Abstract:
System, methods and apparatus for coupling photovoltaic arrays are disclosed. The apparatus may include a first input adapted to couple to a neutral line of a first photovoltaic array; a second input adapted to couple to a neutral line of a second photovoltaic array; a contactor configured to switchably couple the neutral line of a first photovoltaic array to the a neutral line of a second photovoltaic array, the contactor being controllable by an electric control signal; and a control input adapted to couple the switch to a remotely located controller so as to enable the controller to control the first switch by sending the electric control signal.

Description:
PRIORITY 
     The present application is a Divisional Application of U.S. application Ser. No. 12/184,535, filed Aug. 1, 2008 entitled System, Method, and Apparatus for Coupling Photovoltaic Arrays, which is a continuation-in-part of U.S. application Ser. No. 12/022,147, filed Jan. 29, 2008 entitled System and Method for Ground Fault Detection and Interruption and claims priority to provisional patent application No. 60/953,875, filed Aug. 3, 2007, entitled: High Power Photovoltaic System and Method, both of which are incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to apparatus and methods for converting solar energy to electrical energy, and more specifically to apparatus and methods for coupling photovoltaic arrays with energy conversion and/or energy distribution equipment. 
     BACKGROUND OF THE INVENTION 
     Renewable energy is capturing an increasing amount of attention. And among renewable energy sources, the use of solar energy for generating electricity is now a viable option for many electrical energy needs, and solar energy will become more and more viable relative to other applications. In the context of electrical generation systems (e.g., photovoltaic systems greater than 100 kW), the performance, reliability and regulatory aspects of three-phase grid-tie photovoltaic (PV) inverters and the arrays to which they are connected are issues that will continue to garner attention. 
     Development of this class of equipment for the North American market over recent years has resulted in a set of commonly encountered characteristics. These attributes, while acquired through experience and adversity, have led to the present-day condition where the dominant indices of performance, particularly energy efficiency, have plateaued, and as a consequence, new solutions and approaches are needed to provide performance improvement. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention that are shown in the drawings are summarized below. These and other embodiments are more fully described in the Detailed Description section. It is to be understood, however, that there is no intention to limit the invention to the forms described in this Summary of the Invention or in the Detailed Description. One skilled in the art can recognize that there are numerous modifications, equivalents and alternative constructions that fall within the spirit and scope of the invention as expressed in the claims. 
     In one embodiment, the invention may be characterized as a photovoltaic energy conversion system that includes a first photovoltaic array configured to generate direct current (DC) power, the first photovoltaic array is disposed above ground potential and includes a positive rail and a first neutral line; a second photovoltaic array configured to generate direct current (DC) power, the second photovoltaic array is disposed below ground potential and includes a negative rail and a second neutral line; and a power conversion component remotely coupled to both, the positive rail of the first photovoltaic array and the negative rail of the second photovoltaic array, the power conversion component adapted to convert a voltage between the positive rail of the first photovoltaic array and the negative rail of the second photovoltaic array from one form to another form; and a photovoltaic tie coupled between the first photovoltaic array and the second photovoltaic array, the photovoltaic tie is configured to couple the first neutral line of the first photovoltaic array to the second neutral line of the second photovoltaic array while the first and second photovoltaic arrays are providing power to the power conversion component and to uncouple the first neutral line of the first photovoltaic array from the second neutral line of the second photovoltaic array when the first and second photovoltaic arrays are not providing power to the power conversion component. 
     In accordance with another embodiment, the invention may be characterized as a method for controlling a photovoltaic array, the method including coupling a neutral line of a first photovoltaic array to a neutral line of a second photovoltaic array so as to place the first photovoltaic array above ground potential and the second photovoltaic array below ground potential; converting, remote from a location of the first and second photovoltaic arrays, power from the first and second arrays from one form to another form; and controlling, at least in part from a location remote from the neutral lines of the first and second arrays, the coupling of the neutral line of the first photovoltaic array to the neutral line of the second photovoltaic array. 
     In accordance with yet another embodiment, the invention may be characterized as an apparatus for coupling photovoltaic arrays that includes a first input adapted to couple to a neutral line of a first photovoltaic array; a second input adapted to couple to a neutral line of a second photovoltaic array; a first switch configured to switchably couple the neutral line of a first photovoltaic array to the a neutral line of a second photovoltaic array, the first switch being controllable by an electric control signal; a control input adapted to couple the switch to a remotely located controller so as to enable the controller to control the first switch by sending the electric control signal. 
     As previously stated, the above-described embodiments and implementations are for illustration purposes only. Numerous other embodiments, implementations, and details of the invention are easily recognized by those of skill in the art from the following descriptions and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various objects and advantages and a more complete understanding of the present invention are apparent and more readily appreciated by reference to the following Detailed Description and to the appended claims when taken in conjunction with the accompanying Drawings wherein: 
         FIG. 1  is a block diagram depicting an exemplary embodiment of a photovoltaic energy conversion system; 
         FIG. 2  is another block diagram depicting another exemplary embodiment of a photovoltaic energy conversion system; 
         FIG. 3  is a block diagram depicting yet another exemplary embodiment of a photovoltaic energy conversion system; 
         FIG. 4  is a block diagram depicting a particular embodiment of a photovoltaic energy conversion system; and 
         FIG. 5  is a flowchart depicting a method that may be carried out in connection with the embodiments discussed with reference to  FIGS. 1-4 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , shown is a block diagram of an exemplary embodiment of the present invention. As shown, in this embodiment a first set  102  and a second set  104  of photovoltaic arrays are coupled together by a photovoltaic tie  114  to create a bipolar panel array  106  that is remotely coupled to a power conversion component  108 , which is disposed between the panel array  106  and a distribution system  110 . Also depicted is a photovoltaic tie  114  that is disposed between the a first set  102  and a second set  104  of photovoltaic arrays and is remotely coupled to a control portion  107  by control lines  116  and tie-information lines  118 . 
     The illustrated arrangement of the components depicted in  FIG. 1  is logical and not meant to be an actual hardware diagram; thus, additional components can be added or combined with the components that are depicted in an actual implementation. It should also be recognized that the components, in light of the disclosure herein, may be readily implemented by one of ordinary skill in the art. 
     As an example, the control portion  107  is depicted as a separate functional component from the power conversion component  108 , but the control portion  107  may be realized by components housed within the power conversion component  108  or distributed among the power conversion component  108  and the photovoltaic tie  114 . Moreover, the power conversion component  108  is depicted as coupling directly to the array  106 , but this is certainly not required. In some embodiments, for example, a PV interface is interposed between the array  106  and the power conversion component  108 . In these embodiments, the PV interface generally operates to enable the power conversion component  108 , which may designed to operate at lower voltages than the open-load, rail-to-rail voltage of the array  106 , to be utilized in connection with the PV array  106  that operates at least a portion of the time (e.g., while unloaded) at a voltage that exceeds the designed operating voltage of the power conversion component  108 . U.S. application Ser. No. 11/967,933, entitled Photovoltaic Inverter Interface Device, System and Method, which is incorporated herein by reference, discloses exemplary PV interfaces that may be utilized in connection with one or more embodiments of the present invention. 
     In general, the photovoltaic array  106  converts solar energy to DC electrical power, which may be converted to another form of power (e.g., three-phase AC power or higher-voltage DC power) by the power conversion component  108 . As shown, the power that is output by the power conversion component  108  is applied to the distribution system  110 , which in many embodiments is the three phase distribution system of a demand-side energy consumer (e.g., a commercial entity, industrial entity, or collection of residential units). In other embodiments, however, it is contemplated that the distribution system  110  is on or more portions of a utility distribution system. 
     In some embodiments, the cells in the array  106  include crystalline (e.g., monocrystalline or polycrystalline) silicon that operates in an open load state at 1200 Volts and operates in a loaded state between 660 and 960 Volts. And in other embodiments the array includes cells comprising amorphous silicon that operates in an open load state at 1400 Volts and a loaded state around 900 Volts. One of ordinary skill in the art will appreciate, however, that the photovoltaic array  106  may include a variety of different type photovoltaic cells that are disposed in a variety of different configurations. For example, the photovoltaic cells may be arranged in parallel, in series or a combination thereof. 
     Under traditional ground referencing of either the positive or negative rail of a mono-polar array, to comply with low-voltage regulations (e.g., U.S. National Electric Code (NEC)), the voltage of the mono-polar array is limited to 600 VDC. And given the substantial increase in cost to employ medium-voltage equipment, mono-polar arrays are limited in operational performance. 
     In the present embodiment, however, the center of the arrays  102 ,  104  are tied together in a “bi-polar” configuration, which enables the overall PV voltage to double before violating NEC low-voltage limits Aside from efficiency gains from operating at a higher voltage, a direct conversion (e.g., without a transformer) into 480 VAC is possible; thus eliminating the ratio-changing function of the transformer. U.S. application Ser. No. 12/122,950 entitled COMMON MODE FILTER SYSTEM AND METHOD FOR A SOLAR POWER INVERTER, which is incorporated herein by reference provides additional details that may be utilized in connection with realizing a bipolar array, and for coupling one or more bi-polar arrays to a distribution system without a transformer. 
     In many embodiments, the array  106  provides 1200 VDC maximum differential open-load voltages that do not exceed the 600 VDC-to-ground NEC limits While processing power, PV array  106  ground referencing may be derived from a star-point ground on the AC distribution system through the switching action of the power conversion component  108  (e.g., inverter). 
     In general, the power conversion component  108  converts power that is applied by the array  106  from one form to another form. In some embodiments, the power conversion component  108  includes an inverter to convert DC power provided by the array to AC power. In other embodiments, the power conversion component  108  includes DC to DC power conversion components, which may be used to convert the power from the array  106  to a higher or lower voltage. 
     The distribution system  110  generally operates to distribute power from the array  106  and power conversion component  108  to the premises where the array  106  and power conversion component  108  is located and/or to a utility distribution system. In many embodiments the distribution system  110  includes an AC distribution system and associated AC components such as transformers. In other embodiments, however, it is contemplated that the distribution system  110  may include DC distribution components to distribute DC power to remote locations. 
     One of the most challenging issues for solar PV system designers is placement of the power conversion component  108 . Although it is often desirable to place the power conversion equipment  108  (e.g., an inverter) adjacent to the solar array  106 , this placement is often not physically possible and/or cost efficient. And the greater the distance between the array  106  and the power conversion equipment  108 , the more cost is incurred due to wiring cost and the greater the DC cable losses. 
     In the present embodiment, the photovoltaic tie  114  connects the neutrals  120 ,  122  of the arrays  102 ,  104  without returning the neutrals  120 ,  122  to the power conversion component  108  (e.g., inverter). In many implementations the positive  124  and negative  126  rails of the array  106  are contained in conduit and are coupled to power conversion component  108  by conductors capable of carrying high levels (e.g., 500 Amps) of current, but a third, high-gauge neutral run between the photovoltaic tie and the power conversion component  108  is unnecessary in the present embodiment. Instead, the control  116  and tie-information  118  lines are coupled to the control portion  107  by low gauge (e.g., 16 AWG) wire and the neutrals  120 ,  122  may be uncoupled from the power conversion component  108  while the array  106  is applying power to the power conversion component  108 . As a consequence, the neutral DC home runs, and the long-length and large diameter wires of the neutral home-run legs, not to mention the conduit and installation labor, which can amount to tens of thousands of dollars, are eliminated. As compared to a bi-polar array that is tied together remotely from the array (e.g., adjacent to or within the power conversion component  108 ), the distance of DC transmission current may be reduced two fold. 
     Thus, installation of the photovoltaic tie  114  between the arrays  102 ,  104  enables DC wiring losses to potentially be cut in half, and the power conversion component  108  (e.g., inverter) may be positioned near the entrance of the utility feed to the facility to reduce AC losses. The result is either higher total system efficiency or the opportunity to use fewer panels in the system installation for the same energy harvest. 
     Referring next to  FIG. 2 , it is a schematic view of a portion of an exemplary embodiment of the photovoltaic energy conversion system depicted in  FIG. 1 . As shown, a first and second arrays  202 ,  204  are coupled to a photovoltaic tie  214 , and the photovoltaic tie  214  is coupled remotely by control lines  216  and tie-information lines  218  to a control portion  207 , which in this embodiment, is housed within a power conversion component  208 . In addition, a positive rail  224  of the first array  202  and a negative rail  226  of the second array  204  are switchably coupled to the energy conversion component  208 . 
     As shown, in this embodiment a main contactor  228  is configured to couple and decouple respective neutrals  220 ,  222  of the first and second arrays  202 ,  204 , and auxiliary switches  230 ,  232  of the photovoltaic tie  214 , are configured to couple and decouple the respective neutrals  220 ,  222  of the first and second arrays  202 ,  204  to/from ground at the power conversion component  208 . In particular, while the arrays  202 ,  204  are providing power to the power conversion component  208 , the main contactor  228  is closed so as to couple the neutrals  220 ,  222  together and the auxiliary switches  230 ,  232  are opened so as to decouple the neutrals  220 ,  222  of the arrays  202 ,  204  from the power conversion component  208 . In this state of operation, the neutrals  220 ,  222  are at or near ground potential, which may also be referred to as a “virtual ground,” and there are only two conductors that apply power to the power conversion component  208 : the positive rail  224  of the first array  202  and the negative rail  226  of the second array; thus expensive, high gauge neutral runs between the arrays  202 ,  204  and the power conversion component are eliminated. 
     And when the arrays  202 ,  204  are not providing power to the power conversion component  208  (e.g., at night), the main contactor  228  is open so as to decouple the neutrals  220 ,  222  of the arrays  202 ,  204  and the auxiliary switches  230 ,  232  are closed so as to couple the neutrals  220 ,  222  to ground via low gauge conductors (e.g., less than 5 Amps). 
     In many embodiments, the main switch  228  and auxiliary switches  230 ,  232  are integrated within a single relay device so as to enable the control portion  207  to simultaneously close the main contactor  228  while opening the auxiliary switches  230 ,  232  and vice versa. More specifically, the control portion  207  energizes a relay coil  238  and a sensor  240  provides a feedback signal via the tie-information lines  218  to the control portion  207  to provide status information about the state of the main contactor  228  and auxiliary switches  234 ,  236 . 
     During a fault condition, DC contactors  250 ,  252  may be opened first to remove the virtual ground imposed on the arrays  202 ,  204 , and once the contactors  250 ,  252  are opened, the main contactor  228  may be opened so as to isolate the positive and negative arrays  202 ,  204 . Finally, the neutrals  220 ,  222  of the arrays  202 ,  204  are connected to ground with switches  230 ,  232 . If the ground current is still present, the appropriate fuse  234 ,  236  will open; thus interrupting the ground current and preventing hazardous currents from flowing. 
     U.S. application Ser. No. 12/022,147, entitled System and Method for Ground Fault Detection and Interruption, which is incorporated herein by reference, discloses, among other technical advancements that may be utilized in connection with embodiments of the present invention, a novel structure and method to decouple components of a bipolar photovoltaic array once a ground fault condition requiring system interruption is detected. 
     Referring next to  FIG. 3 , shown is a block diagram of yet another embodiment of the present invention. As shown, in this embodiment two separate bipolar arrays  306 ′,  306 ″ are disposed in parallel so that the positive rails  324 ′,  324 ″ of each of the arrays  306 ′,  306 ″ are coupled together in the power conversion component  308  and the negative rails of each  326 ′,  326 ″ of each of the arrays  306 ′,  306 ″ are remotely coupled to a power conversion component  308 . As depicted, power conversion circuitry (not shown) such as switching components of a DC to DC converter or inverter may couple to nodes  360 ,  362 , which couple to the respective positive  324 ′,  324 ″ and negative  326 ′,  326 ″ rails of the arrays  306 ′,  306 ″. 
     In this embodiment, each of the two arrays  306 ′,  306 ″ may be remotely located relative to the other array and both arrays  306 ′,  306 ″ may be remotely located from the power conversion component  308 . And while each of the two arrays  306 ′,  306 ″ is applying power to the power conversion portion  308 , only four conductors are utilized for carrying current that is produced by the arrays  306 ′,  306 ″ to the power conversion component  308 . As a consequence, an enormous amount of money may be saved because the high-gauge neutral lines that are ordinarily present between the arrays and the power conversion components have been eliminated. 
     As depicted, in this embodiment control lines  316 ′,  316 ″, which are utilized for controlling the switching (e.g., switching to couple the first pair  302 ′,  304 ′ of arrays together and the second pair  302 ″,  304 ″ of arrays together) of the photovoltaic ties  314 ′,  314 ″ are arranged in parallel and are coupled to a control portion  307 , and tie-information lines  318 ′,  318 ″ are arranged in series so that if either of the photovoltaic ties  314 ′,  314 ″ fails to operate (e.g., fails to couple either the first pair  302 ′,  304 ′ of arrays together or the second pair  302 ″,  304 ″ of arrays together), then the control portion  307  does not receive feedback indicating the arrays are online and the control portion  307  will prevent the power conversion component  308  from operating. 
     Referring next to  FIG. 4 , it is a schematic view of a portion of another exemplary embodiment of the photovoltaic energy conversion system depicted in  FIG. 1 . As shown, a first and second arrays  402 ,  404  are coupled to a photovoltaic tie  414 , and the photovoltaic tie  414  is coupled remotely by control lines  416  and tie-information lines  418  to a power conversion component  408 . In addition, a positive rail  424  of the first array  402  and a negative rail  426  of the second array  404  are coupled by disconnect switches  450 ,  452  to the energy conversion component  408 . 
     As shown, in this embodiment a main contactor  428  is configured to couple and decouple respective neutrals  420 ,  422  of the first and second arrays  402 ,  404 , and auxiliary switches  430 ,  432  of the photovoltaic tie  414 , are configured to couple and decouple the respective neutrals  420 ,  422  of the first and second arrays  402 ,  404  to/from a ground contact at the power conversion component  408 . In particular, while the arrays  402 ,  404  are providing power to the power conversion component  408 , the main contactor  428  is closed so as to couple the neutrals  420 ,  422  together and the auxiliary switches  430 ,  432  are open so as to decouple the neutrals  420 ,  422  of the arrays  402 ,  404  from the power conversion component  408 . In this state of operation, the neutrals  420 ,  422  are at or near ground potential, and there are only two conductors that apply power to the power conversion component  408 : the positive rail  424  of the first array  402  and the negative rail  426  of the second array  404 ; thus expensive neutral runs between the arrays  402 ,  404  and the power conversion component are eliminated. 
     Referring next to  FIG. 5 , it is a flowchart depicting an exemplary method that may be carried out in connection with the embodiments depicted in  FIGS. 1-4 . As shown, a neutral line (e.g., neutral line  120 ,  220 ,  320 ′,  320 ″,  420 ) of a first photovoltaic array (e.g., array  102 ,  202 ,  302 ′,  302 ″,  402 ) is coupled (e.g., by photovoltaic tie  114 ,  214 ,  314 ′,  314 ″,  414 ) to a neutral line (e.g., neutral line  122 ,  222 ,  322 ′,  322 ″,  422 ) of a second photovoltaic array (e.g., array  104 ,  204 ,  304 ′,  304 ″,  404 ) so as to place the first photovoltaic array above ground potential and the second photovoltaic array below ground potential (Blocks  502 ,  504 ). As discussed, by coupling neutrals of arrays (e.g., to keep the neutrals at virtual ground), the need to install expensive, heavy gauge neutral wires from the arrays, which are typically located on a roof or another remote location, to an electrical service panel (that is typically in close proximity to where power conversion and distribution equipment is located) is avoided. 
     As depicted in  FIG. 5 , power from the first and second arrays is then converted (e.g., by power conversion component  108 ,  208 ,  308 ,  408 ), remote from a location of the first and second photovoltaic arrays, from one form to another form (Block  505 ). As discussed, in some embodiments DC power from the arrays is converted to DC power at a higher voltage, and in many embodiments, the DC power from the arrays is converted to AC power. 
     In several embodiments, control (e.g., carried out by control portions  107 ,  207 ,  307 ) of the coupling of the neutral line of the first photovoltaic array to the neutral line of the second photovoltaic array is carried out, at least in part, from a location remote from the neutral lines of the first and second arrays (Block  508 ). 
     In conclusion, the present invention provides, among other things, a system and method for coupling photovoltaic arrays with energy conversion and/or energy distribution equipment. Those skilled in the art can readily recognize that numerous variations and substitutions may be made in the invention, its use and its configuration to achieve substantially the same results as achieved by the embodiments described herein. Accordingly, there is no intention to limit the invention to the disclosed exemplary forms. Many variations, modifications and alternative constructions fall within the scope and spirit of the disclosed invention as expressed in the claims.