Abstract:
An improved method of collecting DC power in solar photovoltaic systems is disclosed where series string overcurrent protection is provided at distributed series string terminus locations and a tap connection is made to higher current conductors carrying the combined currents of a number of series strings. A number of related string combiner methods and embodiments are disclosed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    In solar photovoltaic systems, modules are typically connected in series-parallel arrangements. Series-aiding strings of twelve or more modules are common. The number of modules per string sets the system voltage. The number of series strings wired in parallel sets or is proportional to the system power. Large, prior art central-inverter-based systems use two tiers of DC power collection. First, a number of series strings are routed to a field combiner box which provides overcurrent protection, via a fuse, for each string conductor (and series module in that string) and combines, or electrically parallels, all string circuits at a field combiner box output. Second, each field combiner box output is routed to a master combiner box input. The master combiner box provides overcurrent protection for conductors between the field combiner boxes and the master combiner box and combines all the field combiner box outputs into a single DC feed for the central inverter. The master combiner box is typically adjacent to the central inverter and field combiner boxes are distributed throughout the array field. 
         [0002]    Functionally, all fuses in a photovoltaic system are primarily there to protect from backfed currents. For example, if one series string in a megawatt-scale system is shorted, the available fault current will be the short circuit current of the entire megawatt photovoltaic source. The National Electric Code specifies that all conductors in a photovoltaic system be protected from all sources of current; current in the normal direction of flow as well as backfed current. 
         [0003]    Fault currents from photovoltaic sources are very different from fault currents in AC systems because shorted photovoltaic sources behave like current sources. A shorted series string of irradiated photovoltaic modules will typically source only 10% more current than under normal operating conditions. As such, series string fuses are sized to never clear in the direction of forward current. 
         [0004]    The cost of providing overcurrent protection for every series string is significant and more so in systems employing thin-film solar modules. Thin-film module currents are typically 4 to 5 times less than crystalline module currents. As such, the number of fuses and conductor runs to field combiner boxes are typically 4 to 5 times greater. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The invention enables improvements in solar photovoltaic (PV) system DC power collection. 
         [0006]    The preferred embodiment is a sealed sub-combiner unit for combining a small number of series strings, at the field terminus location of these series strings, in order significantly reduce the number of conductors run to the field combiner box location or to the next tier of DC power collection. The sub-combiner is fitted with standard connectors for direct attachment to solar modules. The invention leverages the fact that series string fuses rarely open. As such, the cost and safety issues of providing access to these fuses are eliminated. If a fuse does clear, the small, low cost sub-combiner is designed for plug-and-play replacement. There are, by design, no user-serviceable sub-combiner parts. In one embodiment, the sub-combiner is potted to significantly enhance the environmental integrity of this approach over prior art methods and to further reduce the size and therefore the cost of materials per series string connection. 
         [0007]    A second and closely related embodiment is a method of completely eliminating prior-art string combiner boxes by creating a two-conductor monopolar or three-conductor bipolar DC power collection buss for each master combiner or “home run” circuit. Each series string will tap into the applicable DC buss conductor via a local fused or non-fused (depending on the grounding disposition of the conductor) sub-combiner close to the terminus of two to eight series strings. 
       UTILITY OF THE INVENTION 
       [0008]    The invention finds the greatest utility when used with thin-film photovoltaic cell technologies where the number of series strings, and therefore required string fuses and support infrastructure, is typically 4 to 5 times the cost of that required for crystalline cell technologies. 
         [0009]    The invention can significantly reduce material costs, labor costs and normalized I 2 R losses associated with DC power collection. The total weight of copper in series string-to-combiner circuit runs, in thin-film applications, can be reduced by approximately 67%. The number of field wiring connections can be reduced by an order of magnitude. Unskilled personnel can change out a blown fuse by simply replacing the entire touch-safe sub-combiner. 
         [0010]    The invention leverages two principles. First, the fact that in a well designed PV system only 1 in 5,000 fuses may clear in the lifetime of that system, making ease of fuse replacement a cost ineffective luxury. Second, the maturity of the photovoltaic industry is recognized where ease of circuit access, via a conveniently located combiner box for troubleshooting, now provides less value than does the cost reduction, enhanced reliability, system simplification and enhanced personnel safety enabled by the invention. In lieu of intentionally opening string circuits and probing hot string conductors with a voltmeter, troubleshooting can be accomplished at distributed sub-combiner locations with a clamp-on DC current probe with no exposure to hazardous voltages. 
         [0011]    Typical “failures” in state-of-the-art PV combiner boxes may not be directly caused by the intended clearing of a fuse. In most cases thermal cycling, condensation, particulate contamination, insect intrusion and/or corrosion may cause high resistance contacts between wiring terminals and fuse to fuse holder contact areas. In some cases and over time, this may cause fuses to operate at higher than rated temperatures so that a 10 A fuse effectively becomes a 7 A fuse and may clear. In other cases, power production from a series string may cease with the fuse intact as terminals oxidize and corrode to a point where contacts become insulators. 
         [0012]    The invention provides a sealed environment for fuses and connections. In one embodiment of the invention, fuses and conductors to/from MC4 connectors are soldered to a PCB assembly and the entire assembly is potted with a thermally conductive encapsulant. The resulting IP67 or NEMA 6 rated sub-combiner may provide long term reliability figures ten times that of any state-of-the-art PV string combiner equipment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  illustrates a layout and electrical connections for a two input combiner with input and output DC buss connectors. 
           [0014]      FIG. 2  compares the photovoltaic subarray DC power collection approach for a prior-art solution and a solution using the invention. 
           [0015]      FIG. 3  details the electrical circuit of the solution in  FIG. 2  using the invention. 
           [0016]      FIG. 4  illustrates a layout and electrical connections for a four input combiner with input and output DC buss connectors and an optional insulation displacement DC buss connector. 
           [0017]      FIG. 5  is a not-to-scale schematic drawing that illustrates how the combiner described in  FIG. 4  would be connected in a solar field for maximum benefit. 
           [0018]      FIG. 6  illustrates a layout and electrical connections for a two input combiner with one output. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  shows a preferred embodiment of the invention as a sub-combiner for combining positive conductors from two series strings as well as supplying positive buss input and output connectors. 
         [0020]      FIG. 1A  shows a top view of the sub-combiner. Connectors  1 ,  20 ,  30  are standard male MC4 conductors, the type that mate with the positive connector on a standard PV module. Connector  2  is a standard female MC4 conductor, the type that mates with the negative connector on a standard PV module or, as in this case, the type that are used to extend the conductor length of a positive circuit. Electrical connections  21  and  31  are made from connectors  20  and  30  to one end of fuses  22  and  32 , respectively. The remaining ends of fuses  22  and  32  are electrically connected in common with connectors  1  and  2  as shown by circuit  3 . In practice, connectors  20  and  30  will each be connected to the positive pole of the last module in a unique series string. The sub-combiner is intended for use with other sub-combiners where output connector  2  of the first sub-combiner in a daisy-chain will connect through an appropriately MC4-terminated cable to input connector  1  of the second sub-combiner in the daisy chain and so on, such that circuit  3  is common to all sub-combiners and acts as a positive DC power collection buss. For example, in a four sub-combiner daisy-chain, the current out of terminal  2  of the fourth sub-combiner will be the sum of eight series string currents. Reference designator  7  indicates one of two mounting holes. 
         [0021]      FIG. 1B  shows a side view of the sub-combiner with cover  8  in place. In this embodiment of the invention, cover  8  may be glued, welded or otherwise permanently affixed. Enclosure  6  may also be filled with an electrically insulating potting material  9 , shown in  FIG. 1A , to provide a higher level of environmental integrity, to reduce circuit spacing requirements or to prevent exposure to hazardous voltages. If cover  8  is not used, then enclosure  6  will be filled with an electrically insulating filler to cover all live components. The filler may also be thermally conductive in order to transfer waste heat generated by the fuses. In either case, the sub-combiner will be environmentally sealed to IP67 or NEMA 6, will be touch-safe and will have no user serviceable components. 
         [0022]    In  FIG. 1 , if the grounded conductors of a series string were being combined the sub-combiner would be identical to that in  FIG. 1  except that fuses  22  and  32  would be replaced by short circuits. 
         [0023]      FIG. 2  illustrates the advantages of the invention compared to prior art solutions. 
         [0024]      FIG. 2A  illustrates typical prior art DC power collection from a photovoltaic subarray with eight series strings, A through H, each comprising fourteen thin-film modules. In  FIG. 2A  module-to-module connections are not shown, for clarity. 
         [0025]      FIG. 2C  is a detail of the dashed-line area shown in  FIG. 3A . A pair of conductors must be routed from each of the eight series string circuits to a combiner box location X. Based on the layout shown with 1200 mm by 600 mm module dimensions, the total wire length is  300   m  with a conductor cross section of 4 mm 2  (˜14 AWG). The string current under nominal conditions is 1.32 A. Module dimensions and nominal current specifications are based on a widely used thin-film module. 
         [0026]      FIG. 2D  is a detail of  FIG. 3B  and illustrates an improved DC power collection method using the sub-combiner described in  FIG. 1 . With this method the total wire length is  64   m  with a conductor cross section of 6 mm 2  (˜10 AWG). The ratios of copper length times cross section or therefore volume or weight is 3:1 for the prior art approach vs. the solution enabled by the invention and for the conductors shown in  FIGS. 3A and 3B . 
         [0027]      FIG. 3  is a functional schematic of the power collection methodology illustrated in  FIG. 2D  and includes sub-combiners  100  and  200  as described in  FIG. 1  and the associated  FIG. 1  narrative. All plug-to-plug connections in  FIG. 3  are shown disconnected so that the connector gender is well indicated. Because MC4 connectors are shrouded in order to be touch-safe, the gender descriptions and illustrations may not appear to agree. In  FIG. 3 , the positive DC buss is split into conductors  101  and  102 , and are terminated with female MC4 connector  110  and male MC4 connector  113 , respectively. In a similar way, the negative DC buss is split into conductors  201  and  202 , and are terminated with male MC4 connector  210  and female MC4 connector  213 , respectively. To complete the circuit, positive DC buss connectors  110  and  113  are plugged into sub-combiner high current inputs  111  and  112 , respectively. In a similar way negative DC buss connectors  210  and  213  are plugged into sub-combiner high current inputs  211  and  212 . In this application, the DC busses are split to enable daisy-chained power collections and touch-safe, plug-and-play installation and replacement of sub-combiners  100  and  200 , although it is highly unlikely that non-fused sub-combiner  200  will ever need to be replaced. Series strings G and H are each made up of 14 thin-film solar modules connected series aiding with typical module-to-module interconnections as illustrated by connectors  75 / 72  and  85 / 82 , respectively. Module-to-module connections interior to either string G or H are not shown for clarity. DC power is collected or currents combined by plugging female MC4 connector  71  and  81  from modules G 1  and H 1  into sub-combiner input ports  120  and  130 , respectively. When connected, currents flow from strings G and H through fuses  114  and  115 , respectively, and into DC buss conductor  102 . In a similar way currents are combined by plugging male MC4 connector  76  and  86  from modules G 14  and H 14  into sub-combiner input ports  220  and  230 , respectively. When connected, currents return from strings G and H through DC buss conductor  202 . 
         [0028]    By way of a more quantitative example, if PV modules with short circuit current (I sc ) ratings of ˜2 A and sub-combiners supplied with 3.5 A fuses were used for the application described in  FIGS. 2 and 3 , then eight series strings could be combined into one 16 A (I sc ) circuit and the “DC buss conductors” could simply be 6 mm 2  Type PV wire. In this overall system scenario, three-tiers of DC power collection would be used; a sub-combiner tier, a field combiner box tier with one-eighth the fuses and conductors entering the combiner box and a master combiner box tier. 
         [0029]      FIG. 4  illustrates a logical extension of the sub-combiner described in  FIG. 1  but with four series string input connectors  20 ,  30 ,  40 ,  50  with circuit connections  21 ,  31 ,  41 ,  51  and fuses  22 ,  32 ,  42  and  52 , respectively. In some applications, the daisy-chain function may be extended to currents higher than the standard MC4 connector  30 A limit by deleting or not using input connector  1  and by using cable assembly  60 , with male MC4 connector  61 , conductor  62  and insulation displacement tap connector  63  where tap connector  63  is connected to a high current DC buss conductor. If the DC buss conductor is sized to carry the current of a master combiner or “home run” circuit, the traditional field combiner box can be eliminated. In lieu of an insulation displacement tap connector, a junction box could also facilitate this connection but by breaking the buss conductors and connecting each end to a common terminal block inside the junction box. A lower current connection from the same terminal block will also be made to a bulkhead MC4 connector accessible form the exterior of the junction box. In this way, touch-safe plug-and-play installation and replacement of the combiner box may also be accommodated. 
         [0030]      FIG. 5A  illustrates the optimum solar module arrangement for the sub-combiner illustrated in  FIG. 4  where series strings A through F are arranged in a row so that four positive conductors of four unique series strings terminate at a common location and where four negative conductors of four unique series strings terminate at a common location along the length of a row. The row shown is comprised of eight series strings but could be much longer and is only limited in length by the cross sectional area of DC buss conductors  102  and  202 . 
         [0031]      FIG. 5B  is a detail of the  FIG. 5A  area bounded by dashed lines.  FIG. 5B  is not to scale and is use here to schematically illustrate how sub-combiner  10  is connected to four unique series string circuits using only the standard terminations supplied with the solar modules. Module-to-module connections  18  and  19  are also made as intended with the standard manufacturer-supplied connectors. In this example, the output of combiner  10  is connected to positive DC bus conductor  102  with self-sealing, insulation displacement tap connector  63 . Buss conductor  102  could, for example, be a typical stranded conductor with RHW-2 insulation, an insulated aluminum buss bar or any other method for efficiently conducting higher cumulative currents than could be handled by “standard” MC4 type connectors. 
         [0032]      FIG. 6A  shows the simplest embodiment of the invention as a three port device with male MC4 connector-based lower current input ports  1  and  9  to conduct current through electrical contacts  2  and  8  and through fuses  3  and  7 , respectively, to a common higher current connection point  5  and female MC4 connector  6 . The combiner assembly is injection molded as a single piece from an electrically insulating and physically robust and sunlight resistant plastic.  FIG. 6B  is for connection to negative PV module terminals but is otherwise analogous in function and construction to the combiner described in  FIG. 6A . Each combiner may also have provisions, not shown, for mounting the combiners to the array structure. These provisions may include mounting holes to facilitate mounting with a plastic tie-wrap fastener. 
         [0033]    In an additional photovoltaic combiner embodiment, not illustrated, combiner fuses are replaced by diodes. The diodes serve the same function as the fuses by preventing damaging backfed currents in series string conductors and series connected solar modules. The use of diodes also supports the basic utility of the invention; that of providing a maintenance-free, touch-safe distributed method of combining circuits. Both fuses and diodes generate waste heat under normal operating conditions and, as such, all other parameters and disclosures associated with the fuse-based invention would be equivalent in a diode-based solution with the exception that there would be more heat to be removed using the diode solution.