Abstract:
In one embodiment, a harmonic feedback system is characterized by: a three-phase to two-phase transformer configured to convert an output fundamental frequency current from a distributed power system and to convert the output fundamental frequency current from a three-phase current to a two-phase stationary reference current; a harmonic frequency transformer configured to convert the two-phase stationary reference current to a synchronous reference current, said harmonic frequency transformer having a target synchronous reference frame frequency based upon a reference harmonic current of the output fundamental frequency current; at least one filter device configured to remove predetermined components from the synchronous reference current to output a harmonic feedback current; a summation component to compare the harmonic feedback current with the reference harmonic current and to output a harmonic error current for driving the distributed power system to produce the output current.

Description:
BACKGROUND OF INVENTION 
     1. Technical Field 
     The present invention relates generally to the field of power distribution. More particularly, the present invention relates to a method and system for eliminating certain harmonics in a distributed power system. 
     2. Background Information 
     Distributed power systems are employed in a large number of power generation applications. In particular, the use of small and medium-size distributed power systems in a variety of applications has risen in recent years. 
     Conventional distributed power systems comprise either an alternating current (AC) power source or direct current (DC) power source, or a combined AC and DC power source. Such systems provide power to an end load, e.g., an end user, through an output inverter. End loads comprise either linear loads, or non-linear loads, and/or a combination of both linear and nonlinear loads. The use of nonlinear end loads in such systems has increased over the past several years. For example, the use of adjustable speed drives and uninterruptible power supplies (UPS) has increased a great deal over the past several years. Such nonlinear end loads conventionally comprise a rectifier front end which converts AC power to DC power. 
     The operation of nonlinear end loads often involves the injection of harmonic current into the power system. The presence of such harmonics in the power system reduces the power quality level. Many organizations that set related standards, such as the Institute of Electrical and Electronics Engineers (IEEE) and the International Electrotechnical Commission (IEC), publish standards that restrict harmonics content to a defined level. According to IEEE standard, distributed power systems must be designed to deliver “clean power” to a linear load with very low Total Harmonics Distortion (THD), e.g., less than five percent (5%). In particular, each individual harmonics component should be less than three percent (3%). Conventional distributed power systems are designed to deliver power to a linear end user load with very low THD that is in compliance with the IEEE standard. 
     Referring to FIG. 1, a block diagram of a conventional distributed power system is illustrated. The conventional distributed power system  10  includes an inverter  12  for receiving DC supply current  14  from a power source  16  and producing output current  18  for driving linear or non-linear load  20 . Power source  16  may include AC power generator  22  and/or DC power source  24 . For example, alternating current  26  from AC power generator  22  is rectified into direct current  28  by rectifier  20 , and DC bus  32  delivers the direct current to inverter  12  as supply current  14 . Additionally or alternately, direct current  34  from DC power source  24  is converted by DC/DC converter  36  from one voltage level  34  to another voltage level  38 , and DC bus  32  delivers the direct current to inverter  12  as supply current  14 . Inverter  12  converts the DC supply current into an AC output current  18 . Output current  18  comprises both a fundamental frequency current and a harmonic frequency current. In order to reduce the fundamental frequency error current, controller  40  is operable for receiving the output current  18 , comparing the output current  18  with a reference current (not shown), determining the fundamental frequency error current, and providing an adjustment current thereby adjusting the output current  18  accordingly. 
     Typically, however, when such a conventional distributed power system is connected to a nonlinear end load (e.g., a nonlinear end load at a customer&#39;site), the THD in the power system is unacceptably high, sometimes failing the harmonic standards set forth by standards-providing organizations. The high THD in the power system is, in large part, the result of harmonics components injected into the power system by the nonlinear end load. 
     The harmonics injected into the distributed power system creates a problem for the supplier of the distributed power system. The magnitude of the THD injected by the nonlinear end load of various customers can vary greatly. The unpredictable harmonics injected by a customer&#39;s load makes it difficult for the supplier to provide “clean power” to end loads. Conventional systems do not provide an effective, simple, and practical solution to such a problem. What is needed is an effective, simple, and practical solution for the supplier of distributed power systems to reduce or eliminate harmonic distortion injected by nonlinear loads to distributed power systems. 
     SUMMARY OF THE INVENTION 
     It is a feature of embodiments of the present invention to provide an effective, simple, and practical solution for the supplier of distributed power systems to reduce or eliminate the effect of harmonics injected by nonlinear loads on distributed power systems. It is a further feature of embodiments of the present invention to provide a method and system for improving power quality levels in distributed power systems. 
     It is a further feature of embodiments of the present invention to provide a method and system that reduces or eliminates unwanted harmonics by adding an additional negative feedback control loop in the distributed power system. It is a further feature of embodiments of the present invention to provide a method and system for reducing or eliminating such harmonics that may be used with existing distributed power systems. 
     In one embodiment, a distributed power system comprises an inverter having a reference current, an output current and a summation component. The reference current comprises a reference fundamental frequency current and at least one reference harmonic current. The output current comprises an actual fundamental frequency current and at least one actual harmonic current. The summation component is operable to combine a first signal representative of the output of the fundamental current regulator and a second signal representative of the output of the harmonic current regulator to generate a third signal for driving the inverter to produce the output current. This DPS further comprises a control system in communication with the inverter, the control system operable for receiving a feedback signal representative of the output current and generating the harmonic error current representative of a difference between the reference harmonic current and the actual harmonic current. 
     In another embodiment, a distributed power system comprises an inverter having a reference current, an output current and a summation component. The reference current comprises a reference fundamental frequency current and at least one reference harmonic current. The output current comprises an actual fundamental frequency current and at least one actual harmonic current. The summation component is operable to combine a first signal, representative of the output of the fundamental current regulator, and a second signal, representative of the output of a harmonic current regulator, to generate a third signal for driving the inverter to produce the output current. The DPS further comprises a control system in communication with the inverter. The control system has a fundamental frequency current controller and a harmonics current controller. The fundamental frequency current controller may be operable for receiving a feedback signal representative of the output current and generating the fundamental frequency error current. The harmonics current controller may be operable for receiving the feedback signal and generating the harmonic error current. The fundamental frequency error current is representative of a difference between the reference fundamental frequency current and the actual fundamental frequency current. The harmonic error current is representative of a difference between the reference harmonic current and the actual harmonic current. The DPS further comprises a filtering component within the harmonics controller for removing predetermined harmonics components from the feedback signal and a transformer system for orienting the feedback signal into the same reference frame as the reference current. 
     In yet another embodiment, a method is provided for eliminating or substantially reducing certain harmonics in a distributed power system. The method comprises the steps of receiving an output current from a inverter; filtering predetermined harmonics components from the output current to obtain harmonics feedback current; comparing the harmonics feedback current with a reference harmonics current to obtain a harmonics error current; and adding the output of the harmonics current controller to the fundamental current controller output. 
     In a further embodiment, a method of eliminating or substantially reducing certain harmonics in a distributed power system is provided. The method comprises receiving harmonic current from an output of a power inverter, wherein the power inverter is connected to a nonlinear end load; transforming the output current from stationary frame to synchronous frame at predetermined harmonic frequency; generating a harmonic current regulating signal by summing the negative of the output harmonic current and an associated reference harmonic component; transforming the harmonic current regulating signal from synchronous reference frame back to stationary reference frame of the power inverter; and summing the harmonic current regulating signal and the fundamental current regulating signal in the power inverter. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a block diagram of a conventional distributed power system; 
     FIG. 2 is a block diagram of a distributed power system having an inverter, a fundamental frequency controller and a harmonic frequency controller in accordance with an embodiment of the present invention; 
     FIG. 3 is a block diagram of a fundamental frequency controller and a harmonic frequency controller in accordance with an embodiment of the present invention; and 
     FIG. 4 is a flowchart of a method for controlling certain harmonics in a power distribution system in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale, e.g., some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Referring to FIG. 2, a block diagram of a distributed power system having an inverter, a fundamental frequency controller and a harmonic frequency controller in accordance with an embodiment of the present invention is illustrated. As illustrated, fundamental frequency controller  42  and harmonic frequency controller  44  replace controller  40  of FIG.  1 . Fundamental frequency controller  42  includes a feedback loop for controlling errors between fundamental reference current  43  and output current  18  at the fundamental frequency. Harmonic frequency controller  44  includes a feedback loop for controlling errors between harmonic reference current  45  and output current  18  at one or more harmonic frequencies. 
     Referring to FIGS. 2 and 3, block diagrams of the fundamental frequency controller and the harmonic frequency controller in accordance with an embodiment of the present invention is illustrated. Preferably, the output voltage of inverter  12  in FIGS. 1 and 2 is controlled to be constant. As such, the output power is a function of a three-phase (i a , i b , and i c ) output current  18 . In order to control output current  18  so that it follows the specific two-phase (i d * and i q *) fundamental reference current  43 , the output current  18  is measured to determine a two-phase (i a  and i b ) output current  46 , which includes a fundamental frequency current component and at least one harmonic current component. Within fundamental frequency controller  42 , two-phase output current  46  is first transformed into stationary reference frame current (i α  and i β )  48  by three-phase to two-phase transformer  50 . Then, stationary reference frame current (i α  and i β )  48  is transformed into synchronous reference frame current (i d  and i q )  52  or fundamental frequency feedback current  52 , corresponding to the fundamental frequency component of reference current  14 , by frame rotation transformer  54 . Fundamental frequency feedback current (i d  and i q )  52  represents the feedback of the actual fundamental frequency output current that is compared by summation component  56  with reference fundamental frequency current (i d * and i q *)  58 . Summation component  56  determines fundamental frequency error current  60  that is fed into current regulator  62 . Current regulator  62  regulates the error signals such that the feedback current equals the reference fundamental frequency current (i d * and i q *)  58 . The outputs  64  of the current regulator  62 , which are voltage signals, are first transformed back to stationary reference frame voltage signals  68  by frame rotation transformer  66 , and then transformed by two-phase to three-phase transformer  70  to abc frame voltage signals (S abc )  72 . The output voltage signals  72  control output current  18  by controlling the switching actions in inverter  74  to reduce the errors between reference fundamental frequency current (i d * and i q *)  58  and output current  18 . 
     Referring to FIGS. 3 and 4, the method for controlling certain harmonics in a power distribution system in accordance with an embodiment of the present invention is illustrated. To control the harmonic error in the system, harmonic frequency controller  44  receives output current  18  and, in particular, two-phase output current (i a  and i b )  46 , from inverter  74 . Three-phase to two-phase transformer  50  transforms the two-phase output current (i a  and i b )  46  to stationary reference current (i α  and i β )  48  in a stationary reference frame at steps  110  and  112 , respectively. Harmonic frequency transformer  76  transforms stationary reference current (i α  and i β )  48  to synchronous reference current (i kd  and i kq )  78  in a synchronous reference frame corresponding to a predetermined kth harmonic of reference fundamental frequency current (i d * and i q *)  58  at step  114 . Filtering device  80 , such as a low pass filter (LPF), filters predetermined frequency components, such as alternating current (AC) components, from synchronous reference current (i kd  and i kq )  78  to obtain harmonic feedback current (i kd  and i kq )  82  at step  116 . For example, predetermined frequency components include all AC components in the output  78  of the synchronous frame transformer  76 . Summation component  84  compares harmonic feedback current (i kd  and i kq )  82  with reference harmonic current (i kd*  and i kq* )  86  to obtain harmonic error current  88  at step  118 . Regulator  90 , such as a proportional and integral regulator, operates on harmonic error current  88  to provide regulating signals  92  at step  120 . Then, harmonic frequency transformer  94  transforms regulating signals  92  to stationary reference voltage  96  in stationary reference frame at step  122 . Summation component  98  adds the outputs of both fundamental current regulators  68  and harmonic current regulators  96  in stationary frame to form a combined regulating voltage signal  100  at step  124 . Transformer  70  converts the combined regulating voltage signal  100  from two-phase voltage signals to three-phase voltage signals, thereby generating abc frame voltage signals  72  that are used to control the inverter  74  to form output current  18  at step  124 . Thus, fundamental frequency controller  42  in combination with harmonic frequency controller  44  insure that the fundamental frequency component of the output current  18  follows the fundamental reference current  43  and one or more harmonic components of the output current  18  follow one or more harmonic reference current  45 . 
     Following the IEEE standard, a total harmonic distortion (THD) of less than five percent (5%) and each harmonic component of less than three percent (3%) are required. Preferably, lower order harmonics, such as 5 th  and 7 th  harmonics, should be maintained as low as possible. The maximum allowable level of a particular harmonic component to be controlled is usually given in DC value i kd * and i kq *. 
     The signals measured at the output currents are in AC value and contain all harmonic components that the inverter may generate. The following further describes the AC-DC transformer, filtering, and the harmonic current regulation mentioned above with respect to harmonic frequency controller  44  FIG.  2 . 
     Frame Transformation 
     The three phase feedback or actual output currents measured at the inverter output are in AC value and contains harmonic components. The transformation from abc frame to stationary frame α-β is realized at transformer  50 . Assuming there is a third frame d-q that is rotating at the k th  harmonics frequency, the transformation from the stationary frame α-β to the rotating frame d-q results in the k th  harmonics component being in DC value while all other components are still in AC value. This transformation from two axis stationary frame α-β to two axis rotating frame d-q (often called Synchronous Reference Frame) is realized at harmonics transformer  76 . 
     Low Pass Filter 
     After the frame rotation transformation by harmonics transformer  76 , only the k th  harmonic component is transformed into DC value. The remaining components, in particular, the fundamental component (50/60 Hz) counting for more than 95% of the total feedback current, are still in AC values. To remove the AC components, filtering devices  80 , such as two Low Pass Filters (LPFs), are used. In order to extract the DC components without any phase delay, a sixth-order Butterworth low pass filter is used. In alternate embodiments, other filters are utilized, depending on the predetermined harmonics to be filtered. The way of choosing poles for the Butterworth filter is to spread them out around the unit circle for normalized frequency in such a way as to peak up the dynamic response and give a maximally flat result. In one embodiment, the cutoff frequency of the filter may be about 10 Hz. In alternate embodiments, other cutoff frequencies are utilized depending on the predetermined harmonics to be filtered. 
     Harmonic Current Regulators and Feedback Control 
     The goal with harmonic current regulators and feedback control is to eliminate the k th  harmonic component in the output current. Two proportional and integral (PI) current regulators  90  may be used with harmonic reference currents of i kd * and i kq *. The PI current regulators are designed such that the k th  harmonics (i kd  and i kq ) of the output current  18  at the inverter output is always kept at or below the reference harmonic current (i kd * and i kq *) value down to zero. Harmonic transformer  94  rotates the regulating signal  92  of current regulators  90  back to stationary frame with k times of the fundamental frequency. The transformed signals  96  in the stationary frame are added to the output of the fundamental frequency current regulator  68  to form combined regulating signals. 
     In other embodiments, the harmonic frequency controller  44  can include more than one harmonic current control loop. For example, most nonlinear loads have a rectifier front end. The operation of the rectifier involves the injection of harmonic current into the power system. The dominant portion of the injected harmonics is usually at low frequencies, such as the 5 th  and/or the 7 th  harmonics. If the current feedback signals show more than one dominant harmonic component, such as 5 th  and 7 th  harmonics, and the need to eliminate both harmonics are justified, a second harmonic canceller loop may be added to the system. For example, in the above discussion let k=5 for simplicity. Assuming the second harmonic current control loop, for example, the 7 th  harmonic control loop associated with k=7, is similar to the 5 th  harmonic control loop with the same current feedback as described above, the outputs of the 7 th  harmonic current regulator are also added to the output of the fundamental frequency current regulator  68  at the same points as 5 th  harmonic current regulators” output. The only difference then between 5 th  and 7 th  harmonic controllers is the rotation from stationary reference frame to synchronous reference frame at different frequencies, in this case, at 7 times of the fundamental frequency. Hence, more than two harmonic current control loops can be used in this type of control structure to eliminate extra harmonic components. 
     Thus, the present invention provides systems and methods for eliminating certain or dominant harmonics in a distributed power system. The systems and methods include receiving harmonic current from an output of a power inverter, where the power inverter may be connected to a nonlinear end load. The harmonic current from the inverter output is transformed to the synchronous reference frame in a DC form. The reference harmonic current, also in DC form, is compared with the feedback harmonic current. The difference is used as the input of the harmonic current regulator. The regulating signal at the output of the harmonic current regulator is transformed to the stationary reference frame and is added to the output of the fundamental current regulator in stationary reference frame to control the power inverter to produce the output current. Hence, the harmonic current controller takes the feedback signals from the output of an inverter, compares the feedback signals with a reference signal and generates a regulating signal to eliminate or substantially reduce the corresponding harmonics injected by the user load. 
     Various embodiments of the invention have been described above. It should be recognized, however, that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be apparent to those skilled in the art without departing from the spirit and scope of the present invention.