Abstract:
An apparatus for power conversion. In one embodiment, the apparatus comprises a plurality of AC power sources, wherein each AC power source in the plurality of AC power sources has a phase rotation circuit coupled to a DC/AC inverter for converting DC input power to AC output power, wherein (i) a first phase rotation circuit has first three phase output terminals coupled to second three phase input terminals of a second phase rotation circuit; (ii) the second phase rotation circuit has second three phase output terminals coupled to third three phase input terminals of a third phase rotation circuit; and (iii) the third phase rotation circuit has third three phase output terminals coupled to either fourth three phase input terminals of a fourth phase rotation circuit or to a commercial power grid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention is a continuation of co-pending U.S. patent application Ser. No. 12/075,342, filed Mar. 11, 2008, which is herein incorporated in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the present disclosure generally relate to an apparatus for providing phase rotation for a three-phase AC circuit. 
         [0004]    2. Description of the Related Art 
         [0005]    Solar panels have historically been deployed in mostly remote applications, such as remote cabins in the wilderness or satellites, where commercial power was not available. Due to the high cost of installation, solar panels were not an economical choice for generating power unless no other power options were available. However, the worldwide growth of energy demand is leading to a durable increase in energy cost. In addition, it is now well established that the fossil energy reserves currently being used to generate electricity are rapidly being depleted. These growing impediments to conventional commercial power generation make solar panels a more attractive option to pursue. 
         [0006]    Solar panels, or photovoltaic (PV) modules, convert energy from sunlight received into direct current (DC). The PV modules cannot store the electrical energy they produce, so the energy must either be dispersed to an energy storage system, such as a battery or pumped hydroelectricity storage, or dispersed by a load. One option to use the energy produced is to employ inverters to convert the DC current into an alternating current (AC) and couple the AC current to the commercial power grid. In this type of system, the power produced by the solar panels can be sold to the commercial power company. 
         [0007]    Traditionally, solar systems have used centralized inverters, where many PV modules feed into a single large inverter for the conversion of DC current to AC current in applications such as the one described above. A recent trend has been to decentralize this DC/AC conversion by using micro-inverters. Rather than employing a single large inverter, a micro-inverter is individually coupled to each PV module. Micro-inverters improve the performance of the DC/AC power conversion by optimally extracting the maximum power from each PV module. Micro-inverters also offer the added benefit of using a connective wire bus that carries entirely AC voltage rather than the high voltage DC used in traditional centralized inverter systems, thereby offering improved safety and efficiency. 
         [0008]    Micro-inverters are typically arranged in a string on a branch circuit from a load center. Additionally, there may be multiple branch circuits from the load center, where each branch circuit supports a string of micro-inverters and their associated PV modules. In large scale installations, it is common to use three-phase grid connections from the load center. It is not always economical though to have a true three-phase micro-inverter as it requires a substantially more electronics than a single-phase micro-inverter. Traditional methods of connecting single-phase micro-inverters in a three-phase grid connection requires three strings of micro-inverters, where each string is connected to two of the three power phases. In order to properly balance the load on each phase, an electrician needs to install the same number of micro-inverters on each branch circuit and needs to use equally all phases for all of the branch circuits. This leads to a need for extensive installation planning and longer and more cumbersome installations. 
         [0009]    Therefore, there is a need in the art for an apparatus that can employ single-phase micro-inverters in three-phase grid connections in such a way that micro-inverter installation and load balancing on the three phases are greatly simplified. 
       SUMMARY OF THE INVENTION 
       [0010]    Embodiments of the present invention generally relate to an apparatus for power conversion. In one embodiment, the apparatus comprises a plurality of AC power sources, wherein each AC power source in the plurality of AC power sources has a phase rotation circuit coupled to a DC/AC inverter for converting DC input power to AC output power, wherein (i) a first phase rotation circuit has first three phase output terminals coupled to second three phase input terminals of a second phase rotation circuit; (ii) the second phase rotation circuit has second three phase output terminals coupled to third three phase input terminals of a third phase rotation circuit; and (iii) the third phase rotation circuit has third three phase output terminals coupled to either fourth three phase input terminals of a fourth phase rotation circuit or to a commercial power grid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0012]      FIG. 1  is a block diagram of an exemplary system for power generation in accordance with one embodiment of the present invention; 
           [0013]      FIG. 2  is a block diagram of an exemplary string of micro-inverters coupled in series on a three-phase branch circuit in accordance with one embodiment of the present invention; and 
           [0014]      FIG. 3  is a block diagram of an exemplary string of micro-inverters coupled in series on a three-phase branch circuit in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  is a block diagram of an exemplary system  100  for power generation in accordance with one embodiment of the present invention. This diagram only portrays one variation of the myriad of possible system configurations. The present invention can function in a variety of power generation environments and systems. 
         [0016]    The power generation system  100  comprises a plurality of branch circuits  102   1 ,  102   2  . . .  102   m , from a load center  108 . The load center  108  houses connections between incoming power lines from a commercial power grid distribution system and the plurality of branch circuits  102   1 ,  102   2  . . .  102   m , collectively referred to as branch circuits  102 . A branch circuit  102   m  comprises a plurality of micro-inverters  106   1,m ,  106   2,m  . . .  106   n,m , collectively referred to as micro-inverters  106 , coupled in series. Each micro-inverter  106   1,m ,  106   2,m  . . .  106   n,m  is coupled to a PV module  104   1,m ,  104   2,m  . . .  104   n,m , collectively referred to as PV modules  104 . 
         [0017]    The micro-inverters  106  convert DC power generated by the PV modules  104  into AC power. The micro-inverters  106  meter out current that is in-phase with the AC commercial power grid voltage and generate such current with low distortion. The system  100  couples the generated AC power to the commercial power grid via the load center  108 . 
         [0018]      FIG. 2  is a block diagram of an exemplary string of micro-inverters  106  coupled in series on a three-phase branch circuit  102   1  in accordance with one embodiment of the present invention. A load center  230  comprises four lines L 1 , L 2 , L 3 , and N from, for example, a 277/480V commercial power grid supplying a commercial three-phase AC current (herein known as “commercial AC current”). The line L 1  carries a first phase of the commercial AC current (herein known as “first phase of current”), the line L 2  carries a second phase of the commercial AC current (herein known as “second phase of current”), and the line L 3  carries a third phase of the commercial AC current (herein known as “third phase of current”). The line N is a neutral line that carries a resulting current from the sum of the first, the second, and the third phases of current on the lines L 1 , L 2 , and L 3 . Ideally, the first, the second, and the third phases of current on the lines L 1 , L 2 , and L 3  are equally balanced such that the magnitude of each is the same and the phases are offset from one another by 120 degrees. When the first, the second, and the third phases of current on the lines L 1 , L 2 , and L 3  are equally balanced in this manner, the resulting current on the line N is zero. 
         [0019]    A three-phase circuit breaker  232  is coupled to the load center  230  to create a 4-line branch circuit  102   1 . The branch circuit  102   1  comprises the lines L 1 , L 2 , L 3 , and N, a micro-inverter  106   1 , a micro-inverter  106   2 , and a micro-inverter  106   3 , where the micro-inverters  106   1 ,  106   2 , and  106   3  are coupled in a series configuration to the lines L 1 , L 2 , L 3 , and N. 
         [0020]    The micro-inverter  106   1  comprises a phase rotation circuit  202   1 , a single-phase DC/AC inverter  204   1 , input terminals  206   1 ,  208   1 ,  210   1 , a neutral input terminal  218   1 , output terminals  212   1 ,  214   1 ,  216   1 , and a neutral output terminal  220   1 . The micro-inverter  106   2  and the micro-inverter  106   3  are identical to the micro-inverter  106   1 . Coupling the micro-inverters  106   1 ,  106   2 , and  106   3  in the series configuration is as simple as coupling the output terminals  212 ,  214 ,  216 , and the neutral output terminal  220  of one micro-inverter  106  to the input terminals  206 ,  208 ,  210 , and the neutral input terminal  218  respectively of a next micro-inverter  106  in the series. At the load center  230 , the lines L 1 , L 2 , and L 3  are coupled via the three-phase circuit breaker  232  to the output terminals  212   3 ,  214   3 , and  216   3  respectively of the micro-inverter  106   3 ; the line N is coupled to the neutral output terminal  220   3  of the micro-inverter  106   3 . At the micro-inverter  106   1 , the input terminals  206   1 ,  208   1 ,  210   1 , and the neutral input terminal  218   1  remain uncoupled. Additionally, the micro-inverters  106   1 ,  106   2 , and  106   3  are each coupled to a PV module  104   1 ,  104   2 , and  104   3 , respectively. 
         [0021]    At the micro-inverter  106   1 , the output terminals  212   1 ,  214   1 ,  216   1 , and the neutral output terminal  220   1  are coupled to the lines L 2 , L 3 , L 1 , and N respectively via the micro-inverters  106   2  and  106   3 . The DC/AC inverter  204   1  injects a single phase of AC current through the output terminal  212 , onto the line L 2 . The DC/AC inverter  204   1  matches the phase of the injected AC current to the second phase of current that is present on the line L 2 . 
         [0022]    Downstream of the output of the micro-inverter  106   1 , the lines L 2 , L 3 , L 1 , and N are coupled to the input terminals  206   2 ,  208   2 ,  210   2 , and the neutral input terminal  218   2  respectively of the micro-inverter  106   2 . The phase rotation circuit  202   2  couples the input terminals  206   2 ,  208   2 ,  210   2 , and the neutral input terminal  218   2  to the output terminals  216   2 ,  212   2 ,  214   2 , and the neutral output terminal  220   2  respectively; the lines L 3 , L 1 , L 2 , and N are therefore coupled to the output terminals  212   2 ,  214   2 ,  216   2 , and the neutral output terminal  220   2  respectively. The DC/AC inverter  204   2  injects a single phase of AC current through the output terminal  212   2  onto the line L 3 . The DC/AC inverter  204   2  matches the phase of the injected AC current to the third phase of current that is present on the line L 3 . 
         [0023]    Downstream of the output of the micro-inverter  106   2 , the lines L 3 , L 1 , L 2 , and N are coupled to the input terminals  206   3 ,  208   3 ,  210   3 , and the neutral input terminal  218   3 , respectively, of the micro-inverter  106   3 . The phase rotation circuit  202   3  couples the input terminals  206   3 ,  208   3 ,  210   3 , and the neutral input terminal  218   3  to the output terminals  216   3 ,  212   3 ,  214   3 , and the neutral output terminal  220   3  respectively; the lines L 1 , L 2 , L 3 , and N are therefore coupled to the output terminals  212   3 ,  214   3 ,  216   3 , and the neutral output terminal  220   3  respectively. The DC/AC inverter  204   3  injects a single phase of AC current through the output terminal  212   3  onto the line L 1 . The DC/AC inverter  204   3  matches the phase of the injected AC current to the first phase of current that is present on the line L 1 . 
         [0024]    As described above, each of the phase rotation circuits  202  rotates the first, the second, and the third phases of current between the micro-inverters  106  such that a different phase of AC current, phase-matched to one of the three phases of the commercial AC current, is generated by each of the micro-inverters  106 . Assuming that the PV modules  104  are receiving equivalent levels of solar energy and that the subsequent AC currents produced by the DC/AC inverters  204  are equivalent in magnitude, the branch circuit  102   1  generates an equally balanced three-phase AC current that is phase-matched to the commercial AC current. Therefore, the commercial AC current remains equally balanced when the generated three-phase AC current is injected onto the commercial power grid. In addition, a branch circuit  102  comprising a string of micro-inverters  106  coupled in series, where the total number of micro-inverters  106  in the string is a multiple of three, produces the same result in that the three-phase AC current generated by the branch circuit  102  is equally balanced. This automatic balancing of the three-phase AC current generated by the branch circuit  102  improves the efficiency of the system  100  and greatly simplifies installations. 
         [0025]      FIG. 3  is a block diagram of an exemplary string of micro-inverters  106  coupled in series on a three-phase branch circuit  102   2  in accordance with another embodiment of the present invention. A load center  302  comprises four lines L 1 , L 2 , L 3 , and N from, for example, a 120/208V commercial power grid supplying a commercial three-phase AC current (herein known as “commercial AC current”). The line L 1  carries a first phase of the commercial AC current (herein known as “first phase of current”), the line L 2  carries a second phase of the commercial AC current (herein known as “second phase of current”), and the line L 3  carries a third phase of the commercial AC current (herein known as “third phase of current”). The line N is a neutral line that carries a resulting current from the sum of the first, the second, and the third phases of current on the lines L 1 , L 2 , and L 3 . Ideally, the first, the second, and the third phases of current on the lines L 1 , L 2 , and L 3  are equally balanced such that the magnitude of each is the same and the phases are offset from one another by 120 degrees. When the first, the second, and the third phases of current on the lines L 1 , L 2 , and L 3  are equally balanced in this manner, the resulting current on the line N is zero. 
         [0026]    A three-phase circuit breaker  232  is coupled to the load center  302  to create a 4-line branch circuit  102   2 . The branch circuit  102   2  comprises the lines L 1 , L 2 , L 3 , and N, a micro-inverter  106   1 , a micro-inverter  106   2 , and a micro-inverter  106   3 , where the micro-inverters  106   1 ,  106   2 , and  106   3  are coupled in a series configuration to the lines L 1 , L 2 , L 3 , and N. 
         [0027]    The micro-inverter  106   1  comprises a phase rotation circuit  202   1 , a two-phase DC/AC inverter  304   1 , input terminals  206   1 ,  208   1 ,  210   1 , a neutral input terminal  218   1 , output terminals  212   1 ,  214   1 ,  216   1 , and a neutral output terminal  220   1 . The micro-inverter  106   2  and the micro-inverter  106   3  are identical to the micro-inverter  106   1 . Coupling the micro-inverters  106   1 ,  106   2 , and  106   3  in the series configuration is as simple as coupling the output terminals  212 ,  214 ,  216 , and the neutral output terminal  220  of one micro-inverter  106  to the input terminals  206 ,  208 ,  210 , and the neutral input terminal  218  respectively of a next micro-inverter  106  in the series. At the load center  302 , the lines L 1 , L 2 , and L 3  are coupled via the three-phase circuit breaker  232  to the output terminals  212   3 ,  214   3 , and  216   3  respectively of the micro-inverter  106   3 ; the line N is coupled to the neutral output terminal  220   3 . At the micro-inverter  106   1 , the input terminals  206   1 ,  208   1 ,  210   1 , and the neutral input terminal  218   1  remain uncoupled. Additionally, the micro-inverters  106   1 ,  106   2 , and  106   3  are each coupled to a PV module  104   1 ,  104   2 , and  104   3 , respectively. 
         [0028]    At the micro-inverter  106   1 , the output terminals  212   1 ,  214   1 ,  216   1 , and the neutral output terminal  220   1  are coupled to the lines L 2 , L 3 , L 1 , and N respectively via the micro-inverters  106   2  and  106   3 . The DC/AC inverter  304   1  injects an AC current through each of the output terminals  212   1  and  214   1  onto the lines L 2  and L 3  respectively. The DC/AC inverter  304   1  matches the phases of the injected AC currents to the second and the third phases of current that are present on the lines L 2  and L 3 . 
         [0029]    Downstream of the output of the micro-inverter  106   1 , the lines L 2 , L 3 , L 1 , and N are coupled to the input terminals  206   2 ,  208   2 ,  210   2 , and the neutral input terminal  218   2  respectively of the micro-inverter  106   2 . The phase rotation circuit  202   2  couples the input terminals  206   2 ,  208   2 ,  210   2 , and the neutral input terminal  218   2  to the output terminals  216   2 ,  212   2 ,  214   2 , and the neutral output terminal  220   2  respectively; the lines L 3 , L 1 , L 2 , and N are therefore coupled to the output terminals  212   2 ,  214   2 ,  216   2 , and the neutral output terminal  220   2  respectively. The DC/AC inverter  304   2  injects an AC current through each of the output terminals  212   2  and  214   2  onto the lines L 3  and L 1  respectively. The DC/AC inverter  304   2  matches the phases of the injected AC currents to the third and the first phases of current that are present on the lines L 3  and L 1 . 
         [0030]    Downstream of the output of the micro-inverter  106   2 , the lines L 3 , L 1 , L 2 , and N are coupled to the input terminals  206   3 ,  208   3 ,  210   3 , and the neutral input terminal  218   3  respectively of the micro-inverter  106   3 . The phase rotation circuit  202   3  couples the input terminals  206   3 ,  208   3 ,  210   3 , and the neutral input terminal  218   3  to the output terminals  216   3 ,  212   3 ,  214   3 , and the neutral output terminal  220   3  respectively; the lines L 1 , L 2 , L 3 , and N are therefore coupled to the output terminals  212   3 ,  214   3 ,  216   3 , and the neutral output terminal  220   3  respectively. The DC/AC inverter  304   3  injects an AC current through each of the output terminals  212   3  and  214   3  onto the lines L 1  and L 2  respectively. The DC/AC inverter  304   2  matches the phases of the injected AC currents to the third and the first phases of current that are present on the lines L 1  and L 2 . 
         [0031]    As described above, the phase rotation circuits  202  rotate the first, the second, and the third phases of current between the micro-inverters  106  such that a different set of phases of AC current, where each of the phases is phase-matched to one of the three phases of the commercial AC current, is generated by each of the micro-inverters  106 . Assuming that the PV modules  104  are receiving equivalent levels of solar energy and that the subsequent AC currents produced by the DC/AC inverters  304  are equivalent in magnitude, the branch circuit  102   2  generates an equally balanced three-phase AC current that is phase-matched to the commercial AC current. Therefore, the commercial AC current remains equally balanced when the generated three-phase AC current is injected onto the commercial power grid. In addition, a branch circuit  102  comprising a string of micro-inverters  106  coupled in series, where the total number of micro-inverters  106  in the string is a multiple of three, produces the same result in that the three-phase AC current generated by the branch circuit  102  is equally balanced. This automatic balancing of the three-phase AC current generated by the branch circuit  102  improves the efficiency of the system  100  and greatly simplifies installations. 
         [0032]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.