Patent Publication Number: US-2019173419-A1

Title: Systems and method for electrical power distribution in solar power plants

Description:
BACKGROUND 
     The field of the disclosure relates generally to solar power plants and more particularly to the distribution of electrical power within a solar power plant. 
     Solar power plants harvest sunlight to generate electrical power. Specifically, solar power plants may convert the solar energy in sunlight directly into electrical power using photovoltaic (PV) cells. Alternatively, solar power plants may use the sunlight indirectly as a heat source to produce electrical power. 
       FIG. 1  illustrates a schematic diagram of a layout of a known one hundred megawatt PV solar power plant  10 . Solar power plant  10  is divided into a plurality of PV power blocks  11 . In the example of  FIG. 1 , solar power plant  10  includes forty PV power blocks  11 , each PV power block  11  producing 2.5 megawatts of power. The power from each PV power block  11  is delivered to a power substation  13 , where the power is converted to match the requirements of a power grid (not shown) connected to solar power plant  10 . 
       FIG. 2  illustrates a schematic diagram of a known PV module  12  for use in a PV power plant such as solar power plant  10  (shown in  FIG. 1 ). PV module  12  includes a plurality of PV cells  15  that are electrically coupled in series with one another. The plurality of PV cells  15  are arranged in twelve rows  17  and six columns  19 , for a total of seventy-two PV cells  15  electrically connected in series. Each PV cell  15  is operable to convert photons from received light into electricity. Each PV cell  15  produces a low voltage as photons are absorbed, such as approximately 0.75 volts (V) when exposed to sunlight with no load. Because each PV cell is connected in series, the current is constant throughout the PV cells  15  while the voltage is additive. Thus, PV module  12  may produce an open circuit voltage of approximately 72×0.75 V, or 54 V. At full power, the voltage drops about 70% (e.g., to 38 V) with a current of about 9 amps (A) to produce a nominal maximum power of 342 watts (W) per PV module  12 . 
       FIG. 3  illustrates a schematic diagram of a known PV string  14 . Specifically, each PV module  12  is electrically connected in series with other PV modules  12  to form PV string  14 . PV string  14  may contain twenty-eight PV modules  12  connected in series, resulting in a nominal voltage of about 1491 V with no load. At a maximum power of about 9.1 Kilowatts per PV string  14 , the voltage drops to about 1,042 V with a current of about 9 amps. 
       FIG. 4  illustrates a schematic of first row  21  and a second row  23  that form a portion of PV power block  11 . Each string row  21 ,  23  includes eight PV strings  14  for a total of sixteen PV strings  14 . PV strings  14  are electrically connected in parallel to a combiner box  16 , which combines output currents of PV strings  14 . Each PV string  14  is connected to combiner box  16  using a pair of low voltage direct current (LVDC) cable  2  (for clarity, LVDC cable  2  is shown only for farthest strings  3  and nearest strings  4 ). Thus, combiner box  16  for two rows  21 ,  23  of PV strings  14  requires sixteen pairs of LVDC cables  2 . Because PV strings  14  are connected in parallel, the voltage does not increase, but the current is cumulative. For example, combiner box  16  may combine the output of sixteen PV strings  14  to produce a total output of about 145.6 kilowatts at about 1,042 V and about 140 A. 
       FIG. 5  illustrates a schematic diagram of PV power block  11 . PV power block  11  includes two sets of twenty-four rows of PV strings  14  feeding twelve combiner boxes  16 . An output of the combiner boxes  16  is connected to a block inverter  18  in parallel. Thus, twelve combiner boxes  16  may use twelve pairs of LVDC cables  5  to connect to block inverter  18  (for clarity, only a single pair of LVDC cables  5  is shown). Block inverter  18  converts the DC power produced by the PV cells to AC power, and a block transformer  20  steps up the AC voltage for transmitting to substation  13 . 
     Solar power plant  10  has various drawbacks. U.S. Patent Publication 2016/0099572A1 details the drawbacks for the above mentioned standard utility scale solar plant design. For example, each PV string  14  is connected to combiner box  16  in parallel. This requires using relatively long (tens of meters to hundreds of meters) LVDC cables  2  for each PV string  14 . Furthermore, typical utility scale PV plants have tens of thousands of PV strings  14  each requiring separate LVDC cables  2 . The high number of LVDC cables  2  results in significant costs and resistive power losses. In addition, LVDC cables  5  coupled between the combiner boxes  16  and the block inverter  18  transmit the DC power to block inverter  18 . Due to the relatively low DC voltage, the typical current on these LVDC cables  5  can be relatively high (100 s A), requiring the use of large gauge LVDC cable and incurring significant power loss. 
     BRIEF DESCRIPTION 
     In one aspect, a power generation architecture for a photovoltaic power plant includes a plurality of photovoltaic blocks and a medium voltage direct current collector. Each plurality of photovoltaic blocks includes a plurality of photovoltaic groups and a combiner. Each plurality of photovoltaic groups includes a plurality of photovoltaic strings and a direct current (DC) to DC power converter. Each photovoltaic string is operable to output low voltage, direct current (LVDC) electrical power at a string output. Each DC to DC power converter is electrically coupled to the string output of each photovoltaic string and is operable to convert the LVDC electrical power to medium voltage, direct current (MVDC) electrical power at a converter output. The combiner has a combiner input in electrical communication with each of the converter outputs of the plurality of photovoltaic groups and is operable to combine the MVDC electrical power received at the combiner input to produce a block output. The collector includes a collector input electrically coupled to each combiner output and operable to combine each block output. 
     In another aspect, a power generation architecture for use in a photovoltaic power plant is provided. The architecture includes a first photovoltaic group including a first plurality of photovoltaic strings and a first direct current (DC) to DC converter having an input electrically coupled to each photovoltaic string of the first plurality of photovoltaic strings. The architecture further includes a second photovoltaic group including a second plurality of photovoltaic strings and a second DC to DC converter having an input electrically coupled to each photovoltaic string of the second plurality of photovoltaic strings. The first photovoltaic group and the second photovoltaic group are physically arranged in a row and the first DC to DC power converter and the second DC to DC power converter are connected in a ring electrical connection. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  illustrates a schematic diagram of a known PV power plant. 
         FIG. 2  illustrates a schematic diagram of a known PV power module that may be used with the PV power plant shown in  FIG. 1 . 
         FIG. 3  illustrates a schematic diagram of a known PV string that may be used with the PV power plant shown in  FIG. 1 . 
         FIG. 4  illustrates a schematic diagram of rows of a known PV power block. 
         FIG. 5  illustrates a schematic diagram of a power block. 
         FIG. 6  illustrates a schematic diagram of an exemplary string layout. 
         FIG. 7  illustrates a schematic diagram of rows of a PV power plant implemented using the string layout of  FIG. 6 . 
         FIG. 8  illustrates a schematic diagram of a PV power block of a PV power plant implanted using the string layout of  FIG. 6 . 
         FIG. 9  illustrates a schematic diagram of an exemplary PV power plant. 
         FIG. 10  illustrates an exemplary connector that serves both as a cable termination and as a quick disconnector to local DC/DC converters. 
       Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of the disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of the disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings. 
     The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     Throughout this application, reference will be made to low voltage, medium voltage, and high voltage. Low voltage is considered to be voltage up to approximately 1,500V, medium voltage is considered to be voltage between approximately 1,500 V and 35 kV, and high voltage is considered to be voltage between approximately 35 kV and 230 kV. 
     Throughout this application, reference will be made to a ring electrical connection. A ring electrical connection is a variation of a parallel electric circuit. In place of using radial leads in parallel, the ring connection connects terminals of adjacent sources. For example, a group of batteries would each have their positive terminals electrically coupled to one another and their negative terminals electrically coupled to one another. A single pair of leads may then be used at any battery terminal to tap the electrical power. 
     Embodiments of the present disclosure relate to photovoltaic power plants. The photovoltaic power plants described herein include a configuration of photovoltaic strings that results in reduced material costs for constructing the plant as compared to at least some known string configurations. The string configuration described herein employs medium voltage direct current (MVDC) DC/DC converters to reduce the amount of wiring required. 
       FIG. 6  illustrates a schematic diagram of an exemplary layout  44  of a group of PV strings  14 . A PV string  14 , as used herein, refers to a grouping of PV modules  12  connected in a series electrical connection. A PV module  12 , as used herein, refers to a grouping of PV cells  15  electrically coupled to one another and sharing a common support structure. In the exemplary embodiment, layout  44  includes four PV strings  14  that supply electrical power to one local DC/DC converter  40 . PV strings  14  may be arranged symmetrically, with symmetry about a first axis  31  and a second axis  33 . PV strings  14  may be arranged in a rectangle. Local DC/DC converter  40  may be located between PV strings  14  (e.g. along axes of symmetry  31 ,  33  or at an intersection  35  of first axis  31  and second axis  33 ) minimizing the length of LVDC cables  38  electrically coupling PV strings  14  and local DC/DC converter  40 . Because local DC/DC converter  40  is centrally located and PV strings  14  are arranged symmetrically, each LVDC cable  38  may have the same length. Thus, identical LVDC cables may be used to electrically connect any PV string  14  to local DC/DC converter  40 . Local DC/DC converter  40  subsequently steps up the LVDC voltage output from PV strings  14  to MVDC voltage levels. For example, a 1,042 V DC output of a single PV string  14  may be converted to approximately 20,000 VDC with a current of approximately 0.46 A using local DC/DC converter  40 . Accordingly, the output of four PV strings  14  would be approximately 20,000 VDC with a current of 1.8 A. The higher relative voltage reduces the amount of electric current carried in any subsequent cabling by an order of magnitude. 
       FIG. 7  illustrates a schematic diagram of a layout  47  of two rows  46 ,  48  of a PV block  42  using exemplary layouts  44  of groups of PV strings  14 . A first row  46  includes eight PV strings  14  and a second row  48  includes eight PV strings  14 . Together, rows  46 ,  48  include sixteen PV strings  14  and four local DC/DC converters  40  with each layout  44  of four PV strings  14  electrically coupled to a local DC/DC converter  40 . Local DC/DC converters  40  are each electrically connected in a ring electrical connection using a pair of MVDC cables  49 ,  55 . Connecting local DC/DC converters  40  in a ring electrical connection results in increased current relative to a single DC/DC converter  40 , but reduces the amount of MVDC cable  49  required for a given row. For example, rows  46 ,  48  may together produce an output of approximately 20,000 VDC with a current of 7.3 A. In contrast, traditional PV rows  21 ,  23  (shown in  FIG. 2 ) output approximately 1,042 V and approximately 140 A each, and combined traditional PV rows  21 ,  23  output approximately 1,042 V and approximately 280 A. 
     Furthermore, because a MVDC cable carries less current than a LVDC cable, a MVDC cable experiences significantly lower power losses per length than a LVDC cable. As a result, a length of rows  46 ,  48  may be extended significantly by adding additional layouts  44  without incurring significant voltage drops/power losses. Current rows, such as rows  21 ,  23  are limited in length by the length of LVDC cable  2  required for the farthest PV string  14  (i.e. the PV string  14  farthest from the combiner box  16 ). If PV string  14  is too far away, the power losses in the LVDC cable become excessive. For example, most conventional PV power plants have rows  21 ,  23  of eight PV strings  14 . However, using layout  44 , a farthest string  53  uses the same length of LVDC cable  38  as a nearest string  53 . For example, rows  46 ,  48  could be extended to sixteen PV strings  14  (for thirty-two total strings) without incurring significant voltage drops and/or power losses. 
       FIG. 8  illustrates a schematic diagram of a PV power block  42  using multiple layouts  47  of rows of PV strings  14 . PV power block  42  includes forty-eight rows of PV strings  14  (i.e., twenty-four for each side) and ninety-six local DC/DC converters  40 . In the exemplary embodiment, output of layout  47  is electrically connected to a block collector  50  (only one electrical connection of layout  47  is shown for clarity). Because the output of each layout  47  is MVDC with relatively low current, the output may be collected in a ring electrical connection without requiring a high current capacity of block collector  50 . For example, the total current of forty-eight rows may be approximately 174.7 A, which is comparable to the current of a single row of a traditional PV power plant. This electrical power may be delivered to a power substation  52  using a conventional distribution system  56 . 
       FIG. 9  illustrates a schematic diagram of an exemplary PV power plant  54 . PV power plant  54  includes a plurality of PV power blocks  42 , an electrical distribution system  56 , and a power substation  52 . Each PV power block  42  is electrically coupled to power substation  52  through electrical distribution system  56 . Electrical distribution system  56  may connect each PV power block  42  to power substation  52  in a parallel electrical connection, or in some embodiments, groups of PV power blocks  42  may be connected in a ring electrical connection. 
     Power substation  52  includes an inverter  58  and a transformer  60  in the exemplary embodiment. Inverter  58  includes an input  61  in electrical communication with electrical distribution system  56  and an output  63  in electrical communication with an input  65  of transformer  60 . Inverter  58  is operable to convert DC power received from electrical distribution system  56  to AC power. Inverter  58  may be silicon carbide based to operate at higher frequencies and temperatures compared to silicon based power electronics. Transformer  60  includes an input  65  in electrical communication with inverter  58  and an output  67  configured to connect to a power grid (not shown). Transformer  60  is operable to convert the AC power output by inverter  58  into a voltage compatible with the power grid. 
     Exemplary embodiments require using multiple MVDC cables. Compared to a conventional PV power plant design, MVDC terminations may be a relatively significant cost due to the increased number of MVDC cables in the exemplary embodiments.  FIG. 10  illustrates an exemplary connector  100  that serves both as a cable termination and as a quick disconnector to local DC/DC converters  40 . Connector  100  includes a body portion  104  with a cable aperture  110  sized and shaped to receive MVDC cable  102 . MVDC cable  102  is secured within cable aperture  110  using conventional techniques such as a compression fitting. Connector  100  further includes a head portion  106  having a lug aperture  112  sized and shaped to receive a lug  114  of an electrical component. Lug aperture  112  may be secured to lug  114  using conventional techniques. Head portion  106  includes a lug  108  opposite lug aperture  112 . Lug  108  is sized and shaped to simulate a standard electrical lug  114 . Cable aperture  110 , lug aperture  112 , and lug  108  are each in electrical connection with one another. A MVDC cable  102  may be electrically coupled to an electrical component by coupling MVDC cable  102  to cable aperture  110  and lug aperture  112  to lug  114  of electrical component. 
     In addition to connecting to local DC/DC converters  40 , quick disconnector allows piggybacking of connections as shown in  FIG. 10 . Connector  100  may be coupled to an existing connector  116  by coupling lug aperture  112  of connector  100  to lug  108  of existing connector  116 . 
     Connector  100  offers safe isolation of local DC/DC converter  40  from the rest of the PV plant and allows safe access to local DC/DC converter  40  for maintenance, repair or replacement. The multifunctional nature of connector  100  also further reduces the hardware cost by eliminating the need for a separate junction box. 
     The following table details the distribution cost of an example architecture of a conventional PV power plant  10  having a block inverter  18  and block transformer  20  for each PV power block  11  as shown in  FIGS. 1-5 . The cost is derived as the total cost of the components divided by the rated capacity of a power plant. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 MVAC 
                   
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 MVAC 
                 cable 
                   
                 Tsfm + 
               
               
                   
                 LVDC 
                   
                 LVDC 
                   
                 Ring 
                 cable 
                 section to 
                 MVAC 
                 SF6 + 
               
               
                   
                 cable, 
                 Combiner 
                 cable to 
                 Inverter 
                 main 
                 within 
                 switch 
                 switch 
                 substation + 
               
               
                   
                 Misc 
                 box 
                 skid 
                 skid 
                 unit 
                 section 
                 gear 
                 gear 
                 SCADA 
                 Total 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Cost 
                 4.1 
                 1.1 
                 1.2 
                 6.3 
                 0.1 
                 0.8 
                 0.3 
                 0.4 
                 2.5 
                 16.8 
               
               
                 (¢ per W) 
               
               
                   
               
            
           
         
       
     
     The category “LVDC cable, Misc” includes the LVDC cables that are required to connect the individual photovoltaic strings to the combiner box. This category further includes the cable connectors that are used to quickly connect sections of LVDC cables to enable fast installation. “The combiner box” refers to the electrical combiner box that combine LVDC cables from multiple photovoltaic strings, provides electrical protection such as electrical fuse for each individual photovoltaic string and quick electrical disconnect function to allow fast isolating the string assembly from the rest of the PV plant for troubleshooting or maintenance. The category “LVDC cable to skid” refers to the LVDC cables that connect the combiner boxes to the block inverter/transformer skid. The “Inverter skid” refers to the block inverter and block transformer that are typically collocated on the same skid. The skid further has the additional electrical equipment such as LVDC cable recombiner (combines all LVDC cables from the combiner boxes), auxiliary power supply to supply power for plant control and communication equipment, MVAC switchgear, and ring main unit (RMU) for forming ring electrical connection for MVAC power output. The “MVAC cable within section” refers to the MVAC cables (3 phase MVAC) that form ring electrical connection between the blocks. The “MVAC cable section to switch gear” refers to the MVAC cables from the last RMU in the ring connection to the MVAC power collector in the substation. The “MVAC switchgear” refers to the MVAC power collector in the substation. The “Tsfm+SF6+substation+SCADA” includes the transformer located in the substation that steps up voltage to the grid compatible voltage, the dielectric SF6 gas enabled high voltage switchgear that provide safe protection/disconnection between the substation transformer and the grid, all the infrastructure and equipment in substation that are required, and the Supervisory Control And Data Acquisition (SCADA) that is required for plant control. 
     The cost of the exemplary PV power plant design described in  FIGS. 6-10  is broken out in the follow table. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                   
               
               
                   
                 LVDC 
                   
                 MVDC 
                   
                   
                   
                   
                   
               
               
                   
                 cable, 
                 DC-DC 
                 cable and 
                 Substation 
                 Block 
               
               
                   
                 misc 
                 Converter 
                 connectors 
                 Inverter 
                 Combiner 
                 Substation 
                 Misc. 
                 Total 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Cost 
                 3.87 
                 3.05 
                 1.72 
                 2.06 
                 .19 
                 2.54 
                 .2 
                 13.63 
               
               
                 (c/Wac) 
               
               
                   
               
            
           
         
       
     
     Notably, increasing the number of DC-DC converters and reducing the amount of LVDC cables results in a decrease in the cost of the PV power plant relative to the conventional design from 16.8 ¢/W to 13.63 ¢/W. The following table details the cost if the design is modified to increase the number of strings per row (e.g. from eights strings per row to twenty-four strings per row). As described previously, the length of a row is not limited by the farthest strings, unlike at least some known designs. The following table reflects the costs if twenty-four strings are used per row. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                   
               
               
                   
                 LVDC 
                   
                 MVDC 
                   
                   
                   
                   
                   
               
               
                   
                 cable, 
                 DC-DC 
                 cable and 
                 Substation 
                 Block 
               
               
                   
                 misc 
                 Converter 
                 connectors 
                 Inverter 
                 Combiner 
                 Substation 
                 Misc. 
                 Total 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Cost 
                 .86 
                 4.09 
                 3.13 
                 2.07 
                 .15 
                 2.57 
                 .2 
                 13.07 
               
               
                 (c/Wac) 
               
               
                   
               
            
           
         
       
     
     With twenty-four strings per row, the cost is further reduced to 13.07 ¢/W as shown in the table. Further, if the junction boxes are eliminated using connectors  100  of  FIG. 10 , the costs further decreases as follows. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                   
               
               
                   
                 LVDC 
                   
                 MVDC 
                   
                   
                   
                   
                   
               
               
                   
                 cable, 
                 DC-DC 
                 cable and 
                 Substation 
                 Block 
               
               
                   
                 misc 
                 Converter 
                 connectors 
                 Inverter 
                 Combiner 
                 Substation 
                 Misc. 
                 Total 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Cost 
                 .86 
                 3.74 
                 3.13 
                 2.07 
                 .15 
                 2.57 
                 .2 
                 12.73 
               
               
                 (c/Wac) 
               
               
                   
               
            
           
         
       
     
     Eliminating the junction boxes reduces the cost of each DC-DC converter, further reducing the overall cost of the PV plant. Future reductions in the cost of the MVDC cable are possible. For example, standardized cable lengths and connectors may further reduce costs. If the termination cost is reduced to $100 per cable, the cost is as follows. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                   
               
               
                   
                 LVDC 
                   
                 MVDC 
                   
                   
                   
                   
                   
               
               
                   
                 cable, 
                 DC-DC 
                 cable and 
                 Substation 
                 Block 
               
               
                   
                 misc 
                 Converter 
                 connectors 
                 Inverter 
                 Combiner 
                 Substation 
                 Misc. 
                 Total 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Cost 
                 .86 
                 3.74 
                 2.04 
                 2.07 
                 .15 
                 2.57 
                 .2 
                 11.63 
               
               
                 (c/Wac) 
               
               
                   
               
            
           
         
       
     
     As described above, the cost per watt of a PV power plant may be reduced significantly using the exemplary PV string layout described herein, from 16.8 ¢/W to as low as 11.63 ¢/W. The exemplary PV string layout is advantageous in that it allows longer rows, decreases the amount of LVDC cabling, reduces high amperage losses, and reduces the overall cost of a PV power plant. 
     Exemplary embodiments of a PV string layout and PV power plant are described above in detail. The system is not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the configuration of components described herein may also be used in combination with other processes, and is not limited to practice with the systems and related methods as described herein. Rather, the exemplary embodiments can be implemented and utilized in connection with many applications where monitoring of a power circuit is desired. 
     Although specific features of various embodiments of the present disclosure may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the present disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the embodiments of the present disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the embodiments described herein is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.