Patent Publication Number: US-9431924-B2

Title: Power source inverter for use with a photovoltaic solar panel

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/819,792, filed on May 6, 2013. The entire disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to power source inverter which may be used with a photovoltaic solar panel. 
     BACKGROUND 
     The ever increasing demand for clean energy that is alternative to the conventional fossil-fuel based power generation has been one of the primary driving forces for the recent development of photovoltaic (PV) technologies, especially in the past decade. The majority of new installations of PV generation systems is grid interactive and enjoyed a 70% increase of capacity worldwide in 2008. The power electronic converter is an indispensable subsystem that interfaces the PV modules with electric power systems. 
     As indicated by a rich body of literature records, various new topologies of PV inverters have been proposed and studied. Nonetheless, previous studies suggest that the PV inverter still remains the weak link in PV power generation systems with regard to its reliability. The lifetime of PV inverters is approximately 5-10 years as opposed to the 30-year lifetime of PV panels. The relatively short lifetime of inverters not only increases the effective operating cost, but also poses a significant stress on the environment in light of life cycle analysis, which contradicts one of the primary motivations of developing renewable energy generation, namely to relieve the stress to the environment imposed by the energy production processes. 
     The key issues related to PV inverters that negatively affect the reliability performance include the electrolytic capacitor, which commonly exists in PV inverters using the dominant topology of voltage source inverters (VSIs). The less frequently researched current source inverters (CSIs) could potentially eliminate the electrolytic capacitor while providing additional benefits. The adoption of CSI in PV systems, however, is significantly limited by the availability of suitable power semiconductor devices. Three-phase CSI have also been proposed for application in PV generation systems. Thus far, the single-phase topology is predominant for residential systems with capacity below 10 kW peak power. 
     For single phase dc/ac inverter systems, with no regard to either VSI or CSI topology, the pulsating nature of the instantaneous power flow is in common. The pulsating power flow will manifest itself in the form of either dc-link current ripple or voltage ripple in a single-phase CSI or VSI, respectively. The low-frequency ripple on the dc-link will potentially have adverse consequences. Therefore, it is desirable to develop an improved topology for a power inverter that minimizes the low frequency ripple. 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     An improved topology is presented for a single phase power source inverter that is designed to minimize double-frequency ripple. The inverter circuit includes: a capacitor coupled in parallel across a direct current (DC) voltage source; a bridge circuit having three legs electrically connected in parallel with the voltage source and each other, such that each leg of the bridge circuit is comprised of two switches coupled in series with each other; a low-pass filter electrically connected between the bridge circuit and power grid; an auxiliary inductor having a first terminal electrically connected to one leg of the bridge circuit and a second terminal electrically connected to the power grid and the other leg of the bridge circuit; and a controller that drives the switches of the bridge circuit in a manner that maintains instantaneous power transfer across the bridge circuit constant. 
     In one aspect of this disclosure, the capacitor is a non-electrolytic capacitor. 
     In another aspect of this disclosure, the DC voltage source is implemented by a photovoltaic solar panel. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
         FIG. 1  is a diagram illustrating the low-frequency effect on maximum power point tracking in a single phase power inverter; 
         FIG. 2  is a schematic of a proposed topology for a power inverter; 
         FIG. 3  is a graph depicting the maximum power point for a example PV panel application; 
         FIGS. 4A and 4B  are graphs of simulated waveforms of output currents i a  and i b , respectively, from the proposed power source inverter; and 
         FIG. 5  is a graph of a simulated dc voltage ripple of the proposed inverter in comparison of a conventional inverter arrangement. 
     
    
    
     The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
     In a single-phase system, if the grid voltage v and the current i are given by
 
 v ( t )= V  cos(ω t )
 
 i ( t )= I  cos(ω t +φ)  (1)
 
where V and I are the magnitudes of the grid voltage and the current supplied by the inverter, respectively; ω is the angular frequency of the ac system; φ is the power factor angle. Then the instantaneous power flow p(t) is given by
 
                           p   ⁡     (   t   )       =       ⁢       v   ⁡     (   t   )       ⁢     ⅈ   ⁡     (   t   )                     =       ⁢         1   2     ⁢   VI   ⁢           ⁢     cos   ⁡     (   ϕ   )         +       1   2     ⁢   VI   ⁢           ⁢     cos   ⁡     (       2   ⁢   ω   ⁢           ⁢   t     +   ϕ     )                         (   2   )               
The double-frequency term in (2) will cause dc-link current ripple or voltage ripple in a single-phase CSI or VSI, respectively. The oscillating current or voltage will degrade the performance of maximum power point tracking (MPPT) in PV power generation system.  FIG. 1  illustrates a typical i−v characteristic of a PV panel on the i pv −v pv  plane. A power curve is overlaid on top of the i−v curve in the p pv −v pv  plane. As the power output from the PV panel reaches the maximum P max  on the power curve, the maximum power point on the i−v curve corresponds to the point P labeled in  FIG. 1 . The terminal voltage and current V max  and I max  at which the PV panel delivers the maximum power are not labeled on the i pv−  and v pv− axis for avoidance of cluttering the figure. It is evident that the output power will be typically below P max  if the terminal voltage varies around V max  except for discrete operating points.
 
       FIG. 2  depicts a proposed topology for an inverter circuit  20  electrically coupled to a voltage source  21 . The inverter circuit  20  is comprised generally of a capacitor  22 , a bridge circuit  23 , a low pass filter circuit  24  and an auxiliary inductor  25  connected to one leg of the bridge circuit  23 . A controller  29  can be used to drive each of the switches in the bridge circuit in a manner further described below. In an example embodiment, the voltage source is further defined as a photovoltaic solar panel although other types of voltage sources fall within the scope of this disclosure. Likewise the inverter circuit  20  may have application outside the field of solar panels. 
     In one embodiment, the bridge circuit  23  has a conventional arrangement of three legs  23 A,  23 B,  23 C connected in parallel with the voltage source  21  and each other. Each leg includes two switches  26  coupled in series with each other. The switches  26  may be implemented by insulated-gate bipolar transistors (IGBTs), metal-oxide-semiconductor field-effect transistors (MOSFETs) or another type of switching device. Bridge circuits having other topologies are also contemplated by this disclosure. 
     The low-pass filter  24  is connected between the bridge circuit  23  and the load  27 . In one embodiment, the low pass filter  24  is implemented by an LC circuit. In this case, a primary inductor has one terminal coupled between the two switches of the first leg of the bridge circuit while the other terminal is coupled to the load. The capacitor is coupled in parallel across the load. Other implementations for a low pass circuit are contemplated by this disclosure. In some embodiments, the load  27  can be a power grid for a home although other types of loads also falls within the scope of this disclosure. 
     The auxiliary inductor  25  has one terminal coupled between the two switches which comprise the second leg of the bridge circuit while the other terminal of the auxiliary inductor  25  is coupled to the load  27 . The auxiliary inductor  25  facilitates the use of a capacitor  22  having a capacitance one hundred times less than conventional inverter circuits. In some embodiments, the capacitor  22  may be a non-electrolytic capacitor. Such capacitors are more reliable and have a useful lifetime that more closely correlates with a photovoltaic solar panel, thereby reducing the maintenance and replacement cost for this topology. Moreover, the auxiliary inductor  25  operates to cancel out the double-frequency ripple in the DC output and thereby achieves greater efficiency for the photovoltaic solar panel. 
     To draw the maximum power from a PV panel, a controller must be used to vary the load from the grid such that the current and voltage output of the PV panel are at the maximum power point. The maximum power location is at the bend of the current v. voltage curve as shown in  FIG. 3 . Modern PV panels use maximum power point tracking (MMPT) to adjust the load of the inverter and draw the maximum power from the PV as the irradiance changes throughout the day. Because of AC power flowing back through the inverter to the DC link, a ripple effect demonstrates itself in a double-frequency sinusoidal variation of the DC output voltage. The MPPT cannot match the resulting DC output variation, creating a mismatch between the load and maximum PV output, thereby decreasing efficiency. 
     The key to the operation of the proposed topology lies in the control of the converter bridge such that the instantaneous power transfer across the switching bridge is maintained constant. For the determination of the appropriate phase-b voltage v b , the concept of space vector is utilized. 
     The voltage space vector at the ac terminals of the converter is defined as 
                     v   _     =       2   3     ⁢     (       v   a     +     α   ⁢           ⁢     v   b       +       α   2     ⁢     v   c         )               (   3   )               
where
 
             α   =       ⅇ     j   ⁢       2   ⁢   π     3         .           
The three-phase output voltages of the converter are
 
 v   a   =V   a  cos(ω t )
 
 v   b   =V   b  cos(ω t+φ   vb )
 
 v   c =0  (4)
 
with the voltage magnitude V b  of phase-b and its phase angle φ vb  to be determined. Substituting (4) into (3) yields
 
     
       
         
           
             
               
                 
                   
                     v 
                     _ 
                   
                   = 
                   
                     
                       
                         1 
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                       ⁢ 
                       
                         ( 
                         
                           
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                             ⁢ 
                             
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                             ⁢ 
                             
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                   ( 
                   5 
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     In a similar manner, the current space vector at the ac terminals of the converter is 
                     i   _     =       2   3     ⁢     (       i   a     +     α   ⁢           ⁢     i   b       +       α   2     ⁢     i   c         )               (   6   )               
The three-phase currents are
 
 i   a   =I   a  cos(ω t +φ)
 
 i   b   =I   b  cos(ω t+φ   ib )
 
 i   c   =i   a   −i   b   (7)
 
where φ the power factor angle, which is zero for unity power factor operation. The magnitude I b  and the phase angle φ ib  of phase-b current are to be determined. Substitution of (7) into (6) leads to
 
     
       
         
           
             
               
                 
                   
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                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     The instantaneous power flow across the converter can be expressed in terms of the defined voltage and current space vectors  v  and  i . 
                     p   ⁡     (   t   )       =       3   2     ⁢     Re   ⁡     (       v   _     ⁢       i   _     *       )                 (   9   )               
where Re(·) denotes the real component of a complex quantity and ‘*’ denotes the conjugate of a complex variable. Substituting (8) and (5) into (9) yields
 
                           p   ⁡     (   t   )       =       ⁢         1   2     ⁢     V   a     ⁢     I   a     ⁢     cos   ⁡     (   ϕ   )         +                     ⁢       1   2     ⁡     [         V   a     ⁢     I   a     ⁢     cos   ⁡     (       2   ⁢   ω   ⁢           ⁢   t     +   ϕ     )         +       V   b     ⁢     I   b     ⁢     cos   ⁡     (       2   ⁢   ω   ⁢           ⁢   t     +     ϕ   vb     +     ϕ   ib       )           ]                     (   10   )               
In order for p(t) to be constant, it requires that the time varying term in (10) to be constant (i.e. zero). Based on the relation between the voltage across and the current through the inductor, it follows that
 
                       V   b     =     ω   ⁢           ⁢     L   b     ⁢     I   b         ⁢     
     ⁢       ϕ   vb     =       ϕ   ib     +     π   2                 (   11   )               
The following equations describe the critical condition for maintaining the instantaneous power flow across the converter bridge to be constant.
 
     
       
         
           
             
               
                 
                   
                     
                       V 
                       b 
                     
                     = 
                     
                       
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           L 
                           b 
                         
                         ⁢ 
                         
                           V 
                           a 
                         
                         ⁢ 
                         
                           I 
                           a 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       ϕ 
                       vb 
                     
                     = 
                     
                       
                         1 
                         2 
                       
                       ⁢ 
                       
                         ( 
                         
                           ϕ 
                           - 
                           
                             π 
                             2 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     Once the condition that satisfies the constant instantaneous power flow is known, the reference voltages are known for each phase leg. Assume v* a  is the reference for the output voltage that is applied to the load and v* b  is the reference for the voltage that is applied to the auxiliary inductor L b . The modulation functions for the upper switches in each phase leg are given by 
                       m   ap     =       1   +       v   a   *     /     V   dc         2       ⁢     
     ⁢       m   bp     =         v   b   *       V   dc       +       1   -       v   a   *     /     V   dc         2         ⁢     
     ⁢       m   cp     =       1   -       v   a   *     /     V   dc         2               (   13   )               
It is understood that lower switches in each phase leg are driven in a complementary manner. In this way, the controller  29  can drive the switches of the bridge circuit  23  in a manner that maintains instantaneous power transfer across the bridge circuit constant.
 
     In one embodiment, the functionality of the controller  29  can be implemented by software instructions embedded in a microprocessor. It is understood that only the pertinent steps of the control algorithm are discussed above but that other software instructions may be needed to control operation of the inverter. It is also understood that the functionality of the controller  29  can be implemented by an application specific integrated circuit (ASIC) or other suitable electric circuits. 
     Based on the modulation strategy, a detailed Saber model has been built and simulations have been conducted. The simulated waveforms of the output currents are shown in  FIGS. 4A and 4B . With the DC link capacitance sized 100 times smaller than the one used in conventional H-bridge inverter, the comparative dc link voltage ripple is shown in  FIG. 5 . It is clear that the dc link voltage ripple is significantly reduced when compared to the H-bridge. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.