Power generation system, inverter, and method for switching between grid-connected and off-grid operation of a power generation system

Abstract

This application provides a power generation system, an inverter, and a method for switching the power generation system between grid and off-grid operation. In the power generation system, the inverter includes a DC port and a first AC port, and a backup power box includes a second AC port, a first switch, a second switch, a third AC port, and a fourth AC port. The DC port is connected to a DC power source, and the first AC port is connected to the second AC port. The first switch and the second switch are connected in series between the second AC port and the third AC port, and the series connection point of the first switch and the second switch is connected to the fourth AC port. The third AC port is connected to the power grid, and the fourth AC port is connected to the load. When the voltage at the third AC port is greater than a first voltage threshold, the backup power box controls both the first and second switches to close. When the voltage at the third AC port is less than or equal to the first voltage threshold, the backup power box controls the second switch to open while keeping the first switch closed, truly achieving seamless switching of the inverter between grid-connected and off-grid operating modes.

Description

Technical Field

[0001] This application relates to the field of photovoltaic energy storage technology, and in particular to a power generation system, an inverter, and a method for switching the power generation system from grid to off-grid. Background Technology

[0002] In photovoltaic-storage power generation systems, to meet the requirement of seamless switching between grid-connected and off-grid operating modes, when the grid fails, the controller in the inverter typically stops outputting pulse width modulation (PWM) signals to the switching transistors in the inverter circuit for a period of time (i.e., waveform blocking). During this waveform blocking period, the inverter circuit switches from grid-connected to off-grid operating mode to supply power to the load. However, the presence of a waveform blocking period (e.g., 5-20ms) during the switch from grid-connected to off-grid operating mode prevents truly seamless switching between the two modes, thus compromising uninterrupted power supply to the load and resulting in poor power supply reliability for both grid-connected and off-grid inverters. Summary of the Invention

[0003] This application provides a power generation system, an inverter, and a method for switching the power generation system from grid-connected to off-grid operation. This method can switch the inverter from grid-connected to off-grid operation mode without blocking the inverter's waveform, truly achieving seamless switching between grid-connected and off-grid operation modes, thereby ensuring uninterrupted power supply to the load and improving the reliability of the load's power supply.

[0004] In a first aspect, embodiments of this application provide a power generation system, which includes an inverter and a backup power box. The inverter includes a DC port and a first AC port, with the DC port used to connect to a DC power source. The inverter is used to perform AC-DC conversion between the DC power source and the backup power box. Specifically, the inverter converts the DC power output from the DC power source into AC power, which is then supplied to the power grid and load via the backup power box; alternatively, the backup power box draws power from the power grid, converts the AC power supplied by the grid into DC power, and charges the DC power source. The backup power box includes a second AC port, a first switch, a second switch, a third AC port, and a fourth AC port. The first and second switches are connected in series between the second and third AC ports of the backup power box, and the series connection point of the first and second switches is connected to the fourth AC port of the backup power box. The second AC port of the backup power box is connected to the first AC port of the inverter, the third AC port of the backup power box is used to connect to the power grid, and the fourth AC port of the backup power box is used to connect to the load. When the inverter switches between grid-connected and off-grid operating modes, the backup power box controls both the first and second switches to close when the voltage at the third AC port is greater than a first voltage threshold, so that the load is powered by either the inverter or the grid. At this time, the inverter is in grid-connected operating mode. The voltage at the third AC port is the grid voltage. When the grid experiences a power outage or islanding, the backup power box also controls the second switch to open while keeping the first switch closed when the voltage at the third AC port is less than or equal to the first voltage threshold, so that the backup power box is disconnected from the grid, and the load is powered by the inverter. At this time, the inverter switches from grid-connected to off-grid operating mode. Implementing this embodiment allows the inverter to switch from grid-connected to off-grid operating mode without voltage blocking, truly achieving seamless switching between grid-connected and off-grid operating modes. This reduces the inverter's grid-connected / off-grid switching time, ensures uninterrupted power supply to the load, and improves the reliability of the load's power supply.

[0005] In one possible implementation, after a grid power outage or islanding, the voltages of the first and third AC ports gradually rise to a level greater than or equal to a second voltage threshold. The second voltage threshold is greater than the first voltage threshold. At this time, the inverter also operates in open-loop voltage source mode when the voltage of the third AC port is greater than or equal to the second voltage threshold, thereby reducing the voltage of the third AC port to a level less than or equal to the first voltage threshold. The open-loop voltage source mode refers to the inverter controlling the voltage of the first AC port to a fixed voltage, which is less than or equal to the first voltage threshold. It should be understood that after a grid power outage or islanding, the inverter does not block voltage; instead, it controls the voltage of the first AC port to gradually decrease to a level less than or equal to the first voltage threshold, and the inverter's output remains continuous throughout the process of the voltage decrease at the first AC port. At this time, since the voltage of the first AC port is the same as that of the third AC port, the voltage of the third AC port will gradually decrease to less than or equal to the first voltage threshold. This allows the backup power box to detect that the grid has lost power or an islanding effect has occurred, thereby switching the inverter from grid-connected operation mode to off-grid operation mode. Implementing this embodiment can ensure continuous and uninterrupted inverter output after grid power loss or islanding, thus truly achieving seamless switching between grid-connected and off-grid operation modes. This provides more reliable protection against load damage and enhances the inverter's product competitiveness. In addition, the inverter operates in open-loop voltage source mode, which also avoids the risk of overcurrent or runaway caused by current backflow into the inverter after grid power loss or islanding. This avoids the risk of hardware failure due to overcurrent or runaway, thereby improving the inverter's safety and lifespan.

[0006] In one possible implementation, since the voltage of the first AC port is the same as the voltage of the third AC port, the inverter is used to detect the voltage of the first AC port to obtain the voltage of the third AC port, thereby identifying whether the power grid has lost power or an islanding effect has occurred based on the magnitude of the voltage of the third AC port.

[0007] In one possible implementation, the backup power box is further configured to send a mode switching signal to the inverter after the second switch is opened and the first switch remains closed. For example, this mode switching signal can be a low-level signal. The inverter is also configured to receive the mode switching signal and switch from open-loop voltage source mode to closed-loop voltage source mode. The closed-loop voltage source mode refers to the inverter's operating mode, which adjusts the voltage of the first AC port in real time according to the load voltage. When the inverter switches to closed-loop voltage source mode, the voltage of the first AC port gradually increases to a level greater than a first voltage threshold and less than a second voltage threshold. At this time, since the voltage of the first AC port is consistent with the voltage of the fourth AC port, the voltage of the fourth AC port also gradually increases to a level greater than the first voltage threshold and less than the second voltage threshold, and supplies power to the load. By implementing this embodiment, after the inverter switches to off-grid operating mode, the inverter can flexibly adjust the voltage of the first AC port according to the load power and supply power to the load, thereby ensuring the good operation of the load and improving the inverter's power supply efficiency.

[0008] In one possible implementation, the first voltage threshold is k times the rated voltage of the power grid, where k is less than 1.

[0009] In one possible implementation, the second voltage threshold is m times the rated voltage of the power grid, where m is greater than 1.

[0010] Secondly, embodiments of this application provide an inverter for AC-DC conversion between a DC power source and the power grid or load. Specifically, the inverter converts DC power output from the DC power source into AC power and supplies it to the power grid and load, or converts AC power supplied by the power grid into DC power and charges the DC power source. The inverter includes a DC port, an inverter circuit, a first switch, a second switch, a first AC port, and a second AC port. The DC port of the inverter is connected to the DC power source, the first AC port is connected to the power grid, and the second AC port is connected to the load. The DC terminal of the inverter circuit is connected to the DC port of the inverter. The first switch and the second switch are connected in series between the AC terminal of the inverter circuit and the first AC port of the inverter, and the series connection point of the first switch and the second switch is connected to the second AC port of the inverter. When the inverter switches between grid-connected and off-grid operating modes, if the voltage at the first AC port is greater than a first voltage threshold, the inverter controls both the first and second switches to close, so that the load is powered by the inverter circuit or the power grid. In this case, the inverter is in grid-connected operating mode. In this embodiment, the voltage at the first AC port is the grid voltage. When the grid loses power or islanding occurs, the voltage at the first AC port gradually rises to a level greater than or equal to a second voltage threshold, which is higher than the first voltage threshold. At this time, the inverter also controls the second switch to open and keeps the first switch closed when the voltage at the first AC port is greater than or equal to the second voltage threshold, thereby disconnecting the inverter circuit from the grid and supplying power to the load. At this point, the inverter switches from grid-connected mode to off-grid mode. Implementing this embodiment allows the inverter to switch from grid-connected mode to off-grid mode without voltage blocking, truly achieving seamless switching between grid-connected and off-grid modes. This reduces the inverter's grid-connected / off-grid switching time, ensures uninterrupted power supply to the load, and improves the reliability of the load's power supply.

[0011] In one possible implementation, when the grid fails or islanding occurs, the inverter is further configured to control the inverter circuit to operate in open-loop voltage source mode if the voltage at the first AC port is greater than or equal to a second voltage threshold. Open-loop voltage source mode refers to the inverter controlling the AC terminal voltage of the inverter circuit to a fixed voltage. Further, when the inverter circuit operates in open-loop voltage source mode, the inverter controls the second switch to open while keeping the first switch closed, thereby disconnecting the inverter circuit from the grid and supplying power to the load. At this time, the inverter switches from grid-connected operation mode to off-grid operation mode. Implementing this embodiment, controlling the inverter circuit to operate in open-loop voltage source mode after a grid failure or islanding avoids the risk of overcurrent or runaway caused by current backflow into the inverter after a grid failure or islanding, thus avoiding hardware failure risks due to overcurrent or runaway, and improving the inverter's safety and lifespan. In addition, controlling the inverter circuit to operate in open-loop voltage source mode can ensure that the inverter output is continuous and uninterrupted, thereby truly realizing seamless switching between grid-connected and off-grid operating modes, thus more reliably protecting the load from damage and enhancing the inverter's product competitiveness.

[0012] In one possible implementation, the inverter is further configured to control the inverter circuit to switch from an open-loop voltage source mode to a closed-loop voltage source mode when the second switch is open and the first switch remains closed. The closed-loop voltage source mode refers to the inverter's operating mode, where the inverter adjusts the AC terminal voltage of the inverter circuit in real time according to the load voltage. When the inverter circuit switches to the closed-loop voltage source mode, the AC terminal voltage of the inverter circuit gradually decreases to below a second voltage threshold and above a first voltage threshold. At this time, since the AC terminal voltage of the inverter circuit is consistent with the voltage of the second AC port, the voltage of the second AC port also gradually decreases to below the second voltage threshold and above the first voltage threshold, and supplies power to the load. By implementing the embodiments of this application, after the inverter switches to off-grid operating mode, the inverter can flexibly adjust the AC terminal voltage of the inverter circuit according to the load power and supply power to the load, thereby ensuring the good operation of the load and improving the inverter's power supply efficiency.

[0013] In one possible implementation, the first voltage threshold is k times the rated voltage of the power grid, where k is less than 1.

[0014] In one possible implementation, the second voltage threshold is m times the rated voltage of the power grid, where m is greater than 1.

[0015] Thirdly, embodiments of this application provide a method for switching a power generation system between grid connection and off-grid operation. This method can be executed by a controller within the power generation system. In this method, when the grid voltage is greater than a first voltage threshold, the controller controls both the first and second switches to close, so that the load is powered by the inverter or the grid. At this time, the inverter is in grid-connected operating mode. The power generation system includes an inverter, a first switch, and a second switch. The inverter includes a DC port and a first AC port. The DC port of the inverter is used to connect to a DC power source. The first and second switches are connected in series between the first AC port of the inverter and the grid. The series connection point of the first and second switches is used to connect the load. The controller can also control the second switch to open and keep the first switch closed when the grid voltage is less than or equal to the first voltage threshold, so that the inverter is disconnected from the grid, and the load is powered by the inverter. At this time, the inverter switches from grid-connected operating mode to off-grid operating mode. By implementing the embodiments of this application, the inverter can be switched from grid-connected working mode to off-grid working mode without blocking the inverter waveform. This truly realizes the seamless switching of the inverter between grid-connected and off-grid working modes, thereby reducing the grid-connected / off-grid switching time of the inverter and ensuring uninterrupted power supply to the load, thus improving the power supply reliability of the load.

[0016] In one possible implementation, after a power outage or islanding event, the grid voltage gradually rises to a level greater than or equal to a second voltage threshold, which is higher than a first voltage threshold. At this time, when the grid voltage is greater than or equal to the second voltage threshold, the controller operates the inverter in open-loop voltage source mode to reduce the grid voltage to a level less than or equal to the first voltage threshold. It should be understood that after a power outage or islanding event, the inverter does not block the voltage; instead, it controls the voltage at the first AC port to gradually decrease to a level less than or equal to the first voltage threshold, and the inverter output remains continuous during this gradual decrease. Since the voltage at the first AC port is consistent with the grid voltage, the grid voltage also gradually decreases to a level less than or equal to the first voltage threshold, causing the controller to recognize that the grid has lost power or islanding has occurred, thereby switching the inverter from grid-connected operation mode to off-grid operation mode. Implementing the embodiments of this application ensures continuous and uninterrupted inverter output after grid power outages or islanding, and allows for seamless switching between grid-connected and off-grid operating modes while maintaining uninterrupted output. This truly achieves seamless switching between grid-connected and off-grid operating modes, thus reliably protecting the load from damage and enhancing the inverter's product competitiveness. Furthermore, by operating the inverter in open-loop voltage source mode, the risk of overcurrent or runaway caused by current backflow into the inverter after grid power outages or islanding is avoided. This prevents hardware failures due to overcurrent or runaway, thereby improving the inverter's safety and lifespan.

[0017] In one possible implementation, since the voltage of the first AC port is consistent with the voltage of the power grid, the controller can detect the voltage of the first AC port through the inverter to obtain the voltage of the power grid, thereby identifying whether the power grid has lost power or experienced an islanding effect based on the magnitude of the power grid voltage.

[0018] In one possible implementation, the controller sends a mode switching signal to the inverter after the second switch is opened and the first switch remains closed. This mode switching signal is typically a low-level signal. The controller can receive the mode switching signal through the inverter and switch from open-loop voltage source mode to closed-loop voltage source mode. After the inverter switches to closed-loop voltage source mode, the voltage at the first AC port gradually increases to a level greater than a first voltage threshold and less than a second voltage threshold, and then supplies power to the load through the first switch. By implementing this embodiment, after the inverter switches to off-grid operation mode, it can flexibly adjust the voltage at the first AC port according to the load's power and supply power to the load, thereby ensuring good load operation and improving the inverter's power supply efficiency.

[0019] It should be understood that the implementations and beneficial effects of the above-mentioned aspects of this application can be referenced from each other. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the power generation system provided in an embodiment of this application;

[0021] Figure 2 This is a schematic diagram of a circuit architecture of a power generation system provided in an embodiment of this application;

[0022] Figure 3A This is another circuit architecture diagram of the power generation system provided in the embodiments of this application;

[0023] Figure 3B This is another circuit architecture diagram of the power generation system provided in the embodiments of this application;

[0024] Figure 4 This is another circuit architecture diagram of the power generation system provided in the embodiments of this application;

[0025] Figure 5 This is another circuit architecture diagram of the power generation system provided in the embodiments of this application;

[0026] Figure 6 This is a schematic diagram of a circuit architecture of an inverter provided in an embodiment of this application;

[0027] Figure 7A This is a schematic diagram of another circuit architecture of the inverter provided in the embodiments of this application;

[0028] Figure 7B This is a schematic diagram of another circuit architecture of the inverter provided in the embodiments of this application;

[0029] Figure 8 This is a flowchart illustrating the grid-connected / off-grid switching method for a power generation system provided in an embodiment of this application. Detailed Implementation

[0030] The power generation system provided in this application is applicable to various fields, including new energy smart microgrids, power transmission and distribution, new energy, photovoltaic power generation, photovoltaic-storage power generation, and high-power converters. The specific application can be determined based on the actual application scenario, and no restrictions are imposed here. The power generation system provided in this application can be adapted to different application scenarios, such as photovoltaic power supply scenarios, photovoltaic-storage power supply scenarios, or other application scenarios. The following explanation will use a photovoltaic power supply application scenario as an example; further details will not be elaborated upon here.

[0031] See Figure 1 , Figure 1This is a schematic diagram of a power generation system provided in an embodiment of this application. When the power generation system is applied in a photovoltaic power supply scenario, the DC power source can be a photovoltaic module, and the structure of the power generation system can be as follows. Figure 1 As shown, the power generation system includes an inverter 11 and a backup power box 12. The inverter 11 includes a DC port 111 and a first AC port 112. The backup power box 12 includes a second AC port 121, a third AC port 122, and a fourth AC port 123. The DC port 111 of the inverter 11 is used to connect to the photovoltaic module 10. The first AC port 112 of the inverter 11 is connected to the second AC port 121 of the backup power box 12. The third AC port 122 of the backup power box 12 is used to connect to the power grid 13. The fourth AC port 123 of the backup power box 12 is used to connect to the load 14. It should be understood that the number of photovoltaic modules 10 can be one or more, and no specific limitation is made here.

[0032] The photovoltaic module 10 is used to absorb solar energy and convert it into electrical energy, and output DC power to the inverter 11. The inverter 11 is used to convert the DC power output by the photovoltaic module 10 into AC power and output AC power. At this time, the DC port 111 of the inverter 11 is the input port, and the first AC port 112 of the inverter 11 is the output port.

[0033] The backup power box 12 is used to switch the inverter 11 between grid-connected and off-grid operating modes. When switching the inverter 11 to grid-connected mode, the inverter 11 outputs AC power to the grid 13 and load 14 via the backup power box 12, or the grid 13 outputs AC power to the load 14. It should be understood that when the inverter 11 outputs AC power to the grid 13 and load 14, the second AC port 121 is an input port, and the third AC port 122 and the fourth AC port 123 are both output ports. When the grid 13 outputs AC power to the load 14, the third AC port 122 is an input port, and the fourth AC port 123 is an output port. When the grid 13 loses power or experiences islanding, the backup power box 12 is also used to switch the inverter 11 from grid-connected mode to off-grid mode, in which case the inverter 11 outputs AC power to the load 14. The islanding effect refers to the effect that the power generation system continues to supply power to the adjacent lines in the power grid 13 when the power grid 13 suddenly loses power.

[0034] When the power generation system is used in a photovoltaic-storage power supply application scenario, the DC power supply includes photovoltaic modules 10 and an energy storage unit (not shown in the figure). The energy storage unit is connected to the DC port 111 of the inverter 11. The inverter 11 is used to convert the DC power output from the energy storage unit into AC power and output AC power. At this time, the DC port 111 of the inverter 11 is the input port, and the first AC port 112 of the inverter 11 is the output port. The inverter 11 is also used to draw power from the grid 13 through the backup power box 12, convert the AC power provided by the grid 13 into DC power, and charge the energy storage unit. At this time, the DC port 111 of the inverter 11 and the second AC port 121 of the backup power box 12 are both output ports, and the first AC port 112 of the inverter 11 and the third AC port 122 of the backup power box 12 are both input ports.

[0035] The following will combine Figures 2 to 7B The circuit architecture and working principle of the power generation system and inverter provided in the embodiments of this application are described.

[0036] See Figure 2 , Figure 2 This is a schematic diagram of a circuit architecture for a power generation system provided in an embodiment of this application. The power generation system may contain only one inverter, in which case the circuit architecture can be as follows: Figure 2 As shown, the power generation system 2 includes an inverter 20a and a backup power box 21. The inverter 20a includes a DC port dc0 and a first AC port ac0. The DC port dc0 of the inverter 20a is used to connect to a DC power supply 3a. The inverter 20a is used to perform AC-DC conversion between the DC power supply 3a and the backup power box 21. For details, please refer to the above description. Figure 1 The corresponding embodiments will not be described in detail here. The backup power box 21 includes a second AC port ac1, a first switch K1, a second switch K2, a third AC port ac2, and a fourth AC port ac3. The first switch K1 and the second switch K2 are connected in series between the second AC port ac1 and the third AC port ac2 of the backup power box 21, and the series connection point of the first switch K1 and the second switch K2 is connected to the fourth AC port ac3 of the backup power box 21. The second AC port ac1 of the backup power box 21 is connected to the first AC port ac0 of the inverter 20a, the third AC port ac2 of the backup power box 21 is used to connect to the power grid 40, and the fourth AC port ac3 of the backup power box 21 is used to connect to the load 41.

[0037] When inverter 20a switches between grid-connected and off-grid operating modes, the backup power box 21 controls both the first switch K1 and the second switch K2 to close when the voltage at the third AC port ac2 is greater than a first voltage threshold, so that the load 41 is powered by either inverter 20a or the grid 40. At this time, inverter 20a is in grid-connected operating mode. The voltage at the third AC port ac2 is the same as the voltage at the grid 40. When the grid 40 loses power or experiences islanding, backup power box 21 also controls the second switch K2 to open and keeps the first switch K1 closed when the voltage at the third AC port ac2 is less than or equal to the first voltage threshold, so that the backup power box 21 is disconnected from the grid 40, and the load 41 is powered by inverter 20a. At this time, inverter 20a switches from grid-connected to off-grid operating mode.

[0038] By implementing the embodiments of this application, the inverter 20a can be switched from grid-connected working mode to off-grid working mode without blocking the waveform, which truly realizes the seamless switching of the inverter 20a between grid-connected working mode and off-grid working mode, thereby reducing the grid-connected / off-grid switching time of the inverter 20a and ensuring uninterrupted power supply to the load 41, thus improving the power supply reliability of the load 41.

[0039] The aforementioned first switch K1 and second switch K2 can be relays, controllable switches, or other types of switches with load breaking capability, and the number of each of the first switch K1 and second switch K2 can be one or more. Load breaking capability refers to the ability of both the first switch K1 and the second switch K2 to break current and voltage simultaneously.

[0040] The power of the load 41 is greater than the output power of the inverter 20a (i.e., heavy load), or the power of the load 41 is less than or equal to the output power of the inverter 20a (i.e., light load), which is not limited here. It should be understood that when the load 41 is under light or heavy load, the embodiments of this application can truly achieve seamless switching between grid-connected and off-grid operating modes of the inverter 20a, making it more applicable.

[0041] When inverter 20a switches between grid-connected and off-grid operating modes, the backup power box 21 is also used to detect the voltage of the third AC port ac2. If the voltage of the third AC port ac2 is greater than a first voltage threshold, it controls both the first switch K1 and the second switch K2 to close, so that load 41 is powered by either inverter 20a or the grid 40. When load 41 is powered by inverter 20a, the voltage of the first AC port ac0 is greater than the first voltage threshold. This first voltage threshold is k times the rated voltage of the grid 40, where k is less than 1. For example, k is 0.7, 0.8, or other values. It should be understood that after both the first switch K1 and the second switch K2 are closed, the voltage of the first AC port ac0 and the voltage of the third AC port ac2 remain consistent, such as the rated voltage of the grid 40.

[0042] When the power grid 40 experiences a power outage or islanding, the voltage at the first AC port ac0 and the voltage at the third AC port ac2 will gradually rise to a level greater than or equal to a second voltage threshold, which is greater than the first voltage threshold. The second voltage threshold is m times the rated voltage of the power grid 40, where m is greater than 1. For example, m could be 1.15 times or other values. The inverter 20a is also used to detect the voltage at the first AC port ac0 to obtain the voltage at the third AC port ac2. Furthermore, when the voltage at the third AC port ac2 is greater than or equal to the second voltage threshold, the inverter 20a is also used to operate in open-loop voltage source mode to reduce the voltage at the third AC port ac2 to a level less than or equal to the first voltage threshold. The open-loop voltage source mode refers to the operating mode in which the inverter 20a controls the voltage at the first AC port ac0 to a fixed voltage, which is less than or equal to the first voltage threshold. For example, the open-loop voltage source mode can be a voltage source droop control mode, a voltage source virtual synchronous generator (VSG) mode, or other voltage source modes, without limitation. It should be understood that when the grid 40 experiences a power outage or islanding, the inverter 20a will not block the output, but will control the voltage of the first AC port ac0 to gradually decrease to less than or equal to a first voltage threshold. During this gradual decrease in voltage at the first AC port ac0, the output of the inverter 20a will remain continuous. Since the voltage at the first AC port ac0 is the same as the voltage at the third AC port ac2, the voltage at the third AC port ac2 will also gradually decrease to less than or equal to the first voltage threshold, causing the backup power box 21 to recognize that the grid 40 has experienced a power outage or islanding, thereby switching the inverter 20a from grid-connected operation mode to off-grid operation mode.

[0043] By implementing the embodiments of this application, the output of inverter 20a can be guaranteed to be continuous and uninterrupted after the power grid 40 fails or experiences an islanding effect. This avoids the inverter 20a blocking its output for a period of time and switching to off-grid operation mode during the blocking process. It truly achieves seamless switching between grid-connected and off-grid operation modes for inverter 20a, thereby more reliably protecting load 41 from damage and enhancing the product competitiveness of inverter 20a. Furthermore, since inverter 20a operates in open-loop voltage source mode, it also avoids the risk of overcurrent or runaway caused by current backflow into inverter 20a after the power grid 40 fails or experiences an islanding effect. This avoids the risk of hardware failure due to overcurrent or runaway in inverter 20a, thus improving the safety and lifespan of inverter 20a.

[0044] Taking the open-loop voltage source mode as an example of voltage source droop control mode, the inverter 20a is used to collect the voltage of the first AC port ac0, and the proportional-integral (PI) controller adjusts the voltage of the first AC port ac0 according to the difference between the voltage of the first AC port ac0 and the aforementioned fixed voltage, so that the voltage of the first AC port ac0 is consistent with the fixed voltage, gradually reducing it to less than or equal to a first voltage threshold. For example, the fixed voltage can be 0.7 times the rated voltage of the power grid 40.

[0045] After inverter 20a switches to off-grid operation mode, the backup power box 21 is also used to send a mode switching signal to inverter 20a when the second switch K2 is open and the first switch K1 remains closed. The mode switching signal instructs inverter 20a to switch from open-loop voltage source mode to closed-loop voltage source mode. For example, this mode switching signal can be a low-level signal. Inverter 20a is also used to receive the mode switching signal and switch from open-loop voltage source mode to closed-loop voltage source mode. The closed-loop voltage source mode refers to the operating mode in which inverter 20a adjusts the voltage of the first AC port ac0 in real time according to the voltage of load 41. For example, this closed-loop voltage source mode can be voltage source droop control mode, VSG mode, or other voltage source modes, and is not limited here. After inverter 20a switches to closed-loop voltage source mode, the voltage of the first AC port ac0 will gradually increase to a level greater than a first voltage threshold and less than a second voltage threshold. At this time, since the voltage of the first AC port ac0 and the voltage of the fourth AC port ac3 are the same, the voltage of the fourth AC port ac3 will gradually increase to be greater than the first voltage threshold and less than the second voltage threshold, and supply power to the load 41. Implementing this embodiment, after the inverter 20a switches to off-grid operation mode, the inverter 20a can flexibly adjust the voltage of the first AC port ac0 according to the power of the load 41 and supply power to the load 41, thereby ensuring the good operation of the load 41 and improving the power supply efficiency of the inverter 20a.

[0046] The following explanation uses a closed-loop voltage source mode with voltage source droop control as an example. The aforementioned backup power box 21 is also used to detect the voltage of the fourth AC port ac3 (i.e., the voltage of load 41) in real time and output the voltage of load 41 to the inverter 20a. At this time, the inverter 20a uses a PI controller to adjust the voltage of the first AC port ac0 according to the difference between the voltage of load 41 and the reference voltage, so that the voltage of the first AC port ac0 is consistent with the reference voltage, thereby gradually increasing the voltage of the first AC port ac0 to a level greater than a first voltage threshold and less than a second voltage threshold. For example, the reference voltage can be the rated voltage of the power grid 40.

[0047] The inverter 20a described above can be a single-phase inverter or a three-phase inverter. For example, when the inverter 20a is a single-phase inverter, the circuit architecture of the power generation system 2 can be as follows: Figure 3AAs shown in the diagram. DC port dc0 is a single-phase DC port, and the first AC port ac0, second AC port ac1, third AC port ac2, and fourth AC port ac3 are all single-phase AC ports. Each of the first switch K1 and the second switch K2 is a relay with two contacts, and both contacts are simultaneously closed or open. Optionally, each of the first switch K1 and the second switch K2 includes two controllable switches or two relays. For example, when inverter 20a is a three-phase inverter, the circuit architecture of the power generation system 2 can be as follows: Figure 3B As shown in the diagram. The DC port dc0 is a single-phase DC port, while the first AC port ac0, the second AC port ac1, the third AC port ac2, and the fourth AC port ac3 are all three-phase AC ports. Each of the first switch K1 and the second switch K2 is a relay with four contacts, and all four contacts are simultaneously closed or open. Optionally, each of the first switch K1 and the second switch K2 may include four controllable switches or four relays.

[0048] See Figure 4 , Figure 4 This is a schematic diagram of another circuit architecture of the power generation system provided in an embodiment of this application. For example... Figure 4 As shown above, Figure 3B The third AC port ac2 of the backup power box 21 shown is also used to connect the load 42. When both the first switch K1 and the second switch K2 are closed, the load 42 receives AC power output from the inverter 20a or AC power provided by the power grid 40. The load 41 can also be called a backup load, and the load 42 can also be called a non-backup load. In this case, the third AC port ac2 can be a non-backup output port, and the fourth AC port ac3 can be a backup output port. For example, when the power generation system 2 is a residential power generation system, the backup load is an important load in the user's home, and the non-backup load is other loads in the user's home. The backup load has a higher power supply priority than the non-backup load in the user's home, thus ensuring that the inverter 20a can still supply power to important loads when the power grid 40 fails or islanding occurs, thereby improving the user experience.

[0049] The aforementioned backup power box 21 also includes a controller 210, and the inverter 20a includes an inverter circuit 201 and a controller 202. The controller 210 can be wired or wirelessly connected to the controller 202 and is used to send signals to the controller 202, such as mode switching signals. For example, the controller 210 includes a communication interface 2100, and the controller 202 includes a communication interface 2020. The communication interface 2100 is connected to the communication interface 2020 and is used for communication between the controller 210 and the controller 202. The communication interface 2100 and the communication interface 2020 can be a recommended standard (RS) 485 communication interface, an input / output (IO) communication interface, a digital output (DO) communication interface, or other types of interfaces.

[0050] The controller 202 is used to send pulse width modulation (PWM) signals to the switching transistors in the inverter circuit 201 to enable the inverter 20a to output AC power. The controller 210 is used to detect the voltage at the third AC port ac2. When the voltage at the third AC port ac2 is greater than a first voltage threshold, it controls both the first switch K1 and the second switch K2 to close, so that the load 41 is powered by either the inverter 20a or the power grid 40. Furthermore, when the load 41 is powered by the inverter 20a, the voltage at the first AC port ac0 is greater than the first voltage threshold. At this time, the inverter 20a is in grid-connected operation mode. The voltage at the third AC port ac2 can be detected by the voltage detection circuit inside the controller 210, or detected by an external voltage detection device and sent to the controller 210; no limitation is made here.

[0051] When the power grid 40 experiences a power outage or islanding, controller 202 detects the voltage at the first AC port ac0 to obtain the voltage at the third AC port ac2. Furthermore, controller 202 also controls inverter circuit 201 to operate in open-loop voltage source mode when the voltage at the third AC port ac2 is greater than or equal to a second voltage threshold, thereby reducing the voltage at the first AC port ac0 to less than or equal to the first voltage threshold. Simultaneously, the voltage at the third AC port ac2 also decreases to less than or equal to the first voltage threshold. The voltage at the first AC port ac0 can be detected by the voltage detection circuit inside controller 202, or by an external voltage detection device detecting the voltage at the first AC port ac0 and sending it to controller 202; no limitation is made here. Controller 210 detects the voltage at the third AC port ac2. When the voltage at the third AC port ac2 is less than or equal to the first voltage threshold, it controls the second switch K2 to open and keeps the first switch K1 closed, thereby disconnecting the backup power box 21 from the power grid 40, and the load 41 is powered by inverter 20a. At this time, inverter 20a switches from grid-connected working mode to off-grid working mode.

[0052] After inverter 20a switches to off-grid operating mode, controller 210 is also used to send a mode switching signal to communication interface 2020 through communication interface 2100 after the second switch K2 is opened and the first switch K1 remains closed. Controller 202 is also used to receive the mode switching signal through communication interface 2020 and control inverter circuit 201 to switch from open-loop voltage source mode to closed-loop voltage source mode, so that the voltage of the first AC port ac0 gradually increases to be greater than the first voltage threshold and less than the second voltage threshold. At this time, inverter 20a has switched from grid-connected operating mode to off-grid operating mode, and the output of inverter 20a is continuous and uninterrupted during this grid-connected / off-grid switching process, that is, there is no time-limited blocking process.

[0053] By implementing the embodiments of this application, the blocking of the waveform for a period of time and the switching to the off-grid working mode during the blocking process can be avoided, and the inverter 20a can be truly switched seamlessly between the grid-connected working mode and the off-grid working mode (such as 0ms switching), thereby reducing the delay time of the inverter 20a switching from grid connection to off-grid.

[0054] See Figure 5 , Figure 5 This is a schematic diagram of another circuit architecture of the power generation system provided in this application embodiment. The number of inverters in the above power generation system can also be multiple, and the circuit architecture of the power generation system can be as follows: Figure 5As shown, the power generation system 2 includes inverters 20a to 20n. The DC port of inverter 20a is connected to DC power supply 3a, ..., and the DC port of inverter 20n is connected to DC power supply 3n. The first AC ports of inverters 20a to 20n are all connected in parallel to the second AC port ac1 of the backup power box 21. When the power grid 40 experiences a power outage or islanding, inverters 20a to 20n operate in open-loop voltage source mode, ensuring that the voltages at their first AC ports are all fixed. In this mode, inverters 20a to 20n operate in parallel. It should be understood that when the voltages at the first AC ports of inverters 20a to 20n are all fixed, the voltage difference between the first AC ports of any two inverters 20a to 20n is less than or equal to a fluctuation threshold. This fluctuation threshold can be set by the user or determined by the internal electronics of the inverter. By implementing the embodiments of this application, when the output voltages of inverters 20a to 20n are all at the fixed voltage, it can prevent current backflow into inverters 20a to 20n during power outages or islanding effects in the grid 40, and also avoid circulating current between inverters 20a to 20n, thereby improving the safety and service life of inverters 20a to 20n. It should be understood that the specific circuit structure of the other inverters in inverters 20a to 20n and the specific process of switching between grid-connected and off-grid operating modes can be found in the above-described embodiments. Figures 2 to 4 The specific circuit structure of the inverter 20a and the description of its switching between grid-connected and off-grid operating modes in the corresponding embodiments will not be repeated here.

[0055] See Figure 6 , Figure 6 This is a schematic diagram of the circuit architecture of an inverter provided in an embodiment of this application. When the inverter is an integrated inverter with load breaking capability and simultaneously supports grid-connected and off-grid operation, the inverter's circuit architecture can be as follows: Figure 6 As shown, inverter 5 is used to connect between DC power supply 3a and grid 40 or load 41 for AC-DC conversion. Inverter 5 includes a DC port dc1, an inverter circuit 50, a first switch S1, a second switch S2, a first AC port ac4, and a second AC port ac5. Inverter 5 is used to convert the DC power output from DC power supply 3a into AC power and supply power to grid 40 and load 41. In this case, DC port dc1 is the input port, and the first AC port ac4 and the second AC port ac5 are both output ports. Inverter 5 is also used to convert AC power provided by grid 40 into DC power and charge DC power supply 3a. For example, DC power supply 3a can be an energy storage unit. In this case, DC port dc1 is the output port, and the first AC port ac4 is the input port.

[0056] In this inverter, the DC port DC1 is connected to the DC power supply 3a to receive or supply DC power to the DC power supply 3a. The first AC port AC4 of the inverter 5 is connected to the power grid 40 to supply or receive AC power from the power grid 40. The second AC port AC5 of the inverter is connected to the load 41 to supply AC power to the load 41. The DC terminal DC2 of the inverter circuit 50 is connected to the DC port DC1 of the inverter 5. The first switch S1 and the second switch S2 are connected in series between the AC terminal AC6 of the inverter circuit 50 and the first AC port AC4 of the inverter 5. The series connection point of the first switch S1 and the second switch S2 is connected to the second AC port AC5 of the inverter 5.

[0057] When inverter 5 switches between grid-connected and off-grid operating modes, it controls both the first switch S1 and the second switch S2 to close when the voltage at the first AC port ac4 is greater than a first voltage threshold, so that the load 41 is powered by either the inverter circuit 50 or the grid 40. In this case, inverter 5 is in grid-connected operating mode. The first voltage threshold is k times the rated voltage of the grid 40, where k is less than 1. For example, k can be 0.7, 0.8, or other values. The voltage at the first AC port ac4 is the same as the voltage at the grid 40.

[0058] When the power grid 40 experiences a power outage or islanding, the voltage at the first AC port ac4 gradually rises to a level greater than or equal to a second voltage threshold, which is greater than the first voltage threshold. The second voltage threshold is m times the rated voltage of the power grid 40, where m is greater than 1. For example, m could be 1.15 times or other values. At this time, the inverter 5 is also used to control the second switch S2 to open and keep the first switch S1 closed when the voltage at the first AC port ac4 is greater than or equal to the second voltage threshold, thereby disconnecting the inverter circuit 50 from the power grid 40, and supplying power to the load 41 from the inverter circuit 50. At this time, the inverter 5 switches from grid-connected operation mode to off-grid operation mode.

[0059] By implementing the embodiments of this application, the operating mode of inverter 5 can be switched from grid-connected operating mode to off-grid operating mode without blocking the waveform. This truly realizes the seamless switching between grid-connected and off-grid operating modes of inverter 5, thereby reducing the grid-connected / off-grid switching time of inverter 5 and ensuring uninterrupted power supply to load 41, thus improving the power supply reliability of load 41.

[0060] The first switch S1 and the second switch S2 mentioned above can be a relay, a controllable switch or other type of switch with load breaking capability, and the number of each switch in the first switch S1 and the second switch S2 can be one or more.

[0061] The power of the load 41 is greater than the output power of the inverter 5 (i.e., heavy load), or the power of the load 41 is less than or equal to the output power of the inverter 5 (i.e., light load), which is not limited here. It should be understood that when the load 41 is under light or heavy load, the embodiments of this application can truly realize the seamless switching of the inverter 5 between grid-connected and off-grid operating modes, making it more applicable.

[0062] The output power of the DC power supply 3a is usually greater than the power of the load 41, so that the output power of the DC power supply 3a can be quickly released to the load 41 through the inverter 5, thereby ensuring that the power supply requirements of the load 41 are met when the power grid 40 fails or an islanding effect occurs.

[0063] The first AC port ac4 of the inverter 5 is also used to connect other loads (not shown in the figure). When both the first switch S1 and the second switch S2 are closed, the other loads are used to receive AC power output from the inverter 5 or AC power provided by the power grid 40.

[0064] When the power grid 40 experiences a power outage or islanding, the inverter 5 also detects the voltage at the first AC port ac4. If the voltage at the first AC port ac4 is greater than or equal to a second voltage threshold, the inverter circuit 50 is controlled to operate in open-loop voltage source mode. The voltage at the first AC port ac4 can be detected by the voltage detection circuit inside the inverter 5, or detected by an external voltage detection device and transmitted to the inverter 5. This open-loop voltage source mode refers to the operating mode in which the inverter 20a controls the AC terminal ac6 voltage of the inverter circuit 50 to a fixed voltage. For example, the open-loop voltage source mode can be a voltage source droop control mode, VSG mode, or other voltage source mode, and is not limited here. Furthermore, when the inverter circuit 50 operates in open-loop voltage source mode, the inverter 5 also controls the second switch S2 to open while keeping the first switch S1 closed, so that the connection between the inverter circuit 50 and the power grid 40 is disconnected, and the load 41 is powered by the inverter circuit 50. At this time, inverter 5 switches from grid-connected operating mode to off-grid operating mode. Implementing this embodiment, controlling inverter circuit 50 to operate in open-loop voltage source mode after grid 40 loses power or experiences islanding avoids the risk of overcurrent or runaway caused by current backflow into inverter 5 after grid 40 loses power or experiences islanding. This avoids hardware failure risks caused by overcurrent or runaway in inverter 5, thereby improving the safety and lifespan of inverter 5. Furthermore, controlling inverter circuit 50 to operate in open-loop voltage source mode ensures continuous and uninterrupted output from inverter 5, truly achieving seamless switching between grid-connected and off-grid operating modes. This more reliably protects load 41 from damage, enhancing the product competitiveness of inverter 5.

[0065] It is understandable that when the inverter circuit 50 operates in open-loop voltage source mode, the AC terminal AC6 voltage of the inverter circuit 50 can be adjusted or not, depending on the magnitude of the fixed voltage, which is not limited here. In specific implementation, since the voltage of the first AC port AC4 is the same as the AC terminal AC6 voltage of the inverter circuit 50, when the fixed voltage is also greater than or equal to the second voltage threshold, even if the inverter circuit 50 operates in open-loop voltage source mode, the AC terminal AC6 voltage of the inverter circuit 50 can remain unchanged, keeping it greater than or equal to the second voltage threshold. Optionally, when the fixed voltage is less than the second voltage threshold, the inverter circuit 50 operates in open-loop voltage source mode and will adjust the AC terminal AC6 voltage of the inverter circuit 50 to reduce it to less than the second voltage threshold.

[0066] After inverter 5 switches to off-grid operation mode, it is also used to control inverter circuit 50 to switch from open-loop voltage source mode to closed-loop voltage source mode when the second switch S2 is open and the first switch S1 remains closed. The closed-loop voltage source mode refers to the operating mode in which inverter 20a adjusts the AC terminal ac6 voltage of inverter circuit 50 in real time according to the voltage of load 41. For example, the closed-loop voltage source mode can be voltage source droop control mode, VSG mode, or other voltage source modes, and is not limited here. When inverter circuit 50 switches to closed-loop voltage source mode, the AC terminal ac6 voltage of inverter circuit 50 will gradually decrease to below the second voltage threshold and above the first voltage threshold. At this time, since the AC terminal ac6 voltage of inverter circuit 50 is the same as the voltage of the second AC port ac5, the voltage of the second AC port ac5 will also gradually decrease to below the second voltage threshold and above the first voltage threshold, and supply power to load 41. By implementing the embodiments of this application, when the inverter 5 switches to the off-grid working mode, the inverter 5 can flexibly adjust the AC terminal ac6 voltage of the inverter circuit 50 according to the power of the load 41 and supply power to the load 41, thereby ensuring the good operation of the load 41 and improving the power supply efficiency of the inverter 5.

[0067] Taking the closed-loop voltage source mode as an example of voltage source droop control mode, the inverter 5 is used to detect the voltage of the second AC port ac5 (i.e., the voltage of the load 41) in real time, and adjusts the AC terminal ac6 voltage of the inverter circuit 50 according to the difference between the voltage of the load 41 and the reference voltage through the PI controller, so that the AC terminal ac6 voltage of the inverter circuit 50 is consistent with the reference voltage, thereby gradually reducing the AC terminal ac6 voltage of the inverter circuit 50 to less than the second voltage threshold and greater than the first voltage threshold. For example, the reference voltage can be the rated voltage of the power grid 40.

[0068] It should be understood that all operations performed by inverter 5 during the switching process between grid-connected and off-grid operating modes can also be performed by controllers set inside or outside inverter 5, which will not be elaborated here.

[0069] In some feasible implementations, the inverter 5 described above can be a single-phase inverter or a three-phase inverter. For example, when the inverter 5 is a single-phase inverter, its circuit architecture can be as follows: Figure 7A As shown in the diagram. DC port dc1 is a single-phase DC port, and the first AC port ac4 and the second AC port ac5 are both single-phase AC ports. Each of the first switch S1 and the second switch S2 is a relay with two contacts, and both contacts are simultaneously closed or open. Optionally, each of the first switch S1 and the second switch S2 includes two controllable switches or two relays. When the inverter 5 is a three-phase inverter, the circuit architecture of the inverter 5 can be as follows: Figure 7B As shown. DC port dc1 is a single-phase DC port, while the first AC port ac4 and the second AC port ac5 are both three-phase AC ports. Each of the first switch S1 and the second switch S2 is a relay with four contacts, and all four contacts are simultaneously closed or open. Optionally, each of the first switch K1 and the second switch K2 includes four controllable switches or four relays.

[0070] See Figure 8 , Figure 8 This is a schematic flowchart of a grid-connected / off-grid switching method for a power generation system provided in an embodiment of this application. This grid-connected / off-grid switching method can be executed by a controller in the power generation system. Figure 8 As shown, the on-grid / off-grid handover method includes the following steps S101 to S102.

[0071] In step S101, when the voltage of the power grid is greater than the first voltage threshold, the controller controls both the first switch and the second switch to close, so that the load is powered by the inverter or the power grid.

[0072] In step S101, the first voltage threshold is k times the rated voltage of the power grid, where k is less than 1. For example, k can be 0.7, 0.8, or other values. The power generation system includes an inverter, a first switch, and a second switch. The inverter includes a DC port and a first AC port. The DC port of the inverter is used to connect to a DC power source. The first and second switches are connected in series between the first AC port of the inverter and the power grid. The series connection point of the first and second switches is used to connect a load.

[0073] It should be understood that when the first switch and the second switch are located in the backup power box outside the inverter, the controller can be the controller 210 in the backup power box 21. When the first switch and the second switch are located inside the inverter, the controller can be a controller located outside the inverter.

[0074] When the grid experiences a power outage or islanding, the grid voltage gradually rises to a level greater than or equal to a second voltage threshold, which is higher than the first voltage threshold. At this time, the controller can detect the voltage at the first AC port via the inverter to obtain the grid voltage. Furthermore, when the grid voltage is greater than or equal to the second voltage threshold, the controller can operate the inverter in open-loop voltage source mode to reduce the grid voltage to a level less than or equal to the first voltage threshold. It should be understood that when the grid experiences a power outage or islanding, the inverter does not block the output; instead, it controls the voltage at the first AC port to gradually decrease to a level less than or equal to the first voltage threshold, and the inverter's output remains continuous throughout this process. Since the voltage at the first AC port is consistent with the grid voltage, the grid voltage will also gradually decrease to a level less than or equal to the first voltage threshold, allowing the controller to recognize that the grid has lost power or islanded, thus switching the inverter from grid-connected mode to off-grid mode. Implementing the embodiments of this application ensures continuous and uninterrupted inverter output after grid power outages or islanding, and allows for seamless switching between grid-connected and off-grid operating modes while maintaining uninterrupted output. This truly achieves seamless switching between grid-connected and off-grid operating modes, thus reliably protecting the load from damage and enhancing the inverter's product competitiveness. Furthermore, by operating the inverter in open-loop voltage source mode, the risk of overcurrent or runaway caused by current backflow into the inverter after grid power outages or islanding is avoided. This prevents hardware failures due to overcurrent or runaway, thereby improving the inverter's safety and lifespan.

[0075] In step S102, when the grid voltage is less than or equal to the first voltage threshold, the controller controls the second switch to open and keeps the first switch closed, so that the inverter is disconnected from the grid and the load is powered by the inverter.

[0076] When the inverter switches to off-grid operation mode, the controller sends a mode switching signal to the inverter after the second switch is open and the first switch remains closed. This mode switching signal is typically a low-level signal. The controller can receive the mode switching signal through the inverter and switch from open-loop voltage source mode to closed-loop voltage source mode. After the inverter switches to closed-loop voltage source mode, the voltage at the first AC port gradually increases to a level greater than a first voltage threshold and less than a second voltage threshold, and then supplies power to the load through the first switch. By implementing this embodiment, after the inverter switches to off-grid operation mode, it can flexibly adjust the voltage at the first AC port according to the load's power and supply power to the load, thereby ensuring good load operation and improving the inverter's power supply efficiency.

[0077] In specific implementation, for more operations performed by the controller in the grid-connected / off-grid switching method of the power generation system provided in this application, and their corresponding beneficial effects, please refer to the above. Figures 2 to 5 The implementation method and corresponding beneficial effects of the controllers (such as controller 210 and controller 202) in the power generation system 2 shown and its working principle will not be elaborated here.

[0078] The grid-connected / off-grid switching method provided in this application can switch the inverter from grid-connected working mode to off-grid working mode without blocking the inverter's waveform. This truly achieves seamless switching between grid-connected and off-grid working modes, thereby reducing the grid-connected / off-grid switching time of the inverter and ensuring uninterrupted power supply to the load, thus improving the power supply reliability of the load.

[0079] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A power generation system, characterized in that, The power generation system includes an inverter and a backup power box; The inverter includes a DC port and a first AC port. The DC port of the inverter is used to connect to a DC power supply, and the inverter is used to perform AC-DC conversion between the DC power supply and the backup power box. The backup power box includes a second AC port, a first switch, a second switch, a third AC port, and a fourth AC port. The first switch and the second switch are connected in series between the second AC port and the third AC port of the backup power box. The series connection point of the first switch and the second switch is connected to the fourth AC port of the backup power box. The second AC port of the backup power box is connected to the first AC port of the inverter. The third AC port of the backup power box is used to connect to the power grid. The fourth AC port of the backup power box is used to connect to the load. The backup power box is used to control both the first switch and the second switch to close when the voltage of the third AC port is greater than the first voltage threshold, so that the inverter is in grid-connected working mode, and the voltages of the first AC port, the third AC port and the fourth AC port are kept consistent in the grid-connected working mode. The inverter is used to control the voltage of the first AC port to a fixed voltage to operate in open-loop voltage source mode when the voltage of the third AC port is greater than or equal to the second voltage threshold, so that the voltage of the third AC port is reduced to less than or equal to the first voltage threshold, wherein the second voltage threshold is greater than the first voltage threshold, and the fixed voltage is less than or equal to the first voltage threshold. The backup power box is also used to control the second switch to open and keep the first switch closed when the voltage of the third AC port is less than or equal to the first voltage threshold, so that the inverter switches from the grid-connected working mode to the off-grid working mode. In the off-grid working mode, the backup power box is disconnected from the grid and the load is powered by the inverter.

2. The power generation system according to claim 1, characterized in that, The inverter is used to detect the voltage of the first AC port in order to obtain the voltage of the third AC port.

3. The power generation system according to claim 1, characterized in that, The backup power box is also used to send a mode switching signal to the inverter when the second switch is opened and the first switch is kept closed; The inverter is also used to receive the mode switching signal, switch from the open-loop voltage source mode to the closed-loop voltage source mode, and adjust the voltage of the first AC port according to the voltage of the load in the closed-loop voltage source mode, so that the voltage of the first AC port rises to a level greater than the first voltage threshold and less than the second voltage threshold.

4. The power generation system according to any one of claims 1-3, characterized in that, The first voltage threshold is k times the rated voltage of the power grid, where k is less than 1.

5. The power generation system according to any one of claims 1-3, characterized in that, The second voltage threshold is m times the rated voltage of the power grid, where m is greater than 1.

6. An inverter for connecting between a DC power source and the power grid to perform AC-DC conversion, characterized in that, The inverter includes a DC port, an inverter circuit, a first switch, a second switch, a first AC port, and a second AC port; wherein, the DC port of the inverter is used to connect to the DC power supply, the first AC port of the inverter is used to connect to the power grid, and the second AC port of the inverter is used to connect to the load. The DC terminal of the inverter circuit is connected to the DC port of the inverter. The first switch and the second switch are connected in series between the AC terminal of the inverter circuit and the first AC port of the inverter. The series connection point of the first switch and the second switch is connected to the second AC port of the inverter. The inverter is used for: When the voltage at the first AC port is greater than the first voltage threshold, both the first switch and the second switch are controlled to close to operate in grid-connected mode. In grid-connected mode, the voltages at the AC terminal, the first AC port, and the second AC port of the inverter circuit remain consistent. When the voltage at the first AC port is greater than or equal to the second voltage threshold, the voltage at the AC terminal of the inverter circuit is controlled to be a fixed voltage to operate in open-loop voltage source mode, so that the voltage at the first AC port is reduced to less than or equal to the first voltage threshold, wherein the second voltage threshold is greater than the first voltage threshold, and the fixed voltage is less than or equal to the first voltage threshold. When the voltage at the first AC port is less than or equal to the first voltage threshold, the second switch is controlled to open while the first switch remains closed, so that the inverter switches from the grid-connected working mode to the off-grid working mode. In the off-grid working mode, the inverter circuit is disconnected from the power grid, and the load is powered by the inverter circuit.

7. The inverter according to claim 6, characterized in that, The inverter is also used for: When the second switch is opened and the first switch remains closed, the inverter circuit is controlled to switch from the open-loop voltage source mode to the closed-loop voltage source mode. In the closed-loop voltage source mode, the voltage of the AC terminal of the inverter circuit is adjusted according to the voltage of the load so that the AC terminal voltage of the inverter circuit is increased to be greater than the first voltage threshold and less than the second voltage threshold.

8. The inverter according to any one of claims 6-7, characterized in that, The first voltage threshold is k times the rated voltage of the power grid, where k is less than 1.

9. The inverter according to any one of claims 6-7, characterized in that, The second voltage threshold is m times the rated voltage of the power grid, where m is greater than 1.

10. A method for switching between grid connection and off-grid operation in a power generation system, characterized in that, The grid-connected / off-grid switching method is executed by a controller in the power generation system. The power generation system includes an inverter, a first switch, and a second switch. The inverter includes a DC port and a first AC port. The DC port of the inverter is used to connect to a DC power source. The first switch and the second switch are connected in series between the first AC port of the inverter and the power grid. The series connection point of the first switch and the second switch is used to connect a load. The method includes: When the voltage of the power grid is greater than a first voltage threshold, both the first switch and the second switch are closed to put the inverter into grid-connected operation mode. In the grid-connected operation mode, the voltage of the first AC port and the voltage of the power grid are kept consistent. When the voltage of the power grid is greater than or equal to the second voltage threshold, the inverter operates in open-loop voltage source mode, and controls the voltage of the first AC port to gradually decrease to less than or equal to the first voltage threshold, so that the voltage of the power grid is reduced to less than or equal to the first voltage threshold, wherein the second voltage threshold is greater than the first voltage threshold. When the voltage of the power grid is less than or equal to the first voltage threshold, the second switch is controlled to open while the first switch remains closed, so that the inverter switches from the grid-connected operating mode to the off-grid operating mode. In the off-grid operating mode, the inverter is disconnected from the power grid and the load is powered by the inverter.

11. The on-grid / off-grid handover method according to claim 10, characterized in that, The method further includes: The voltage of the power grid is obtained by detecting the voltage at the first AC port through the inverter.

12. The on-grid / off-grid handover method according to claim 11, characterized in that, The method further includes: When the second switch is opened and the first switch remains closed, a mode switching signal is sent to the inverter; The inverter receives the mode switching signal and switches from the open-loop voltage source mode to the closed-loop voltage source mode. In the closed-loop voltage source mode, the inverter adjusts the voltage of the first AC port according to the voltage of the load so that the voltage of the first AC port rises to a level greater than the first voltage threshold and less than the second voltage threshold.

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