# Patent

## Title
Dual use photovoltaic system

## Abstract

A system for providing power from a direct current (DC) source to the power grid. The system includes a first inverter with an input and an output. The input is adapted to connect to the DC source. A first switch disposed between the output and the power grid. A second inverter with a DC terminal and an AC terminal, the AC terminal is adapted to connect in parallel with the output of the first inverter. A battery adapted to connect to the DC terminal of the second inverter. A second switch connected between the DC terminal of the second inverter and the input of the first inverter. The second switch also operatively connects the DC source to the battery. The system may further include a charging circuit adapted to be disposed between the input and the DC terminal and a load adapted to connect to the output.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. patent application Ser. No. 12/957,490, filed Dec. 1, 2010, which claims the benefit of priority from U.S. Provisional Patent Application No. 61/265,734, which was filed on Dec. 1, 2009, the disclosures of which are incorporated herein by reference.


BACKGROUND
1. Technical Field
The present invention relates to a hybrid photovoltaic system which features both power grid tied operation and battery charging operation.
2. Description of Related Art
Utility networks provide an electrical power system to utility customers. The distribution of electric power from utility companies to customers utilizes a network of utility lines connected in a grid-like fashion, referred to as an power grid. The power grid may consist of many independent energy sources energizing the grid in addition to utility companies energizing the grid, with each independent energy source being referred to as a distributed power (DP) generation system. The modern utility network includes the utility power source, consumer loads, and the distributed power generation systems which also supply electrical power to the network. The number and types of distributed power generation systems is growing rapidly and can include photovoltaics, wind, hydro, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitor types, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines. These distributed power generation systems are typically connected to the utility network such that they operate in parallel with the utility power sources.
The term “DC source” as used herein refers to and includes DC available from photovoltaics cells, wind powered DC generators, hydro powered turbines, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitor types, and mechanical devices including conventional and variable speed diesel engines, stirling engines, gas turbines, and micro-turbines.
BRIEF SUMMARY
According to an embodiment of the present invention there is provided a system for providing power from a direct current (DC) source to the power grid. The system includes a first inverter with an input and an output. The input is adapted to connect to the DC source. A first switch is disposed between the output and the power grid. A second inverter has a DC terminal and an alternating current (AC) terminal. The AC terminal is adapted to connect in parallel with the output of the first inverter. A battery is adapted to connect to the DC terminal of the second inverter. A second switch connects between the DC terminal of the second inverter and the input of the first inverter. The second switch also connects the DC source to the battery. The system may further include a charging circuit adapted to be disposed between the input and the DC terminal. A load connects to the output.
According to an embodiment of the present invention there is provided a method featuring both grid tied operation and battery charging operation, the method using a hybrid photovoltaic system. Two inverters are used during off grid operation of the hybrid photovoltaic system. The hybrid photovoltaic system includes the first inverter with an input and an output. A direct current (DC) source is connected to the input. A first switch is disposed between the output and the power grid. A second inverter has a DC terminal and an AC terminal. The AC terminal is connected in parallel with the output of the first inverter. A battery connects to the DC terminal. A second switch is disposed between the DC terminal and the input. The method senses absence of the power grid. Upon sensing the absence of the power grid, the power grid is disconnected from the output and the AC terminal with the first switch. The DC source is connected to the DC terminal with the second switch, thereby shunting the input of the first inverter with the DC terminal of the second inverter. Power on the DC terminal of the second inverter may typically be converted to an output power on the AC terminal of the second inverter. The output power typically provides a local grid to the load and the output of the first inverter. Voltage of the input of the first inverter may be controlled to a voltage value based on the local grid and/or the output power provided by the second inverter. Connection to the DC terminal with the second switch may be disconnected when a difference between voltage of the input and the voltage value exceeds a predetermined value. Connection to the DC terminal with the second switch typically allows the DC source to charge the battery. The control of voltage of the input of the first inverter may be performed by the first inverter. The control of voltage of the input of the first inverter may be by adjusting a parameter of the local grid. The local grid parameter may be adjusted by the backup inverter. The parameter may be a frequency, a phase angle, a voltage or a current. The control of voltage of the input of the first inverter may alternatively be performed by a circuit connected in series between the second switch and the DC terminal of the second inverter.
During on grid operation the method senses presence of the power grid. Based on the presence of the grid the DC source is disconnected from the DC terminal of the second inverter with the second switch, when the power grid is present. The output and the AC terminal are connected to the power grid with the first switch. Power on the input of the first inverter is typically converted to an output power on the output of the first inverter. The output power preferably supplies the power grid and/or a load. Power on the AC terminal of the second inverter is typically converted to a power on the DC terminal of the second inverter. The power on the DC terminal preferably charges the battery.
According to an embodiment of the present invention there is provided a power system which includes a power inverter, a backup inverter and a direct current (DC) source. The power inverter and the backup inverter are interconnected at respective outputs. A switch may be adapted to interconnect the inputs of the power inverter and the backup inverter and to connect the inputs to the DC source when a power grid is not available. When the power grid is available the switch may be adapted to disconnect the input of the backup inverter from the DC source.
The power system may further include a second switch adapted to connect the outputs of the power inverter and the backup inverter to the power grid when the power grid is available. When the power grid is not available the second switch adapted to disconnect the outputs of the backup inverter and the power inverter from the power grid. A charging circuit is typically adapted to be disposed between the DC source and the input of the backup inverter. A battery may be connected in parallel at the input of the backup inverter.
These, additional, and/or other aspects and/or advantages of the present invention are set forth in the detailed description which follows; possibly inferable from the detailed description; and/or learnable by practice of the present invention.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:
FIG. 1a shows a hybrid photovoltaic system according to an embodiment of the present invention;
FIG. 1b shows further details of a DC supply according to an exemplary embodiment of the present invention;
FIG. 2a shows a method for performing off grid or on grid operation of the hybrid photovoltaic system shown in FIGS. 1a and 1b, according to an embodiment of the present invention;
FIG. 2b shows more detailed method steps for off grid operation shown in FIG. 2a, according to an embodiment of the present invention; and
FIG. 2c more detailed method steps for on grid operation shown in FIG. 2a, according to an embodiment of the present invention.



The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.
DETAILED DESCRIPTION
Reference will now be made in detail to aspects of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The aspects are described below to explain the present invention by referring to the figures.
Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.
By way of introduction, aspects of the present invention are directed primarily towards off grid operation of a hybrid photovoltaic system and possible benefits of using a lower power rated and cheaper standard backup inverter to provide a local grid. The local grid may additionally be used to derive additional circuit control function benefits from an offline main grid tied inverter which is typically higher power rated, more expensive and provides more functionality.
The term “hybrid” as used herein refers off-grid and/or on grid operation of a photovoltaic system. Equivalently, the term “hybrid” as used herein may refer to a photovoltaic system which is both/either grid connected or battery connected. Operation of a “hybrid photovoltaic system” may typically include supplying AC power to a power grid and/or load and/or DC power to the battery. AC power present on the power grid, leads to an “on grid” operation of the hybrid photovoltaic system. Otherwise the absence of AC power on the power grid leads to an “off grid” operation of the hybrid photovoltaic system. Another operating factor may include nighttime or daytime operation of the hybrid photovoltaic system in either off grid or on grid operation. Also consideration of whether or not the photovoltaic array is illuminated or not or illuminated to a sufficient level leads to the load and/or power grid being supplied by a number of combinations. The number of combinations for example may be to supply the load solely from the photovoltaic array, the photovoltaic array and power grid, the power grid alone, solely from the battery, the photovoltaic array and the battery or the photovoltaic array and the battery and the power grid.
Referring now to the drawings, reference is now made to FIG. 1a which shows a hybrid photovoltaic system 10 according to an embodiment of the present invention. System 10 typically includes a direct current (DC) source 104, grid tied inverter 150, load 120, grid switch controller 126, backup switch 122, charger 124, battery 128 and backup inverter 152. DC source 104 may typically be a bank of interconnected batteries, photovoltaic arrays, DC generator or DC producing fuel cells. DC source is connected at nodes A and B to an input of grid tied inverter 150. Grid tied inverter 150 typically may be rated to perform a conversion of 10000 watts of DC power provided by DC source 104 to substantially 10000 watts of alternating current (AC) power on the output of inverter 150 at nodes C and D. The conversion typically involves a maximum power point tracking (MPPT) algorithm utilized by inverter 150 such that inverter 150 sets the input voltage (V1) to inverter 150.
Output of inverter 150 at nodes C and D is connected to the input of grid switch controller 126, load 120 and the alternating current (AC) side of backup inverter 152. Backup inverter 152 is typically rated at 1000 watts. Load 120 typically is an AC supply to a domestic residence. The domestic supply receives the AC supply from power grid 140 and/or AC voltage (V2) provided by inverter 150. Grid switch controller 126 typically senses the 25 presence or absence of power grid 140 to provide a status of grid 140. The status of grid 140 is typically conveyed to grid tied inverter 150 using bi-directional control line CTRL3. Grid switch controller 126 typically connects and reconnects grid 140 to nodes C and D. Grid switch controller 126 typically includes a double pole switch which connects and reconnects grid 140 to nodes C and D.
An input to backup switch 122 connects at nodes A and B. The output of switch 122 connects to the input of charging circuit 124. Backup switch 122 typically connects and reconnects the 5 output of DC supply 104 to the input of charging circuit 124. Charging circuit 124 typically may be a DC to DC converter or a typically charge circuit which uses a linear regulator. According to one aspect of the present invention, charging circuit 124 may not be required and backup switch 122 typically connects and reconnects the output of DC supply 104 to the input of backup inverter 152. Placed in parallel across the input of backup inverter 152 is a 10 battery 128. Backup inverter 152 typically operates in a bi-directional mode of power conversion. The bi-directional mode of inverter 152 converts AC power to DC power and vice versa. When grid 140 is not present (i.e. off grid operation), DC power from DC source 104 and/or battery 128 is converted to AC power on nodes C and D. When grid 140 is present (i.e. on grid operation), backup inverter 152 converts AC power from grid 140 and/or AC power from grid tied inverter 150 to a DC power which charges battery 128. The DC power which charges battery 128 in on grid mode, is typically applied with the input of charging circuit isolated from DC source 104 via backup switch 122. The direction of power conversion of backup inverter 152 is from a control signal applied to control line CTRL2 from inverter 150. The operation of backup switch 122 is from a control signal applied to control line CTRL3 from inverter 150. Control signals between grid switch controller 126, grid tied inverter 150, backup switch 122 and backup inverter 152 may be wireless or by power line communications.
Reference is now made to FIG. 1b which shows further details of DC supply 140 according to an exemplary embodiment of the present invention. DC power is supplied by DC source 140 at positive and negative nodes A and B respectively, by parallel connected photovoltaic strings 120. Details of only one of strings 120 are shown explicitly. In each of strings 120, direct current power sources 116 are serially connected. Each direct current power source 116 includes a photovoltaic panel 101 connected to an electronic module or photovoltaic module 102. Outputs of photovoltaic modules 102 are connected in series to form serial string 120. Photovoltaic modules 102 may be direct current (DC) to DC converters such as a buck circuit, boost circuit or buck-boost circuit. Attached to photovoltaic modules 102 is a processor 132 which accesses a memory 130. A transceiver 108 is attached to the output of electronic module 102 and to processor 132. According to a feature of the present invention, one of photovoltaic modules 102 referenced 102a is a master electronic module 102a of string 120 and controls and communicates with the other modules, i.e. slave modules 102 via power line communications or wireless link. According to this exemplary use of DC source 140, control of input voltage V1 may be an independent control such that inverter 150 sets the input voltage (V1) to inverter 150 and/ or that inverter 150 instructs (via power line communication) DC-DC converters 102 to set the input voltage (V1) to inverter 150.
Reference is now made to FIG. 2a which shows a method 201 according to an embodiment of the present invention. Method 201 performs off grid or on grid operation of hybrid photovoltaic system 10 shown in FIGS. 1a and 1b. During off grid operation, DC source 104 typically supplies battery 128 and/or load 120 using backup inverter 152. During on grid operation, DC source 104 typically supplies power grid 140 and/or load 120 using grid tied inverter 150 and with battery 128 being charged using backup inverter 152.
Typically grid switch controller 126 senses the presence or absence of power grid 140 in step 203. In decision 205, if power grid 140 absent, connection to grid 140 to nodes C and D is disconnected using switch controller 126 (step 207). Grid switch controller 126 typically isolates both live and neutral of power grid 140 from nodes C and D using a double pole single throw switch. Once grid 140 is disconnected in step 207, off grid operation of system 10 continues by use of method 221 applied to system 10.
In decision 205, if power grid 140 is present, connection to grid 140 at nodes C and D using switch controller 126 typically disconnects DC source 104 from the input of backup inverter 152 and/or charge circuit 124 (step 209). With DC source 104 disconnected from the input of backup inverter 152 and/ or charge circuit 124 on grid operation of system 10 continues by use of method 251 applied to system 10.
Reference is now made to FIG. 2b which shows method 221 in greater detail, according to an embodiment of the present invention. With connection to grid 140 disconnected using switch controller 126 in step 207, DC source 104 is connected (step 225) to the input of charger circuit 124. According to one aspect of the present invention, charging circuit 124 may not be required and backup switch 122 typically connects the output of DC supply 104 to the input or DC terminal of backup inverter 152. DC power from the output of charger circuit 124 or DC source 104 on the input of backup inverter 152, is then converted to an AC voltage (V2) power on the output or AC terminal of backup inverter 152 (step 227). The AC voltage (V2) power now serves as a local grid. The local grid may be sensed by grid tied inverter 150 on the AC side of grid tied inverter 150. Typically, the frequency of the local grid may be made to vary by backup inverter 152 as an indication to grid tied inverter 150 of loading on backup inverter 152. The frequency of the local grid may be made to vary in a range from 49 hertz to 51 hertz in increments of 0.1 hertz. Alternatively the phase angle, current or voltage of the local grid may be made to vary by backup inverter 152 as an indication to grid tied inverter 150 of loading on backup inverter 152. Loading on backup inverter 152 may be dependent on the state of charge/discharge of battery 128, power demand by load 120 and the amount of power supplied by DC source 104. The indication is typically used by grid tied inverter 150 to control a maximum level of input voltage (V1) on the input of inverter 150 and the input of charger circuit 124. The level of input voltage (V1) on the input of inverter 150 typically ranges from 120 volts to an upper maximum of 140 volts. In decision 235 if voltage level (V1) exceeds the upper maximum of 140 volts, DC source 104 is disconnected from the input of charger 124 (step 237) using backup switch 122. Otherwise sensing presence of power grid 140 continues with step 203.
Reference is now made to FIG. 2c which shows method 251 in greater detail according to an aspect of the present invention. With DC source 104 disconnected from the input of backup inverter 152 and/or charge circuit 124 in step 209, the output of grid tied inverter 150 at nodes C and D is connected to power grid 140 via switch controller 126. DC power on the input of grid tied inverter 150 is converted to an output AC power on nodes C and D (step 255). The output AC power supplies power 140 and/or load 120. The output AC power is additionally converted by backup inverter 152 to a DC power on the DC side of backup inverter 152 (step 257). The DC power charges battery 128 is typically controlled by back up inverter 152 and/or grid tied inverter 150. Sensing of power grid 140 continues with step 203.
By way of numerical example a DC-DC module 102 has a typical maximum output current of 16.4 A. Assuming minimum charger 124 voltage will be 120V, the total power on one string 120 is limited to 16.4 A*120V≈2000 W. If more than 2000 Watts are installed on one string 120, system 10 will still work. Module 102 will be current limited and the total number of strings will not supply more than 2000 Watts during off-grid/charge mode.
By way of another numerical example, a 3240 Kilowatt PV system 10 which has 2 strings 120 of 9 modules 102, where each module 102 is rated at 180 watts. Each string 120 power will be 9*180 W=1620 W. In on grid mode, V1=250−500V, total string current=6.48−12.96 A and each string 120 current=3.24−6.48 A. In on grid mode, V1=120−140 V, total string current=23.1−27 A and each string 120 current=11.5−13.5 A.
The term “DC source” as used herein refers to and includes DC available from photovoltaics cells, wind powered DC generators, hydro powered turbines, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitor types, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines.
The term “comprising” as used herein, refers to an open group of elements for example, comprising an element A and an element B means including one or more of element A and one or more of element B and other elements other than element A and element B may be included.
The terms “sensing” and “measuring” are used herein interchangeably.
The definite articles “a”, “an” is used herein, such as “an inverter”, “a switch” have the meaning of “one or more” that is “one or more inverters or “one or more switches”.
Examples of various features/aspects/components/operations have been provided to facilitate understanding of the disclosed embodiments of the present invention. In addition, various preferences have been discussed to facilitate understanding of the disclosed embodiments of the present invention. It is to be understood that all examples and preferences disclosed herein are intended to be non-limiting.
Although selected embodiments of the present invention have been shown and described individually, it is to be understood that at least aspects of the described embodiments may be combined. Also although selected embodiments of the present invention have been shown and described, it is to be understood the present invention is not limited to the described embodiments. Instead, it is to be appreciated that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and the equivalents thereof.

## Claims

1. A system comprising a first inverter, a second inverter, a switch, and direct-current power nodes; wherein alternating-current terminals of the first inverter are connected to alternating-current terminals of the second inverter; and wherein the direct-current power nodes are connected to direct-current terminals of the first inverter and connected through the switch to direct-current terminals of the second inverter.

2. The system of claim 1, further comprising a second switch and alternating-current power nodes, wherein the alternating-current power nodes are connected through the second switch to the alternating-current terminals of the first inverter and the alternating-current terminals of the second inverter.

3. The system of claim 2, further comprising a controller configured to:
close the second switch in response to a presence of an external energy source energizing the alternating-current power nodes; and
open the second switch in response to an absence of the external energy source energizing the alternating-current power nodes.

4. The system of claim 2, further comprising a controller configured to:
open the switch in response to a presence of an external energy source energizing the alternating-current power nodes; and
close the switch in response to an absence of the external energy source energizing the alternating-current power nodes.

5. The system of claim 2, the second inverter being configured to:
convert alternating-current power on the alternating-current terminals of the second inverter to direct-current power on the direct-current terminals of the second inverter in response to a presence of an external energy source energizing the alternating-current power nodes; and
convert direct-current power on the direct-current terminals of the second inverter to alternating-current power on the alternating-current terminals of the second inverter in response to an absence of the external energy source energizing the alternating-current power nodes.

6. The system of claim 2, wherein the system is configured to operate in a plurality of modes when a load is connected to the alternating-current terminals of the first inverter and the alternating-current terminals of the second inverter, a battery is connected to the direct-current terminals of the second inverter, a power source is connected to the direct-current power nodes, and a power grid is connected to the alternating-current power nodes, the plurality of modes including at least one of:
the system being configured to supply power to the load solely from the power source;
the system being configured to supply power to the load solely from the power grid;
the system being configured to supply power to the load solely from the battery;
the system being configured to supply power to the load solely from the power source and the power grid;
the system being configured to supply power to the load solely from the power source and the battery; and
the system being configured to supply power to the load solely from the power source, the battery, and the power grid.

7. The system of claim 1, further comprising a battery connected to the direct-current terminals of the second inverter.

8. The system of claim 7, further comprising a charging circuit connected between the battery and the direct-current power nodes through the switch.

9. The system of claim 1, further comprising a power source connected to the direct-current power nodes.

10. The system of claim 1, further comprising a load connected to the alternating-current terminals of the first inverter and the alternating-current terminals of the second inverter.

11. A method comprising:
connecting direct-current terminals of a first inverter to direct-current terminals of a second inverter in response to sensing a power grid not being energized, the direct-current terminals of the first inverter being connected to a power source, and alternating-current terminals of the first inverter and alternating-current terminals of the second inverter being connected to a load; and
disconnecting the direct-current terminals of the first inverter from the direct-current terminals of the second inverter in response to sensing the power grid being energized.

12. The method of claim 11, further comprising:
connecting the alternating-current terminals of the first inverter and the alternating-current terminals of the second inverter to the power grid in response to the sensing of the power grid being energized; and
disconnecting the alternating-current terminals of the first inverter and the alternating-current terminals of the second inverter from the power grid in response to the sensing of the power grid not being energized.

13. The method of claim 11, further comprising:
the sensing of the power grid being energized and not being energized.

14. The method of claim 11, further comprising:
converting, with the first inverter, direct-current power received from the power source to alternating-current power output to the power grid and to the load in response to the sensing of the power grid being energized.

15. The method of claim 11, further comprising:
converting, with the second inverter, direct-current power received from the power source to alternating-current power output to the load in response to the sensing of the power grid not being energized.

16. The method of claim 11, further comprising:
converting, with the second inverter, direct-current power received from a battery to alternating-current power output to the load in response to the sensing of the power grid not being energized.

17. The method of claim 11, further comprising:
charging a battery connected the direct-current terminals of the second inverter with power provided from the power source or from the power grid.

18. The method of claim 17, the charging comprising:
converting, with the second inverter, alternating-current power provided from the first inverter or from the power grid to direct-current power output to the battery in response to the sensing of the power grid being energized.

19. The method of claim 17, the charging comprising:
charging the battery from the power source through a connection of the direct-current terminals of the second inverter to the direct-current terminals of the first inverter made in response to the sensing the power grid not being energized.

20. The method of claim 19, further comprising:
disconnecting the battery and the direct-current terminals of the second inverter from the power source and from the direct-current terminals of the first inverter in response to sensing a voltage across the direct-current terminals of the first inverter exceeding a predetermined threshold.