Abstract:
Methods and apparatus are provided for controlling a stand-alone four-leg three-phase inverter. The inverter three-phase output is converted from AC domain elements to corresponding DC domain elements. The DC domain elements are processed into combined regulating and imbalance compensating signals, including over-current limiting. The compensating signals are restored to corresponding AC domain signals, and are processed into control inputs for the inverter, in order to stabilize the inverter output when connected to an unbalanced load. The inverter controller can be implemented entirely in software as a control algorithm.

Description:
TECHNICAL FIELD 
     The present invention generally relates to three-phase voltage source inverters, and more particularly relates to the control of 4-leg three-phase voltage source inverters. 
     BACKGROUND 
     Three-phase voltage source inverters (VSI&#39;s) are generally used to convert DC power into three-phase AC power. Typically, the three-phase output voltages are sinusoidal waveforms spaced 120 degrees apart, to be compatible with a wide variety of applications requiring conventional AC power. In general, the output power frequencies commonly used are 50, 60, and 400 hertz, but other frequencies could be used as well. One current example of an inverter application is the electric or hybrid automobile, where a DC power source, such as a battery, fuel cell array, or other equivalent device, is converted into an AC power supply for various internal control functions, including the propulsion system. 
     The quality of an inverter is generally determined by its output voltage and frequency stability, and by the total harmonic distortion of its output waveforms. In addition, a high quality inverter should maintain its output stability in the presence of load current variations and load imbalances. 
     In the case of unbalanced loads, the 4-leg three-phase inverter topology is generally considered to offer superior performance than a 3-leg three-phase topology. That is, with an unbalanced load, the three-phase output currents from an inverter will generally not add up to zero, as they would in a 3-leg balanced load situation. Therefore, a fourth (neutral) leg is typically added to accommodate the imbalance in current flow caused by an unbalanced load. If a neutral is not used with an unbalanced load, voltage imbalances may occur at the load terminals, and the output power quality may be adversely affected. 
     The operational functions of a typical inverter are generally controlled by drive signals from an automatic controller. The controller and inverter are usually implemented as a closed loop control system, with the inverter output being sampled to provide regulating feedback signals to the controller. The feedback signals typically include samples of the output voltage and current signals, and can also include harmonics of the fundamental output frequency. 
     The ability of an inverter control system to compensate for undesirable harmonics is generally limited by the bandwidth of the system voltage control loop, which may not be adequate for compensating high frequency harmonic distortion. For example, in a typical cascaded voltage/current regulator configuration, the voltage loop bandwidth is generally limited to approximately 1/100 th  of the sampling frequency. Due to technical factors, the sampling frequency is usually limited to a range of 5 to 20 kHz, thus limiting the voltage loop bandwidth to a range of 50 to 200 Hz. Therefore, harmonic compensation and transient response would be limited to frequencies within this range. 
     Moreover, the transient response characteristics of an inverter control system may also be limited by the overall execution time of the regulating loop software modules. That is, the larger the number of software modules, the greater the execution time, and the slower the transient response. 
     Accordingly, it is desirable to provide an inverter controller with a relatively high voltage control loop bandwidth, for improved harmonic compensation and transient response. In addition, it is desirable to provide an inverter controller with a minimal quantity of software modules, in order to speed up execution and reduce throughput time. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY 
     According to various exemplary embodiments, methods and devices are provided for controlling a multi-phase stand-alone inverter. One method comprises the following steps:
         a) converting the multi-phase inverter output from an AC domain to a DC domain equivalent, where the DC equivalent includes feedback voltage elements and associated feedback current elements, each voltage element and associated current element corresponding to one phase of the multi-phase output;   b) comparing each of the feedback voltage elements to a corresponding reference voltage to create corresponding difference voltage signals;   c) processing the difference voltage signals to create voltage regulating signals, where each of the voltage regulating signals includes a fundamental compensating component combined with an imbalance compensating component;   d) limiting the voltage regulating signals with a current limiting factor derived from the feedback current elements;   e) converting the voltage regulating signals to AC domain equivalents;   f) processing the AC domain equivalents to produce a set of control inputs to the inverter;   g) providing the set of control inputs to the inverter to enable compensating regulation of the fundamental and imbalance characteristics of the multi-phase output of the inverter.       

     An exemplary embodiment of a device for controlling a multi-phase stand-alone inverter includes:
         a converter configured to transform the alternating current multi-phase output to a direct current equivalent, where the direct current equivalent includes feedback voltage elements and associated feedback current elements, each voltage element and its associated current element corresponding to one phase of the multi-phase output;   a set of regulators, each regulator corresponding to a respective feedback voltage element, with each regulator configured to compare its respective feedback voltage element to a corresponding reference voltage to create a difference voltage signal, and to process the difference voltage signal into a voltage regulating signal, including a fundamental compensating component and an imbalance compensating component;   a set of limiters, each limiter corresponding to one of the voltage regulating signals and configured to limit its respective voltage regulating signal with a current limiting factor derived from the feedback current elements;   an inverse converter configured to inverse transform the voltage regulating signals into alternating current equivalents;   an inverter driver configured to process the alternating current equivalents to produce control inputs to the inverter that enable compensating regulation of the fundamental and imbalance characteristics of the multi-phase output.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a block diagram of an exemplary four-leg three-phase inverter system; 
         FIG. 2  is a simplified block diagram of an exemplary inverter controller; and 
         FIG. 3  is a detailed block diagram of an exemplary embodiment of an inverter controller. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Various embodiments of the present invention pertain to the area of voltage source inverters operating in a stand-alone mode. Generally, this type of inverter is used to convert DC power available at a selected voltage into AC power with fixed voltage and frequency. Ideally, the output voltage and frequency stability of an inverter should be independent of load variations and imbalances. To provide this type of stabilization, an inverter controller may be used in a closed loop feedback configuration to provide regulating and imbalance compensating signals to the inverter. The inverter controller may be implemented in hardware or software, or any combination of the two. 
     As previously noted in the Background section, the four-leg inverter topology is generally used for quality AC power generation into a three-phase unbalanced load application. The fourth leg provides a return path for the neutral imbalance current of a three-phase load. 
     A three-leg inverter configuration typically connects the load neutral to the mid-point of two series-connected capacitors across the DC voltage source. In this configuration, the AC output voltage would be approximately 0.5Vdc, whereas the four-leg configuration provides an AC output voltage of approximately 0.578Vdc. A further advantage of the four-leg configuration is that a smaller, single capacitor can be used instead of the two required for the three-leg approach. 
     According to an exemplary embodiment of a four-leg three-phase inverter system  100 , shown in  FIG. 1 , a DC voltage source  102  supplies a selected level of voltage (Vdc) to an inverter/filter  104  connected to a three-phase four-wire load  106 . Inverter/filter  104  typically comprises an input (link) capacitor C L  connected across source  102 , and in parallel with four sets of switching circuits  103 , which generate a three-phase output signal via L-C filter  105  to the load  106 . Inductor L n  represents the inductance of the neutral line. 
     An inverter controller  108  is typically configured to receive voltage and frequency command signals from a control unit (not shown in FIG.  1 ), and to also receive feedback signals from the input Vdc and from the outputs of inverter/filter  104  at the inputs to load  106 . Inverter controller  108  processes the command and feedback signals to create output drive signals for the inverter/filter  104  switching circuits  103 . The inverter controller  108  output drive signals may include voltage and current regulating elements as well as load imbalance compensating elements. 
       FIG. 2  depicts a simplified block diagram of inverter controller  108  within the closed loop four-leg three-phase inverter system  100 . In this embodiment, an external control unit  110  typically provides reference signals, such as voltage, current, frequency, etc., to inverter controller  108  to establish the desired output voltage and frequency values of inverter/filter  104 . In an alternate embodiment, control unit  110  could be integrated within inverter controller  108 . 
     Voltage regulator blocks  112 ,  114 ,  116  receive voltage reference signals from control unit  110  while a current limiting block  126  receives a current reference signal from control unit  110 . Samples of the voltage and current outputs from L-C filter  105  are transformed from the AC domain to the DC domain in block  124 , which receives a frequency reference signal from control unit  110 . Voltage feedback signals from block  124  are fed to corresponding voltage regulator blocks  112 ,  114 ,  116 , and current feedback signals from block  124  are fed to current limiting block  126 . A current limiting signal from block  126  is applied to voltage regulator blocks  112 ,  114 ,  116 . 
     Voltage regulating blocks  112 ,  114 ,  116  generate regulating signal outputs that are limited by the output of current limiting block  126 . The regulating signal outputs are inverse transformed from the DC domain to the AC domain in block  120 , which receives a frequency reference signal from control unit  110 . The transformed regulating signals are then processed by block  122  into driving signals for the inverter  104  switching circuits  103 . A more detailed description of the operation of inverter controller  108  is given below in conjunction with FIG.  3 . 
     An exemplary embodiment of an inverter controller  108  for a four-leg three-phase inverter/filter  104  is shown in a more detailed block diagram form in FIG.  3 . In this embodiment, the block functions within inverter controller  108  are implemented in software modules to constitute a control algorithm for inverter/filter  104 . This approach utilizes the Park transformation, as is known in the electrical machine art (see “Analysis of Electric Machinery” by Krause, Paul C., Wasynczuk, Oleg and Sudhoff, Scott D.; IEEE Press 1995, Institute of Electrical and Electronics Engineers, Inc.), to convert the sampled output signals from an AC domain to a DC domain in order to simplify the mathematical processes implemented within inverter controller  108 . An inverse Park transformation is then used to convert the processed DC domain signals back to the AC domain for the control inputs to the inverter switching circuits  103 . Other techniques for converting from the AC domain to the DC domain could be used in a wide array of equivalent embodiments. 
     The basic concept of the Park transformation is known as the synchronous reference frame approach. That is, a rotating reference frame is utilized in order to make the fundamental frequency quantities appear as DC values. A common convention is to label the AC domain (stationary reference frame) quantities, such as phase voltages and currents, as “abc”, and to label the corresponding Park-transformed DC domain (synchronous reference frame) quantities as “dq0”. This labeling convention will be followed throughout the following discussion. 
     Reference values for voltage, current and frequency are generally determined within a control unit  110  to establish desired values of inverter output voltage and frequency within a maximum current limit. The voltage references are shown in  FIG. 3  as V* d , V* q , V* 0 , which are typically calculated Park transformations of predetermined reference three-phase voltage values. The maximum current limit value is shown in  FIG. 3  as I inv     —     max , and the reference frequency is represented as ω*. 
     The inverter/filter  104  three-phase output voltages and currents may be measured by any conventional method to create feedback signals to inverter controller  108 . The voltage feedback signals are typically measured between phase and neutral, and are designated herein as V an , V bn , V cn . The current feedback signals can be measured by line sensors on each phase, and are designated herein as I a , I b , I c . 
     Voltage feedback signals V an , V bn , V cn  are converted from AC domain to DC domain equivalents via the Park transformation in block  124 . The reference angle used for this transformation is designated θ*, and is generated by an integrator block  23  from the reference signal ω*. The transformed voltage feedback signals are designated V d , V q , V 0  and are fed back with changed sign to adders  1120 ,  1140  and  1160 , respectively. The reference voltage signals V* d , V* q , V* 0  are also inputted to adders  1120 ,  1140  and  1160 , respectively, to generate voltage error signals (V* d −V d , V* q −V q , V* 0 −V 0 ) at the outputs of the respective adders  1120 ,  1140 ,  1160 . 
     The voltage error signals V* d −V d , V* q −V q , V* 0 −V 0  are routed through proportional-integral (PI) controller blocks  1122 ,  1142 , and  1162 , respectively, for amplifying and smoothing. At the same time, voltage error signals V* d −V d , V* q −V q , V* 0 −V 0  are also routed through band pass filter blocks  1128 ,  1148 , and  1168 , respectively. 
     Referring now to the d-axis voltage regulator ( 112 ) in this embodiment, block  1128  is configured as a second order band pass filter with an adjustable gain. The center frequency of filter  1128  is set at twice the reference frequency ω*, in order to provide a high gain for the d-axis voltage controller at this particular frequency. This is intended to compensate for an unbalanced inverter output voltage condition, where a voltage component at twice the fundamental frequency appears in the voltage feedback signal. By placing band pass filter  1128  in a parallel path within the d-axis voltage controller  112 , the loop gain can be increased at 2*ω* without affecting the phase and gain margin of the system. 
     The output signals from blocks  1122  and  1128  are combined in adder  1124 , along with a quantity −ω*LI q . This latter quantity is a feed-forward term, which may be obtained from control unit  110  by transforming the steady-state equations of the filter  105  from the stationary reference frame to the synchronous reference frame. The feed-forward term −ω*LI q  is used in this embodiment to improve the transient response of the d-axis voltage regulator  112 , and to reduce the cross-channel coupling between the d-axis and q-axis controllers ( 112  and  114 ). For the q-axis controller  114 , the corresponding feed-forward term is ω*LI d . 
     The q-axis voltage regulator  114  operates in essentially the same manner as the d-axis voltage regulator  112 , except for the feed-forward term, as noted above. 
     The  0 -axis voltage regulator  116  differs from the d-axis and q-axis regulators ( 112 ,  114 ) in that its associated band pass filter  1168  is tuned to ω*, rather than 2*ω*. This is due to the fact that an unbalanced output voltage condition will generally produce a fundamental frequency component on the  0 -axis feedback signal. Also, there is generally no need for a feed-forward signal in the  0 -axis channel. 
     The outputs of adders  1124 ,  1144  and  1164  are routed through limiter blocks  1126 ,  1146 , and  1166 , respectively. Limiter blocks  1126 ,  1146 ,  1166  also receive a common input signal from current limiter  126 , as will be described below. The limited output signals of blocks  1126 ,  1146 ,  1166  are then processed in block  120  from DC domain (dq 0 ) to equivalent AC domain (abc) by means of an inverse Park transformation, using the reference angle θ*. 
     The regulating output signals from block  120  are designated V a , V b , V c , and are normalized in block  122  by a multiplication factor (√3/V dc ), which is the inverse of the maximum achievable inverter phase output voltage for a given DC input voltage (V dc ). The normalized regulating voltages may be used to control the pulse train duty cycles of a conventional Pulse Width Modulator (PWM) within block  122 , or through any other technique. The duty cycle modulated pulse trains, designated as d abcn , are configured as the drive signals for the switching circuits  103  in inverter/filter  104 . The switching devices in switching circuits  103 , as depicted in  FIG. 1 , may be MOSFET&#39;s, IGBT&#39;s (Insulated Gate Bipolar Transistor), or any type of switching device with appropriate speed and power capabilities. 
     Referring now to the operation of current limiting block  126 , current feedback signals I a , I b , I c  are converted from AC domain to DC domain equivalents via the Park transformation in block  124 . The transformed current feedback signals are designated I d , I q , I 0  and are fed into a summing block  1260  within current limiting block  126 . The amplitude of inverter/filter  104  output current I inv  is calculated in summing block  1260 , based on the square root of the sum of the squares of the current feedback signals I d , I q , I 0 . This calculated value (I inv ) is combined with the maximum current limit value I inv     —     max  in adder  1262  to form a difference signal (I inv     —     max −I inv ). This difference signal is then amplified and smoothed in a PI block  1264 , so that the dynamics of the regulator are adequate for a fast reacting over-current protection. Block  1266  processes the output of block  1264  into a limiting factor, such as in the range of 0 to 1, where 1 corresponds to the maximum current limit. This limiting factor is then applied to the three limiting blocks  1126 ,  1146 ,  1166  as a multiplier, to add over-current protection to the voltage limiting function of blocks  1126 ,  1146 ,  1166 . 
     It should be noted that the PI controllers ( 1122 ,  1142 ,  1162 ,  1264 ) in  FIG. 3  each receive a feedback signal from their respective limiting modules ( 1126 ,  1146 ,  1166 ,  1266 ). This feedback scheme, known in the art as “integrator anti-wind-up”, improves the transient behavior of the PI controllers. 
     The previously described drive signals from controller  108  to the switching circuits  103  provide the desired regulating control for the multi-phase output of inverter/filter  104 . As such, controller  108  and inverter/filter  104  constitute a closed-loop feedback system for maintaining the stability and quality of the inverter/filter output. 
     In summary, the architecture of the inverter control algorithm, as disclosed in the exemplary embodiment of  FIG. 3 , provides a combination of voltage regulation, imbalance compensation, and over-current protection, with fast transient response, short execution time, and high harmonic suppression. Verification tests have demonstrated a voltage loop bandwidth capability of approximately 600 Hz for a sampling frequency of 12 kHz. Tests have also shown that voltage regulation (approximately 1%) and total harmonic distortion (approximately 2%) are essentially the same for a 100% unbalanced load operation as they are for a 100% balanced load operation. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.