A standby power system and a starting control method thereof

Abstract

This application provides a backup power system and its startup control method. When the grid-connected controller is powered down, the first control circuit provides a low-level signal to the second control circuit of the inverter as an off-grid signal. This allows the inverter to prioritize off-grid black start when the DC side voltage meets the inverter startup voltage condition. After startup, the off-grid operating output AC voltage quickly supplies power to the backup load. If, before the inverter outputs AC voltage for off-grid startup, the AC side voltage meets the minimum off-grid output voltage condition and a high-level signal is received as a grid-connected signal, the inverter can directly switch to grid-connected startup for rapid startup. When the grid voltage meets the undervoltage protection voltage condition, the grid-connected controller can directly start up to grid-connected operation, closing the first switching circuit and sending a high-level signal as a grid-connected signal to quickly supply power to the backup load and simultaneously provide voltage to the AC side of the inverter.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a backup power system and its startup control method. Background Technology

[0002] A whole-house backup power system mainly consists of an inverter and a grid-connected / off-grid controller. It can be powered by various sources, including photovoltaic modules, energy storage devices, and the power grid, to ensure a stable power supply for household loads. Because the backup power system includes multiple power electronic devices such as inverters and grid-connected / off-grid controllers, these devices need to coordinate to ensure rapid startup and stable operation to supply power to the household loads. To achieve a rapid and continuous stable power supply to the loads, the backup power system must be able to start quickly under different power supply conditions to power the household loads. Summary of the Invention

[0003] This application provides a backup power system and its startup control method to enable the backup power system to start up quickly and supply power to household loads.

[0004] In one aspect, this application provides a backup power system, including: an inverter and a grid-connected / off-grid controller; the grid-connected / off-grid controller includes: a first switching circuit and a first control circuit, and the inverter includes: a second control circuit. The grid side of the first switching circuit is connected to the power grid, the inverter side of the first switching circuit is connected to the AC side of the inverter, the DC side of the inverter is used to connect to a DC source, and the first control circuit is connected to the second control circuit. The first control circuit is used to: provide a low-level signal to the second control circuit when the grid-connected / off-grid controller is powered down; and when the grid-connected / off-grid controller is powered on, detect whether the voltage of the power grid is less than or equal to the undervoltage protection voltage value and whether the voltage on the inverter side is greater than or equal to the minimum off-grid output voltage value. The undervoltage protection voltage value is generally set to the voltage value of the first-level undervoltage protection point, for example, it can be set to 0.7Un, where Un is the rated value of the power grid voltage. The grid-connected / off-grid controller can only operate in grid-connected mode when the power grid voltage is greater than the undervoltage protection voltage value; once the power grid voltage is less than the undervoltage protection voltage value, the grid-connected / off-grid controller cannot operate in grid-connected mode and can only switch to off-grid mode. The minimum off-grid output voltage is the minimum voltage output by the inverter during off-grid operation, such as 50V. When the voltage on the inverter side of the on / off-grid controller is greater than or equal to the minimum off-grid output voltage, the inverter has completed off-grid startup and operation. If the grid voltage is greater than the undervoltage protection voltage and the inverter side voltage is less than the minimum off-grid output voltage, the first switching circuit is closed, providing a high-level signal to the second control circuit. If the grid voltage is less than or equal to the undervoltage protection voltage or the inverter side voltage is greater than or equal to the minimum off-grid output voltage, the first switching circuit is opened, providing a low-level signal to the second control circuit. The second control circuit is used to: when the inverter is powered on, detect whether the DC side voltage meets the inverter startup voltage. The inverter startup voltage is the minimum DC side voltage at which the inverter can start, such as 120V. If the DC side voltage is greater than the inverter startup voltage, the inverter meets the startup conditions. If the DC side voltage is greater than the inverter startup voltage, the AC side voltage is less than the minimum off-grid output voltage, and a low-level signal is received, the inverter is controlled to operate off-grid; if the DC side voltage is greater than the inverter startup voltage, the AC side voltage is greater than or equal to the minimum off-grid output voltage, and a high-level signal is received, the inverter is controlled to operate on-grid.

[0005] In the backup power system of this application, corresponding grid connection and off-grid start-up conditions are set for the inverter and the grid connection / off-grid controller. When the corresponding start-up conditions are met, the inverter and the grid connection / off-grid controller can start up in grid connection or off-grid independently to achieve rapid power supply to the backup load.

[0006] Specifically, in the backup power system of this application, since the first control circuit can provide a low-level signal as an off-grid signal to the second control circuit when the grid-connected controller is powered off (i.e., when neither the inverter side nor the grid side of the grid-connected controller is powered on), the inverter can prioritize off-grid black start when the DC side voltage meets the inverter start-up voltage condition. After start-up, the off-grid operating output AC voltage quickly supplies power to the backup load. The inverter does not need to wait for the control signal indicating grid-connected or off-grid black start sent by the grid-connected controller or system controller after power-on before starting the start-up procedure. The start-up method of this application allows the inverter to start off-grid as soon as the DC side voltage meets the condition after power-on, resulting in fast start-up and rapid power supply to the backup load. Furthermore, after the inverter starts off-grid operation, it supplies power to the grid-connected controller, causing the grid-connected controller to start off-grid, maintain the low-level signal as an off-grid signal, and disconnect the first switching circuit, enabling the backup power system to operate off-grid. If, before the inverter starts outputting AC voltage off-grid, the voltage on the AC side of the inverter meets the minimum off-grid output voltage condition and a high-level signal is received as a grid-connected signal, it indicates that the off-grid controller has started operating and is supplying power to the backup load. In this case, the inverter can directly switch to grid-connected start-up.

[0007] Specifically, in the backup power system of this application, when the grid voltage meets the undervoltage protection voltage condition, the grid-connected / off-grid controller can directly start up to grid-connected operation, close the first switching circuit and send a high-level signal as the grid connection signal to quickly supply power to the backup load, while simultaneously providing voltage to the AC side of the inverter. At this time, if the DC side voltage of the inverter meets the inverter start-up voltage condition, the inverter starts up to grid-connected operation, and the backup power system operates in grid-connected mode. If, during the grid-connected / off-grid controller's grid-connected start-up process, the voltage on the inverter side meets the minimum off-grid output voltage condition before the grid-connected / off-grid controller closes the first switching circuit, it indicates that the inverter has already started operating and supplying power to the backup load. In this case, the grid-connected / off-grid controller can switch to off-grid start-up, and subsequently, when the grid voltage meets the grid connection condition, it can switch from off-grid to grid-connected operation.

[0008] In some embodiments of this application, the first control circuit includes a first control chip and a signal generation circuit. The signal generation circuit includes a second switching circuit and a first resistor, which are connected in series between the ground port and the signal output port. The second switching circuit is closed by default. When the first switching circuit in the grid-connected controller is open, it indicates that the grid-connected controller is operating off-grid. The first control chip needs to close the second switching circuit to output a low-level signal at the signal output port, indicating off-grid operation. When the first switching circuit in the grid-connected controller is closed, it indicates that the grid-connected controller is operating in grid-connected mode. The first control chip needs to open the second switching circuit to output a high-level signal at the signal output port, indicating grid connection.

[0009] In some embodiments of this application, the second control circuit includes a second control chip and a signal detection circuit. The signal detection circuit includes an optocoupler, a second resistor, a first power supply, and a second power supply. The first end of the optocoupler is connected to the first power supply, the second end of the optocoupler is connected to a signal input port, the signal input port of the signal detection circuit is connected to the signal output port of the signal generation circuit, the third end of the optocoupler is connected to the second power supply, and the fourth end of the optocoupler is connected to the input terminal of the second control chip. The second resistor is connected in series between the input terminal and the ground terminal of the second control chip. When the first switching circuit in the grid-connected controller is open, it indicates that the grid-connected controller is operating off-grid. The first control chip needs to close the second switching circuit to output a low-level signal at the signal output port, indicating off-grid operation. At this time, the optocoupler in the signal detection circuit is turned on, and the input terminal of the second control chip receives a high-level signal, indicating that the grid-connected control command is off-grid. The inverter then starts off-grid operation, controlling the inverter to operate off-grid. When the first switch circuit in the grid-connected controller is closed, it indicates that the grid-connected controller is operating in grid-connected mode. The first control chip needs to disconnect the second switch circuit, and the optocoupler in the signal detection circuit is disconnected, so that the signal output port outputs a high-level signal, indicating grid connection. At this time, the input terminal of the second control chip receives a low-level signal, indicating that the grid-connected control command is grid-connected. Then the inverter starts grid connection and controls the inverter to operate in grid-connected mode.

[0010] The above circuit structure can be implemented in hardware so that the first control circuit provides a low-level signal as an off-grid signal when the grid-connected controller is not powered on, so that the second control circuit can control the inverter to start off-grid according to the off-grid signal. The inverter operates in voltage source mode off-grid and prioritizes power supply to the backup load.

[0011] In this application, the backup power system has both grid and DC power sources. At this time, the grid voltage is greater than the undervoltage protection voltage and the DC side voltage of the inverter is greater than the inverter startup voltage. The grid-connected controller and the inverter each execute their respective startup procedures according to the conditions provided in this application. During startup, the inverter can decide whether to switch startup modes based on the changes in AC side voltage and grid-connected / off-grid signals. The grid-connected / off-grid controller can decide whether to switch startup modes based on the changes in inverter side voltage. This allows the backup power system to adapt to changes in power supply and adjust the startup mode during startup, prioritizing power supply to the backup load.

[0012] In some embodiments of this application, if the grid voltage is greater than the undervoltage protection voltage and the DC side voltage is greater than the inverter start-up voltage, the first control circuit is further configured to: control the grid-connected controller to start up when the grid-connected controller is powered on; if the inverter side voltage is less than the minimum off-grid output voltage, close the first switch circuit and send a high-level signal to the second control circuit so that the second control circuit controls the inverter to operate in grid-connected mode when the DC side voltage is greater than the inverter start-up voltage; if the inverter side voltage is greater than or equal to the minimum off-grid output voltage, open the first switch circuit and send a low-level signal to the second control circuit.

[0013] When the grid-connected controller is powered on, the first control circuit detects that the grid voltage meets the undervoltage protection voltage condition, and the grid-connected controller starts up. Before closing the first switching circuit, it checks whether the voltage on the inverter side meets the minimum off-grid output voltage condition. If the voltage on the inverter side is less than the minimum off-grid output voltage, it indicates that the inverter has not completed off-grid startup and is supplying power to the backup load during off-grid operation. Therefore, the first control circuit closes the first switching circuit and sends a high-level signal to the second control circuit as a grid-connection signal. The grid-connected controller then operates in grid-connection mode, supplying power to the backup load and the AC side of the inverter. This ensures that when the DC side voltage of the inverter's second control circuit is greater than the inverter's startup voltage, it controls the inverter to operate in grid-connection mode, thus enabling the backup power system to operate in grid-connection mode. If the voltage on the inverter side is greater than or equal to the minimum off-grid output voltage, it means that the inverter has completed off-grid startup and is supplying power to the backup load during off-grid operation. In this case, the first control circuit does not need to close the first switch circuit, but keeps the first switch circuit open and continues to provide a low-level signal to the second control circuit as an off-grid signal. The off-grid controller then starts off-grid operation, and the backup power system begins off-grid operation.

[0014] In some embodiments of this application, if the grid voltage is greater than the undervoltage protection voltage and the DC side voltage is greater than the inverter start-up voltage, the second control circuit is further configured to: receive a low-level signal when the inverter is powered on, control the inverter to start off-grid; if the AC side voltage is less than the minimum off-grid output voltage and a low-level signal is received, control the inverter to operate off-grid; if the AC side voltage is greater than or equal to the minimum off-grid output voltage and a high-level signal is received, control the inverter to operate in grid-connected mode.

[0015] When the inverter is powered on, the second control circuit directly receives a low-level signal as an off-grid signal. When the second control circuit detects that the DC-side voltage meets the inverter's startup voltage requirement, the inverter starts off-grid. It then checks whether the AC-side voltage is greater than or equal to the minimum off-grid output voltage and whether a high-level signal is received as a grid-connected signal. Before the inverter outputs AC voltage during off-grid startup, if the AC-side voltage is greater than or equal to the minimum off-grid output voltage and a high-level signal is received, it indicates that the grid-connected / off-grid controller has completed grid-connected startup and is supplying power to the backup load. The inverter is then switched to grid-connected operation, and the backup power system is now in grid-connected operation. After the inverter completes its off-grid startup, it supplies power to the load and the inverter side of the grid-connected / off-grid controller, and the grid-connected / off-grid controller operates off-grid, thus completing the off-grid operation of the backup power system.

[0016] In some embodiments of this application, if the grid voltage is greater than the undervoltage protection voltage and the DC-side voltage is less than or equal to the inverter startup voltage, the first control circuit is further configured to: close the first switching circuit and send a high-level signal to the second control circuit when the grid-connected controller is powered on. When the backup power system is powered only by the grid, when the grid-connected controller is powered on, and the first control circuit of the grid-connected controller detects that the grid voltage meets the undervoltage protection voltage condition, it performs grid-side startup, closes the first switching circuit, and connects to the grid to supply power to the backup load and the AC side of the inverter. Furthermore, the first control circuit sends a high-level signal to the second control circuit as a grid-connection signal so that when the DC-side voltage of the inverter meets the inverter startup voltage, the second control circuit controls the inverter to operate in grid-connected mode.

[0017] In some embodiments of this application, if the DC side voltage is greater than the inverter startup voltage and the grid voltage is less than or equal to the undervoltage protection voltage, the second control circuit is further configured to: control the inverter to operate off-grid when the inverter is powered on. The backup power system is powered only by a DC source. When the inverter is powered on, the second control circuit of the inverter directly receives a low-level signal as an off-grid signal. When the second control circuit detects that the DC side voltage meets the inverter startup voltage condition, it executes the inverter off-grid startup, controlling the inverter to operate off-grid to supply power to the backup load and the inverter side of the grid-connected / off-grid controller.

[0018] In some embodiments of this application, a power cable and a signal cable are also included. The power cable is used to connect the inverter side of the first switching circuit to the AC side of the inverter, and the signal cable is used to connect the first control circuit and the second control circuit. The signal cable is independent of the power cable, which helps to improve the reliability of signal transmission. The signal cable is used to transmit the grid-connected control signal, and the transmission of the grid-connected signal can be independent of the power state of the grid-connected controller. Specifically, the first control circuit can provide a low-level signal as a grid-connected signal to the second control circuit when the grid-connected controller is powered off, so that the second control circuit can control the inverter to start up in grid-connected mode according to the grid-connected signal. The inverter operates in voltage source mode in grid-connected mode, giving priority to powering the backup load.

[0019] In some embodiments of this application, the off-grid controller includes: a first power input port, a second power input port, an inverter-side port, and a grid-side port; the first power input port is used to connect to a backup power load, the second power input port is used to connect to a non-backup power load, the inverter-side port is connected to the AC side of the inverter, and the grid-side port is used to connect to the power grid. The first switching circuit includes: a first relay and a second relay connected in series; the first relay is connected to both the first power input port and the inverter-side port, and the second relay is connected to both the second power input port and the grid-side port, enabling the backup power load to be powered through the power grid or the inverter, and the non-backup power load to be powered through the power grid.

[0020] Secondly, this application provides a startup control method for a backup power system, including:

[0021] When the grid-connected controller is powered down, a low-level signal is provided to the second control circuit of the inverter.

[0022] When the grid-connected controller is powered on, if the grid voltage is greater than the undervoltage protection voltage and the inverter voltage is less than the minimum off-grid output voltage, the first switch circuit in the grid-connected controller is closed, and a high-level signal is sent to the second control circuit; if the grid voltage is less than or equal to the undervoltage protection voltage or the inverter voltage is greater than or equal to the minimum off-grid output voltage, the first switch circuit is opened, and a low-level signal is sent to the second control circuit.

[0023] When the inverter is powered on, if the DC side voltage is greater than the inverter startup voltage, the AC side voltage is less than the minimum off-grid output voltage, and a low-level signal is received, the inverter is controlled to operate off-grid; if the DC side voltage is greater than the inverter startup voltage, the AC side voltage is greater than or equal to the minimum off-grid output voltage, and a high-level signal is received, the inverter is controlled to operate on-grid.

[0024] In some embodiments of this application, it also includes:

[0025] If the grid voltage is greater than the undervoltage protection voltage and the DC side voltage is greater than the inverter startup voltage, the grid-connected controller will start up when powered on. If the inverter side voltage is less than the minimum off-grid output voltage, the first switching circuit will be closed, and a high-level signal will be sent to the second control circuit so that the second control circuit will control the inverter to operate in grid-connected mode when the DC side voltage is greater than the inverter startup voltage. If the inverter side voltage is greater than or equal to the minimum off-grid output voltage, the first switching circuit will be opened, and a low-level signal will be sent to the second control circuit.

[0026] In some embodiments of this application, it also includes:

[0027] If the grid voltage is greater than the undervoltage protection voltage and the DC side voltage is greater than the inverter startup voltage, a low-level signal is received when the inverter is powered on, controlling the inverter to start off-grid. If the AC side voltage is less than the minimum off-grid output voltage and a low-level signal is received, the inverter is controlled to operate off-grid. If the AC side voltage is greater than or equal to the minimum off-grid output voltage and a high-level signal is received, the inverter is controlled to operate on-grid.

[0028] In some embodiments of this application, it also includes:

[0029] If the grid voltage is greater than the undervoltage protection voltage and the DC side voltage is less than or equal to the inverter startup voltage, the first switching circuit is closed and a high-level signal is sent to the second control circuit when the grid-connected controller is powered on.

[0030] In some embodiments of this application, it also includes:

[0031] If the DC side voltage is greater than the inverter startup voltage and the grid voltage is less than or equal to the undervoltage protection voltage, the inverter will be controlled to operate off-grid when powered on. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the backup power system.

[0033] Figure 2 This is a schematic diagram of the backup power system provided in an embodiment of this application;

[0034] Figure 3a A schematic diagram of the circuit structure of a single-phase grid-connected controller provided in an embodiment of this application;

[0035] Figure 3b A schematic diagram of the circuit structure of a three-phase grid-connected controller provided in an embodiment of this application;

[0036] Figure 4 This is a schematic diagram of the circuit structure of the backup power system provided in the embodiments of this application;

[0037] Figure 5A schematic flowchart illustrating the startup control method of the backup power system on the grid-connected / off-grid controller side provided in the embodiments of this application;

[0038] Figure 6 A flowchart illustrating the startup control method of the backup power system on the inverter side provided in the embodiments of this application;

[0039] Figure 7 This is a flowchart illustrating the startup control method for a backup power system provided in an embodiment of this application. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this application clearer, a further detailed description of this application will be provided below in conjunction with the accompanying drawings. However, the exemplary embodiments can be implemented in various forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided to make this application more comprehensive and complete, and to fully convey the concept of the exemplary embodiments to those skilled in the art. The same reference numerals in the drawings denote the same or similar structures, and therefore repeated descriptions of them will be omitted. Terms describing position and direction as described in this application are illustrative based on the accompanying drawings, but changes may be made as needed, and all such changes are included within the scope of protection of this application. The accompanying drawings of this application are for illustrating relative positional relationships only and do not represent actual scale.

[0041] It should be noted that specific details are set forth in the following description to provide a full understanding of this application. However, this application can be implemented in many ways other than those described herein, and those skilled in the art can make similar extensions without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below. The following descriptions are preferred embodiments for carrying out this application; however, these descriptions are for the purpose of illustrating the general principles of this application and are not intended to limit the scope of this application. The scope of protection of this application shall be determined by the appended claims.

[0042] To facilitate understanding of the embodiments of this application, the relevant technologies involved in the embodiments of this application will be introduced first below.

[0043] The terminology used in the following embodiments is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. As used in the specification and appended claims of this application, the singular expressions “a,” “an,” and “the” are intended to also include expressions such as “one or more,” unless the context clearly indicates otherwise. It should also be understood that in the following embodiments of this application, “at least one” means one, two, or more.

[0044] References to "one embodiment" and similar terms used in this specification mean that one or more embodiments of this application include a particular feature, structure, or characteristic described in connection with that embodiment. Therefore, phrases such as "in one embodiment," "in some embodiments," and "in other embodiments" appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "having," and variations thereof mean "including, but not limited to," unless otherwise specifically emphasized.

[0045] Reference Figure 1 The backup power system mainly includes an inverter and a grid-connected / off-grid controller. The grid-connected / off-grid controller is connected between the inverter and the power grid. The inverter converts the DC power provided by photovoltaic modules or energy storage devices into AC power. The grid-connected / off-grid controller includes a first power input port, a second power input port, an inverter-side port, and a grid-side port. The first power input port and the inverter-side port are directly connected, the second power input port and the grid-side port are directly connected, the inverter-side port is connected to the AC side of the inverter, and the grid-side port is used to connect to the power grid. Multiple inverters can be installed in the backup power system, and correspondingly, the grid-connected / off-grid controller can be configured with inverter-side ports that correspond one-to-one with the AC side of each inverter. Household loads connected to the grid-connected / off-grid controller include backup loads and non-backup loads. The first power input port is used to connect to the backup load, and the second power input port is used to connect to the non-backup load. The backup load can be powered by the power grid or the inverter, while the non-backup load is powered by the power grid.

[0046] The grid-connected / off-grid controller includes a control chip and relays. The control chip mainly controls the opening and closing of the relays to achieve the grid-connected and off-grid switching functions of the inverter. When the grid is normal, the control chip controls the relays to close, and the backup power system operates in grid-connected mode to supply power to the backup load. When the grid fails, the control chip controls the relays to open, and the inverter operates off-grid to supply power to the backup load, thereby ensuring that the backup load can always operate normally.

[0047] Based on the power supply conditions when the backup power system starts, the backup power system can be divided into two startup modes: one is that when the voltage of the grid meets the threshold condition, the backup power system performs grid-side startup and grid-connected operation; the other is that when the DC voltage provided by the photovoltaic modules or energy storage devices meets the threshold condition, the backup power system performs inverter-side black startup and off-grid operation.

[0048] Currently, the backup power system starts up as follows: the grid-connected or off-grid controller initiates grid-connected or off-grid startup directly upon power-up based on communication commands. After completing the closing or opening of the relays, the controller sends a corresponding startup signal to the inverter, which then starts up based on the AC side's detection results and outputs AC power. This approach requires waiting for the inverter to complete startup before supplying power to the backup load during off-grid startup, failing to provide a rapid and stable power supply. Furthermore, if the power supply changes during the backup power system's startup process, failure to adjust the startup method promptly will hinder the system's ability to quickly complete startup and ensure power supply to the backup load, impacting customer experience.

[0049] Based on this, the backup power system and its startup control method provided in this application can enable the backup power system to start up quickly, providing a continuous and stable power supply to the backup load. Furthermore, if the power supply changes during the startup process, the startup mode can be switched promptly to ensure power supply to the backup load.

[0050] Reference Figure 2 The backup power system provided in this application embodiment includes an inverter and a grid-connected / off-grid controller. The backup power system may also include a system controller, mainly used for system communication transmission, power dispatch, and information reporting functions. Generally, the system controller is connected to the inverter and the grid-connected / off-grid controller via an RS485 communication cable. The inverter is connected between the DC source and the grid-connected / off-grid controller. The DC side of the inverter is used to connect to the DC source, which can be a photovoltaic module or an energy storage device. The AC side of the inverter is connected to the inverter side of the grid-connected / off-grid controller. The inverter is used to convert the DC power provided by the photovoltaic module or energy storage device into AC power. The inverter may specifically include an inverter circuit and a DC-DC converter circuit. The DC-DC converter circuit is used to perform power conversion on the DC power provided by the photovoltaic module, and the inverter circuit is used to convert the DC power provided by the DC-DC converter circuit or the DC power provided by the energy storage device into AC power. The grid-connected / off-grid controller may specifically include a first switching circuit. The grid side of the first switching circuit is used to connect to the power grid, and the inverter side of the first switching circuit is connected to the AC side of the inverter. To meet electrical safety requirements, the first switching circuit generally includes two-stage switches: a first relay T1 and a second relay T2 connected in series. (Refer to...) Figure 3a and Figure 3bA typical grid-connected / off-grid controller includes four ports: a first power input port, a second power input port, an inverter-side port, and a grid-side port. The first power input port connects to the backup load, the second power input port connects to the non-backup load, the inverter-side port connects to the AC side of the inverter, and the grid-side port connects to the power grid. A first relay T1 connects to both the first power input port and the inverter-side port, and a second relay T2 connects to both the second power input port and the grid-side port. Multiple inverters can be configured in the backup power system; correspondingly, the grid-connected / off-grid controller can be configured with inverter-side ports that connect one-to-one with the AC side of each inverter. (Refer to...) Figure 3a The grid-connected controller provided in this application can be a single-phase grid-connected controller, or, as per [reference], [other options]. Figure 3b The grid-connected controller provided in this application can also be a three-phase grid-connected controller. Furthermore, the grid-connected controller may also include an NPE relay T3 connected between the neutral line N and the ground line PE, and a bypass switch T4 connected between the inverter side port and the grid side port.

[0051] The grid-connected / off-grid controller also includes a first control circuit, and the inverter also includes a second control circuit, which are connected together. The first and second control circuits can be connected via a signal cable independent of the power cable to improve signal transmission reliability. The power cable is the cable connecting the inverter side of the first switching circuit to the AC side of the inverter. The signal cable is used to transmit the grid-connected / off-grid control signal, and the transmission of the off-grid signal is independent of the power supply state of the grid-connected / off-grid controller. Specifically, the first control circuit can provide a low-level signal as an off-grid signal to the second control circuit when the grid-connected / off-grid controller is not powered on (i.e., when neither the inverter side nor the grid side of the grid-connected / off-grid controller is powered on). This allows the second control circuit to control the inverter to start off-grid based on the off-grid signal. During off-grid operation, the inverter operates in voltage source mode, prioritizing power supply to the backup load. When the grid-connected and off-grid controller is powered on, the first control circuit can provide a high-level signal as a grid-connected signal or a low-level signal as an off-grid signal to the second control circuit, depending on the voltage conditions of the grid. This allows the second control circuit to control the inverter to operate in grid-connected mode in current source mode or to operate in off-grid mode in voltage source mode, depending on the grid-connected signal.

[0052] When there are multiple inverters in the backup power system, each inverter is connected to the corresponding inverter side port of the grid-connected controller through a corresponding power cable, and the second control circuit of each inverter is connected to the first control circuit of the grid-connected controller through a corresponding signal cable.

[0053] In this application, in order to enable the first control circuit to provide a low-level signal as an off-grid signal by default when the grid-connected controller is not powered on, a signal generation circuit for outputting the grid-connected signal can be set in the first control circuit. Correspondingly, a signal detection circuit for receiving the grid-connected signal can be set in the second control circuit.

[0054] Reference Figure 4 The first control circuit specifically includes a first control chip and a signal generation circuit, while the second control circuit includes a second control chip and a signal detection circuit. The signal generation circuit specifically includes a second switch circuit K2 and a first resistor R1. The second switch circuit K2 and the first resistor R1 are connected in series between the ground port CGND and the signal output port. The second switch circuit K2 is closed by default and can be a normally closed relay. The signal output port can provide a digital output (DO) signal. When the second switch circuit K2 is in the default closed state, the ground port CGND and the signal output port form a path, and the DO signal is low, indicating off-grid operation. After the first control chip controls the second switch circuit K2 to open, the signal output port provides a high-level signal, i.e., the DO signal is high, indicating grid connection. Correspondingly, the signal detection circuit specifically includes an optocoupler OC, a second resistor R2, a first power supply, and a second power supply. In this circuit, the first terminal of the optocoupler OC is connected to a first power supply, such as a 12V power supply. The second terminal of the optocoupler OC is connected to a signal input port, which is connected to the signal output port of the signal detection circuit. The signal input port is used to receive DO signals. The third terminal of the optocoupler OC is connected to a second power supply, such as a 3.3V power supply. The fourth terminal of the optocoupler OC is connected to the input terminal of the second control chip. A second resistor is connected in series between the input terminal of the second control chip and the ground terminal AGND. The input terminal of the second control chip is used to receive digital input (DI) signals. Optionally, other resistors can be placed on the line between the ground terminal CGND and the signal output port in the signal generation circuit, and other resistors can also be placed on the line between the signal input port and the input terminal of the second control chip in the signal detection circuit, for example... Figure 4 The resistor is indicated by the dashed line.

[0055] When the first switching circuit in the grid-connected / off-grid controller is open, it indicates that the controller is operating off-grid. The first control chip needs to close the second switching circuit K2, causing the signal output port to output a low-level DO signal, indicating off-grid operation. At this time, the optocoupler OC in the signal detection circuit is turned on, and the input terminal of the second control chip receives a high-level DI signal, indicating that the grid-connected / off-grid control command is off-grid. The inverter then initiates off-grid startup, controlling its off-grid operation. Conversely, when the first switching circuit in the grid-connected / off-grid controller is closed, it indicates that the controller is operating in grid-connected mode. The first control chip needs to open the second switching circuit K2, causing the optocoupler OC in the signal detection circuit to open, causing the signal output port to output a high-level DO signal, indicating grid connection. At this time, the input terminal of the second control chip receives a low-level DI signal, indicating that the grid-connected / off-grid control command is grid connection. The inverter then initiates grid-connected startup, controlling its grid-connected operation.

[0056] Reference Figure 5 When the grid-connected / off-grid controller is powered on, the first control circuit detects whether the grid voltage is less than or equal to the undervoltage protection voltage and whether the inverter-side voltage is greater than or equal to the minimum off-grid output voltage. The undervoltage protection voltage is typically set to the voltage of the first-level undervoltage protection point, for example, 0.7Un, where Un is the rated grid voltage. The grid-connected / off-grid controller can only operate in grid-connected mode when the grid voltage is greater than the undervoltage protection voltage. If the grid voltage is less than the undervoltage protection voltage, the controller cannot operate in grid-connected mode and must switch to off-grid mode. The minimum off-grid output voltage is the minimum voltage output by the inverter during off-grid operation, for example, 50V. When the voltage on the inverter side of the grid-connected / off-grid controller is greater than or equal to the minimum off-grid output voltage, it indicates that the inverter has completed off-grid startup and is operating in off-grid mode. If the grid voltage is less than or equal to the undervoltage protection voltage or the inverter voltage is greater than or equal to the minimum off-grid output voltage, the first control circuit disconnects the first switching circuit and maintains a low-level signal to the second control circuit as an off-grid signal. The off-grid controller then enters off-grid operation mode, and the backup power system operates off-grid to supply power to the backup load. If the grid voltage is greater than the undervoltage protection voltage and the inverter voltage is less than the minimum off-grid output voltage, the first switching circuit closes, providing a high-level signal to the second control circuit, and the off-grid controller enters grid-connected operation mode. At this time, if the DC voltage of the inverter meets the inverter startup voltage condition, the inverter will also operate grid-connected, putting the backup power system in grid-connected operation to supply power to the backup load.

[0057] Reference Figure 5 Optionally, when the off-grid controller is powered on, it can first determine whether the communication with the inverter is normal. If the communication is normal, the relevant system detection is performed. If the communication is abnormal, the system detection is skipped and the backup load is prepared to be powered first.

[0058] Reference Figure 6 When the inverter is powered on, the second control circuit detects whether the DC-side voltage meets the inverter's startup voltage requirement. If the DC-side voltage is greater than the startup voltage (the minimum DC-side voltage at which the inverter can start, e.g., 120V), the inverter meets the startup conditions. The second control circuit then detects whether the AC-side voltage is less than the minimum off-grid output voltage and receives a low-level signal as an off-grid signal. If the AC-side voltage is less than the minimum off-grid output voltage and a low-level signal is received, the second control circuit controls the inverter to operate off-grid, supplying power to the grid-connected / off-grid controller and the backup load. If the AC-side voltage is greater than or equal to the minimum off-grid output voltage and a high-level signal is received as a grid-connected signal, the grid-connected / off-grid controller is already operating in grid-connected mode, supplying power to the backup load, and the second control circuit controls the inverter to operate in grid-connected mode. If other detection results are obtained, such as the AC-side voltage being less than the minimum off-grid output voltage and a low-level signal being received, it indicates a mismatch between the DO signal and the operating state of the grid-connected / off-grid controller, triggering an alarm and shutdown.

[0059] In the backup power system of this application, since the first control circuit can provide a low-level signal as an off-grid signal to the second control circuit when the grid-connected controller is powered off, the inverter can preferentially perform off-grid black start when the DC side voltage meets the inverter start-up voltage condition. After start-up, it outputs AC voltage for off-grid operation, quickly supplying power to the backup load. Furthermore, after the inverter operates off-grid, it supplies power to the grid-connected controller, causing the controller to perform off-grid start-up, maintaining a low-level signal as an off-grid signal and disconnecting the first switching circuit, thus enabling the backup power system to operate off-grid. If, before the inverter outputs AC voltage for off-grid start-up, the inverter's AC side voltage meets the minimum off-grid output voltage condition and a high-level signal as a grid-connected signal is received, the inverter can directly switch to grid-connected start-up.

[0060] In the backup power system of this application, when the grid voltage meets the undervoltage protection voltage condition, the grid-connected / off-grid controller can directly start up to grid-connected operation, close the first switching circuit and send a high-level signal as the grid connection signal to quickly supply power to the backup load, while simultaneously providing voltage to the AC side of the inverter. At this time, if the DC side voltage of the inverter meets the inverter start-up voltage condition, the inverter starts up to grid-connected operation, and the backup power system operates in grid-connected mode. If, during the grid-connected / off-grid controller's grid-connected start-up process, the voltage on the inverter side meets the minimum off-grid output voltage condition before the controller closes the first switching circuit, the controller can switch to off-grid start-up. Subsequently, when the grid voltage meets the grid connection condition, it can switch from off-grid to grid-connected operation.

[0061] Specifically, the backup power system provided in this application has the following power supply scenarios during startup:

[0062] 1. The backup power system is supplied solely by the grid. In this case, the grid voltage is greater than the undervoltage protection voltage, and the DC-side voltage of the inverter is less than or equal to the inverter's startup voltage. When the grid-connected controller is powered on, if the first control circuit of the grid-connected controller detects that the grid voltage meets the undervoltage protection voltage condition, it executes grid-side startup, closing the first switching circuit to operate in grid-connected mode, supplying power to the backup load and the AC side of the inverter. Furthermore, the first control circuit sends a high-level signal to the second control circuit as a grid-connection signal, so that when the DC-side voltage of the inverter meets the inverter's startup voltage, the second control circuit controls the inverter to operate in grid-connected mode.

[0063] 2. The backup power system is powered only by a DC source. At this time, the voltage on the DC side is greater than the inverter start-up voltage and the grid voltage is less than or equal to the undervoltage protection voltage. When the inverter is powered on, the second control circuit of the inverter directly receives a low-level signal as an off-grid signal. When the second control circuit detects that the voltage on the DC side meets the inverter start-up voltage condition, it executes the inverter off-grid start-up and controls the inverter to operate off-grid to supply power to the backup load and the inverter side of the grid-connected and off-grid controller.

[0064] 3. The backup power system has both grid and DC power sources. In this case, the grid voltage is greater than the undervoltage protection voltage and the DC side voltage of the inverter is greater than the inverter startup voltage. The grid-connected controller and the inverter each execute their respective startup procedures according to the conditions provided in this application. During startup, the inverter can decide whether to switch startup modes based on the changes in AC side voltage and DO signal. The grid-connected controller can decide whether to switch startup modes based on the changes in inverter side voltage. This allows the backup power system to adapt to changes in power supply and adjust the startup mode during startup, prioritizing power supply to the backup load. The detailed startup switching process is as follows.

[0065] Reference Figure 7When the grid-connected controller is powered on, if the first control circuit detects that the grid voltage meets the undervoltage protection voltage condition, the grid-connected controller starts up. Before closing the first switching circuit, it checks whether the voltage on the inverter side meets the minimum off-grid output voltage condition. If the voltage on the inverter side is less than the minimum off-grid output voltage, it indicates that the inverter has not completed off-grid startup and is supplying power to the backup load during off-grid operation. Therefore, the first control circuit closes the first switching circuit and sends a high-level signal to the second control circuit as a grid-connection signal. The grid-connected controller then operates in grid-connection mode, supplying power to the backup load and the AC side of the inverter. This ensures that when the DC side voltage of the inverter's second control circuit is greater than the inverter's startup voltage, it controls the inverter to operate in grid-connection mode, thus enabling the backup power system to operate in grid-connection mode. If the voltage on the inverter side is greater than or equal to the minimum off-grid output voltage, it means that the inverter has completed off-grid startup and is supplying power to the backup load during off-grid operation. In this case, the first control circuit does not need to close the first switch circuit, but keeps the first switch circuit open and continues to provide a low-level signal to the second control circuit as an off-grid signal. The off-grid controller then starts off-grid operation, and the backup power system begins off-grid operation.

[0066] Reference Figure 7 When the inverter is powered on, the second control circuit directly receives a low-level signal as an off-grid signal. When the second control circuit detects that the DC-side voltage meets the inverter's start-up voltage condition, the inverter starts off-grid and checks whether the AC-side voltage is greater than or equal to the minimum off-grid output voltage and whether a high-level signal is received as a grid-connected signal. Before the inverter outputs AC voltage for off-grid start-up, if the AC-side voltage is greater than or equal to the minimum off-grid output voltage and a high-level signal is received, it indicates that the grid-connected controller has completed grid-connected start-up and is supplying power to the backup load in grid-connected operation. The inverter is then switched to grid-connected operation, and the backup power system is now in grid-connected operation. Optionally, before switching the inverter to grid-connected operation, it can be determined again whether the DC-side voltage of the inverter meets the inverter start-up voltage condition. After determining that the DC-side voltage is greater than the inverter start-up voltage, the inverter switches to grid-connected operation. Before the inverter starts to output AC voltage off-grid, if the voltage on the AC side is less than the minimum off-grid output voltage or a low-level signal is continuously received, the inverter will start to supply power to the load and the inverter side of the on-grid / off-grid controller. The on-grid / off-grid controller will then start to operate off-grid, and the backup power system will then start to operate off-grid.

[0067] Based on the same inventive concept, this application also provides a startup control method for the above-mentioned backup power system, including the following steps:

[0068] When the grid-connected controller is powered down, a low-level signal is provided to the second control circuit of the inverter.

[0069] When the grid-connected controller is powered on, if the grid voltage is greater than the undervoltage protection voltage and the inverter voltage is less than the minimum off-grid output voltage, the first switch circuit in the grid-connected controller is closed, and a high-level signal is sent to the second control circuit; if the grid voltage is less than or equal to the undervoltage protection voltage or the inverter voltage is greater than or equal to the minimum off-grid output voltage, the first switch circuit is opened, and a low-level signal is sent to the second control circuit.

[0070] When the inverter is powered on, if the DC side voltage is greater than the inverter startup voltage, the AC side voltage is less than the minimum off-grid output voltage, and a low-level signal is received, the inverter is controlled to operate off-grid; if the DC side voltage is greater than the inverter startup voltage, the AC side voltage is greater than or equal to the minimum off-grid output voltage, and a high-level signal is received, the inverter is controlled to operate on-grid.

[0071] In some embodiments of this application, it also includes:

[0072] If the grid voltage is greater than the undervoltage protection voltage and the DC side voltage is greater than the inverter startup voltage, the grid-connected controller will start up when powered on. If the inverter side voltage is less than the minimum off-grid output voltage, the first switching circuit will be closed, and a high-level signal will be sent to the second control circuit so that the second control circuit will control the inverter to operate in grid-connected mode when the DC side voltage is greater than the inverter startup voltage. If the inverter side voltage is greater than or equal to the minimum off-grid output voltage, the first switching circuit will be opened, and a low-level signal will be sent to the second control circuit.

[0073] In some embodiments of this application, it also includes:

[0074] If the grid voltage is greater than the undervoltage protection voltage and the DC side voltage is greater than the inverter startup voltage, a low-level signal is received when the inverter is powered on, controlling the inverter to start off-grid. If the AC side voltage is less than the minimum off-grid output voltage and a low-level signal is received, the inverter is controlled to operate off-grid. If the AC side voltage is greater than or equal to the minimum off-grid output voltage and a high-level signal is received, the inverter is controlled to operate on-grid.

[0075] In some embodiments of this application, it also includes:

[0076] If the grid voltage is greater than the undervoltage protection voltage and the DC side voltage is less than or equal to the inverter startup voltage, the first switching circuit is closed and a high-level signal is sent to the second control circuit when the grid-connected controller is powered on.

[0077] In some embodiments of this application, it also includes:

[0078] If the DC side voltage is greater than the inverter startup voltage and the grid voltage is less than or equal to the undervoltage protection voltage, the inverter will be controlled to operate off-grid when powered on.

[0079] The backup power system and its startup control method provided in this application, when the grid-connected controller is powered down, the first control circuit provides a low-level signal to the second control circuit of the inverter as an off-grid signal. This allows the inverter to prioritize off-grid black start when the DC side voltage meets the inverter startup voltage condition. After startup, the off-grid operating output AC voltage quickly supplies power to the backup load. If, before the inverter outputs AC voltage for off-grid startup, the AC side voltage meets the minimum off-grid output voltage condition and a high-level signal is received as a grid-connected signal, the inverter can directly switch to grid-connected startup to complete a rapid startup. When the grid voltage meets the undervoltage protection voltage condition, the grid-connected controller can directly start up to grid-connected operation, close the first switching circuit and send a high-level signal as a grid-connected signal to quickly supply power to the backup load, while simultaneously providing voltage to the AC side of the inverter.

[0080] 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. An electrical backup system, characterized by, include: Inverters and grid-connected / off-grid controllers; The grid-connected / off-grid controller includes: a first switching circuit and a first control circuit; the inverter includes: a second control circuit. The grid side of the first switching circuit is connected to the power grid, the inverter side of the first switching circuit is connected to the AC side of the inverter, the DC side of the inverter is used to connect to a DC source, and the first control circuit is connected to the second control circuit. The first control circuit is used to: provide a low-level signal to the second control circuit when the grid-connected controller is powered down; When the grid-connected controller is powered on, if the voltage of the grid is greater than the undervoltage protection voltage value and the voltage on the inverter side is less than the minimum off-grid output voltage value, the first switching circuit is closed to provide a high-level signal to the second control circuit; if the voltage of the grid is less than or equal to the undervoltage protection voltage value or the voltage on the inverter side is greater than or equal to the minimum off-grid output voltage value, the first switching circuit is opened to provide the low-level signal to the second control circuit. The second control circuit is used to: when the inverter is powered on, if the voltage on the DC side is greater than the inverter start-up voltage value, the voltage on the AC side is less than the minimum off-grid output voltage value, and a low-level signal is received, control the inverter to operate off-grid; if the voltage on the DC side is greater than the inverter start-up voltage value, the voltage on the AC side is greater than or equal to the minimum off-grid output voltage value, and a high-level signal is received, control the inverter to operate on-grid.

2. The backup power system as described in claim 1, characterized in that, The first control circuit includes: a first control chip and a signal generation circuit; The signal generating circuit includes: a second switching circuit and a first resistor, wherein the second switching circuit and the first resistor are connected in series between the ground port and the signal output port, and the second switching circuit is in the closed state by default. The first control chip is used to disconnect the second switch circuit when the first switch circuit is closed, so that the signal output port outputs the high-level signal, and to close the second switch circuit when the first switch circuit is disconnected, so that the signal output port outputs the low-level signal.

3. The backup power system as described in claim 2, characterized in that, The second control circuit includes: a second control chip and a signal detection circuit; The signal detection circuit includes: an optocoupler, a second resistor, a first power supply, and a second power supply. The first end of the optocoupler is connected to the first power supply, the second end of the optocoupler is connected to the signal input port, the signal input port is connected to the signal output port, the third end of the optocoupler is connected to the second power supply, and the fourth end of the optocoupler is connected to the input terminal of the second control chip. The second resistor is connected in series between the input terminal and the ground terminal of the second control chip. The second control chip is used to receive a low-level signal when the signal output port outputs the high-level signal, and to control the inverter to operate in grid-connected mode if the voltage on the DC side is greater than the inverter start-up voltage and the voltage on the AC side is greater than or equal to the minimum off-grid output voltage; and to receive a high-level signal when the signal output port outputs the low-level signal, and to control the inverter to operate off-grid mode if the voltage on the DC side is greater than the inverter start-up voltage and the voltage on the AC side is less than the minimum off-grid output voltage.

4. The backup power system as described in any one of claims 1-3, characterized in that, If the voltage of the power grid is greater than the undervoltage protection voltage and the voltage on the DC side is greater than the inverter startup voltage; the first control circuit is further configured to: When the grid-connected controller is powered on, the grid-connected controller is started. If the voltage on the inverter side is less than the minimum off-grid output voltage value, the first switching circuit is closed and the high-level signal is sent to the second control circuit. If the voltage on the inverter side is greater than or equal to the minimum off-grid output voltage, the first switching circuit is disconnected, and the low-level signal is sent to the second control circuit.

5. The backup power system as described in any one of claims 1-4, characterized in that, If the voltage of the power grid is greater than the undervoltage protection voltage and the voltage on the DC side is greater than the inverter start-up voltage; the second control circuit is further configured to: When the inverter is powered on, a low-level signal is received, and the inverter is controlled to start off-grid. If the voltage on the AC side is less than the minimum off-grid output voltage value and the low-level signal is received, the inverter is controlled to operate off-grid. If the voltage on the AC side is greater than or equal to the minimum off-grid output voltage value and the high-level signal is received, the inverter is controlled to operate on-grid.

6. The backup power system as described in any one of claims 1-5, characterized in that, If the voltage of the power grid is greater than the undervoltage protection voltage value and the voltage on the DC side is less than or equal to the inverter startup voltage value; the first control circuit is further configured to: When the grid-connected controller is powered on, the first switching circuit is closed, and the high-level signal is sent to the second control circuit.

7. The backup power system as described in any one of claims 1-6, characterized in that, If the voltage on the DC side is greater than the inverter startup voltage and the voltage of the power grid is less than or equal to the undervoltage protection voltage, the second control circuit is further configured to: When the inverter is powered on, control the inverter to operate off-grid.

8. The backup power system as described in any one of claims 1-7, characterized in that, Also includes: Power cables and signal cables; The power cable is used to connect the inverter side of the first switching circuit to the AC side of the inverter, and the signal cable is used to connect the first control circuit to the second control circuit.

9. The backup power system as described in any one of claims 1-8, characterized in that, The grid-connected / off-grid controller includes: a first power input port, a second power input port, an inverter-side port, and a grid-side port; the first power input port is used to connect to a backup power load, the second power input port is used to connect to a non-backup power load, the inverter-side port is connected to the AC side of the inverter, and the grid-side port is used to connect to the power grid; The first switching circuit includes: a first relay and a second relay connected in series; the first relay is connected to the first power supply port and the inverter side port respectively, and the second relay is connected to the second power supply port and the grid side port respectively.

10. A startup control method for a backup power system, characterized in that, include: When the grid-connected controller is powered down, a low-level signal is provided to the second control circuit of the inverter. When the grid-connected controller is powered on, if the grid voltage is greater than the undervoltage protection voltage value and the voltage on the inverter side of the grid-connected controller is less than the minimum off-grid output voltage value, the first switching circuit in the grid-connected controller is closed, and a high-level signal is sent to the second control circuit. If the voltage of the power grid is less than or equal to the undervoltage protection voltage value or the voltage on the inverter side is greater than or equal to the minimum off-grid output voltage value, the first switching circuit is disconnected and the low-level signal is sent to the second control circuit. When the inverter is powered on, if the DC side voltage of the inverter is greater than the inverter startup voltage, the AC side voltage of the inverter is less than the minimum off-grid output voltage, and a low-level signal is received, the inverter is controlled to operate off-grid; if the DC side voltage is greater than the inverter startup voltage, the AC side voltage is greater than or equal to the minimum off-grid output voltage, and a high-level signal is received, the inverter is controlled to operate in grid-connected mode.

11. The start-up control method as described in claim 10, characterized in that, Also includes: If the voltage of the grid is greater than the undervoltage protection voltage and the voltage on the DC side is greater than the inverter start-up voltage, when the grid-connected controller is powered on, the grid-connected controller is controlled to start up in grid-connected mode. If the voltage on the inverter side is less than the minimum off-grid output voltage, the first switch circuit is closed and the high-level signal is sent to the second control circuit. If the voltage on the inverter side is greater than or equal to the minimum off-grid output voltage, the first switching circuit is disconnected, and the low-level signal is sent to the second control circuit.

12. The start-up control method as described in claim 10 or 11, characterized in that, Also includes: If the voltage of the power grid is greater than the undervoltage protection voltage and the voltage on the DC side is greater than the inverter startup voltage, a low-level signal is received when the inverter is powered on, and the inverter is controlled to start off-grid. If the voltage on the AC side is less than the minimum off-grid output voltage and the low-level signal is received, the inverter is controlled to operate off-grid. If the voltage on the AC side is greater than or equal to the minimum off-grid output voltage and the high-level signal is received, the inverter is controlled to operate in grid-connected mode.

13. The start-up control method according to any one of claims 10-12, characterized in that, Also includes: If the voltage of the power grid is greater than the undervoltage protection voltage and the voltage on the DC side is less than or equal to the inverter start-up voltage, when the grid-connected controller is powered on, the first switching circuit is closed and the high-level signal is sent to the second control circuit.

14. The start-up control method according to any one of claims 10-13, characterized in that, Also includes: If the voltage on the DC side is greater than the inverter startup voltage and the voltage of the grid is less than or equal to the undervoltage protection voltage, the inverter is controlled to operate off-grid when powered on.

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