1  plots frequency along the horizontal axis and gain along the vertical axis. As shown in  FIG. 1 , the controller can reduce the magnitude of the fundamental frequency, negative sequence, negative fifth harmonic, positive seventh harmonic, etc. However, the controller does not eliminate these components as the controller does not have infinite gain at these frequencies. Accordingly, whatever remains of these components in the output voltage can lead to an increase in total harmonic distortion and voltage unbalance in the output voltage of the inverter. 
     Synchronous reference frame control schemes can operate based on a d-q transformation (e.g., performed using a Park transformation) that transforms the output voltage and current waveforms into a reference frame that rotates synchronously with the output voltage. Multiple synchronous reference frame controllers can include a plurality of control structures to introduce infinite gain at each of the frequencies of interest (e.g., fundamental, −11, −5, +7 and +13 of the fundamental frequency). 
     However, a condition of synchronous reference frame control schemes is that the controllers in the control structure for each frequency of interest have gains that cross each other below 0 dB. As a result the bandwidth of each controller used in the synchronous reference frame control scheme can be limited, leading to gaps in the control action of each controller.  FIG. 2  depicts a graphical representation of the gains of various current and voltage controllers at each of the frequencies of interest for a known synchronous reference frame control scheme  60 .  FIG. 2  plots frequency along the horizontal axis and gain along the vertical axis. The solid lines represent gains associated with the current controllers. The dashed lines represent gains associated with the voltage controllers. As shown, the synchronous reference frame control scheme can provide infinite gains at the fundamental frequency as well as at selected frequencies of interest. However, due to the limited bandwidth of the controllers, the negative sequence attenuation (e.g., at the −1 frequency) is still required to be performed with the control structure used to attenuate the fundamental frequency. As a result, the control structure does not provide infinite gain for the negative sequence. 
       FIG. 3  provides a graphical depiction  70  of the gains associated with the regulation of the zero sequence component.  FIG. 3  plots frequency along the horizontal axis and gain along the vertical axis. The solid lines represent gains associated with current controllers. The dashed lines represent gains associated with voltage controllers. The zero sequence component does not rotate, and thus it is not controlled using a synchronous reference controller. Rather, the zero sequence is controlled using controllers with finite gains at the fundamental frequency. As a result, typical multiple synchronous reference frame controllers do not completely regulate out the zero sequence and negative sequence components, leading to increased total harmonic distortion and voltage unbalance in the output voltage. 
     Thus, a need exists for a control scheme for regulating a DC to AC inverter that can reduce output voltage total harmonic distortion and voltage unbalance by providing improved regulation of the zero sequence and negative sequence components generated, for instance, by powering single phase loads with the DC to AC inverter. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of embodiments of the present disclosure will be set forth in part in the following description, or may be learned from the description, or may be learned through practice of the embodiments. 
     One example aspect of the present disclosure is directed to a control system for a DC to AC inverter configured to provide a multiphase output at a fundamental frequency. The control system includes a plurality of synchronous reference frame control structures. Each synchronous reference frame control structure includes a voltage controller configured to provide a current reference based on a voltage feedback signal. The control system further includes a shared current controller configured to determine a voltage command for controlling one or more bridge circuits of the DC to AC inverter. The shared current controller is disposed as an inner control loop relative to the voltage controller of each of the plurality of synchronous reference frame control structures. 
     Another example aspect of the present disclosure is directed to a method for controlling a DC to AC inverter configured to provide a multiphase output at a fundamental frequency. The method includes determining a voltage reference signal using an outer voltage control loop having a voltage controller with infinite gain at a frequency associated with a negative sequence component. The method further includes determining a voltage error signal based at least in part on the voltage reference signal and determining a current reference signal based at least in part on the voltage error signal at a voltage controller of a synchronous reference frame control structure. The method includes generating, with a current controller, a voltage command for controlling one or more bridge circuits based at least in part on the current reference. 
     Yet another example aspect of the present disclosure is directed to a control system for a DC to AC inverter configured to provide a multiphase output at a fundamental frequency. The control system includes a plurality of synchronous reference frame control structures. Each synchronous reference frame control structure can be associated with a different frequency. At least one of the plurality of synchronous reference frame control structures can be associated with the fundamental frequency. Each synchronous reference frame control structure includes a voltage controller configured to provide a current reference based at least in part on a voltage feedback signal. The control system further includes an outer voltage control loop in communication with the synchronous reference frame control structure associated with the fundamental frequency. The outer voltage control loop includes an outer loop voltage controller having an infinite gain at a frequency associated with a negative sequence component. The voltage controller of the synchronous reference frame control structure associated with the fundamental frequency is configured to determine a current reference based at least in part on a voltage reference provided by the outer loop voltage controller. 
     Variations and modifications can be made to these example aspects of the present disclosure. 
     These and other features, aspects and advantages of various embodiments 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 present disclosure and, together with the description, serve to explain the related principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Detailed discussion of embodiments directed to one of ordinary skill in the art are set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  depicts a graphical representation of the gain associated with controllers used in a natural reference frame control scheme for a DC to AC inverter; 
         FIG. 2  depicts a graphical representation of the gains associated with controllers used in a synchronous reference frame control scheme; 
         FIG. 3  depicts a graphical representation of the gains associated with controllers used to regulate a zero sequence component; 
         FIG. 4  depicts a circuit diagram of an example DC to AC inverter; 
         FIG. 5  depicts a multiple synchronous reference frame control scheme according to example embodiments of the present disclosure; 
         FIG. 6  depicts a graphical representation of the gains associated with controllers used in a synchronous reference frame control scheme according to example embodiments of the present disclosure; 
         FIG. 7  depicts a graphical representation of the gains associated with controllers used to regulate a zero sequence component in a control scheme according to example embodiments of the present disclosure; and 
         FIG. 8  depicts a flow diagram of an example method according to example embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     Example aspects of the present disclosure are directed to control systems and methods for controlling power converters, for instance, to reduce total harmonic distortion and voltage unbalance in the output voltage of the power converter. More particularly, a synchronous reference frame control scheme can be employed to regulate a DC to AC inverter to attenuate error at various frequencies of interest for the DC to AC inverter. According to particular example aspects of the present disclosure, the synchronous reference frame control scheme can include control structures to provide for attenuation of the negative sequence and zero sequence currents, leading to reduced total harmonic distortion and voltage unbalance in the output voltage of the inverter. 
     More particularly, a control topology according to example aspects of the present disclosure can include a plurality of synchronous reference frame control structures. As used herein, a control structure refers to control logic or circuitry configured to implement one or more control functions, such as one or more of the controller functions, control loop functions, transform functions, and other control functions disclosed herein. In some implementations, a control structure can refer to a set of instructions stored in a memory device that when executed by one or more control devices (e.g., microprocessor(s), microcontroller(s), etc.), cause the one or more control devices to provide the desired control functionality. 
     Each of the plurality of synchronous reference frame control structures can be implemented in a d-q reference frame that rotates measured quantities (e.g., three phase voltage or three-phase current) into a rotating reference frame. Each of the plurality of synchronous reference frame control structures can be associated with a different frequency and can include a voltage controller having an infinite gain at a frequency of interest. For instance, in one embodiment, the plurality of synchronous reference frame control structures can include a control structure associated with the fundamental (1) frequency (e.g., 400 Hz in aviation applications), a control structure associated with a frequency corresponding to the negative fifth (−5) harmonic, and a control structure associated with a frequency corresponding to the seventh (7) harmonic. The control structures for regulating the −5 and 7 frequencies can be particularly suitable for six pulse DC to AC inverters, which can generate harmonics at the −5 and 7 frequencies. 
     In some embodiments, if the switching frequency of bridge circuits in the inverter are fast enough (e.g., SiC transistors), the control scheme can include a control structure associated with a frequency corresponding to the negative eleventh (−11) harmonic and a control structure associated with a frequency corresponding to the thirteenth (13) harmonic. The control structures for regulating the −11 and 13 frequencies can be particularly suitable for twelve pulse DC to AC inverters, which can generate harmonics at the −11 and 13 frequencies. 
     Each synchronous reference frame control structure can include a voltage controller configured to generate a current reference command based at least in part on a voltage error signal. The voltage controller for each synchronous reference frame control structure can have an infinite gain at the frequency the synchronous reference frame control structure is intended to regulate. For instance, the voltage controller of the synchronous reference frame control structure associated with the fundamental frequency can have an infinite gain at the fundamental frequency. 
     In some embodiments of the present disclosure, the control scheme can employ a shared current controller to provide current regulation for each of the plurality of synchronous reference frame control structures. The shared current controller can be configured as an inner control loop relative to the voltage controllers of each of the plurality of synchronous reference frame control structures. The shared current controller can be a wide bandwidth current controller that is configured to determine a voltage command for controlling one or more bridge circuits of the DC to AC inverter based on a current error signal. The current error signal can be determined based at least in part on an aggregated current reference signal determined from the current reference signals provided by each of the voltage controllers of each of the plurality of synchronous reference frame control structures. 
     Because the voltage controllers of the plurality of synchronous reference frame control signals provide infinite gain at the frequencies of interest, the shared current controller does not need to have an infinite gain at each of the frequencies of interest. As a result, the shared current loop can provide for wideband control that does not leave uncontrolled frequency gaps. 
     In some embodiments of the present disclosure, the control scheme can include an outer voltage control loop relative to at least one of the plurality of synchronous reference frame control structures to provide for attenuation of the negative sequence component. For instance, the outer voltage control loop can include an outer voltage controller having an infinite gain at a frequency associated with the negative sequence (e.g., the −1 frequency). The outer voltage control loop can be configured to provide a voltage reference to the synchronous reference frame control structure associated with the fundamental frequency. This configuration can allow for wide bandwidth control of the fundamental synchronous reference frame control structure and can achieve infinite gain at both the fundamental frequency and the frequency associated with the negative sequence. 
     In some embodiments of the present disclosure, the control scheme can further include a neutral control structure for controlling one or more bridge circuits associated with a neutral leg of the DC to AC inverter. The neutral control structure can be a non-rotating controller (e.g. not implemented in the d-q space). A pseudo d-q controller can be used in the neutral leg to provide an infinite gain at the fundamental frequency. The neutral control structure can further include a voltage controller configured as an inner control loop relative to the pseudo d-q controller. This voltage controller can be configured to provide an infinite gain at a frequency associated with the zero sequence component. In this manner, the neutral control structure can achieve infinite gain at both the fundamental frequency and a frequency associated with the zero sequence component, leading to reduced total harmonic distortion and voltage unbalance in the output voltage of the DC to AC inverter. 
     With reference now to the FIGS., example embodiments of the present disclosure will now be set forth.  FIG. 4  depicts an example circuit diagram of a DC to AC inverter  100 . The inverter  100  is configured to convert a DC power source V DC  to an output three-phase voltage at a fundamental frequency. Other multiphase outputs are contemplated without deviating from the scope of the present disclosure. In example embodiments for aviation applications, the fundamental frequency is about 400 Hz. The use of the term about in conjunction with a numerical value is intended to refer to within 30% of the specified amount. 
     The inverter can receive the DC power from a DC bus  102 . The DC bus  102  can include a DC bus capacitor  105 . The DC power on the DC bus  102  can be converted to an output three-phase AC power using a plurality of bridge circuits, such as bridge circuit  104 , bridge circuit  106 , and bridge circuit  108 . Each bridge circuit  104 ,  106 , and  108  can include an upper switching element and a lower switching element coupled in series. The switching elements can be semiconductor devices, such as insulated gate bipolar transistors (IGBTs), silicon carbide (SiC) transistors, or other suitable switching elements. Each switching element can be coupled in parallel with a diode. 
     The switching elements can be controlled using gate timing commands to convert the DC power from the DC power source V DC  into an output three-phase AC voltage including an A-phase  114 , a B-phase  116 , and a C-phase  118 . The gate timing commands for the switching elements can be determined based at least in part on commands provided from the synchronous reference frame control scheme according to example aspects of the present disclosure as will be discussed in more detail below. 
     The inverter  100  can further include a neutral leg  110 . The neutral leg  110  can similarly include a bride circuit having an upper switching element and a lower switching element. The switching elements of the neutral leg  110  can be controlled based at least in part one commands provided from a neutral control structure according to example embodiments of the present disclosure to provide a neutral reference  120 . The neutral leg  110  can be eliminated in some embodiments by connecting a neutral between the split DC capacitor or by using a neutral forming transformer. 
     Single phase loads can be powered by the inverter  100  by coupling the load between the neutral reference  120  and one of the A-phase  114 , B-phase  116 , or C-phase  118 . Powering single phase loads can lead to negative sequence and zero sequence components in the output voltage of the inverter  100 . Example aspects of the present disclosure provide a control scheme using multiple synchronous reference frame control structures that regulate the negative sequence and zero sequence components to reduce total harmonic distortion and voltage unbalance in the output voltage of the inverter  100 . 
       FIG. 5  depicts an example control scheme  200  according to example embodiments of the present disclosure. The control scheme  200  can be implemented using one or more control devices. In one implementation, the control scheme  200  can be implemented by one or more processors executing computer-readable instructions stored in one or more memory devices. 
     The control scheme  200  includes a plurality of synchronous reference frame control structures, including synchronous reference frame control structure  210 , synchronous reference frame control structure  220 , synchronous reference frame control structure  230 , synchronous reference frame control structure  240 , and synchronous reference frame control structure  250 . 
     Each synchronous reference frame control structure is associated with a different frequency. For instance, control structure  210  is associated with the fundamental frequency. Control structure  220  is associated with the negative fifth harmonic. Control structure  230  is associated with the seventh harmonic. Control structure  240  is associated with the negative eleventh harmonic. Control structure  250  is associated with the thirteenth harmonic. More or fewer control structures associated with varying frequencies can be used in the control scheme  200  without deviating from the scope of the present disclosure. 
     Each control structure  210 ,  220 ,  230 ,  240 , and  250  is a synchronous reference frame control structure that includes a voltage control loop implemented in the d-q space. More specifically, each control structure  210  can include a voltage control loop configured to receive a voltage feedback signal transformed to the d-q space at an appropriate rotational frequency. The voltage feedback signal can be used to determine an error signal based on a voltage reference. The error signal can be provided to a voltage controller having an infinite gain at the frequency associated with the control structure. The voltage controller can determine a current command. The voltage controllers in the synchronous reference frame control structures can be proportional integral controllers or other suitable controllers (e.g. PD controllers, PID controllers, etc.). 
     More particularly, synchronous reference frame control structure  210  associated with the fundamental frequency can include a rotation transform  212  configured to transform a three-phase voltage feedback signal to a voltage feedback signal in the d-q space associated with the fundamental frequency. As shown, the transformed voltage feedback signal can be used to determine an error signal based on a voltage reference V REF . The error signal can be provided to the voltage controller  214 , which can determine a current reference based at least in part on the error signal. The voltage controller  214  can have an infinite gain at the fundamental frequency. 
     Synchronous reference frame control structure  220  can include a rotation transform  222  configured to transform a three-phase voltage feedback signal to a voltage feedback signal in the d-q space associated with the negative fifth harmonic. The transformed voltage feedback signal can be used to determine an error signal based on a reference. In some implementations, the reference for the negative fifth harmonic can be [0, 0] to reduce components at the negative fifth harmonic. The error signal can be provided to voltage controller  224 , which can determine a current reference based at least in part on the error signal. The voltage controller  224  can have an infinite gain at the negative fifth harmonic. The control structure  220  can include a transform  226  to transform the current reference from the d-q space associated with the negative fifth harmonic to the d-q space associated with the fundamental frequency. 
     Synchronous reference frame control structure  230  can include a rotation transform  232  configured to transform a three-phase voltage feedback signal to a voltage feedback signal in the d-q space associated with the seventh harmonic. The transformed voltage feedback signal can be used to determine an error signal based on a reference. In some implementations, the reference for the seventh harmonic can be [0, 0] to reduce components at the seventh harmonic. The error signal can be provided to voltage controller  234 , which can determine a current reference based at least in part on the error signal. The voltage controller  234  can have an infinite gain at the seventh harmonic. The control structure  230  can include a transform  236  to transform the current reference from the d-q space associated with the seventh harmonic to the d-q space associated with the fundamental frequency. 
     Synchronous reference frame control structure  240  can include a rotation transform  242  configured to transform a three-phase voltage feedback signal to a voltage feedback signal in the d-q space associated with the negative eleventh harmonic. The transformed voltage feedback signal can be used to determine an error signal based on a reference. In some implementations, the reference for the negative eleventh harmonic can be [0, 0] to reduce components at the negative eleventh harmonic. The error signal can be provided to voltage controller  244 , which can determine a current reference based at least in part on the error signal. The voltage controller  244  can have an infinite gain at the negative eleventh harmonic. The control structure  240  can include a transform  246  to transform the current reference from the d-q space associated with the negative eleventh harmonic to the d-q space associated with the fundamental frequency. 
     Synchronous reference frame control structure  250  can include a rotation transform  252  configured to transform a three-phase voltage feedback signal to a voltage feedback signal in the d-q space associated with the thirteenth harmonic. The transformed voltage feedback signal can be used to determine an error signal based on a reference. In some implementations, the reference for the thirteenth harmonic can be [0, 0] to reduce components at the thirteenth harmonic. The error signal can be provided to voltage controller  254 , which can determine a current reference based at least in part on the error signal. The voltage controller  254  can have an infinite gain at the thirteenth harmonic. The control structure  250  can include a transform  256  to transform the current reference from the d-q space associated with the thirteenth harmonic to the d-q space associated with the fundamental frequency. 
     As depicted in  FIG. 5 , the control scheme  200  can include a shared current controller  260  in the control structure  210  for the fundamental frequency. The shared current controller  260  can be configured as an inner control loop relative to the voltage control loops associated with the control structures  210 ,  220 ,  230 ,  240 , and  250 . The shared current controller  260  can be a proportional integral controller or other suitable controller (e.g., PD controller, PID controller, etc.). 
     The shared current controller  260  can determine a voltage command  270  for controlling one or more bridge circuits  290  associated with the DC to AC inverter based at least in part on an aggregated current reference  262 . As shown in  FIG. 5 , the aggregated current reference  262  can be determined as the sum of the current references determined by each of the voltage controllers (after being transformed to the d-q-space associated with the fundamental frequency) associated with the different control structures  210 ,  220 ,  230 ,  240 , and  250 . 
     The shared current control loop can receive a current feedback signal I FB  transformed to the d-q space associated with the fundamental frequency by transform  212 . A current error signal  264  can be determined based at least in part on a difference between the aggregated current reference  262  and the current feedback signal I FB . The current error signal  264  can be provided to the shared current controller  260 , which can be a wideband controller having an infinite gain at the fundamental frequency. The shared current controller  260  can determine the voltage command  270  based on the current error signal  264 . The voltage command  270  can be provided to transform  216  which converts the voltage command  270  from the d-q-space to a voltage command  272  associated with the natural reference frame space. 
     The voltage command  272  can be provided to control logic which determines gate timing commands for switching elements in the bridge circuits  290  of the DC to AC inverter based on the voltage command  272 . The bridge circuits  290  can be controlled in accordance with the determined gate timing commands to generate a three-phase output voltage at the fundamental frequency. 
     To regulate negative sequence components, the control scheme  200  further includes an outer voltage control loop  280  in communication with the control structure  210  associated with the fundamental frequency. The outer voltage control loop  280  can be configured as an outer control loop relative to the voltage control loop and current control loop of the control structure  210  associated with the fundamental frequency. 
     More particularly, the outer voltage control loop  280  can include a rotation transform  282  configured to transform a three-phase voltage feedback signal to a voltage feedback signal in the d-q space associated with a frequency corresponding to the negative sequence component. The transformed voltage feedback signal can be used to determine an error signal based on a reference. In some implementations, the reference for the negative sequence component can be [0, 0] to reduce the negative sequence component. The error signal can be provided to voltage controller  284 , which can determine a voltage reference based at least in part on the error signal. The voltage controller  284  can have an infinite gain at a frequency associated with the negative sequence. The voltage controller  284  can be a proportional integral controller or other suitable controller (e.g., PD controller, PID controller, etc.). 
     The outer voltage control loop  280  can include a transform  286  to transform the voltage reference from the d-q space associated with the negative sequence to the d-q space associated with the fundamental frequency. The voltage reference can be used to adjust the voltage reference V REF  provided to the control structure  210  associated with the fundamental frequency. In this way, the outer voltage control loop  280  in combination with the control structure  210  can regulate both the fundamental and negative sequence components by including a voltage controller  284  having an infinite gain at the frequency corresponding to the negative sequence component and a voltage controller  214  having an infinite gain corresponding to the fundamental frequency. 
       FIG. 6  provides a graphical representation  400  of the gains associated with the controllers used in the control scheme  200  according to example embodiments of the present disclosure.  FIG. 6  plots frequency of interest along the horizontal axis and gain along the vertical axis. The solid curve  460  represents gain attributable to the shared current regulator  260 . Dashed curve  414  represents gain attributable to the control structure  210  in combination with the outer voltage control loop  280 . Dashed curve  424  represents gain attributable to the voltage regulator  224  in control structure  220 . Dashed curve  434  represents gain attributable to the voltage regulator  234  in control structure  230 . Dashed curve  444  represents gain attributable to the voltage regulator  244  in control structure  240 . Dashed curve  454  represents gain attributable to the voltage regulator  254  in control structure  250 . 
     As depicted, the control scheme  200  can achieve infinite gains at each of the frequencies of interest (e.g., 1, −1, −5, 7, −11, and 13) by virtue of the voltage controllers. The shared current controller provides a wideband gain that is infinite at the fundamental frequency to reduce gaps in the control scheme. 
     Referring back to  FIG. 5 , the control scheme  200  can further include a neutral control structure  300  for determining a neutral command  272  to control one or more bridge circuits associated with a neutral leg of the DC to AC inverter. The neutral control structure  300  can include a Z-transform to transform a measured current feedback signal and a voltage feedback signal to the Z-space. The voltage feedback signal can be provided to a pseudo d-q controller  312 . The pseudo d-q controller can determine a voltage reference  322  based on the voltage feedback signal. The pseudo d-q controller  312  can provide infinite gain at the fundamental frequency. The pseudo d-q controller  312  can have 0 gain at all other frequencies. 
     The voltage reference  322  determined by the pseudo d-q controller can be used to adjust a voltage reference (in some implementations [0,0]) to an adjusted voltage reference  324 . The adjusted voltage reference  324  can be provided to an outer voltage controller  314 , which can be configured to provide an infinite gain at the frequency associated with the zero sequence component. The outer voltage controller can determine a voltage reference  326  for the neutral control structure based on the adjusted voltage reference  324 . In this way, the combination of the pseudo d-q controller  312  and the outer voltage controller  314  in the neutral control structure  300  can regulate both the fundamental frequency and frequency associated with the zero sequence component. The voltage reference  326  can be used by voltage controller  316  and current controller  318  to generate the neutral command  372  for the neutral leg of the DC to AC inverter. 
       FIG. 7  depicts a graphical representation of gains associated with the controllers used in the neutral control leg  300  according to example aspects of the present disclosure.  FIG. 7  plots frequency along the horizontal axis and gain along the vertical axis. Curve  516  depicts gains provided by the voltage controllers of the neutral control structure  300 . As shown, the voltage controllers provide infinite gains at both the fundamental frequency and the frequency associated with the zero sequence component. 
       FIG. 8  depicts a flow diagram of an example method ( 600 ) for controlling a DC to AC inverter according to example embodiments of the present disclosure. The method ( 600 ) can be implemented using any suitable control scheme, such as the control scheme  200  depicted in  FIG. 5 .  FIG. 8  depicts steps performed in a particular order for purposes of illustration and discussion of one example embodiment of the present disclosure. Those of ordinary skill in the art, using the disclosures provided herein will understand that the steps of any of the methods described herein can be combined, modified, expanded, omitted, rearranged or otherwise adapted in various ways without deviating from the scope of the present disclosure. 
     At ( 602 ), the method can include determining a voltage reference signal with an outer voltage control loop. For instance, the outer voltage control loop  280  of  FIG. 5  can be used to determine a voltage reference based on a voltage feedback signal. The outer voltage control loop can include a voltage controller with infinite gain at a frequency associated with a negative sequence component. 
     At ( 604 ) of  FIG. 8 , an error signal is determined based on the voltage reference. For instance, as shown in  FIG. 5 , the voltage reference provided by the outer voltage control loop  280  can be used to adjust a voltage reference V REF  for the control structure  210  associated with the fundamental frequency. A voltage error signal can be determined based at least in part on a difference between the adjusted voltage reference V REF  and the voltage feedback signal V FB . 
     At ( 606 ) of  FIG. 8 , a current reference signal is determined using an inner voltage control loop relative to the outer voltage control loop. For instance, the voltage error signal can be provided to the voltage controller  214  of  FIG. 5  associated with the control structure  210 . The voltage controller  214  have an infinite gain at the fundamental frequency. The voltage controller  214  can determine a current reference based at least in part on the voltage error signal. 
     At ( 608 ), an aggregated current reference signal is determined. For instance, the aggregated current reference  262  can be determined by aggregating current references generated by each voltage controller of the plurality of synchronous reference frame control structures  210 ,  220 ,  230 ,  240 , and  250  of  FIG. 5 . 
     At ( 610 ), a voltage command for controlling one or more bridge circuits of an AC to DC converter is determined using a shared current controller. For instance, the shared current controller  260  of  FIG. 5  can determine a voltage command for the one or more bridge circuits  290  based at least in part on the aggregated current reference  262 . The shared current controller  260  can be configured as an inner control loop relative to the voltage controller of each of the plurality of synchronous reference frame control structures  210 ,  220 ,  230 ,  240 , and  250 . The shared current  260  regulator can be a wideband current controller having an infinite gain at the fundamental frequency. 
     At ( 612 ), the method can include determining a neutral command for a neutral leg of the DC to AC inverter using a neutral control structure. For instance, the neutral control structure  300  of  FIG. 5  can be used to determine a neutral command  372  for controlling a bridge circuit associated with the neutral leg of the DC to AC inverter. 
     In particular implementations, determining the neutral command can include determining a first voltage reference  322  using a pseudo d-q controller  312  configured to provide an infinite gain at the fundamental frequency and determining a second voltage reference  324  based on the first voltage reference using a voltage controller  314  configured to provide an infinite gain at a frequency associated with a zero sequence component. The neutral command  372  can be determined based on the second voltage reference  324  using voltage controller  316  and current controller  318 . 
     At ( 614 ), the method can include controlling bridge circuits based at least in part on the voltage command and the neutral command. For instance, control logic can be accessed to determine gate timing commands for the switching elements of the bridge circuits of the DC to AC inverter based on the voltage command and the neutral command. The switching elements of the bridge circuits can be driven based on the gate timing commands to provide a multiphase output at the fundamental frequency. 
     Although specific features of various embodiments may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the present disclosure, 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.