Photovoltaic inverter, control method thereof and micro-grid system

Abstract

The invention provides a photovoltaic inverter, a control method thereof and a micro-grid system, the direct current end of the photovoltaic inverter is connected with a photovoltaic module, the alternating current end of the photovoltaic inverter is used for being connected with an external power grid through a grid-connected switch after being connected with an energy storage converter in parallel, and the energy storage converter is in a grid construction working mode. The photovoltaic inverter comprises a controller and an inverter circuit, and under the condition that the grid-connected switch is switched off, the controller is used for adjusting active power output by the inverter circuit to target power under the condition that the voltage amplitude of the alternating current end drops to be smaller than an amplitude threshold value at the first moment; wherein the magnitude of the target power is in direct proportion to the voltage amplitude of the alternating current end at the first moment, and is in inverse proportion to the magnitude of the target reactive current of the inverter circuit at the first moment. The photovoltaic inverter is applied to the micro-grid system, can reduce voltage oscillation generated by the alternating current bus when the micro-grid system operates in an off-grid mode and the voltage of the alternating current bus of the micro-grid system drops, and is high in applicability.

Description

Technical Field

[0001] The present application relates to the technical field of power electronics, and particularly relates to a photovoltaic inverter, a control method thereof, and a microgrid system. Background Art

[0002] A microgrid is a small-scale localized power system integrating new energy power generation, energy storage systems, load management, and control technologies, which can realize local production, flexible scheduling, and efficient utilization of energy. The microgrid has two operating modes: grid-connected mode and off-grid mode. The grid-connected mode means that under normal circumstances, the microgrid is connected to the main grid and operates in parallel, supplying excess electric energy to the main grid or supplying power to local loads from the main grid and charging the battery. The off-grid mode means that when a fault in the main grid or unsatisfactory power quality is detected, the microgrid disconnects from the main grid to form an island state, and the distributed power sources and batteries in the microgrid supply power to local loads.

[0003] When a short-circuit fault occurs in the microgrid or a large-capacity asynchronous machine is connected to the microgrid, the voltage amplitude of the microgrid bus drops, and the photovoltaic inverter in the microgrid enters the low-voltage ride-through stage. At this time, if the microgrid is operating in grid-connected mode, the bus voltage of the microgrid is determined by the external grid and can be kept stable under the action of the external grid. If the microgrid is operating independently in off-grid mode, the bus voltage of the microgrid cannot be stabilized by the external grid, and the conventional photovoltaic inverter has limited external regulation ability during the low-voltage ride-through stage, which may cause the voltage of the microgrid bus to oscillate, thereby affecting the safe operation of the load and reducing the power supply stability of the microgrid. Therefore, how to reduce the voltage oscillation generated by the microgrid bus when the microgrid is operating in off-grid mode and the voltage amplitude of the microgrid bus drops is a technical problem that those skilled in the art urgently need to solve. Summary of the Invention

[0004] The present application provides a photovoltaic inverter, a control method thereof, and a microgrid system. The photovoltaic inverter is applied to the microgrid system and can reduce the voltage oscillation generated by the AC bus when the microgrid system is operating in off-grid mode and the AC bus voltage of the microgrid system drops, and has high reliability and strong applicability.

[0005] In a first aspect, the present application provides a photovoltaic inverter. The DC side of the photovoltaic inverter is used to connect to photovoltaic modules, and the AC side of the photovoltaic inverter is used to connect to an external power grid through a grid connection switch after being connected in parallel with an energy storage converter. The energy storage converter is in a grid-forming operation mode. The photovoltaic inverter includes a controller and an inverter circuit. The controller is configured to control the inverter circuit to convert the direct current input from the DC side into alternating current and output it to the AC side. When the grid connection switch is open, the controller is configured to: when the voltage amplitude at the AC side drops below the amplitude threshold at a first moment, adjust the active power output by the inverter circuit to a target power; wherein, the magnitude of the target power is proportional to the voltage amplitude at the AC side at the first moment and inversely proportional to the magnitude of the target reactive current of the inverter circuit at the first moment.

[0006] In this embodiment, when the PV inverter is applied to a microgrid system, the AC side of the PV inverter is connected in parallel with the energy storage converter to the AC bus of the microgrid system, and this AC bus is connected to the external grid through a grid connection switch. When the grid connection switch is disconnected and the microgrid system operates in an off-grid mode, the energy storage converter is in a grid-forming operation mode, and the PV inverter is in a grid-following operation mode. It can be understood that since the AC side of the PV inverter is electrically connected to the AC bus, the voltage amplitude of this AC side is the same as that of the AC bus. Therefore, the PV inverter can determine the voltage amplitude of the AC bus by detecting the voltage amplitude of the AC side through the controller. When the PV inverter detects through the controller that the voltage amplitude of the AC side drops below the amplitude threshold at the first moment, it indicates that the voltage amplitude of the AC bus drops below the amplitude threshold at the first moment. Among them, the amplitude threshold refers to the maximum value of the voltage amplitude of the AC bus when the PV inverter enters the low-voltage ride-through stage, and this amplitude threshold can be specifically obtained according to the rated voltage amplitude of the AC bus. Therefore, when the voltage amplitude of the AC side drops below the amplitude threshold at the first moment, it indicates that the PV inverter enters the low-voltage ride-through stage at the first moment. At this time, the PV inverter adjusts the active power output by the inverter circuit to the target power through the controller, aiming to keep the active power output by the inverter circuit stable, and further reduce the active power oscillation of the inverter circuit output. Since the AC side is electrically connected to the AC bus, the active power output by the inverter circuit and the voltage of the AC bus affect each other. When the controller controls the reduction of the active power oscillation of the inverter circuit output, the voltage oscillation of the AC bus will also decrease. In addition, when the voltage of the AC bus drops, the controller needs to preferentially increase the reactive current output by the inverter circuit to help boost the voltage of the AC bus, and the smaller the voltage amplitude of the AC bus drops, the larger the reactive current that the inverter circuit needs to output. To avoid overloading the inverter circuit, the PV inverter of this application can control the inverter circuit to use the remaining capacity to output stable active power after ensuring that the output of the reactive current meets the requirements. Thus, it can be seen that the target value of the active power output by the inverter circuit in this application is inversely proportional to the target reactive current of the inverter circuit (i.e., the target value of the reactive current that the inverter circuit needs to output), and the magnitude of this target reactive current is inversely proportional to the voltage amplitude of the AC side. Therefore, the magnitude of the active power output by the inverter circuit is directly proportional to the voltage amplitude of the AC side. Thus, it can be seen that the PV inverter can determine the target value of the active power output by the inverter circuit, that is, the above-mentioned target power, according to the voltage amplitude of the AC side and the magnitude of the target reactive current. Further, when the PV inverter enters the low-voltage ride-through stage at the first moment, the PV inverter obtains the above-mentioned target power according to the voltage amplitude of the AC side at the first moment and the target reactive current, and quickly controls the active power output by the inverter circuit to the target power through the controller, which can quickly control the reduction of the voltage oscillation of the AC bus after entering the low-voltage ride-through stage.Generally speaking, the photovoltaic inverter of the present application can operate off-grid in a microgrid system. When the voltage of the AC bus cannot be stabilized by the external power grid, the controller detects the voltage amplitude at the AC side to determine the voltage amplitude of the AC bus. When entering the low-voltage ride-through stage, the controller controls the active power output by the inverter circuit to the target power, so as to reduce the oscillation of the active power output by the inverter circuit and at the same time reduce the voltage oscillation generated by the AC bus. In addition, the photovoltaic inverter in the present application obtains the target power based on the voltage amplitude at the AC side at the first moment and the target reactive current, which can avoid overloading the inverter circuit when adjusting the active power output by the controller. At the same time, after entering the low-voltage ride-through stage, it can quickly control the reduction of the voltage oscillation of the AC bus, avoid the influence of the AC bus voltage oscillation on the stable operation of the load, and improve the power supply safety of the microgrid system.

[0007] In a possible implementation manner, the target power is less than the magnitude of the active power output by the inverter circuit in the first time period, and the first time period is a period of time before the voltage amplitude at the AC side drops to less than the amplitude threshold. In this implementation manner, after entering the low-voltage ride-through stage, the photovoltaic inverter reduces the active power output by the controller, which can release capacity for reactive power output and avoid overloading the inverter circuit. Further, by adjusting the active power output by the inverter circuit to the target power, the controller can keep the active power stable and at the same time reduce the voltage oscillation generated by the AC bus.

[0008] In a possible implementation manner, the target power satisfies:

[0009]

[0010] where I max is the rated current of the photovoltaic inverter, I qrefmax is the target reactive current, U tmin is the voltage amplitude at the AC side at the first moment, U max is the rated voltage amplitude of the photovoltaic inverter, S max is the rated apparent power of the photovoltaic inverter. In this implementation manner, the controller can calculate the target power by sampling the voltage amplitude at the AC side, combining with the target reactive current of the inverter circuit, and the rated voltage amplitude, rated current and rated apparent power of the photovoltaic inverter. The calculation process is simple and easy to implement.

[0011] In a possible implementation, the magnitude of the target reactive current is inversely proportional to the voltage amplitude of the AC terminal at the first moment and directly proportional to the magnitude of the reactive current output by the inverter circuit during the first time period, where the first time period is a period of time before the voltage amplitude of the AC terminal drops to less than the amplitude threshold. In this implementation, after the PV inverter enters the low-voltage ride-through stage at the first moment, in order to help boost the voltage of the AC bus, the PV inverter needs to control the controller to increase the reactive current output by the inverter circuit. Moreover, the smaller the voltage amplitude of the AC bus drops at the first moment, the greater the reactive current that the inverter circuit needs to output, that is, the greater the target reactive current of the inverter circuit at the first moment. At the same time, if the reactive current output by the inverter circuit during the first time period is greater, then after entering the low-voltage ride-through stage, in order to increase the reactive current output, the target reactive current of the inverter circuit at the first moment is greater. Therefore, in this application, the PV inverter determines the target reactive current of the inverter circuit based on the voltage amplitude of the AC terminal at the first moment and the magnitude of the reactive current output by the inverter circuit during the first time period, which can ensure that the reactive current output by the inverter circuit can be used to boost the voltage of the AC bus.

[0012] In a possible implementation, the target reactive current satisfies:

[0013]

[0014] where U tmin is the voltage amplitude of the AC terminal at the first moment, U max is the rated voltage amplitude of the PV inverter, I q0 is the reactive current output by the inverter circuit during the first time period, I N is the rated current of the PV inverter, and K1 is the reactive current support coefficient of the PV inverter during low-voltage ride-through. The first time period is a period of time before the voltage amplitude of the AC terminal drops to less than the amplitude threshold. In this implementation, the controller can calculate the target reactive current based on sampling the voltage amplitude of the AC terminal, the reactive current of the inverter circuit, and combining the rated voltage amplitude, rated current, and reactive current support coefficient of the PV inverter. The calculation process is simple and easy to implement.

[0015] In a possible implementation, the voltage amplitude of the AC terminal at the first moment is less than the voltage amplitudes of the AC terminal at other moments during the low voltage ride through period. In this implementation, after the microgrid system enters the low voltage ride through stage at the first moment, as the voltage amplitude of the AC terminal increases, the reactive current required for the inverter circuit to boost the AC bus voltage will decrease, and the active power that the remaining capacity of the inverter circuit can output will increase. It can be seen that the active power that the inverter circuit can output at other moments after the first moment is greater than the active power that can be output at the first moment, that is, greater than the target power. Therefore, when the controller of the present application adjusts the active power output by the inverter circuit with this target power during the entire low voltage ride through stage, it will not affect the requirement for the reactive current output by the inverter circuit (the requirement for the reactive current is getting lower and lower). At the same time, the controller adjusts the active power output by the inverter circuit to a fixed target power, which can make the active power output stable and reduce the voltage oscillation generated by the AC bus.

[0016] In a second aspect, the present application further provides a microgrid system, which includes an energy storage converter and a photovoltaic inverter. The DC terminal of the photovoltaic inverter is used to connect to a photovoltaic module, and the AC terminal of the photovoltaic inverter is used to connect to an external power grid through a grid connection switch after being connected in parallel with the energy storage converter. The energy storage converter is in a grid forming working mode. The photovoltaic inverter includes a controller and an inverter circuit. The controller is used to control the inverter circuit to convert the direct current input from the DC terminal into alternating current and output it to the AC terminal. In the case where the grid connection switch is disconnected, the controller is used to: when the voltage amplitude of the AC terminal drops below the amplitude threshold at the first moment, adjust the active power output by the inverter circuit to a target power; wherein, the magnitude of the target power is directly proportional to the voltage amplitude of the AC terminal at the first moment and inversely proportional to the magnitude of the target reactive current of the inverter circuit at the first moment.

[0017] In a third aspect, the present application further provides a control method for a photovoltaic inverter. The DC terminal of the photovoltaic inverter is used to connect to a photovoltaic module, and the AC terminal of the photovoltaic inverter is used to connect to an external power grid through a grid connection switch after being connected in parallel with the energy storage converter. The energy storage converter is in a grid forming working mode. The photovoltaic inverter includes an inverter circuit, and the inverter circuit is used to convert the direct current input from the DC terminal into alternating current and output it to the AC terminal. In the case where the grid connection switch is disconnected, the method includes:

[0018] When the voltage amplitude of the AC terminal drops below the amplitude threshold at the first moment, adjust the active power output by the inverter circuit to a target power; wherein, the magnitude of the target power is directly proportional to the voltage amplitude of the AC terminal at the first moment and inversely proportional to the magnitude of the target reactive current of the inverter circuit at the first moment.

[0019] In a possible implementation, the target power is less than the active power output by the inverter circuit during a first time period, where the first time period is a period of time before the voltage amplitude at the AC side drops to less than the amplitude threshold.

[0020] In a possible implementation, the target power satisfies:

[0021]

[0022] where I max is the rated current of the PV inverter, I qrefmax is the target reactive current, U tmin is the voltage amplitude at the AC side at a first moment, U max is the rated voltage amplitude of the PV inverter, S max is the rated apparent power of the PV inverter.

[0023] In a possible implementation, the magnitude of the target reactive current is inversely proportional to the voltage amplitude at the AC side at the first moment and directly proportional to the reactive current output by the inverter circuit during the first time period, where the first time period is a period of time before the voltage amplitude at the AC side drops to less than the amplitude threshold.

[0024] In a possible implementation, the target reactive current satisfies:

[0025]

[0026] where U tmin is the voltage amplitude at the AC side at the first moment, U max is the rated voltage amplitude of the PV inverter, I q0 is the reactive current output by the PV inverter during the first time period, I N is the rated current of the PV inverter, and K1 is the reactive current support coefficient of the PV inverter during low voltage ride-through. The first time period is a period of time before the voltage amplitude at the AC side drops to less than the amplitude threshold.

[0027] In a possible implementation, the voltage amplitude at the AC side at the first moment is less than the voltage amplitudes at other moments during low voltage ride-through of the AC side.

[0028] The solutions provided in the above second aspect and third aspect are used to implement or cooperate with the PV inverter provided in the above first aspect. Therefore, the same or corresponding beneficial effects can be achieved as those of the corresponding PV inverter in the first aspect, and details are not described herein again. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 FIG. 1 is a schematic diagram of an application scenario of the microgrid system provided in the present application;

[0030] Figure 2a It is a schematic diagram of the voltage amplitude at the AC terminal of the PV inverter provided by the embodiment of the present application;

[0031] Figure 2b It is a schematic diagram of the active power of the inverter circuit provided by the embodiment of the present application;

[0032] Figure 2c It is a schematic diagram of the reactive power of the inverter circuit provided by the embodiment of the present application;

[0033] Figure 3 It is a schematic diagram of the frame of the PV inverter provided by the embodiment of the present application;

[0034] Figure 4a It is another schematic diagram of the voltage amplitude at the AC terminal of the PV inverter provided by the embodiment of the present application;

[0035] Figure 4b It is another schematic diagram of the active power of the inverter circuit provided by the embodiment of the present application;

[0036] Figure 4c It is another schematic diagram of the reactive power of the inverter circuit provided by the embodiment of the present application;

[0037] Figure 5 It is a block diagram of the closed-loop control of the active power of the inverter circuit provided by the embodiment of the present application;

[0038] Figure 6 It is a schematic flow diagram of the control method of the PV inverter provided by the embodiment of the present application. Detailed implementation manners

[0039] The microgrid system provided by the present application can be applied to different application scenarios, such as photovoltaic power supply scenarios, photovoltaic-storage hybrid power supply scenarios, photovoltaic-wind hybrid power supply scenarios, etc. Hereinafter, the microgrid system will be described by taking the photovoltaic-storage hybrid power supply scenario as an example.

[0040] As a small power system, the microgrid system can rely on internal distributed power sources and energy storage devices to provide independent power supply to local loads when the external power grid fails. Exemplarily, in remote areas or on islands, the microgrid system uses photovoltaic devices and energy storage devices to supply power to local loads, thereby realizing off-grid power supply and solving the problem of difficult grid extension. In addition, the microgrid system can also be connected to the external power grid for parallel operation. Specifically, please refer to Figure 1 , Figure 1 It is a schematic diagram of an application scenario of the microgrid system provided by the present application. As shown in Figure 1As shown, the microgrid system includes a grid connection switch through which the microgrid system is connected to the external power grid. When the grid connection switch is closed, the microgrid system operates synchronously with the external power grid and can transmit electrical energy to the external power grid or receive electrical energy from the external power grid for utilization. For example, the microgrid system can transmit the excess electrical energy generated by photovoltaic power generation during strong sunlight during the day to the external power grid to improve the electrical energy utilization efficiency, and obtain electrical energy from the external power grid when the photovoltaic power generation is insufficient at night to meet the electricity demand of local loads. When a fault occurs in the external power grid or the microgrid system is disconnected from the grid according to a preset time, the grid connection switch is disconnected and the microgrid system operates independently off the grid.

[0041] In Figure 1 the application scenario shown, when the microgrid system operates independently off the grid, the microgrid system can supply power to local loads through the energy storage converter and the photovoltaic inverter together. As Figure 1 shown, the microgrid system includes an energy storage converter and a photovoltaic inverter. Among them, the DC side of the energy storage converter is connected to the energy storage battery, and the AC side of the energy storage converter is connected to the AC bus in the microgrid system. The energy storage converter converts the DC power from the energy storage battery into AC power and outputs it to the AC bus to supply power to the local loads connected to the AC bus. At the same time, the energy storage converter operates in the grid-forming mode and can adjust the amplitude, frequency, and waveform of the output voltage to maintain stable power supply. The DC side of the photovoltaic inverter is connected to the photovoltaic module, and the AC side of the photovoltaic inverter is connected to the AC bus. The photovoltaic inverter converts the DC power from the photovoltaic module into AC power and outputs it to the AC bus to supply power to local loads. At the same time, the photovoltaic inverter operates in the grid-following mode and can adjust the output current to achieve power transmission.

[0042] In practical applications, the microgrid system will also be provided with a box-type transformer, which is used to adjust and distribute the input AC voltage and then output it to meet the voltage requirements of different electrical energy transmission nodes. Exemplarily, the voltage amplitude of the AC power transmitted by the AC bus is usually relatively high. For this reason, the photovoltaic inverter and the energy storage converter can transmit the AC power to the box-type transformer for boosting and then provide it to the AC bus. In addition, the AC power transmitted on the AC bus can be stepped down by the box-type transformer and provided to the load. Or, if the rated voltage amplitude of the load is suitable for the AC power voltage amplitude transmitted on the AC bus, the AC bus can also directly supply power to the load. When the grid connection switch is closed and the microgrid system is connected to the grid for operation, the AC power transmitted on the AC bus can be stepped up by one or more box-type transformers and then output to the external power grid. The specific installation location and quantity of the box-type transformer can be flexibly adjusted according to the actual application scenario, and the embodiments of the present application do not limit this.

[0043] It should be noted that in Figure 1In the application scenario shown, when the load connected to the AC bus is a large-capacity asynchronous machine (such as a motor with a rated power of 500 kW), since the large-capacity asynchronous machine will generate an instantaneous large current during startup, when the large-capacity asynchronous machine is connected to the AC bus, it will cause the voltage of the AC bus to drop. Or, when a short-circuit fault occurs in the microgrid system, the AC bus of the microgrid system is equivalent to being connected to the ground wire through a resistor with a small resistance, which will also cause the voltage of the AC bus to drop. Generally, when the voltage amplitude of the AC bus drops below 90% of the rated voltage amplitude, the PV inverter enters the low-voltage ride-through stage. At this time, the PV inverter can maintain grid-connected operation by adjusting the current output to prevent large-scale disconnection from the grid. However, as described in the background art, in the case of the microgrid system operating independently off-grid, since the AC bus is disconnected from the external grid, the voltage of the AC bus cannot be stabilized by the external grid. At the same time, the external adjustment capabilities of the energy storage converter and the PV inverter during the low-voltage ride-through stage are limited, which may cause voltage oscillations in the AC bus.

[0044] Exemplarily, please refer to Figure 2a 、 Figure 2b and Figure 2c , Figure 2a which is a schematic diagram of the voltage amplitude at the AC terminal of the PV inverter provided by an embodiment of the present application, Figure 2b and Figure 2c is a schematic diagram of the active power of the inverter circuit provided by an embodiment of the present application. Among them, the voltage amplitude of the AC bus drops below 90% of the rated voltage amplitude at the first moment T1, that is, the PV inverter enters the low-voltage ride-through stage at the first moment T1. Due to many limitations in the conventional adjustment methods adopted by the PV inverter during the low-voltage ride-through stage, obvious oscillations occur in both the active power and the reactive power output by the PV inverter, as shown in Figure 2b and Figure 2c . At the same time, since the voltage of the AC bus cannot be stabilized by the external grid, obvious oscillations also occur in the AC bus, as shown in Figure 2a . It can be understood that the oscillations generated by the voltage of the AC bus will affect the safe operation of the load and reduce the power supply stability. Therefore, how to reduce the voltage oscillations generated by the AC bus when the microgrid system operates off-grid and the voltage of the AC bus of the microgrid system drops is a technical problem that those skilled in the art urgently need to solve.

[0045] For this reason, the present application provides a PV inverter, which is applied to the above-mentioned microgrid system, and can reduce the voltage oscillations generated by the AC bus when the microgrid system operates off-grid and the voltage of the AC bus of the microgrid system drops, causing the PV inverter to enter the low-voltage ride-through stage, and has high reliability and strong applicability.

[0046] The above is only an example of the application scenarios of the photovoltaic inverter provided in this application, rather than an exhaustive list. This application does not limit the application scenarios.

[0047] The photovoltaic inverter provided in this application can be applied to microgrid systems in different power supply scenarios. For the convenience of understanding, the following content takes the photovoltaic inverter applied in the Figure 1 photovoltaic-storage hybrid power supply scenario shown above as an example for illustration. Please refer to Figure 3 , Figure 3 which is a frame schematic diagram of the photovoltaic inverter provided in an embodiment of this application. As Figure 3 shown, the DC terminal of the photovoltaic inverter is electrically connected to the photovoltaic module, and the AC terminal of the photovoltaic inverter is electrically connected to the local load through the AC bus. At the same time, the AC terminal of the photovoltaic inverter is also connected to the external power grid through the grid connection switch. Among them, the photovoltaic inverter includes a controller and an inverter circuit, and the inverter circuit is used to convert the direct current input at the DC terminal into alternating current and output it through the AC terminal. It can be understood that when the grid connection switch is closed, the microgrid system where the photovoltaic inverter is located operates in parallel; when the grid connection switch is opened, the microgrid system where the photovoltaic inverter is located operates independently off-grid. For the convenience of understanding the implementation principle of how the photovoltaic inverter provided in this application reduces the voltage oscillation generated by the AC bus when the microgrid system operates independently off-grid, this application will describe the application scenario where the microgrid system where the photovoltaic inverter is located operates independently off-grid, that is, the grid connection switch connected to the AC terminal of the photovoltaic inverter is opened, in the following content. The application scenario where the microgrid system where the photovoltaic inverter is located operates in parallel, that is, the grid connection switch is closed, will not be elaborated here.

[0048] As can be seen from the above content, when the grid connection switch is opened, when the voltage amplitude of the AC bus drops, causing the photovoltaic inverter to enter the low voltage ride-through stage, the AC bus may generate voltage oscillation. Therefore, in order to timely control the voltage oscillation generated by the AC bus, the photovoltaic inverter can detect the voltage amplitude of the AC bus through the controller to determine whether the photovoltaic inverter enters the low voltage ride-through stage.

[0049] Specifically, when the photovoltaic inverter detects that the voltage amplitude of the AC bus drops below the amplitude threshold through the controller, it can determine that the photovoltaic inverter enters the low-voltage ride-through stage. Among them, the amplitude threshold can be obtained from the rated voltage amplitude of the AC bus. Exemplarily, assuming that the rated voltage amplitude of the AC bus is 220 volts, when the voltage amplitude of the AC bus drops below 90% of the rated voltage amplitude, the photovoltaic inverter enters the low-voltage ride-through stage. Therefore, the controller can set the above amplitude threshold to 198 volts (220 volts × 90%), and when it detects that the voltage amplitude of the AC bus is less than 198 volts, it can determine that the photovoltaic inverter enters the low-voltage ride-through stage. Among them, the specific value of the amplitude threshold can be flexibly adjusted according to the requirements of the actual application scenario, and the embodiments of the present application do not limit this.

[0050] It should be noted that since the AC terminal is electrically connected to the AC bus, the voltage amplitude of the AC bus can be equivalent to the voltage amplitude of the AC terminal. Then the controller can use the voltage amplitude of the AC terminal as the voltage amplitude of the AC bus and determine whether the photovoltaic inverter enters the low-voltage ride-through stage according to the voltage amplitude of the AC terminal. For this reason, when the controller detects that the voltage amplitude of the AC terminal is less than the above amplitude threshold, it can determine that the voltage amplitude of the AC bus is less than the amplitude threshold, and further determine that the photovoltaic inverter enters the low-voltage ride-through stage. For the convenience of description, in the following content, the voltage amplitude of the AC bus is equivalently expressed as the voltage amplitude of the AC terminal.

[0051] In some feasible embodiments, the controller can be electrically connected to the AC terminal and obtain the voltage amplitude of the AC terminal by sampling the electrical signal of the AC terminal. Alternatively, the controller can obtain the voltage amplitude of the AC terminal through an external sampling module. The sampling module is electrically connected to the AC terminal, can sample the electrical signal of the AC terminal, and then obtain the voltage amplitude of the AC terminal. At the same time, the sampling module is communicatively connected to the controller and can send the sampled voltage amplitude of the AC terminal to the controller. It can be understood that the above is only an example, and the embodiments of the present application do not limit the implementation manner of the controller to obtain the voltage amplitude of the AC terminal.

[0052] It can be understood that when the grid-connected switch is disconnected and the AC terminal outputs alternating current, the controller continuously detects the voltage amplitude of the AC terminal. When the controller detects at the first moment that the voltage amplitude of the AC terminal drops below the amplitude threshold, it indicates that the photovoltaic inverter enters the low-voltage ride-through stage at the first moment. Further, after the photovoltaic inverter enters the low-voltage ride-through stage at the first moment, the controller can control the active power output by the inverter circuit to remain stable to reduce the voltage oscillation generated by the AC bus.

[0053] It should be noted that during the low-voltage ride-through phase, the controller can reduce the oscillation of the active power output by the inverter circuit by controlling the active power output by the inverter circuit to be stable. At the same time, since the active power output by the inverter circuit and the reactive power output by the inverter circuit affect each other, when the oscillation of the active power output by the inverter circuit decreases, the oscillation of the reactive power output by the inverter circuit will also decrease. Further, since the AC side is electrically connected to the AC bus, the active power and reactive power output by the inverter circuit interact with the voltage of the AC bus. When the oscillations of the active power output by the inverter circuit and the reactive power output by the inverter circuit decrease, the voltage oscillation of the AC bus will also decrease. Thus, after the PV inverter enters the low-voltage ride-through phase at the first moment, by controlling the active power output by the inverter circuit to be stable through the controller, the voltage oscillation generated by the AC bus can be reduced, thereby reducing the impact of the voltage oscillation on the operation of the load.

[0054] Exemplarily, please refer to Figure 4a 、 Figure 4b and Figure 4c , Figure 4a which is another schematic diagram of the voltage amplitude of the AC side of the PV inverter provided by the embodiment of the present application, Figure 4b which is another schematic diagram of the active power of the inverter circuit provided by the embodiment of the present application, Figure 4c which is another schematic diagram of the reactive power of the inverter circuit provided by the embodiment of the present application. Among them, as Figure 4a shown, when the first moment T1 arrives, the PV inverter detects through the controller that the voltage amplitude of the AC side drops to be less than the amplitude threshold, that is, the PV inverter enters the low-voltage ride-through phase at the first moment T1. In order to reduce the voltage oscillation generated by the AC bus after the PV inverter enters the low-voltage ride-through phase, the PV inverter controls the active power output by the inverter circuit to be stable through the controller. As Figure 4b shown, the active power output by the inverter circuit basically does not generate oscillation. At the same time, when the active power output by the inverter circuit remains stable, the oscillation of the reactive power output by the inverter circuit decreases. As Figure 4c shown, the oscillation of the reactive power output by the inverter circuit is significantly reduced compared to the reactive power shown in Figure 2c . Further, when the oscillations of both the active power output by the inverter circuit and the reactive power output by the inverter circuit are significantly reduced, the voltage oscillation generated by the AC bus also decreases. As Figure 4a shown, the voltage oscillation generated by the AC bus (i.e., the voltage oscillation generated by the AC side) is significantly reduced compared to the voltage oscillation generated by the AC bus shown in Figure 2a .

[0055] It can be seen that after the photovoltaic inverter provided by the embodiment of the present application enters the low-voltage ride-through stage at the first moment, by controlling the active power output by the inverter circuit to remain stable, the oscillation of the reactive power output by the inverter circuit can be reduced, and at the same time, the voltage oscillation generated by the AC bus can be reduced, ensuring the safe and stable power supply to the load.

[0056] Further, the controller can control the active power output by the inverter circuit to remain stable by adjusting the active power output by the inverter circuit to the target power. Among them, the target power can be understood as the target value of the active power output by the inverter circuit. The controller dynamically controls the switching tubes of each arm in the inverter circuit based on the target power, adjusts the actually output active power of the inverter circuit to be equal to the target power, or the difference from the target power can be ignored, so as to make the active power output by the inverter circuit remain stable. For this purpose, before controlling the active power output by the inverter circuit to remain stable, the controller needs to first obtain the target power of the inverter circuit.

[0057] It should be noted that in order to quickly reduce the voltage oscillation generated by the AC bus during the low-voltage ride-through stage, when the photovoltaic inverter enters the low-voltage ride-through stage at the first moment, the controller can obtain the target power of the inverter circuit at the first moment. Further, the controller controls the active power output by the inverter circuit based on the target power at the first moment, and can quickly control the reduction of the voltage oscillation of the AC bus after entering the low-voltage ride-through stage.

[0058] Specifically, the controller can obtain the target power of the inverter circuit at the first moment based on the voltage amplitude at the AC side at the first moment and the magnitude of the target reactive current of the inverter circuit at the first moment.

[0059] It should be noted that during the low-voltage ride-through stage, in order to help the voltage of the AC bus recover, the inverter circuit needs to inject reactive current into the AC bus, and this reactive current can generate an effect of voltage increase under the action of the line impedance, so as to compensate for the voltage drop of the AC bus. It can be understood that the smaller the voltage amplitude at the AC side at the first moment, the more the voltage drop of the AC bus is characterized, and the more reactive current the inverter circuit needs to output to compensate the voltage of the AC bus, that is, the larger the target reactive current of the inverter circuit. The target reactive current refers to the target value of the reactive current output by the inverter circuit. On the contrary, the larger the voltage amplitude at the AC side at the first moment, the less the voltage drop of the AC bus is characterized, and the smaller the reactive current the inverter circuit needs to output, that is, the smaller the target reactive current of the inverter circuit. At the same time, during the low-voltage ride-through stage, the inverter circuit can also inject active power into the AC bus to maintain the frequency stability of the AC bus.

[0060] It can be understood that, in order to compensate for the voltage drop of the AC bus as much as possible, during the low-voltage ride-through phase, the inverter circuit needs to prioritize ensuring the output of reactive power. At the same time, to avoid output overload, the inverter circuit needs to reduce the output of active power. Therefore, when the voltage amplitude at the AC side is smaller at the first moment, that is, when the voltage drop of the AC bus is more, the reactive power that the inverter circuit needs to output at the first moment is larger (the target reactive current is larger), and the active power that the remaining capacity of the inverter circuit can output is smaller. On the contrary, if the voltage amplitude of the AC bus is larger at the first moment, the reactive power that the inverter circuit needs to output at the first moment is smaller (the target reactive current is smaller), and the active power that the remaining capacity of the inverter circuit can output is larger. Thus, it can be seen that the target value of the active power that the inverter circuit can output at the first moment (i.e., the target power at the first moment) can be understood as the maximum active power that the remaining capacity of the inverter circuit can output after prioritizing ensuring that the output of reactive power at the first moment meets the requirements. Therefore, the magnitude of the target power of the inverter circuit at the first moment is inversely proportional to the magnitude of the reactive power that the inverter circuit needs to output at the first moment, that is, inversely proportional to the target reactive current of the inverter circuit at the first moment.

[0061] At the same time, since the target reactive current of the inverter circuit at the first moment is inversely proportional to the voltage amplitude at the AC side at the first moment, the magnitude of the target power of the inverter circuit at the first moment is directly proportional to the voltage amplitude at the AC side at the first moment.

[0062] Exemplarily, as Figure 4b shown, assume that the controller detects that the voltage amplitude at the AC side drops below the amplitude threshold at the first moment T1, that is, the PV inverter enters the low-voltage ride-through phase at the first moment T1. First, the controller can determine the target reactive current of the inverter circuit at the first moment T1 and adjust the reactive current output by the inverter circuit based on this target reactive current to boost the voltage of the AC bus. At the same time, the controller can determine the target power P1 based on the magnitude of this target reactive current and the voltage amplitude at the AC side at the first moment. When the voltage amplitude at the AC side is relatively small at the first moment, the voltage drop of the AC bus is relatively large, so the target reactive current of the inverter circuit at the first moment is relatively large, which makes the active power that the inverter circuit can output relatively small, that is, the target power P1 is relatively small. At this time, the inverter circuit is mainly used to output reactive power to help boost the voltage of the AC bus. When the voltage amplitude at the AC side is relatively large at the first moment, the voltage drop of the AC bus is relatively small, so the target reactive current of the inverter circuit at the first moment is relatively small, which makes the active power that the inverter circuit can output relatively large, that is, the target power P1 is relatively large. At this time, the inverter circuit reduces the output of reactive power and increases the output of active power, so that it can help boost the voltage of the AC bus while maintaining the frequency stability of the AC voltage.

[0063] It can be understood that after the controller obtains the target power, it can dynamically control the switching tubes of each arm in the inverter circuit to adjust the active power actually output by the inverter circuit to the target power, so as to keep the active power output by the inverter circuit stable.

[0064] Exemplarily, please refer to again Figure 4b , assuming that the photovoltaic inverter is in the low-voltage ride-through stage during the period from the first moment T1 to the second moment T2, and the target power set by the controller is P1, then during the period from the first moment T1 to the second moment T2, the photovoltaic inverter can control the switching tubes of each arm in the inverter circuit through the controller, so that the active power output by the inverter circuit is equal to the target power P1, or fluctuates slightly around the target power P1, so as to keep the active power output by the inverter circuit stable during the low-voltage ride-through stage.

[0065] Generally speaking, the photovoltaic inverter of the present application can detect the voltage amplitude of the AC side through the controller to determine the voltage amplitude of the AC bus when the microgrid system is operating off-grid and the voltage of the AC bus cannot be stabilized by the external power grid. When entering the low-voltage ride-through stage, the controller controls the active power output by the inverter circuit to the target power to reduce the oscillation of the active power output by the inverter circuit and at the same time reduce the voltage oscillation generated by the AC bus. In addition, the photovoltaic inverter in the present application can obtain the target power based on the voltage amplitude of the AC side at the first moment and the target reactive current, which can avoid overloading the inverter circuit when adjusting the active power output by the controller, and can quickly control the reduction of the voltage oscillation of the AC bus after entering the low-voltage ride-through stage, avoiding the influence of the AC bus voltage oscillation on the stable operation of the load and improving the power supply safety of the microgrid system.

[0066] As can be seen from the above, after the photovoltaic inverter enters the low-voltage ride-through stage, due to the voltage drop of the AC bus, the photovoltaic inverter usually needs to control the reduction of the active power output by the inverter circuit and increase the reactive power output to help the voltage of the AC bus rise.

[0067] In some feasible embodiments, after the photovoltaic inverter enters the low-voltage ride-through stage, when the controller in the photovoltaic inverter uses a conventional regulation method to control the reduction of the active power output by the inverter circuit, it usually causes obvious oscillation of the active power output by the inverter circuit. Exemplarily, as Figure 2b shown, before the first moment T1, the controller controls the active power output by the inverter circuit to be P2. After the first moment T1, the controller controls the reduction of the active power output by the inverter circuit. However, due to many limitations of the conventional regulation method, obvious oscillation of the active power output by the inverter circuit occurs, which will affect the voltage stability of the AC bus.

[0068] After the photovoltaic inverter provided by the embodiment of the present application enters the low voltage ride-through stage, the photovoltaic inverter adjusts the active power output by the inverter circuit to the target power through the controller, and the target power is less than the active power output by the inverter circuit in the first time period. The first time period is a period of time before the voltage amplitude at the AC side drops to less than the amplitude threshold. Exemplarily, in the case of strong sunlight during the day, assuming that the photovoltaic module stably supplies direct current to the inverter circuit at the maximum power, and at the same time, the output power of the inverter circuit remains unchanged, then before the voltage of the AC bus drops, the active power output by the inverter circuit remains stable. The reactive power that remains stable can be understood as the magnitude of the active power output by the inverter circuit in the first time period. As Figure 4b shown, before the arrival of the first moment T1, the magnitude of the active power output by the inverter circuit remains basically unchanged, and the magnitude of the active power before the first moment T1 is the magnitude of the active power output by the inverter circuit in the first time period described above. The above is only an example and does not constitute a limitation to the embodiment of the present application.

[0069] It can be understood that after the photovoltaic inverter provided by the embodiment of the present application enters the low voltage ride-through stage, the photovoltaic inverter reduces the active power output by the inverter circuit through the controller, and the capacity can be released for outputting reactive power. At the same time, the controller reduces the active power output by the inverter circuit to the target power, so that the active power can be kept stable.

[0070] Exemplarily, as Figure 4b shown, the controller controls the active power output by the inverter circuit after the first moment T1 to be reduced to the target power P1, the target power P1 is less than the active power P3 output by the inverter circuit before the first moment T1, and the active power output by the inverter circuit remains stable, thereby reducing the generation of active power oscillation.

[0071] In some feasible embodiments, as can be seen from the above, the smaller the voltage amplitude at the AC side of the inverter circuit at the first moment, the more the voltage drop of the AC bus is characterized, and the greater the reactive current that the inverter circuit needs to output to compensate the voltage of the AC bus, that is, the greater the target reactive current of the inverter circuit. On the contrary, the larger the voltage amplitude at the AC side at the first moment, the less the voltage drop of the AC bus is characterized, and the smaller the reactive current that the inverter circuit needs to output, that is, the smaller the target reactive current of the inverter circuit. Therefore, the controller can determine the target reactive current of the inverter circuit at the first moment based on the voltage amplitude at the AC side at the first moment. It can be understood that the voltage amplitude at the AC side at the first moment is inversely proportional to the target reactive current of the inverter circuit at the first moment.

[0072] Further, the controller can obtain the above-mentioned target reactive current based on the voltage amplitude of the AC side at the first moment and the magnitude of the reactive current output by the inverter circuit in the first time period. Herein, the first time period is a period of time before the voltage amplitude of the AC side drops to be less than the amplitude threshold. Exemplarily, in the case of strong sunlight during the day, assuming that the photovoltaic module stably supplies direct current to the inverter circuit at the maximum power, and meanwhile, the output power of the inverter circuit remains unchanged, then before the voltage of the AC bus drops, the reactive current output by the inverter circuit remains stable. This stable reactive current can be understood as the magnitude of the reactive current output by the inverter circuit in the first time period. Exemplarily, as Figure 4c shown, before the arrival of the first moment T1, the magnitude of the reactive power output by the inverter circuit remains basically unchanged, and then the reactive current output by the inverter circuit also remains basically unchanged. The controller can use the magnitude of the reactive current before the first moment T1 as the magnitude of the reactive current output by the inverter circuit in the first time period. The above is only an example and does not constitute a limitation on the embodiments of the present application.

[0073] In some feasible embodiments, specifically, the controller can calculate the magnitude of the target reactive current of the inverter circuit at the first moment based on the following formula (1), and the formula (1) is specifically expressed as:

[0074]

[0075] wherein, I qref is the target reactive current of the inverter circuit at the first moment, U tmin is the voltage amplitude of the AC side at the first moment, U max is the rated voltage amplitude of the photovoltaic inverter, I q0 is the reactive current output by the photovoltaic inverter in the first time period, I N is the rated current of the photovoltaic inverter, and K1 is the reactive current support coefficient of the photovoltaic inverter during low-voltage ride-through.

[0076] In some feasible embodiments, after the controller obtains the target reactive current of the inverter circuit at the first moment, further, the controller can calculate the target power of the inverter circuit based on the following formula (2), and the formula (2) is specifically expressed as:

[0077]

[0078] wherein, P ref is the target power of the inverter circuit, I max is the rated current of the photovoltaic inverter, I qrefmax is the target reactive current of the inverter circuit at the first moment, U tmin is the voltage amplitude of the AC side at the first moment, U maxis the rated voltage amplitude of the PV inverter, S max is the rated apparent power of the PV inverter.

[0079] In some feasible embodiments, after the controller obtains the target power of the inverter circuit at the first moment based on the above, it can adopt Figure 5 the process shown to adjust the active power output by the inverter circuit to the target power. Specifically, please refer to Figure 5 , Figure 5 which is a block diagram of the closed-loop control of the active power of the inverter circuit provided by the embodiment of the present application. As Figure 5 shown, first, the controller samples the actual active power P output by the inverter circuit and obtains the difference between the sampled active power P and the target power Pref. Further, the controller can calculate the difference between the active power P and the target power Pref through a proportional-integral controller PI to obtain the target active current Idref. This target active current Idref can be understood as the target value of the active current output by the inverter circuit. The controller samples the actual active current Id output by the inverter circuit and obtains the difference between the sampled active current Id and the target active current Idref. Further, the controller can calculate the difference between the active current Id and the target active current Idref through a proportional-integral controller PI to obtain the modulation reference voltage Ud. Based on this modulation reference voltage Ud, the controller controls the switching tubes of each arm in the inverter circuit, which can gradually reduce the difference between the actual active power output by the inverter circuit and the target power, so as to adjust the actual active power output by the inverter circuit to be equal to the target power, or the difference from the target power can be ignored. It should be noted that the specific method adopted by the controller to adjust the active power output by the inverter circuit to the target power can be flexibly adjusted according to the actual application scenario, Figure 5 the example shown is only for illustration and does not constitute a limitation to the embodiments of the present application.

[0080] In some feasible embodiments, in the embodiments of the present application, the voltage amplitude of the AC side at the first moment is less than the voltage amplitude of the AC side at other moments during the low-voltage ride-through period, that is, the voltage amplitude of the AC bus drops to the minimum value at the first moment. Exemplarily, as Figure 4a shown, during the period from the first moment T1 to the second moment T2, the PV inverter is in the low-voltage ride-through stage. The voltage amplitude of the AC side drops to the minimum value at the first moment T1, and after the first moment T1, the voltage amplitude of the AC side gradually increases.

[0081] It should be noted that, as can be seen from the above formulas (1) and (2), the magnitude of the voltage amplitude at the AC terminal is inversely proportional to the magnitude of the reactive current that the inverter circuit needs to output, and the magnitude of the active power that the inverter circuit can output is directly proportional to the magnitude of the voltage amplitude at the AC terminal and inversely proportional to the magnitude of the reactive current that the inverter circuit needs to output. After the first moment, as the voltage amplitude at the AC terminal increases, the reactive current that the inverter circuit needs to output will decrease, and the magnitude of the active power that the remaining capacity of the inverter circuit can output will increase. Thus, it can be seen that the active power that the inverter circuit can output at other moments after the first moment is greater than the active power that can be output at the first moment, that is, greater than the target power of the inverter circuit at the first moment.

[0082] Therefore, after the controller of the embodiment of the present application obtains the target power of the inverter circuit at the first moment, even if the fixed target power is used to adjust the active power output by the inverter circuit throughout the low-voltage ride-through stage, it will not affect the subsequent satisfaction of the reactive power output requirement of the inverter circuit (the requirement for reactive power is getting lower and lower). At the same time, the controller adjusts the active power output by the inverter circuit to remain the target power unchanged, which can make the active power output stable.

[0083] Exemplarily, assume that the target power of the inverter circuit obtained by the controller at the first moment is 100 watts, that is, after the inverter circuit preferentially outputs reactive power at the first moment, the active power that the remaining capacity can output is 100 watts. Then the controller can control the active power output by the inverter circuit to 100 watts throughout the low-voltage ride-through stage. Further, after the first moment, as the voltage amplitude at the AC terminal gradually increases, the reactive power that the inverter circuit needs to preferentially output decreases, and the active power that the remaining capacity can output increases to 150 watts. At this time, the controller continues to control the active power output by the inverter circuit to 100 watts, which will not affect the reactive power that the inverter circuit needs to preferentially output, and at the same time can keep the active power output stable.

[0084] It can be understood that during the low-voltage ride-through stage, the voltage amplitude at the AC terminal can gradually increase or fluctuate up and down. The embodiment of the present application does not limit this.

[0085] In some feasible embodiments, the controller and the PV inverter can be independent devices, and the controller can be arranged inside or outside the PV inverter. Or, in some other cases, the controller can also be a relevant control device within the PV inverter, such as an integrated circuit (IC), etc. The above-mentioned controller can be a digital signal processing (DSP) unit, a field programmable gate array (FPGA), a microcontroller unit (MCU), or other devices with computing and control functions.

[0086] When implementing the above functions, the control module contains corresponding algorithms, and the corresponding circuit modules (such as the inverter circuit) adopt appropriate types. If other new functions need to be implemented, new circuit modules can be added or the types of circuit modules can be changed appropriately, and corresponding algorithms can be set in the control module.

[0087] Generally speaking, after the PV inverter of the present application enters the low voltage ride-through stage at the first moment, the controller can adjust the active power output by the inverter circuit to decrease, so as to release capacity for outputting reactive power and boost the AC bus voltage, and avoid overloading of the inverter circuit. After the first moment, as the voltage of the AC bus increases and the voltage amplitude at the AC side increases, the reactive current required for the inverter circuit to boost the AC bus voltage will decrease, and then the magnitude of the active power that the remaining capacity of the inverter circuit can output will increase. Therefore, when the controller of the present application adjusts the active power output by the inverter circuit with this target power during the entire low voltage ride-through stage, it will not affect the requirement for the inverter circuit to output reactive current. At the same time, the controller adjusts the active power output by the inverter circuit to the target power, which can make the active power output stable and reduce the voltage oscillation generated by the AC bus. In addition, after entering the low voltage ride-through stage, in order to help boost the voltage of the AC bus, the PV inverter needs to control the reactive current output by the inverter circuit to increase through the controller, and the smaller the voltage amplitude of the AC bus drops at the first moment, the larger the reactive current that the inverter circuit needs to output, that is, the larger the target reactive current of the inverter circuit at the first moment. Therefore, the PV inverter of the present application determines the target reactive current of the inverter circuit based on the voltage amplitude at the AC side at the first moment and the magnitude of the reactive current output by the inverter circuit during the first time period, which can ensure that the reactive current output by the inverter circuit can be used to boost the voltage of the AC bus. At the same time, the principle for the controller of the present application to calculate the target reactive current and the target power is simple and easy to implement.

[0088] Please refer to Figure 6 , Figure 6A flowchart diagram of a control method for a photovoltaic inverter provided by an embodiment of the present application. The control method for the photovoltaic inverter provided by the embodiment of the present application is applicable to Figure 3 and Figure 5 the controllers shown. Specifically, the control method for the photovoltaic inverter may include the steps:

[0089] S101. Obtain the voltage amplitude at the AC side.

[0090] It can be understood that when the photovoltaic inverter is applied in a microgrid system, the AC side of the photovoltaic inverter and the energy storage converter are commonly connected to the AC bus of the microgrid system. The AC bus is connected to the external grid through a grid connection switch. When the grid connection switch is disconnected, the microgrid system operates in an off-grid mode, the energy storage converter is in a grid-forming working mode, and the photovoltaic inverter is in a grid-following working mode. It can be understood that since the AC side of the photovoltaic inverter is electrically connected to the AC bus, the voltage amplitude at this AC side is the same as the voltage amplitude of the AC bus. Therefore, by detecting the voltage amplitude at the AC side through the controller of the photovoltaic inverter, the voltage amplitude of the AC bus can be determined.

[0091] For the specific implementation manner of the above S101, reference can be made to the implementation manner executed by the controller in the above Figures 3 to 5 , which will not be elaborated in this embodiment of the present application.

[0092] S102. When the voltage amplitude at the AC side drops below the amplitude threshold at the first moment, adjust the active power output by the inverter circuit to the target power.

[0093] It can be understood that when the photovoltaic inverter detects through the controller that the voltage amplitude at the AC terminal drops below the amplitude threshold at the first moment, it indicates that the voltage amplitude of the AC bus drops below the amplitude threshold at the first moment. Among them, the amplitude threshold refers to the maximum value of the voltage amplitude of the AC bus when the photovoltaic inverter enters the low-voltage ride-through stage, and this amplitude threshold can be specifically obtained according to the rated voltage amplitude of the AC bus. Therefore, when the voltage amplitude at the AC terminal drops below the amplitude threshold at the first moment, it indicates that the photovoltaic inverter enters the low-voltage ride-through stage at the first moment. At this time, the photovoltaic inverter adjusts the active power output by the inverter circuit to the target power through the controller, aiming to keep the active power output by the inverter circuit stable, thereby reducing the oscillation of the active power output by the inverter circuit. Since the AC terminal is electrically connected to the AC bus, the active power output by the inverter circuit and the voltage of the AC bus affect each other. When the controller controls the reduction of the oscillation of the active power output by the inverter circuit, the voltage oscillation of the AC bus will also decrease. In addition, when the voltage of the AC bus drops, the controller needs to preferentially increase the reactive current output by the inverter circuit to help boost the voltage of the AC bus, and the smaller the voltage amplitude of the AC bus drops, the larger the reactive current that the inverter circuit needs to output. To avoid overloading the inverter circuit, the photovoltaic inverter of the present application can use the remaining capacity of the inverter circuit to output stable active power after ensuring that the output of the reactive current meets the requirements. It can be seen that the target value of the active power output by the inverter circuit in the present application is inversely proportional to the target reactive current of the inverter circuit (that is, the target value of the reactive current that the inverter circuit needs to output), and the magnitude of this target reactive current is inversely proportional to the voltage amplitude at the AC terminal. Therefore, the magnitude of the active power output by the inverter circuit is directly proportional to the voltage amplitude at the AC terminal. It can be seen that the photovoltaic inverter can determine the target value of the active power output by the inverter circuit, that is, the above-mentioned target power, according to the voltage amplitude at the AC terminal and the magnitude of the target reactive current.

[0094] Furthermore, when the photovoltaic inverter enters the low-voltage ride-through stage at the first moment, the photovoltaic inverter obtains the above-mentioned target power according to the voltage amplitude at the AC terminal and the target reactive current at the first moment, and quickly controls the active power output by the inverter circuit to the target power through the controller, which can quickly control the reduction of the voltage oscillation of the AC bus after entering the low-voltage ride-through stage.

[0095] The specific implementation manner of the above S102 can refer to the implementation manner executed by the controller in the above Figures 3 to 5 and will not be elaborated in this embodiment of the present application.

[0096] In an optional implementation manner, the target power is less than the magnitude of the active power output by the inverter circuit in the first time period, and the first time period is a period of time before the voltage amplitude at the AC terminal drops below the amplitude threshold.

[0097] It can be understood that after entering the low-voltage ride-through stage, the photovoltaic inverter reduces the active power output by the controller to adjust the inverter circuit, which can release capacity for reactive power output and avoid overloading of the inverter circuit. Further, by adjusting the active power output of the inverter circuit to the target power, the controller can keep the active power stable and reduce the voltage oscillation generated by the AC bus at the same time.

[0098] In an alternative embodiment, the target power satisfies:

[0099]

[0100] Wherein, I max is the rated current of the photovoltaic inverter, I qrefmax is the target reactive current, U tmin is the voltage amplitude at the AC terminal at the first moment, U max is the rated voltage amplitude of the photovoltaic inverter, S max is the rated apparent power of the photovoltaic inverter.

[0101] It can be understood that the controller can calculate the target power by sampling the voltage amplitude at the AC terminal and combining the target reactive current of the inverter circuit, as well as the rated voltage amplitude, rated current and rated apparent power of the photovoltaic inverter. The calculation process is simple and easy to implement.

[0102] In an alternative embodiment, the magnitude of the target reactive current is inversely proportional to the voltage amplitude at the AC terminal at the first moment and directly proportional to the magnitude of the reactive current output by the inverter circuit in the first time period, and the first time period is a period of time before the voltage amplitude at the AC terminal drops to less than the amplitude threshold.

[0103] It can be understood that when the photovoltaic inverter enters the low-voltage ride-through stage at the first moment, in order to help improve the voltage of the AC bus, the photovoltaic inverter needs to increase the reactive current output by the controller to control the inverter circuit, and the smaller the voltage amplitude of the AC bus drops at the first moment, the greater the reactive current that the inverter circuit needs to output, that is, the greater the target reactive current of the inverter circuit at the first moment. At the same time, if the reactive current output by the inverter circuit in the first time period is greater, then after entering the low-voltage ride-through stage, in order to increase the reactive current output, the reactive current that the inverter circuit needs to output will also be greater. Therefore, in this application, the photovoltaic inverter determines the target reactive current of the inverter circuit based on the voltage amplitude at the AC terminal at the first moment and the magnitude of the reactive current output by the inverter circuit in the first time period, which can ensure that the reactive current output by the inverter circuit can be used to improve the voltage of the AC bus.

[0104] In an alternative embodiment, the target reactive current satisfies:

[0105]

[0106] Among them, U tmin is the voltage amplitude of the AC side at the first moment, U max is the rated voltage amplitude of the PV inverter, I q0 is the reactive current output by the inverter circuit during the first time period, I N is the rated current of the PV inverter, K1 is the reactive current support coefficient of the PV inverter during low-voltage ride-through, and the first time period is a period of time before the voltage amplitude of the AC side drops to less than the amplitude threshold.

[0107] It can be understood that the controller can calculate the target reactive current based on sampling the voltage amplitude of the AC side, the reactive current of the inverter circuit, and combining the rated voltage amplitude, rated current, and reactive current support coefficient of the PV inverter. The calculation process is simple and easy to implement.

[0108] In an optional embodiment, the voltage amplitude of the AC side at the first moment is less than the voltage amplitudes of the AC side at other moments during low-voltage ride-through.

[0109] It can be understood that the microgrid system enters the low-voltage ride-through stage at the first moment. After the first moment, as the voltage amplitude of the AC side increases, the reactive current required for the inverter circuit to boost the AC bus voltage will decrease, and the active power that the remaining capacity of the inverter circuit can output will increase. Thus, it can be seen that the active power that the inverter circuit can output at other moments after the first moment is greater than the active power that can be output at the first moment, that is, greater than the target power. Therefore, when the controller of the present application adjusts the active power output by the inverter circuit with this target power during the entire low-voltage ride-through stage, it will not affect the demand for the reactive current output by the inverter circuit (the demand for reactive current is getting lower and lower). At the same time, the controller adjusts the active power output by the inverter circuit to remain constant at the target power, which can make the active power output stable and reduce the voltage oscillation generated by the AC bus.

[0110] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the above-mentioned claims.

Claims

1. A photovoltaic inverter, characterized in that, The DC side of the PV inverter is used to connect to PV modules, and the AC side of the PV inverter is used to connect to the external grid through a grid connection switch after being paralleled with the energy storage converter. The energy storage converter is in the grid-forming operation mode. The PV inverter includes a controller and an inverter circuit. The controller is used to control the inverter circuit to convert the DC power input from the DC side into AC power and output it to the AC side. When the grid connection switch is off, the controller is used to: When the voltage amplitude at the AC side drops below the amplitude threshold at a first moment, adjust the active power output by the inverter circuit to a target power; wherein, the magnitude of the target power is proportional to the voltage amplitude at the AC side at the first moment and inversely proportional to the magnitude of the target reactive current of the inverter circuit at the first moment.

2. The photovoltaic inverter according to claim 1, wherein The target power is less than the magnitude of the active power output by the inverter circuit in a first time period, and the first time period is a period of time before the voltage amplitude at the AC side drops below the amplitude threshold.

3. The photovoltaic inverter according to claim 1 or 2, characterized in that, The target power satisfies: Wherein, I max is the rated current of the PV inverter, I qrefmax is the target reactive current, U tmin is the voltage amplitude at the AC side at the first moment, U msc is the rated voltage amplitude of the PV inverter, S max is the rated apparent power of the PV inverter.

4. The photovoltaic inverter according to any one of claims 1 to 3, characterized in that The magnitude of the target reactive current is inversely proportional to the voltage amplitude at the AC side at the first moment and proportional to the magnitude of the reactive current output by the inverter circuit in a first time period, and the first time period is a period of time before the voltage amplitude at the AC side drops below the amplitude threshold.

5. The photovoltaic inverter according to any one of claims 1 to 4, characterized in that, The target reactive current satisfies: Wherein, U tmin is the voltage amplitude of the AC terminal at the first moment, and U max is the rated voltage amplitude of the PV inverter. I q0 is the reactive current output by the inverter circuit in the first time period, and I N is the rated current of the PV inverter. K1 is the reactive current support coefficient of the PV inverter during low voltage ride-through. The first time period is a period of time before the voltage amplitude of the AC terminal drops to less than the amplitude threshold.

6. The photovoltaic inverter according to any one of claims 1 to 5, characterized in that, The voltage amplitude at the AC side at the first moment is less than the voltage amplitudes at other moments during the low voltage ride-through period of the AC side.

7. A microgrid system, characterized in that, The microgrid system includes an energy storage converter and a PV inverter. The DC side of the PV inverter is used to connect to PV modules, and the AC side of the PV inverter is used to connect to the external grid through a grid connection switch after being paralleled with the energy storage converter. The energy storage converter is in the grid-forming operation mode. The PV inverter includes a controller and an inverter circuit. The controller is used to control the inverter circuit to convert the DC power input from the DC side into AC power and output it to the AC side. When the grid connection switch is off, the controller is used to: When the voltage amplitude at the AC side drops below the amplitude threshold at a first moment, adjust the active power output by the inverter circuit to a target power; wherein, the magnitude of the target power is proportional to the voltage amplitude at the AC side at the first moment and inversely proportional to the magnitude of the target reactive current of the inverter circuit at the first moment.

8. A control method for a photovoltaic inverter, characterized in that, The DC side of the PV inverter is used to connect to PV modules, and the AC side of the PV inverter is used to connect to the external grid through a grid connection switch after being paralleled with the energy storage converter. The energy storage converter is in the grid-forming operation mode. The PV inverter includes an inverter circuit. The inverter circuit is used to convert the DC power input from the DC side into AC power and output it to the AC side. When the grid connection switch is off, the method includes: When the voltage amplitude at the AC terminal drops below the amplitude threshold at a first moment, adjust the active power output by the inverter circuit to a target power; wherein, the magnitude of the target power is directly proportional to the voltage amplitude at the AC terminal at the first moment and inversely proportional to the magnitude of the target reactive current of the inverter circuit at the first moment.

9. The control method according to claim 8, wherein The target power is less than the magnitude of the active power output by the inverter circuit during a first time period, and the first time period is a period of time before the voltage amplitude at the AC terminal drops below the amplitude threshold.

10. The control method according to claim 8 or 9, characterized in that, The target power satisfies: Wherein, I max is the rated current of the photovoltaic inverter, I qrefmax is the target reactive current, U tmin is the voltage amplitude at the AC terminal at the first moment, U msc is the rated voltage amplitude of the photovoltaic inverter, S max is the rated apparent power of the photovoltaic inverter.

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