Patent Application
20190173419
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.
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.
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.
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:
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.
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.
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.
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.
In addition to connecting to local DC/DC converters 40, quick disconnector allows piggybacking of connections as shown in
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
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
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.
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
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.
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.
