Abstract:
A string inverter for use with a photovoltaic array includes a string-level DC input channel for receiving DC power from a photovoltaic array. The input channel performs channel-level maximum power point tracking. An input-output channel connects the string inverter to a battery pack. A DC to DC buck-boost circuit between the at least one DC input channel and the at least one input-output channel prevents more than a predetermined amount of DC voltage from reaching the battery pack. A DC to AC inverter circuit having an AC output serving as an output of the string inverter. A revenue grade power meter is configured to measure the AC output of the string inverter.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Application No. 62/151,257, filed Apr. 22, 2015, which is hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     Due to decreasing costs, state and federal tax incentives, and increased evidence and awareness of the correlation between CO 2  emissions and climate change, photovoltaic or “solar” power systems are becoming increasingly popular with consumers, businesses and utility companies. A basic solar power system consists of an array of solar panels connected together on one or more strings, a combiner for combining the outputs of the one or more strings, one or more string inverters for converting the combined direct current (DC) output from the strings to alternating current (AC), and a physical interface to AC grid power—typically on the load side of the utility meter, between the meter and the customer&#39;s main electrical panel. 
     The next step in the evolution of solar power system is on-site energy storage. Energy storage is important for a number of reasons. First, it provides a potential source of power when the grid is unavailable (outage). Second, in states and/or countries where the customer is unable to be compensated for sending power back to the grid or is compensated below the retail rate, it allows the customer to store the energy generated during the day—specifically when the solar array is generating the most power—and then consume that power after the sun has set reducing the customer&#39;s peak demand. Third, it allows the customer to supply power back to the grid at a time when the grid needs power the most. Localized energy storage can help utilities stabilize the grid by supplying power to enhance demand response, shave demand peaks, and shift loads to times of lower demand. Fourth, it provides a mechanism for storing grid power when demand is lower (i.e., when there is a surplus of power), smoothing utility companies&#39; power demand curve from the bottom up. Fifth, by enabling customers to store energy onsite, it may be possible to bill customers for energy supplied to back-up loads when the grid is unavailable (e.g., during an outage). 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with one embodiment, a string inverter for use with a photovoltaic array includes a string-level DC input channel for receiving DC power from a photovoltaic array. The input channel performs channel-level maximum power point tracking. An input-output channel connects the string inverter to a battery pack. A DC to DC buck-boost circuit between the at least one DC input channel and the at least one input-output channel limits the amount of DC voltage from reaching the battery pack so that it does not exceed a predetermined threshold. A DC to AC inverter circuit having an AC output serving as an output of the string inverter. A revenue grade power meter is configured to measure the AC output of the string inverter. 
     In one embodiment, the string inverter includes a three position switch, wherein in a first position, AC power is permitted to flow from the string inverter to a load side of a customer utility meter and one or more back-up loads, in a second position, power is permitted to flow from the load side of a customer utility meter to the one or more back-up loads bypassing the string inverter, and in a third position, all circuits tied to the string inverter&#39;s output are electrically disconnected from each other. 
     In another embodiment, the string inverter further includes a first circuit for metering the total DC power received at the at least one DC input channel from the photovoltaic array, a second circuit for metering a total DC power received at the at least one input-output channel from the battery pack, and a controller programmed to determine the portion of total AC output measured by the revenue grade power meter attributable to the at least one DC input channel and the at least one input-output channel. 
     In another embodiment, the string inverter is operable to receive AC power, to rectify the AC power to DC power, and to deliver the rectified DC power to the battery pack. 
     In accordance with another embodiment, a string inverter for use with a photovoltaic array includes at least one string-level DC input channel for receiving DC power from a photovoltaic array, at least one input-output channel for connecting the string inverter to a battery pack, a DC to AC inverter circuit having an AC output serving as an output of the string inverter, and a switch controlling a flow of power through the string inverter. In one variation, the switch is configured so that in a first state, AC power is permitted to flow from the string inverter to a load side of a customer utility meter and one or more back-up loads, in a second state, power is permitted to flow from the load side of a customer utility meter to the one or more back-up loads bypassing the string inverter, and in a third state, all circuits tied to the string inverter&#39;s output are electrically disconnected from the inverter. 
     In accordance with another embodiment, a string inverter for use with a photovoltaic array includes at least one string-level DC input channel for receiving DC power from a photovoltaic array, at least one input-output channel for connecting the string inverter to a battery pack, a DC to AC inverter circuit having an AC output serving as an output of the string inverter, and a revenue grade power meter configured to measure the AC output of the string inverter. In one variation, the revenue grade power meter includes a first circuit for metering a total DC power received at the at least one DC input channel from the photovoltaic array, a second circuit for metering a total DC power received at the at least one input-output channel from the battery pack, and a controller programmed to determine the portion of total AC output measured by the revenue grade power meter attributable to the at least one DC input channel and the at least one input-output channel. 
     In accordance with still another embodiment, a string inverter for use with a photovoltaic array includes at least one string-level DC input channel for receiving DC power from a photovoltaic array, at least one input-output channel for connecting the string inverter to a battery pack, a DC to DC buck-boost circuit coupled to the at least one DC input channel, the DC to DC buck-boost being configured to prevent more than a predetermined amount of voltage from reaching the battery pack, and a DC to AC inverter circuit having an AC output serving as an output of the string inverter. 
     In accordance with yet another embodiment, a string inverter for use with a photovoltaic array includes at least one string-level DC input channel for receiving DC power from a photovoltaic array, at least one input-output channel for connecting the string inverter to a battery pack, a DC to DC buck-boost circuit coupled to the at least one input-output channel, the DC to DC buck-boost being configured to prevent more than a predetermined amount of voltage from reaching the battery pack, and a DC to AC inverter circuit having an AC output serving as an output of the string inverter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of an exemplary solar energy generation system according to one embodiment; 
         FIG. 2  illustrates a more detailed block diagram of the system shown in  FIG. 1 ; 
         FIG. 3  illustrates two sides of an inverter box according to one embodiment; 
         FIG. 4  illustrates a technique for metering the amount of power generated by the system; and 
         FIGS. 5-9  illustrate various possible power flow states of the inverter system according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to unlock the full potential of energy storage devices in solar energy generation systems and to ensure safe and efficient operation, there is a need for more sophisticated control systems and related circuitry that are capable of interfacing with high voltage, on-site energy storage systems. To that end, this disclosure teaches systems, methods, devices and related circuits that improve the operation of solar energy generation systems that incorporate localized energy storage. 
       FIG. 1  illustrates a block diagram of an exemplary solar energy generation system according to various embodiments disclosed herein, and  FIG. 2  illustrates a more detailed block diagram of the same system showing additional internal components, overall system wiring and inverter wiring compartment interconnections. 
     In the system of  FIGS. 1 and 2 , a pair of photovoltaic (PV) strings  102  are input to Inverter PCS (power control system)  104 . Each string may comprise a plurality of PV panels (not shown) connected serially with an additive direct current (DC) voltage somewhere between 100 and 1000 volts, depending on such factors as the number of panels, their efficiency, their output rating, ambient temperature and irradiation on each panel. Also, each MPPT (maximum power-point tracking) channel input may receive the output of two or more separate strings connected in parallel (i.e., a two (or more)-to-one combiner at each MPPT channel input). 
     In some embodiments, when the high voltage DC line from each string is input to the inverter, it is subject to maximum power-point tracking (MPPT) at the string level (e.g., dual MPPT in the exemplary system of  FIGS. 1 &amp; 2 ). Alternatively, each module, or a number of individual modules in the respective strings, may include a DC optimizer that performs MPPT at the module level or N-module level output, rather than at the string level. The various embodiments are compatible with either centralized or distributed MPPT. 
     In some embodiments, the inverter may include a DC/DC conversion stage  106  at the PV input side. DC/DC stages are commonly employed to insure that the voltage supplied to the DC/AC stage  108  is sufficiently high for inversion. However, unlike conventional inverters, the inverter of  FIGS. 1 and 2  also includes a DC link bus attached to a battery pack  110  so that the DC power coming from the strings can be used to deliver DC power to battery pack  110  to “charge the battery.” The DC link bus is represented by the capacitor bank shown between the two DC-DC converters and the DC-AC section in  FIG. 1 . Battery pack  110  has a minimum and maximum associated operating voltage window. Because battery pack  110  has a maximum exposed input voltage limit that, in many cases, is lower than the theoretical maximum DC voltage coming off of the strings (open circuit voltage, V OC ), various embodiments of the invention include a buck-boost circuit  112  between the string-level PV input of inverter  104  and the DC-link connection to the battery pack. The inclusion of buck-boost circuit  112  will prevent voltages above a safe threshold from being exposed to battery pack  110  thereby eliminating the possibility of damage to battery pack  110  from overvoltage stress. 
     It should be appreciated that inverter  104  may have more than one mode of operation. In some modes, no power may be flowing from PV strings  102  to battery pack  110 , while in other modes power may be flowing exclusively to the battery pack, while in still further modes power may be flowing to a combination of the battery pack and the AC grid. In a first mode, illustrated in  FIG. 7 , all available PV power may go to battery pack  110  as priority, with any surplus power being supplied to DC/AC stage  108  ( FIG. 1 ) of inverter  104  to be supplied to the grid  114  or delivered to back-up loads  116 . In a second mode, illustrated in  FIG. 5 , all generated power may be supplied to DC/AC stage  108  of inverter  104  and either used to power back-up loads  116 , or supplied to the grid  114 . In yet other modes, illustrated in  FIGS. 6 and 8 , battery pack  110  may be discharged to DC/AC stage  108  of inverter  104  alone ( FIG. 8 ) and/or with PV power from the strings  102  ( FIG. 6 ) to supply power to the AC grid  114  and/or back-up loads  116 . In a further mode, illustrated in  FIG. 9 , power may come from the grid  114 , through DC/AC inverter  108  ( FIG. 1 ) to charge battery pack  110 , for example, at a time when the PV array  102  is not generating power and demand for power is at its lowest point (e.g., after sunset). In various embodiments, the selection of mode may be controlled by logic in battery pack  110 , in inverter  104 , or in both, or selection could be based on signals from an external source. The various modes of operation are described in greater detail further below in the context of  FIGS. 5-9 . 
     With continued reference to the exemplary solar energy generation system of  FIG. 1 , in this figure, there are two blocks  106 / 112  labeled “DC/DC (Buck-Boost)”. These blocks  106 / 112  represent alternative embodiments. In the first embodiment, the buck-boost circuit is located in the DC-link at the front end of inverter  104  (as depicted by block  106 ) so that the DC input(s) coming from PV strings  102  are always subject to buck or boost, keeping the voltage at DC link bus sufficiently high level for inversion while also preventing too high of a voltage from being presented to battery pack  110 . In this embodiment, there is no need for a second buck-boost circuit anywhere else. In the second embodiment, the buck-boost circuit is located between the DC link bus of Inverter  104  and battery pack  110  (as depicted by block  112 ) such that the high voltage DC inputs from strings  102  only go through the buck-boost whenever voltage is exposed to battery pack  110 . In this alternative embodiment, there may be an additional DC-DC boost stage at the input to the inverter but no need for a second buck circuit anywhere else. Either embodiment will prevent battery pack  110  from being exposed to excessively high voltages generated by the PV array. The voltage from the array could be as high as 500 Volts, or even 750 Volts in the case of a 1 kV PV system. 
     It should be appreciated that battery pack  110  in  FIGS. 1 and 2  may be an exemplary commercially available residential li-ion battery pack with its own battery  120  only or battery  120  with DC/DC boost converter  118  or other topologies. Alternatively, battery  120  may be a lead acid battery, advanced lead acid battery, flow battery, organic battery, or other battery type. The various embodiments disclosed herein are compatible with numerous different battery chemistries. Various disclosed embodiments will work with other commercially available battery packs as well, however, the embodiments may have particular utility for systems that use high voltage battery packs (e.g., &gt;48 volts) such as 200V-750V battery packs. As depicted by the dashed line boxing inverter PCS  104  and battery pack  110  in  FIG. 2 , inverter PCS  104  and battery pack  110  may be housed in a wall-mounted housing located inside or outside of a residence or a commercial building. Alternatively, battery pack  110  and inverter PCS  104  may be located in separate housings. 
     Referring to  FIG. 3 , this figure illustrates two sides of an inverter box according to various embodiments of the invention. The left side, labeled “inverter,” includes internal components that are generally in a fixed configuration and not intended to be modified by the installer or operator. The right hand side, labeled “wiring box,” includes wire interfaces to AC grid  114  as well as a connection to protected or back-up home loads  116 . For example, as shown in  FIG. 2 , back-up loads  116  could include an AC compressor, fan, and/or clothes washer. A refrigerator/freezer combination could be another back-up load. These are just examples and are not intended to be limiting. The particular back-up load may be at the discretion of the installer or homeowner need by wiring the inverter&#39;s AC output directly to one or more breakers in the home owner&#39;s or business&#39;s main electric panel. Providing a separate connection via the inverter PCS wiring box for back-up loads may enable the battery pack to serve as back-up power for certain loads in cases where the grid power is lost. It is noted that the grid standard depicted in  FIG. 2  (240V L-L/120V L-N) is merely exemplary. Other grid standards, such as  208  1-ph, 3-ph/ 277  1-ph/ 480  3-ph, may be integrated with the various techniques described herein. 
     In a typical solar power generation system, the inverter includes a high accuracy alternating current (AC) revenue grade meter (RGM) at the output so that the solar provider and/or customer can ascertain how much power the system is generating at any given moment and over time, and in some cases so that the customer can be billed or compensated with energy credit. Typically, this information is transmitted wirelessly from the inverter to a wireless router located in the home or business so that it can be viewed on a local or remote graphical user interface. However, with the addition of a battery, it may be desirable to have the ability to make a more granular measurement of not only the inverter&#39;s output to the AC grid or back-up loads, but also the respective outputs of the photovoltaic system and the battery (e.g., what percentage of the total AC power is attributed to each source). In certain cases, such as when there is an outage of grid, it may be desirable to bill a customer for the power supplied to their back-up loads via the battery pack or PV power, since ordinarily when the grid is down, a string inverter stops outputting power. 
     In order to accomplish this, as depicted in  FIGS. 3 and 4 , revenue grade meter  312 , in certain embodiments of the invention, makes separate DC measurements of power coming into the inverter from the PV system and the battery. Measurement circuit  410  accurately meters a total DC power received from photovoltaic array  102  by measuring the current (I), voltage (V) and power (P) at the DC input channel. Measurement circuit  412  meters a total DC power received from battery pack  110  by measuring the current (I), voltage (V) and power (P) at the input-output channel. A controller  414  may be programmed to determine the portion of total AC output measured by the revenue grade power meter attributable to photovoltaic array  102  and battery pack  110 . By doing this, the combined AC output power measured by revenue grade meter  312  can be separately apportioned into power being generated by the PV system and the power being supplied by the battery pack. Also, as seen in  FIGS. 2 and 3 , in various embodiments, the wiring box side of inverter PCS  104  may include a single DC disconnect that enables an operator to manually shut off all DC power from battery and PV system. 
     An additional feature of the embodiment illustrated in  FIG. 3  is a bypass switch  318  built into or external to the inverter PCS wiring box. In some embodiments, the switch may be a three-position switch. In a first position, switch  318  may connect the inverter to grid  114  and also connect the inverter to back-up loads  116  using an internal relay, such as relay  316  depicted in  FIG. 3 . In the second position, switch  318  may bypass back-up loads  116  directly to grid  114 . The third position of the switch may open all circuits so that everything is disconnected, meaning, AC grid  114  and back-up loads  116  are disconnected from the inverter. This may be useful, for example, if the inverter side of the inverter PCS fails and needs to be serviced or replaced. As depicted in  FIG. 3 , the internal components of the inverter may include anti-islanding relay  314  and protected load relay  316 . Relays  314  and  316  together with bypass switch  318  route power between the inverter and grid  114  and back-up loads  116  based at least in part on the position of bypass switch  318 . 
       FIGS. 5-9  illustrate various possible power flow states of the inverter PCS system according to various embodiments. The various numerical values indicated in  FIGS. 5-9  are exemplary and are provided solely for the purpose of more clearly conveying the various exemplary power flow states. In state 1, illustrated in  FIG. 5 , inverter  104  will deliver a maximum power output equivalent to the maximum power rating of PV array  102  less any conversion losses attributable to inverter  104 . In this full PV inverter mode, battery pack  110  is on standby (not charging or discharging). In this state, the AC output power may, for example, be ˜6 kW to grid  114  or back-up loads  116 . 
     Referring to  FIG. 6 , this figure illustrates state 2, a combined PV and battery inverter mode, where the combined output of PV array  102  and battery pack  110  is inverted and supplied to AC grid  114  or back-up loads  116 . In the example of  FIG. 6 , inverter  104  delivers a maximum AC output of, for example, ˜6 kW, but only partial power (e.g., 4 kW) of it originates with solar power generation system  102  and the remaining power (e.g., 2 kW) is supplied by discharge capacity of battery pack  110 . The AC output power may be delivered to grid  114  or back-up loads  116 . 
       FIG. 7  illustrates state 3, another partial inverter mode, where PV array  102  is generating its theoretical maximum output (e.g., 6 kW+), which is supplied to inverter PCS  104 . Instead of inverting all of that power, primarily partial power (e.g., 2 kW) is utilized to charge battery  110 , with the remainder inverted and supplied to AC grid  114  or back-up loads  116 . In an alternate exemplary embodiment where PV array  102  generates 8 kW+, battery pack  110  can be charged with 2 kW with a full 6 kW being provided to the AC grid/Back-up loads. 
       FIG. 8  illustrates state 4, called full battery inverter mode. In this mode, all the power supplied by inverter PCS  104  to AC grid  114  or back-up loads  116  originates from discharging of battery  110 . This may occur, for example, at night or when PV system  102  is otherwise unable to generate power. In this example, the discharging battery  110  is, for example, only supplying ˜2 kW of AC power. Typically, the power capacity of battery  110  will be less than or equal to the maximum output of PV array  102 , though the disclosed embodiments are not intended to be limited as such. This mode may be useful to help level load sharing/moving situations and with peak shaving. 
       FIG. 9  illustrates a 5th state of power flow. In this state, like state 4, PV system  102  generates no power, however, grid  114  is supplying power back through the bi-directional inverter to charge battery  110 . This could be done, for example, at a time when grid power demand is relatively low and less expensive. Then, later in the day, when demand quickly rises, the system could shift to mode 4 or a variant of that, where battery  110  either supplies power to grid  114  or to back-up loads  116 . 
     The embodiments described herein are not to be limited in scope by the specific embodiments described above. Indeed, various modifications of the embodiments, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Further, although some of the embodiments have been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that their usefulness is not limited thereto and that they can be beneficially implemented in any number of environments for any number of purposes. Accordingly, the disclosure should be construed in view of the full breath and spirit of the embodiments as disclosed herein.