Microinverters for photovoltaic applications

Abstract

A method for controlling power converters in an energy management system is provided and comprises calculating an average AC voltage value, calculating a magnitude of a difference between the average AC voltage value and a current AC voltage in a cycle of operation of the power converters, determining if the calculated magnitude exceeds a threshold, and if the calculated magnitude exceeds the threshold, adding an extra cycle of delay without producing power so that the power converters are desynchronized.

Description

MICROINVERTERS FOR PHOTOVOLTAIC APPLICATIONSBACKGROUNDField of the Disclosure

[0001] Embodiments of the present disclosure relate generally to power conversion systems, and for example, to microinverters that are configured to desynchronize when in one or more modes of operation.Description of the Related Art

[0002] Power converters (microinverters) play an indispensable role in various electrical power applications, such as converting DC from renewable energy resources to power-grid compliant AC. Some microinverters are operable in one or more modes. For example, some microinverters are operable in two modes of operation. For example, a microinverter can be operable in a first mode of operation for normal conditions (e.g., adequate light) and can be operable in a second mode of operation for less than normal conditions (e.g., low-light). In the second mode of operation, a microinverter can be configured to capture energy in low-light, such as when shadows or clouds are passing over a solar array. In the second mode of operation, however, the microinverters can sometimes synchronize with each other, which can cause flicker in an energy management system, which, in turn, can cause disturbance on a grid.

[0003] Therefore, there is a need in the art for improved microinverters that are configured to desynchronize when in one or more modes of operation.SUMMARY

[0004] In accordance with at least some aspects of the disclosure, there is provided a method for controlling power converters in an energy management system comprising calculating an average AC voltage value, calculating a magnitude of a difference between the average AC voltage value and a current AC voltage in a cycle of operation of the power converters, determining if the calculated magnitude exceeds a threshold, and if the calculated magnitude exceeds the threshold, adding an extra cycle of delay without producing power so that the power converters are desynchronized.

[0005] In accordance with at least some aspects of the disclosure, there is provided a non-transitory computer readable storage medium having instructions stored thereon that when executed by a processor perform a method for controlling power converters in an energy management system. The method comprises calculating an average AC voltage value, calculating a magnitude of a difference between the average AC voltage value and a current AC voltage in a cycle of operation of the power converters, determining if the calculated magnitude exceeds a threshold, and if the calculated magnitude exceeds the threshold, adding an extra cycle of delay without producing power so that the power converters are desynchronized.

[0006] In accordance with at least some aspects of the disclosure, there is provided an energy management system comprising a plurality of DC power sources, a plurality of corresponding power converters connected to the plurality of DC power sources and configured to calculate an average AC voltage value, calculate a magnitude of a difference between the average AC voltage value and a current AC voltage in a cycle of operation of the plurality of corresponding power converters, determine if the calculated magnitude exceeds a threshold, and if the calculated magnitude exceeds the threshold, add an extra cycle of delay without producing power so that the plurality of corresponding power converters are desynchronized.

[0007] These and other features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

[0009] Figure 1 is a block diagram of an energy management system, in accordance with one or more embodiments of the present disclosure;

[0010] Figure 2 is a block diagram of a controller, in accordance with one or more embodiments of the present disclosure;

[0011] Figure 3 is a diagram of a correlation of AC voltage with power production, in accordance with one or more embodiments of the present disclosure;

[0012] Figure 4 is a flowchart of a method for desynchronizing power converters when the power converters are in a burst mode of operation, in accordance with one or more embodiments of the present disclosure; and

[0013] Figure 5 is a sequencing diagram of the method of Figure 4, in accordance with one or more embodiments of the present disclosure.DETAILED DESCRIPTION

[0014] Methods and apparatus described herein provide improved microinverters that are configured to desynchronize when in one or more modes of operation. For example, a method for controlling power converters in an energy management system can comprise calculating an average AC voltage value, calculating a magnitude of a difference between the average AC voltage value and a current AC voltage in a cycle of operation of the power converters, determining if the calculated magnitude exceeds a threshold, and if the calculated magnitude exceeds the threshold, adding an extra cycle of delay without producing power so that the power converters are desynchronized. The methods and apparatus described herein reduce, if not eliminate, unwanted synchronization of microinverters which allows an energy management system to operate without causing disturbance to a grid.

[0015] Figure 1 is a block diagram of a system 100 (energy management system) for power conversion using one or more embodiments of the present disclosure. This diagram only portrays one variation of the myriad of possible system configurations and devices that may utilize the present disclosure.

[0016] The system 100 is a microgrid that can operate in both an islanded state (off- grid state) and in a grid-connected state (i.e. , when connected to another power grid (such as one or more other microgrids and / or a commercial power grid). The system 100 comprises a plurality of power converters 102-1 , 102-2, ... ,102-N, 102-N+1 , and 102-N+M collectively referred to as power converters 102 (which also may be called power conditioner units (PCU)); a plurality of DC power sources 104-1 , 104-2, ...,104-N, collectively referred to as DC power sources 104 (e.g., resources); a plurality of energy storage devices / delivery devices 120-1 , 120-2, ....120-M collectively referred to as energy storage / delivery devices 120; a system controller 106; a plurality of BMUs 190-1 , 190-2, ... ,190-M (battery management units) collectively referred to as BMUs 190; a system controller 106; a bus 108; a load center 110; and an HD 140 (island interconnect device) (which may also be referred to as a microgrid interconnect device (MID)). In some embodiments, the energy storage / delivery devices are rechargeable batteries (e.g., multi-C-rate collection of AC batteries) which may be referred to as batteries 120, although in other embodiments the energy storage / delivery devices may be any other suitable device for storing energy and providing the stored energy. Generally, each of the batteries 120 comprises a plurality cells that are coupled in series, e.g., eight cells coupled in series to form a battery.

[0017] Each power converter 102-1 , 102-2....102-N is coupled to a DC power source 104-1 , 104-2....104-N, respectively, in a one-to-one correspondence, although in some other embodiments multiple DC power sources may be coupled to one or more of the power converters 102. The power converters 102-N+1 , 102-N+2... 102- N+M are respectively coupled to plurality of energy storage devices / delivery devices 120-1 , 120-2... 120-M via BMUs 190-1 , 190-2.. ,190-M to form AC batteries 180-1 , 180-2. ,.180-M, respectively. Each of the power converters 102-1 , 102-2...102-N+M comprises a controller 114-1 , 114-2...114-N+M (collectively referred to as the inverter controllers 114) for controlling operation of the power converters 102-1 , 102- 2...102-N+M.

[0018] In some embodiments, such as the embodiment described below, the DC power sources 104 are DC power sources and the power converters 102 are bidirectional inverters such that the power converters 102-1 ...102-N convert DC power from the DC power sources 104 to grid-compliant AC power that is coupled to the bus 108, and the power converters 102-N+1...102-N+M convert (during energy storage device discharge) DC power from the batteries 120 to grid-compliant AC power that is coupled to the bus 108 and also convert (during energy storage devicecharging) AC power from the bus 108 to DC output that is stored in the batteries 120 for subsequent use. The DC power sources 104 may be any suitable DC source, such as an output from a previous power conversion stage, a battery, a renewable energy source (e.g., a solar panel or photovoltaic (PV) module (solar cells), a wind turbine, a hydroelectric system, or similar renewable energy source), or the like, for providing DC power. In other embodiments the power converters 102 may be other types of converters (such as DC-DC converters), and the bus 108 is a DC power bus.

[0019] The power converters 102 are coupled to the system controller 106 via the bus 108 (which also may be referred to as an AC line or a grid). The system controller 106 generally comprises a CPU coupled to each of support circuits and a memory that comprises a system control module for controlling some operational aspects of the system 100 and / or monitoring the system 100 (e.g., issuing certain command and control instructions to one or more of the power converters 102, collecting data related to the performance of the power converters 102, and the like). The system controller 106 is capable of communicating with the power converters 102 by wireless and / or wired communication (e.g., power line communication) for providing certain operative control and / or monitoring of the power converters 102.

[0020] In some embodiments, the system controller 106 may be a gateway that receives data (e.g., performance data, MTTP data, etc.) from the power converters 102 and communicates (e.g., via the Internet) the data and / or other information to a remote device or system, such as a master controller (not shown). Additionally (or alternatively), the gateway may receive information from a remote device or system (not shown) and may communicate the information to the power converters 102 and / or use the information to generate control commands that are issued to the power converters 102.

[0021] The power converters 102 are coupled to the load center 110 via the bus 108, and the load center 110 is coupled to the power grid via the IID 140. When coupled to the power grid (e.g., a commercial grid or a larger microgrid) via the IID 140, the system 100 may be referred to as grid-connected; when disconnected from the power grid via the IID 140, the system 100 may be referred to as islanded. The IID 140 determines when to disconnect from / connect to the power grid (e.g., the IID 140may detect a grid fluctuation, disturbance, outage or the like) and performs the disconnection / connection. Once disconnected from the power grid (off-grid), the system 100 can continue to generate power as an intentional island, without imposing safety risks on any line workers that may be working on the grid, using one or more conventional droop control techniques. The HD 140 comprises a disconnect component (e.g., a disconnect relay) for physically disconnecting / connecting the system 100 from / to the power grid. In some embodiments, the HD 140 may additionally comprise an autoformer for coupling the system 100 to a split-phase load that may have a misbalance in it with some neutral current. In certain embodiments, the system controller 106 comprises the IID 140 or a portion of the IID 140.

[0022] The power converters 102 convert the DC power from the DC power sources 104 and discharge the batteries 120 to grid-compliant AC power and couple the generated output power to the load center 110 via the bus 108. The power is then distributed to one or more loads (for example to one or more appliances) and / or to the power grid (when connected to the power grid). Additionally (or alternatively), the generated energy may be stored for later use, for example using batteries, heated water, hydro pumping, F -to-hydrogen conversion, or the like. Generally, the system 100 is coupled to the commercial power grid, although in some embodiments the system 100 is completely separate from the commercial grid and operates as an independent microgrid.

[0023] In some embodiments, the AC power generated by the power converters 102 is single-phase AC power. In other embodiments, the power converters 102 generate three-phase AC power.

[0024] Figure 2 is a block diagram of the inverter controllers 114 in accordance with one or more embodiments of the present disclosure. The inverter controllers 114 comprises support circuits 204 and a memory 206, each coupled to a CPU 202 (central processing unit). The CPU 202 may comprise one or more conventionally available microprocessors or microcontrollers. Alternatively, the CPU 202 may include one or more application specific integrated circuits (ASICs). In other embodiments, the CPU 202 may be a microcontroller comprising internal memoryfor storing controller firmware that, when executed, provides the controller functionality described herein.

[0025] The support circuits 204 are well known circuits used to promote functionality of the CPU 202. Such circuits include, but are not limited to, a cache, power supplies, clock circuits, buses, input / output (I / O) circuits, and the like. The inverter controllers 114 may be implemented using a general-purpose computer that, when executing particular software, becomes a specific purpose computer for performing various embodiments of the present disclosure.

[0026] The memory 206 may comprise random access memory, read only memory, removable disk memory, flash memory, and various combinations of these types of memory. The memory 206 is sometimes referred to as main memory and may, in part, be used as cache memory or buffer memory. The memory 206 generally stores the OS 208 (operating system), if necessary, of the inverter controllers 114 that can be supported by the CPU capabilities. In some embodiments, the OS 208 may be one of a number of commercially available operating systems such as, but not limited to, LINUX, Real-Time Operating System (RTOS), and the like.

[0027] The memory 206 may store various forms of application software, such as a converter control module 210 for controlling operation of the power converter 102 when executed by the inverter controllers 114. The memory 206 may also store a database 214 for storing data related to the operation of the power converters 102.

[0028] For example, the memory 206 may further store an MPPT module 212 (maximum power point tracking) that, when executed by the inverter controllers 114, determines an operating point for biasing the PV module at a maximum power point (MPP). For example, the MPPT module 212 may comprise a commercially- available MPPT module for providing a global maximum power point tracking for the DC power sources 104 (PV module). For example, the MPPT module 212 algorithm determines a DC input current demand signal in order to operate the DC power sources 104 at a loading that produces a maximum output power for a given solar irradiance. For example, input and output voltages are determined by the DC power sources 104 and AC grid voltages, while a DC input current is determined by a PV MPPT algorithm used to extract a maximum power from the DC power sources 104. In at least some embodiments, the converter control module 210 is responsible fordetermining an AC output current to keep the capacitor voltage regulated to a target value. The MPPT module 212 may comprise control logic for performing MPP tracking. The MPPT module 212 may be implemented in software (e.g., computer- readable program code stored in memory and executed by a processor), hardware (e.g., ASIC, hardwired logic), or combination of hardware and software (e.g., firmware). The MPPT module 212 may be configured to perform a normal MPPT method, which is so named because it is configured to perform MPPT under normal operating conditions. The normal MPPT method may comprise the perturb and observe (P&O) method and the incremental conductance (IncC) method, for example. The MPPT module 212 may be configured to modify the normal MPPT method in response to detecting shading of the plurality of PV modules (solar cells) and / or when certain temporal conditions are met. The modification may involve use of another MPPT method instead of the normal MPPT method, changing a value of a variable (e.g., increment step size or decrement step size) of the normal MPPT method, performing a full l-V curve sweep, and / or other ways of changing the processing behavior, and accordingly the output, of the normal MPPT method. The MPPT module 212 may be configured to receive from a solar tracker controller position information indicative of orientation of the plurality of PV modules relative to solar illumination.

[0029] The MPPT module 212 is implemented in a microinverter and reads the voltage and current of the plurality of PV modules, performs MPPT using the voltage and current levels of the plurality of PV modules to determine a voltage setpoint in the form of a voltage reference Vref, and provides the voltage reference Vref to the inverter circuits. The inverter circuits change the impedance presented to the plurality of PV modules, or perform other actions depending on the inverter, to induce the plurality of PV modules to generate a voltage at a same level as the reference voltage Vref. It is to be noted that the reference voltage Vref serves as a setpoint, but the actual voltage operating point of the photovoltaic system may differ from the reference voltage Vref. As can be appreciated, the functionality of the MPPT module 212 may also be implemented in a device other than a photovoltaic inverter.

[0030] The inverter controllers 114 are also configured to operate the power converters 102 (microinverters) in one or more modes of operation. For example, the inverter controllers 114 are configured to operate the power converters 102 in a first mode of operation, which is suitable for normal conditions (e.g., adequate light). Likewise, the inverter controllers 114 are configured to operate the power converters 102 in a second mode of operation for less than normal conditions (e.g., low-light). In the second mode of operation (sometimes referred to as burst mode of operation), the power converters can be configured to capture energy in low-light, such as when shadows or clouds are passing over a solar array. In accordance with at least some embodiments of the present invention, the power converters 102 employ a burst mode during initial operation. In burst mode, the power converters 102 store energy over one or more AC grid voltage cycles (energy storage periods) and subsequently burst the stored energy to the commercial power grid (burst periods). The length of the energy storage periods is determined such that a ripple voltage overriding the PV module output voltage remains below a desired ripple voltage threshold. As noted above, in the burst mode of operation, however, the power converters can sometimes synchronize with each other, which can cause flicker in the system 100, which, in turn, can cause disturbance on a grid.

[0031] Accordingly, the inventors describe herein improved methods and apparatus that are configured to desynchronize power converters 102 when the power converters 102 are in the burst mode of operation. For example, Figure 3 is a diagram of a correlation of AC voltage with power production, Figure 4 is a flowchart of a method 400 for desynchronizing power converters 102 when the power converters 102 are in a burst mode of operation, and Figure 5 is a sequencing diagram 500 of the method of Figure 4, in accordance with one or more embodiments of the present disclosure.

[0032] In at least some embodiments, the power converters 102 and / or inverter controllers 114 are configured to operate in a manner as described in commonly- owned U.S. Patent No. 7,768,155, titled “Method And Apparatus For Improved Burst Mode During Power Conversion,” and in commonly-owned U.S. Patent No. 7,986,122, titled “Method And Apparatus For Power Conversion With MaximumPower Point Tracking And Burst Mode Capability,” the entire contents of each of these patents is incorporated herein by reference.

[0033] As illustrated in Figure 3, during burst mode of operation, the inverter controllers of the power converters 102 perform the method 400. For illustrative purposes, the average AC voltage 302 (e.g., determined at 402) is assumed to be the same for each cycle of operation (e.g., delta of the average AC voltage 505 of Figure 5 is not applicable for the description of the method 400). Thus, during the first cycle 304, the power converters 102 are not producing power and the inverter controllers, therefore, determine that an AC voltage of the current cycle is less than the average AC voltage 302 (at 404), which is a positive correlation (see 503 of Figure 5). Next, during a second cycle 306, the power converters 102 are producing power and the inverter controllers, therefore, determine that an AC voltage of the current cycle is greater than the average AC voltage 302 (at 404), which is another positive correlation.

[0034] Accordingly, the positive correlations combine and, after a predetermined threshold (e.g., when the filtered correlation values exceed the threshold, a relatively short time passes), the inverter controllers increase the number of cycles between making power from N to N+1 (for 1 period). The inverter controllers continue performing the method 400 until the end of the burst mode of operation.

[0035] In at least some embodiments, as there can be some skipping around as a system settles, the HR filter and predetermined threshold ensure that skipping doesn’t occur very often and does not impact efficiency. That is, the power converters (PCUs) in a system can sometimes spend time exceeding a correlation value, e.g., multiple times before reaching a steady state. Thus, in at least some embodiments, the HR filter and threshold can use a randomization feature. For example, a randomized wait time after a desynchronization event has occurred can be used to stop the desynchronization event synchronizing across multiple PCUs or groups of PCUs. The correlation of an AC voltage with power production works for any NOFF value (number of cycles skipped in burst mode). Additionally, as each inverter controller is configured to perform the method 400 individually, there is no need for communication or co-ordination between the power converters / invertercontrollers. Additionally, the method 400 works when some power converters are 180° out of phase (e.g., AC connector flipped).

[0036] Continuing with reference to Figures 4 and 5, as noted above, the inventors have found that a correlation of an AC voltage with power production exists, and the method 400 can be performed by the inverter controllers 114 on a cycle-by-cycle basis based on the correlation and one or more calculations. For example, the method 400 can comprise waiting for an AC cycle to start. Next, the method 400 can comprise measuring an AC voltage. For example, the inverter controllers 114 can be configured to detect when a next AC cycle starts (see 502 of Figure 5) and measure an AC voltage (see 504). Next, at 402, the method 400 comprises calculating an average AC voltage value (see 506). For example, the inverter controllers 114 can be configured to calculate an average AC voltage value. In at least some embodiments, the average AC voltage value can be calculated using an infinite impulse response (HR). In at least some embodiments, the average AC voltage can be calculated for each cycle of operation of the power converters 102 (see 302 of Figure 3, for example).

[0037] Next, at 404, the method 400 comprises calculating a magnitude of a difference between the average AC voltage value and the current AC voltage in a current cycle, e.g., when the power converters 102 are producing power and when the power converters 102 are not producing power (see 501 , power production status). In at least some embodiments, when the power converters 102 are not producing power, the inverter controllers 114 can multiply the magnitude of the difference between the average AC voltage value and the current AC voltage in the current cycle by -1 . In at least some embodiments, the calculated magnitude can be filtered with the HR, see 508.

[0038] Next, at 406, the method 400 comprises determining if the calculated magnitude exceeds a threshold. For example, at 406, the inverter controllers 114 determine if the value (which can be filtered) of the calculated magnitude exceeds a threshold (e.g., determined by analysis and experimentation, see 510 and 512). Next, at 408, if the value of the calculated magnitude exceeds a threshold, the inverter controllers 114 add an extra cycle of delay so instead of N cycles without producing power, N + 1 cycles without producing power are used (see 514), so thatin the burst mode of operation the power converters 102 are not synched (the power converters 102 are desynchronized). Also at 408, if the value of the calculated magnitude does not exceed a threshold, the inverter controllers 114 do nothing (see 516). After 408, the method 400 is completed (518) and repeats.

[0039] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof.

Claims

WHAT IS CLAIMED IS:1 . A method for controlling power converters in an energy management system, comprising: calculating an average AC voltage value; calculating a magnitude of a difference between the average AC voltage value and a current AC voltage in a cycle of operation of the power converters; determining if the calculated magnitude exceeds a threshold; and if the calculated magnitude exceeds the threshold, adding an extra cycle of delay without producing power so that the power converters are desynchronized.

2. The method of claim 1 , further comprising performing the method when the power converters are in a burst mode of operation.

3. The method of claim 1 , further comprising calculating the average AC voltage value using an infinite impulse response (HR) filter.

4. The method as in any of claims 1 to 3, wherein the infinite impulse response (HR) filter uses a randomized wait time after a desynchronization event has occurred to stop the desynchronization event synchronizing across the power converters.

5. The method of claim 1 , wherein the average AC voltage value is calculated for each cycle of operation of the power converters.

6. The method as in any of claims 1 to 3 or 5, wherein calculating the magnitude of the difference between the average AC voltage value and the current AC voltage is performed when the power converters are producing power and when the power converters are not producing power.

7. A non-transitory computer readable storage medium having instructions stored thereon that when executed by a processor perform a method for controlling power converters in an energy management system, comprising: calculating an average AC voltage value; calculating a magnitude of a difference between the average AC voltage value and a current AC voltage in a cycle of operation of the power converters; determining if the calculated magnitude exceeds a threshold; and if the calculated magnitude exceeds the threshold, adding an extra cycle of delay without producing power so that the power converters are desynchronized.

8. The non-transitory computer readable storage medium of claim 7, further comprising performing the method when the power converters are in a burst mode of operation.

9. The non-transitory computer readable storage medium of claim 7, further comprising calculating the average AC voltage value using an infinite impulse response (HR) filter.

10. The non-transitory computer readable storage medium as in any of claims 7 to 9, wherein the infinite impulse response (HR) filter uses a randomized wait time after a desynchronization event has occurred to stop the desynchronization event synchronizing across the power converters.

11. The non-transitory computer readable storage medium of claim 7, wherein the average AC voltage value is calculated for each cycle of operation of the power converters.

12. The non-transitory computer readable storage medium as in any of claims 7 to 9 or 11 , wherein calculating the magnitude of the difference between the average AC voltage value and the current AC voltage is performed when the power converters are producing power and when the power converters are not producing power.

13. An energy management system, comprising: a plurality of DC power sources; a plurality of corresponding power converters connected to the plurality of DC power sources and configured to: calculate an average AC voltage value; calculate a magnitude of a difference between the average AC voltage value and a current AC voltage in a cycle of operation of the plurality of corresponding power converters; determine if the calculated magnitude exceeds a threshold; and if the calculated magnitude exceeds the threshold, add an extra cycle of delay without producing power so that the plurality of corresponding power converters are desynchronized.

14. The energy management system of claim 13, wherein the plurality of corresponding power converters are further configured to control the plurality of corresponding power converters when the plurality of corresponding power converters are in a burst mode of operation.

15. The energy management system of claim 13, wherein the plurality of corresponding power converters are further configured to calculate the average AC voltage value using an infinite impulse response (HR) filter.

16. The energy management system as in any of claims 13 to 15, wherein the infinite impulse response (HR) filter uses a randomized wait time after a desynchronization event has occurred to stop the desynchronization event synchronizing across the plurality of corresponding power converters.

17. The energy management system of claim 13, wherein the average AC voltage value is calculated for each cycle of operation of the plurality of corresponding power converters.

18. The energy management system as in any of claims 13 to 15 or 17, wherein the plurality of corresponding power converters are further configured to calculate the magnitude of the difference between the average AC voltage value and the current AC voltage when the plurality of corresponding power converters are producing power and when the plurality of corresponding power converters are not producing power.

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