Abstract:
An active filtering method and apparatus for controlling a current generator that sources/sinks an APF current for compensating polluting harmonics on a power line connecting a power source and a load. A feedback loop regulates the APF current by sensing the current output of the current generator and the current flowing through the load. The feedback loop controls the current generator to force the APF current to track a current command signal to effectively limit the APF current to a safe value within the limitations of a particular design.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to power filters in general, and to active power filters in particular. 
   2. Description of the Prior Art 
   The wide use of nonlinear loads has increased the harmonic content of the voltage and current waveforms in alternating current (AC) power distribution systems. In many cases large numbers of such loads are operating, causing a corresponding increase in power line harmonics. Such harmonic currents in conjunction with their associated source impedances produce distortion of the line voltages which can cause equipment to malfunction. 
   To address the above problems, active power filters (APF) have been used for compensation of polluting harmonics on electricity distribution networks. An APF is a device that is connected to a power line and cancels the reactive and harmonic currents from a group of nonlinear loads so that the resulting total current drawn from an AC source is sinusoidal. Ideally, the APF needs to generate just enough reactive and harmonic current to compensate the nonlinear loads on the line, thus it handles only a fraction of the total power to the loads. 
   In one conventional APF design, an open loop scheme is used to control the APF current, as generally described by E. Dallago and M. Passoni in an article titled “Single-Phase Active Power Filter with Only Line Current Sensing”, IEEE Electronics Letters, 20 Jan. 2000, Vol. 36, No. 2, pp. 105-106, and by K. M. Smedley, L. Zhou, and C. Qiao in an article titled “Unified Constant Frequency Control of Active Power Filters-Steady State and Dynamics”, IEEE Transactions on Power Electronics, Vol. 16, No. 3, May 2001, pp. 428-436. In these systems the APF is controlled such that the output voltage of the APF is proportional to the input source current. This causes the impedance seen by the power source to appear resistive, hence maintaining the input current approximately proportional to the input voltage. 
   In another conventional APF design, a closed input current loop with a reference multiplier is used, as described by F. P. de Souza and I. Barbi in an article titled “Single-Phase Active Power Filters for Distributed Power Factor Correction”, IEEE PESC 2000, pp. 500-505. A current loop is used to force the input source current to track the input source voltage, thereby achieving near-unity power factor. 
   However, in the above approaches, only the input source current is measured. This makes it difficult to control the current flow from the APF and the load. Hence, if there is an overload condition at the load, the APF control will attempt to deliver the current demanded, even if the demand is beyond the design limits of the APF. 
   There is, therefore, a need for an APF control method and apparatus that limit the APF current, and allow transition from a normal condition to an overload condition and back again in stable manner, while maintaining the lowest input current distortion within the design limits of the APF. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention addresses the above needs. In one embodiment the present invention provides an active filtering method and apparatus for controlling a current generator that generates an APF current for compensating polluting harmonics on a power line that connects a powersource and a load. A feedback loop regulates the APF current by sensing the current output of the current generator and the current flowing through the load. The feedback loop controls the current generator to force the APF current to track a current command signal to provide near unity power factor (i.e., proportional current), while effectively limiting the APF current to a safe value within the limitations of a particular design. 
   In one implementation, an active filter is provided that can be connected to a power line between a power source and a load. The active filter comprises a current generator that in response to a control signal generates an APF current to compensate for polluting harmonics on the power line. The active filter further includes a controller that controls the current generator to compensate for the polluting harmonics on the power line, such that the APF current does not exceed a selected threshold value. 
   Preferably, the controller further includes a limiter that generates said control signal based on feedback values of the APF current and the load current, to control the current generator such that the APF current does not exceed the selected threshold value. In one example, the active filter can include a first sensor that senses the APF current and provides a corresponding signal to the limiter that represents the feedback value for the APF current, and a second sensor that senses the load current and provides a corresponding signal to the limiter that represents the feedback value for the load current. 
   As such, the limiter is configured to control the current generator such that even if the APF current necessary to compensate for the polluting harmonics on the power line exceeds said selected threshold value, the APF current generated by the current generator is limited to at most the selected threshold value. The present invention provides overload protection of the APF, low input current distortion, and stable operation of the APF into and out of overload conditions while maintaining minimum input current distortion within the bounds of the APF design limits. 
   While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of “means” or “steps” limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an example block diagram of an embodiment of an active power filter (APF) according to the present invention, interconnected in a parallel circuit with a load and a power source; 
       FIG. 2  is an example detailed functional block diagram of an embodiment of the active power filter of  FIG. 1 , interconnected in a parallel circuit with a load and a power source; and 
       FIG. 3  is an example functional block diagram of an example implementation of the active power filter of  FIG. 2 , interconnected in a parallel circuit with a load and a power source. 
   

   The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , an example block diagram of an embodiment of an APF apparatus  10  according to the present invention is shown, interconnected in parallel in a circuit  12  that includes a load  14  having am impedance Z L  and a voltage source  16  providing a voltage v S . The APF  10  generates (i.e., sources or sinks) the current i APF  as necessary to compensate for the polluting harmonics on the power line  11  in the circuit  12 . 
   The APF  10  includes a limiter  18  to control and limit the APF current i APF , to a desirable/safe threshold value. To do so, the APF current i APF  and the load current i L  are measured using two sensors  20 ,  22 , respectively, and the sensed values are used by the limiter  18  to control the APF current i APF  directly. 
   The APF  10  operates in the circuit  12  such that:
 
 i   APF   =i   L   −i   S   (1)
 
   The desired (ideal) input source current i S  is proportional to the input voltage v S  in order to achieve unity input power factor, such that:
 
 i   S   =v   S   /R   EM   (2)
 
   wherein R EM  is the emulation resistance, or the equivalent resistance seen by the input voltage source v S . 
   To achieve the above desired performance, the APF current i APF  is controlled so that:
 
 i   APF   =i   L   −v   S   /R   EM   (3)
 
   If, however, the required APF current (to essentially cancel out said harmonics) exceeds a design limit I max , then the APF current i APF  is limited by the limiter  18  to the selected threshold value I max  according to the following relation: 
               i   APF     =     {               i   L     -       v   S     /     R   EM         ;                    i   L     -       v   S     /     R   EM              &lt;     I   max                   I   max     ;                    i   L     -       v   S     /     R   EM              ≥     I   max                       (   4   )             
 
   To accomplish this control strategy, a first (inner) control loop is closed around the APF current i APF .  FIG. 2  shows an example functional block diagram of an embodiment of the APF  10  according to the present invention, which includes the inner control loop, wherein the APF  10  is shown interconnected in parallel in the circuit  12  including the load  14  and the voltage source  16 . 
   In this example, the APF  10  includes an APF current generator  24 , an APF current controller  26  and a reference current generator  28 . The inner control loop is formed via the APF current controller  26  and the APF current generator  24 , wherein the APF current generator  24  sources or sinks the current i APF  as controlled by the APF current controller  26 . 
   The limiter  18  in the APF current controller  26  provides APF current control based on the measured/feedback values of the currents i APF  and i L . As such, the APF current controller  26  never commands more current than the APF current generator  18  is capable of delivering safely. 
   The APF current generator  24  includes an energy storage device (e.g., capacitor, inductor, etc.) that sinks or sources current as needed to compensate for said polluting harmonics. A second (outer) control loop in the APF  10  maintains the energy level (e.g., capacitor voltage, inductor current, etc.) of the energy storage device at a safe value. The outer control loop is formed via the reference current generator  28  with the APF current generator  24  feeding back the energy level of the energy storage device into the reference current generator  28 . 
   In the example described below, wherein the energy storage device comprises a capacitor, the reference current generator  28  determines the value of R EM  based on the energy storage device voltage, and generates the value v S /R EM  for the APF current controller  26 . As such, the outer control loop determines the value of R EM  that provides an energy balance such that the energy storage source voltage does not grow too large (i.e., the outer control loop provides the proper value of R EM  for scaling V S  in relation  4  above). 
     FIG. 3  is a functional block diagram illustrating an example implementation of the APF  10  of  FIG. 2 , according to an embodiment of the present invention. The APF current generator  24 , the APF current controller  26  and the reference current generator  28  are shown connected in a parallel circuit with the load  14  and the voltage source  16 . 
   In this embodiment, the APF current generator  24  includes an H-bridge that can source or sink current through an inductor. In this example, the H-bridge comprises four controllable switches  30  (e.g., IGBT, MOSFET, etc.) connected in an H pattern. The switches  30  can be controlled by the APF current controller  26  to allow sourcing or sinking current from/to said energy storage device, such as a capacitor  32 , through an inductor  34 . 
   The APF current controller  26  includes a first summer  36 , the limiter  18 , a second summer  38 , a gain amplifier  40  and a modulator  42 . A current command signal i cmd  is generated at the output of the summer  36  using the input values i L  from the current sensor  22  and V S /R EM  from the reference current generator  28 , wherein:
 
 i   cmd   =i   L   −v   S   /R   EM   (5)
 
   The current command signal i cmd  is then bounded by the limiter  18 . The limiter output is a bounded current command signal i cmd   * , which serves as the reference current for the inner current loop. In this example, the bounded current command signal i cmd   *  is generated according to the following relation: 
               i   cmd   *     =     {               i   L     -       v   S     /     R   EM         ;                    i   L     -       v   S     /     R   EM              &lt;     I   max                   I   max     ;                    i   L     -       v   S     /     R   EM              ≥     I   max                       (   6   )             
 
   The bounded command current i cmd   *  and the sensed current i APF  are combined in the summer  38  and passed through the gain amplifier  40 , to provide input control to the modulator  42 . The modulator  42  provides four outputs (A, B, C, D) for the four switches  30  in the APF current generator  24 , respectively, wherein each MOSFET switch  30  is given a duty cycle by the action of the modulator  42  for the capacitor  32  to sink or source current while satisfying relations ( 4 )-( 6 ) above. 
   As such, the wide bandwidth inner control loop is used to control the current generator  24  to force the APF current i APF  to track the bounded command current i cmd   * , thereby achieving the desired performance according to relation ( 6 ) above. The APF current i APF  is sensed directly, while the command current, i L −v S /R EM , is bounded by the limiter  18 . 
   Accordingly, a current control method according to the present invention combines two major feedback/control loops. The first feedback loop (said inner control loop) regulates the APF current i APF  using the two current sensors  20 ,  22 . The first feedback loop forces the APF current to track the bounded current command signal to provide near unity power factor (i.e., proportional current), while effectively limiting the APF current to a safe value within the limitations of a particular design. 
   The second feedback loop (said outer control loop) is provided to regulate the high voltage buss. The second feedback loop has a low crossover frequency below that of the input AC line, and includes the reference current generator  28 . In this example, the reference current generator  28  comprises a summer  44 , a gain amplifier  46  and an analog multiplier  48 . 
   The summer  44  determines the difference between the capacitor voltage v C  in the APF current generator  24  and a reference voltage v REF . The output of the gain amplifier  46  represents an error signal (i.e., indicating the value 1/R EM ) which is fed to the analog multiplier  48 . The analog multiplier  48  uses the error signal to scale the input voltage signal V S  and generates the reference signal V S /R EM . 
   As noted, the difference between the load current signal i L  and the reference signal V S /R EM  is the current command signal i cmd , wherein the APF performance is ideal to the extent that the APF current i APF  tracks the current command signal i cmd . 
   As such, the present invention provides inherent overload protection for the APF  10 , low input current distortion, and stable operation of the APF  10  into and out of overload conditions while maintaining minimum input current distortion within the bounds of the APF design limits. Although in the above examples the load current i L  was sensed to provide necessary shaping of the input current waveform, either the input source current i S  or preferably the load current i L  can be used. 
   Further, though in the description and claims herein the APF current generator is described to generate a current i APF , those skilled in the art recognize that generating the current i APF  in this context means sourcing or sinking a current as necessary to compensate for the polluting harmonics on the power line in accordance with relations ( 4 )-( 6 ) above. 
   In another aspect of the present invention, the current generator is controlled to compensate for the polluting harmonics on the power line, such that the current i APF  is bounded by a selected upper threshold and a selected lower threshold. 
   Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. 
   The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
   The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination. 
   Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
   The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.