Power converter, control method thereof and power supply system

Abstract

The invention provides a power converter, a control method thereof and a power supply system, and belongs to the technical field of power electronics. The controller in the power converter can control the DC / AC conversion circuit to operate in a network construction mode in a grid-connected state and an off-network state. Therefore, it can be ensured that the local load connected with the power converter is not powered off in the grid-connected and off-grid switching process. In other words, seamless grid-connected and off-grid switching of the power converter can be ensured. Moreover, in the grid-connected state, the controller can perform closed-loop control on the output power of the AC end of the DC / AC conversion circuit based on the power reference value, so that the power output by the DC / AC conversion circuit can be ensured to accurately meet the requirements of a power grid.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to a power converter and a control method thereof, and a power supply system. Background Art

[0002] A microgrid is an independent, small-scale power generation and distribution system consisting of energy storage batteries, energy storage converters, and loads. Microgrids are typically connected to the grid via a grid-connected switch and can operate both on-grid and off-grid.

[0003] Typically, when the grid is operating normally, the grid-connected switch is closed, the microgrid is connected to the grid, and the energy storage converter operates in grid-following mode. In this mode, the energy storage converter can track grid parameters such as voltage, frequency, and phase in real time to ensure that the AC power it outputs is consistent with the grid in terms of voltage amplitude, frequency, and phase. In the event of a grid fault, the grid-connected switch is opened, the microgrid switches to off-grid operation, and the energy storage converter switches to grid-building mode. In this mode, the energy storage converter no longer relies on the operating status of the grid. Instead, it can use its own control strategy to build and maintain an independent microgrid and provide power to off-grid loads.

[0004] However, when switching between on-grid and off-grid, it takes a certain amount of time for the energy storage converter to switch its working mode and build the microgrid voltage, during which time the load may be disconnected from the power supply. Summary of the Invention

[0005] The present application provides a power converter and a control method thereof, as well as a power supply system, which can solve the technical problem that when switching between grid and off-grid, the energy storage converter switching working mode may cause load power outage.

[0006] In a first aspect, a power converter is provided. The power converter includes: a controller and a direct current / alternating current (DC / AC) conversion circuit. The DC end of the DC / AC conversion circuit is used to connect to a DC source, the AC end of the DC / AC conversion circuit is used to connect to a power grid, and the DC / AC conversion circuit is used to convert the DC power of the DC source into AC power and output it to the power grid. The controller is used to control the DC / AC conversion circuit to operate in a grid-forming mode when the power converter is in a grid-connected state, and to control the output power of the AC end of the DC / AC conversion circuit to be maintained at a power reference value. That is, the controller is used to perform closed-loop control (also known as differential power tracking) on ​​the output power of the AC end of the DC / AC conversion circuit based on the power reference value in a grid-connected state. The controller is also used to control the DC / AC conversion circuit to operate in a grid-forming mode when the power converter is in an off-grid state.

[0007] In the solution provided in this application, the controller can control the DC / AC conversion circuit to operate in the grid-building mode in both the grid-connected state and the off-grid state, that is, the DC / AC conversion circuit is controlled to operate in the grid-building mode throughout the entire process after startup. As a result, it can be ensured that the local load connected to the power converter will not be powered off during the on-grid and off-grid switching process. In other words, it can be ensured that the power converter can achieve seamless on-grid and off-grid switching. In addition, because in the grid-connected state, the controller can perform closed-loop control of the output power of the AC end of the DC / AC conversion circuit based on the power reference value, it can ensure that the power output of the DC / AC conversion circuit accurately meets the grid requirements.

[0008] Optionally, when the power converter is in an off-grid state, the output power of the AC end of the DC / AC conversion circuit is not controlled by the power reference value. For example, the absolute value of the difference between the output power of the AC end of the DC / AC conversion circuit and the power reference value may be greater than 0. In other words, the controller is configured to no longer perform closed-loop control on the output power of the AC end of the DC / AC conversion circuit in the off-grid state.

[0009] In an off-grid state, if the power converter continues to perform closed-loop power control on its output power based on the power reference value, it may cause its operating point (such as voltage and frequency) to deviate from the rated operating point. Therefore, exiting the power closed-loop control in an off-grid state can ensure that the power converter operates at the rated operating point and provides a stable voltage for the local load.

[0010] Optionally, the AC end of the DC / AC conversion circuit is configured to connect to a power grid via a grid-connected switch. When the grid-connected switch is on, the power converter is in the grid-connected state; when the grid-connected switch is off, the power converter is in the off-grid state. Accordingly, the controller is configured to perform closed-loop control of the output power of the AC end of the DC / AC conversion circuit based on a power reference value when the grid-connected switch is on, so as to maintain the output power at the power reference value. The controller is further configured to cease closed-loop control of the output power of the AC end of the DC / AC conversion circuit when the grid-connected switch is off.

[0011] It is understood that when the grid-connected switch is on (i.e., closed), the power converter is in a grid-connected state. When the grid-connected switch is off (i.e., closed), for example, due to a grid fault, the power converter is in an off-grid state. Therefore, the controller can accurately determine the on-grid and off-grid states of the power converter based on the on-off state of the grid-connected switch and timely adjust its power control strategy based on the on-off state of the grid-connected switch.

[0012] Optionally, the controller is further configured to receive switch status information sent by the system controller. The switch status information is used to indicate the on / off status of the grid-connected switch. Thus, the controller in the power converter can accurately know the on / off status of the grid-connected switch.

[0013] The system controller refers to the system controller of the system in which the power converter is located. For example, if the power converter is used in a microgrid, the system controller may be the microgrid controller. Furthermore, the system controller can transmit switch status information via the controller area network (CAN) bus or Modbus protocol.

[0014] Optionally, the controller is further configured to receive switch status information sent by a relay protection device, wherein the relay protection device can control the grid-connected switch to disconnect when a grid fault is detected.

[0015] Optionally, the controller includes: a sampling circuit, a control circuit, and a drive circuit. The sampling circuit is configured to sample the output power of the AC end of the DC / AC conversion circuit. The control circuit is configured to obtain a power sampling value obtained by the sampling circuit and to control the drive circuit to output a pulse width modulation (PWM) signal to the DC / AC conversion circuit to control the output power of the AC end of the DC / AC conversion circuit to maintain a power reference value. The signal parameters of the PWM signal are determined based on the difference between the power reference value and the power sampling value, and the signal parameters include at least one of pulse width, frequency, and phase.

[0016] Among them, the sampling circuit can sample the output voltage and output current of the AC end of the DC / AC conversion circuit respectively, and calculate the power sampling value of the output power based on the sampled voltage and current values. The control circuit can adjust the modulation signal (such as modulation voltage) output to the drive circuit based on the difference between the power reference value and the power sampling value. For example, the control circuit can use a proportional-integral (PI) algorithm to adjust the modulation signal it outputs. The drive circuit can then adjust the signal parameters of the PWM signal it outputs based on the modulation signal, thereby achieving zero-difference closed-loop control of the output power of the AC end of the DC / AC conversion circuit.

[0017] Optionally, the controller is further configured to receive a power reference value issued by the system controller. That is, the power converter can accept power scheduling from the system controller in a grid-connected state to ensure that the power output by the power converter meets grid requirements.

[0018] In a second aspect, a control method for a power converter is provided, which can be applied to the power converter provided in the first aspect. The power converter includes a DC / AC conversion circuit, wherein the DC end of the DC / AC conversion circuit is used to connect to a DC source, and the AC end of the DC / AC conversion circuit is used to connect to a power grid. The DC / AC conversion circuit is used to convert the DC power of the DC source into AC power and output it to the power grid. The power converter also includes a controller, and the control method can be executed by the controller. The control method includes: when the power converter is in a grid-connected state, controlling the DC / AC conversion circuit to operate in a grid-forming mode, and controlling the output power of the AC end of the DC / AC conversion circuit to maintain at a power reference value; and when the power converter is in an off-grid state, controlling the DC / AC conversion circuit to operate in a grid-forming mode.

[0019] Optionally, when the power converter is in an off-grid state, the output power of the AC end of the DC / AC conversion circuit is not controlled by the power reference value. That is, the control method provided in this application can perform closed-loop control on the output power of the AC end of the DC / AC conversion circuit in a grid-connected state, and not perform closed-loop control on the output power of the AC end of the DC / AC conversion circuit in an off-grid state, i.e., exit power closed-loop control.

[0020] Optionally, the AC end of the DC / AC conversion circuit is used to connect to the power grid via a grid-connected switch. When the grid-connected switch is on, the power converter is in a grid-connected state; when the grid-connected switch is off, the power converter is in an off-grid state. Accordingly, when the power converter is in a grid-connected state, the process of controlling the output power of the AC end of the DC / AC conversion circuit to maintain at a power reference value includes: when the grid-connected switch is on, performing closed-loop control on the output power of the AC end of the DC / AC conversion circuit based on the power reference value to maintain the output power at the power reference value. Furthermore, the control method further includes: when the grid-connected switch is off, ceasing closed-loop control of the output power of the AC end of the DC / AC conversion circuit.

[0021] Optionally, the control method further includes: receiving switch status information sent by the system controller, where the switch status information is used to indicate an on / off state of the grid-connected switch.

[0022] Optionally, the process of controlling the output power of the AC end of the DC / AC conversion circuit to maintain at a power reference value includes: obtaining a power sampling value obtained by sampling the output power of the AC end of the DC / AC conversion circuit, and outputting a PWM signal to the DC / AC conversion circuit to control the output power of the AC end of the DC / AC conversion circuit to maintain at the power reference value. A signal parameter of the PWM signal is determined based on a difference between the power reference value and the power sampling value, and the signal parameter includes at least one of a pulse width, a frequency, and a phase.

[0023] Optionally, the control method further includes: receiving a power reference value sent by a system controller.

[0024] In a third aspect, a chip is provided, which includes a programmable logic circuit and / or program instructions, and when the chip is running, executes the control method provided in the second aspect.

[0025] In a fourth aspect, a power supply system is provided, comprising: an energy storage battery; and the power converter provided in the first aspect above, which is also referred to as an energy storage converter or a power conversion system (PCS). The DC end of the DC / AC conversion circuit in the power converter is connected to the energy storage battery, and the AC end of the DC / AC conversion circuit is used to connect to the power grid. The DC / AC conversion circuit is used to convert the DC power of the energy storage battery into AC power and output it to the power grid. The DC / AC conversion circuit is also used to convert the AC power of the power grid into DC power for charging the energy storage battery.

[0026] Optionally, the power supply system may further include a DC / DC converter circuit, with the DC end of the DC / AC converter circuit connected to the energy storage battery via the DC / DC converter circuit. The DC / DC converter circuit is used to boost the DC power from the DC / AC converter circuit's energy storage battery before transmitting it to the DC / AC converter circuit, and to step down the DC power output by the DC / AC converter circuit before transmitting it to the energy storage battery. The DC / DC converter circuit may be located within the PCS or may be independent of the PCS.

[0027] Optionally, the power supply system can be an energy storage system or a microgrid, such as a power station microgrid or an industrial and commercial microgrid. In the case of a microgrid, the power supply system can also include photovoltaic panels and a photovoltaic inverter. The DC terminal of the photovoltaic inverter is connected to the photovoltaic panel, and the AC terminal is used to connect to the power grid. The photovoltaic inverter is used to convert the DC power from the photovoltaic panel into AC power for output to the power grid.

[0028] In summary, the present application provides a power converter, a control method thereof, and a power supply system. The power converter provided in the present application includes a controller and a DC / AC conversion circuit. The controller can control the DC / AC conversion circuit to operate in a grid-building mode in both the grid-connected state and the off-grid state, that is, control the DC / AC conversion circuit to operate in the grid-building mode throughout the entire process after startup. Thus, it can be ensured that the local load connected to the power converter will not be powered off during the grid-connected and off-grid switching process. In other words, it can be ensured that the power converter can achieve seamless grid-connected and off-grid switching. Moreover, since in the grid-connected state, the controller can perform closed-loop control of the output power of the AC end of the DC / AC conversion circuit based on the power reference value, it can be ensured that the power output of the DC / AC conversion circuit accurately meets the grid requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a schematic diagram of an application scenario of a power converter provided in an embodiment of the present application;

[0030] Figure 2 is a schematic diagram of another application scenario of a power converter provided in an embodiment of the present application;

[0031] Figure 3 1 is a schematic structural diagram of a power converter provided in an embodiment of the present application;

[0032] Figure 4 This is a schematic structural diagram of a control circuit in a power converter provided in an embodiment of the present application;

[0033] Figure 5 Schematic diagram of the voltage and current output by a power converter before and after an unplanned on-grid and off-grid switching according to an embodiment of the present application;

[0034] Figure 6 Schematic diagram of the voltage and current output by another power converter according to an embodiment of the present application before and after unplanned on-grid and off-grid switching;

[0035] Figure 7 Schematic diagram of the voltage and current output by a power converter when it is converted from discharging to charging according to an embodiment of the present application;

[0036] Figure 8 1 is a schematic diagram of the output voltage and current of a power converter when it switches from charging to discharging according to an embodiment of the present application;

[0037] Figure 9 This is a flow chart of a method for controlling a power converter provided in an embodiment of the present application;

[0038] Figure 10 This is a schematic diagram of another application scenario of a power converter provided in an embodiment of the present application. DETAILED DESCRIPTION

[0039] The power converter and its control method and power supply system provided by the embodiments of the present application are described in detail below with reference to the accompanying drawings. First, the key terms involved in the embodiments of the present application are introduced.

[0040] Photovoltaic panel: also known as photovoltaic module, is a device used to convert solar energy into direct current electricity through the photovoltaic effect.

[0041] Energy storage battery: An electrochemical device that can store electrical energy and release it when needed.

[0042] Power converter: Also known as an inverter, this device converts DC power from a DC source (such as a photovoltaic panel or energy storage battery) into AC power. A power converter connected to a photovoltaic panel is also called a photovoltaic inverter, while a power converter connected to an energy storage battery is also called a power storage converter or PCS.

[0043] Microgrid: An independent, small-scale power generation and distribution system typically includes distributed power sources (DGSs), energy storage devices, energy conversion devices, loads, and monitoring and protection devices. The DGSs are typically photovoltaic panels, and the energy storage devices are batteries. Energy conversion devices typically include power converters (such as PV inverters and power supply systems) and transformers. Monitoring and protection devices are typically implemented by a microgrid controller, also known as a microgrid central controller (MGCC) or microgrid controller.

[0044] Grid-building mode: A working mode of a power converter. In grid-building mode, the power converter can simulate the function of a synchronous generator and actively build the operating characteristics of the power grid and maintain the stability of the grid by actively controlling its own output power and parameters such as voltage and frequency.

[0045] Grid-following mode: A working mode of the power converter. In grid-following mode, the power converter can follow the operating characteristics of the power grid and adjust its own operating status based on parameters such as the grid voltage and frequency.

[0046] Virtual synchronous generator (VSG): A technology that simulates the electromechanical transient characteristics of a synchronous generator set, giving a converter-based power source (such as photovoltaic panels and energy storage batteries) the external grid-connected operating characteristics of a synchronous generator set, such as inertia, damping, primary frequency regulation, and reactive voltage regulation.

[0047] Grid-connected state: refers to the state in which the energy storage system, distributed power system or microgrid is connected to the external large power grid, exchanges electric energy and interacts with the large power grid through transmission lines, and operates in a coordinated manner to meet power demand.

[0048] Off-grid state: also known as island operation state, refers to the state in which the energy storage system, distributed power system or microgrid operates independently of the main power grid, does not exchange electricity with the external power grid, and acts as an independent power system to provide power supply to internal loads.

[0049] On-grid and off-grid switching: the action of switching between on-grid and off-grid states.

[0050] Seamless on-grid and off-grid switching: During the on-grid and off-grid switching process, the switching action cannot be perceived in terms of external characteristics (such as output power and AC voltage).

[0051] Figure 1 Schematic diagram of an application scenario of a power converter provided by an embodiment of the present application, the application scenario may be a microgrid. Figure 1 As shown, the application scenario includes: a power converter 10, a DC source 20, a microgrid controller 30, a transformer T and a grid-connected switch S. The DC end of the power converter 10 is connected to the DC source 20, and the AC end is connected to the grid through the transformer T and the grid-connected switch S. The DC source 20 is used to provide DC power, and the power converter 10 is used to convert the DC power into AC power and output it to the transformer T. The transformer T can boost the AC power and transmit it to the grid through the grid-connected switch S. The microgrid controller 30 establishes communication connections with the grid-connected switch S and the power converter 10 respectively. The microgrid controller 30 is used to detect the switching state of the grid-connected switch S and send it to the power converter 10 so that the power converter 10 adjusts the working mode based on the switching state. Figure 1 It can also be seen that the microgrid also includes a local load 40. The local load 40 is connected to the grid-side port of the transformer T. When the grid-connected switch S is closed, that is, when the microgrid is grid-connected, the local load 40 is powered by the power converter 10 and / or the grid. When the grid-connected switch S is open, that is, when the microgrid is off-grid, the local load 40 is powered by the power converter 10.

[0052] In the embodiments of the present application, the DC source 20 may include at least one of a photovoltaic panel and an energy storage battery. If the DC source 20 includes a photovoltaic panel, the power converter 10 includes a photovoltaic inverter connected to the photovoltaic panel. If the DC source 20 includes an energy storage battery, the power converter 10 includes a PCS connected to the energy storage battery. It is understood that the PCS can also convert AC power from the grid into DC power and output it to the energy storage battery, thereby charging the energy storage battery.

[0053] Figure 2 Schematic diagram of another application scenario of a power converter provided by an embodiment of the present application, the application scenario may be a microgrid. Figure 2 As shown, the microgrid may include multiple PCSs, the DC end of each PCS is connected to at least one energy storage battery, and the AC ends of the multiple PCSs are connected in parallel to a transformer T. In addition, the microgrid also includes multiple photovoltaic inverters (INV), the DC end of each photovoltaic inverter is connected to at least one photovoltaic panel, and the AC ends of the multiple photovoltaic inverters are connected in parallel to the transformer T.

[0054] In some embodiments, when the grid is operating normally, the grid-connected switch S is closed. At this point, the microgrid is connected to the grid (also known as the external main grid) and the power converter 10 within the microgrid operates in a grid-following mode and can operate according to the dispatch of the power station controller, performing maximum self-generation and self-consumption, fixed power charging and discharging, and microgrid backflow prevention. If a short circuit or open circuit occurs in the grid, the grid-connected switch S is disconnected. At this point, the microgrid operates independently off-grid, and the power converter 10 within the microgrid operates in a grid-building mode to establish an independent microgrid voltage, thereby continuing to supply power to the local load 40.

[0055] During the above-mentioned on-grid and off-grid switching process, the power converter 10 in the microgrid needs to switch its operating mode from the grid-following mode to the grid-building mode and re-establish the grid voltage. The above-mentioned switching of the operating mode and the speed of establishing the grid voltage will affect the power supply continuity of the local load 40. If the local load 40 cannot continuously receive the voltage, frequency, etc. that continuously meet its operating conditions, the local load 40 will actively cut off the power and shut down to protect its own safety. If the microgrid can switch from on-grid operation to off-grid operation in a coherent manner, that is, switch to off-grid operation in a timely manner when a grid fault occurs and establish the microgrid voltage in the grid-building mode, it can achieve seamless on-grid and off-grid switching, continuously provide stable voltage, frequency, etc. to the local load 40, and ensure that the local load 40 in the microgrid can maintain continuous operation without shutting down. However, due to factors such as transient mode switching and asynchronous switching between parallel machines, the microgrid voltage cannot be quickly established after the grid switch S is disconnected, which in turn causes the local load 40 to lose power.

[0056] In microgrid scenarios that include both PCS and PV inverters, the PCS can switch to grid-building mode and establish the microgrid voltage when off-grid, while the PV inverter typically continues to operate in grid-following mode. Therefore, if the PCS fails to quickly establish the microgrid voltage during the on-grid to off-grid switching process, the PV inverter may shut down.

[0057] It is understandable that the microgrid generally also includes a relay protection device (also known as a relay protection device or a relay protection device), which can control the grid switch S to disconnect after detecting a grid fault. After the microgrid controller 30 detects that the grid switch S is disconnected, it can transmit the switching state of the grid switch S to the power converter 10 via a high-speed communication line. The power converter 10 can then switch from a closed state to an open state in response to the switching state of the grid switch S, and switch its operating mode from a grid-following mode to a grid-building mode. However, the timeliness of the power converter 10 switching the operating mode in this solution will depend on the speed at which the grid switch S is disconnected, and the communication performance of the switching state of the grid switch S transmitted to the power converter 10.

[0058] The relay protection device usually determines the state of the power grid by island detection, and executes the action of disconnecting the grid switch S when a grid fault is detected. Therefore, the timeliness of the disconnection of the grid switch S will be affected by the detection characteristics of the relay protection device. If the detection speed of the relay protection device is slow, the grid switch S will be delayed in disconnecting. Since the grid switch S is a mechanical switch, it takes a certain amount of time to disconnect. Therefore, there is a certain delay when the microgrid controller 30 detects that the grid switch S is disconnected and transmits the switching state of the grid switch S. It can be seen that when the power grid fails, there is a certain lag in the power converter 10 switching its working mode, which makes the microgrid switching on and off the grid seamless. In addition, since the switching state of the grid switch S needs to be transmitted to the power converter 10 through high-speed communication broadcast, if there is a delay or interruption in the communication, it will directly lead to the failure of the working mode switching of the power converter 10, and then the failure of the microgrid voltage establishment will be affected, affecting the reliability of the system operation. For the scenario where the microgrid includes multiple power converters 10 in parallel (i.e., a scenario where multiple machines are connected in parallel), for example Figure 2 In the scenario shown above where multiple PCSs are connected in parallel, the synchronization of the switching state broadcast transmission of the grid-connected switch S will also affect the consistency of the operating mode switching time of multiple power converters 10. If the operating mode switching of multiple power converters 10 is not synchronized, it is easy to cause power circulation between different power converters 10. Based on the above analysis, it can be seen that the seamless on-grid and off-grid switching in the above scheme has high requirements for the detection characteristics of the relay equipment, the switching speed of the grid-connected switch S, and the reliability and synchronization of communication.

[0059] The embodiment of the present application provides a power converter that can achieve seamless on-grid and off-grid switching, that is, it can ensure that the load is not disconnected during the on-grid and off-grid switching process. Figure 1 or Figure 2 The application scenario shown is, for example, applied in a microgrid. Moreover, the power converter can be a PCS or a photovoltaic inverter. Figure 3 As shown, the power converter includes a DC / AC conversion circuit 11 and a controller 12. The DC end of the DC / AC conversion circuit 11 is connected to a DC source 20, and the AC end of the DC / AC conversion circuit 11 is connected to the power grid. The DC / AC conversion circuit 11 is used to convert the DC power from the DC source 20 into AC power and output it to the power grid. It is understood that if the power converter is a PCS, the DC source 20 can be an energy storage battery. If the power converter is a photovoltaic inverter, the DC source 20 can be a photovoltaic panel.

[0060] In the embodiment of the present application, the controller 12 is configured to control the DC / AC conversion circuit 11 to operate in a grid-forming mode when the power converter is in a grid-connected state, and to control the output power of the AC end of the DC / AC conversion circuit 11 to be maintained at a power reference value. That is, in the grid-connected state, the controller 12 can perform closed-loop control of the output power of the AC end of the DC / AC conversion circuit 11 based on the power reference value. Closed-loop control is also known as zero-error tracking.

[0061] The controller 12 is also used to control the DC / AC conversion circuit 11 to operate in the grid-forming mode when the power converter is in the off-grid state. That is, the controller 12 can always control the DC / AC conversion circuit 11 to operate in the grid-forming mode in both the on-grid and off-grid states.

[0062] It is understandable that the above-mentioned power reference value is also called power command value, dispatch power or power dispatch value, and the power reference value can be issued by the system controller (such as microgrid controller 30) of the system where the power converter is located. When the power converter is in the grid-connected state, the controller 12 performs closed-loop control on the output power of the AC end of the DC / AC conversion circuit 11 based on the power reference value, so that the output power of the AC end of the DC / AC conversion circuit 11 is maintained at the power reference value, that is, the zero-difference tracking of the power reference value (dispatching power) is achieved. Based on this, the power converter can operate as a power source in the grid-connected state, and then can accurately control its power output according to the real-time demand of the power grid to ensure that the power output can accurately meet the grid requirements. Accordingly, the power converter provided in the embodiment of the present application can meet the grid-connected regulatory requirements and meet the functional requirements of the system controller (such as microgrid controller) in the grid-connected state. Among them, the functional requirements of the system controller in the grid-connected state include at least the requirement for scheduling the power output of the power converter.

[0063] In summary, an embodiment of the present application provides a power converter. The controller is capable of controlling the DC / AC conversion circuit to operate in a grid-forming mode both in a grid-connected state and in an off-grid state, that is, controlling the DC / AC conversion circuit to operate in a grid-forming mode throughout the entire process after startup. This ensures that the local load connected to the power converter will not be powered off during the on-grid and off-grid switching process. In other words, it ensures that the power converter achieves seamless on-grid and off-grid switching. Furthermore, since the controller can perform closed-loop control of the output power of the AC end of the DC / AC conversion circuit based on a power reference value in the grid-connected state, it ensures that the power output of the DC / AC conversion circuit accurately meets the grid requirements.

[0064] It is understandable that in the grid-forming mode, the main controlled variable of the power converter is the AC voltage. Ideally, the AC voltage output is constant, and its equivalent output impedance is zero.

[0065] Optionally, when the power converter is in an off-grid state, the output power of the AC end of the DC / AC conversion circuit 11 is not controlled by the power reference value. For example, the absolute value of the difference between the output power of the AC end of the DC / AC conversion circuit 11 and the power reference value can be greater than 0. That is, in the off-grid state, the controller 12 does not perform closed-loop control on the output power of the AC end of the DC / AC conversion circuit 11.

[0066] It is understandable that in the off-grid state, the power converter operates independently of the power grid as a voltage source. At this time, if the power converter continues to perform power closed-loop control on its output power based on the power reference value, it may cause its operating point (such as voltage and frequency) to shift relative to the rated working state. Therefore, in the off-grid state, the power converter no longer performs power closed-loop control based on the power reference value, that is, exits power closed-loop control, which can ensure that it operates at the rated working point and provides a stable voltage for the local load, such as providing a stable microgrid voltage.

[0067] Alternatively, as Figure 3 As shown, the AC end of the DC / AC conversion circuit 11 is used to connect to the grid through the grid-connected switch S. For example, Figure 3 In the illustrated scenario, the AC end of the DC / AC conversion circuit 11 is connected to one end of a grid-connected switch S via a line inductor L and a transformer T. The other end of the grid-connected switch S is connected to the power grid. The grid-connected switch S is also called a grid-connected point switch or an off-grid switch. When the grid-connected switch S is on, the power converter is in a grid-connected state; when the grid-connected switch S is off, the power converter is in an off-grid state.

[0068] Accordingly, the controller 12 is configured to: when the grid-connected switch S is turned on, perform closed-loop control on the output power of the AC end of the DC / AC conversion circuit 11 based on the power reference value so as to maintain the output power at the power reference value; and when the grid-connected switch S is turned off, stop performing closed-loop control on the output power of the AC end of the DC / AC conversion circuit 11.

[0069] It can be understood that when the power converter is in a grid-connected state, that is, when the power converter is grid-connected, the grid-connected switch S is turned on. When the power converter is in an off-grid state, that is, when the power converter is off-grid, the grid-connected switch S is turned off. Therefore, in the embodiment of the present application, the controller 12 can accurately determine the grid-connected and off-grid state of the DC / AC conversion circuit 11 based on the on-off state of the grid-connected switch S. That is, it accurately determines whether the DC / AC conversion circuit 11 undergoes grid-connected and off-grid switching. Moreover, when the controller 12 detects that the grid-connected switch S is switched from the on state to the off state, it determines that the DC / AC conversion circuit 11 is switched from the grid-connected state to the off-grid state, and can then stop the closed-loop control of the output power of the AC end of the DC / AC conversion circuit 11.

[0070] It can also be understood that the system (such as a microgrid) in which the power converter is located generally also includes a relay device that can detect the operating status of the power grid and can control the grid-connected switch S to disconnect when a grid fault is detected. Thus, the power converter can be switched from a grid-connected state to an off-grid state. The above-mentioned on-grid and off-grid switching caused by a grid fault is also called unplanned on-grid and off-grid switching. The power converter provided in the embodiment of the present application can achieve seamless on-grid and off-grid switching in the scenario of unplanned on-grid and off-grid switching.

[0071] Optionally, the system where the power converter is located also includes a system controller. For example, referring to Figures 1 to 3 As shown, assuming that the system in which the power converter is located is a microgrid, the system controller is the microgrid controller 30. The controller 12 in the power converter establishes a communication connection with the system controller and is used to receive switch status information issued by the system controller (such as the microgrid controller 30). This switch status information is used to indicate the on / off status of the grid-connected switch S.

[0072] For example, the system controller can send switch status information via the CAN bus or Modbus protocol. Furthermore, upon detecting a switch in the on / off state of the grid-connected switch S (e.g., from on to off), the system controller can send the switch status information to the controller 12 in the power converter to update (or synchronize) the on / off state of the grid-connected switch S to the controller 12. This ensures that the controller 12 accurately knows the on / off state of the grid-connected switch S.

[0073] It is understood that a system controller such as the microgrid controller 30 typically has the function of detecting the on / off state of the grid-connected switch S and transmitting the on / off state to the controller 12 in the power converter. Therefore, in the embodiment of the present application, the controller 12 in the power converter can directly determine the on / off state based on the on / off state of the grid-connected switch S transmitted by the system controller, and when it is determined to be in the off-grid state, it exits the closed-loop control of the output power of the DC / AC conversion circuit 11, that is, exits the zero-error tracking of the dispatched power.

[0074] It can also be understood that, in the off-grid state, if the power converter continues to perform power closed-loop control on its output power based on the power reference value, the power converter will operate at another steady-state operating point outside the rated operating point. At this point, although the operating point of the power converter is offset, it is also possible to stably establish a power grid for the local load (or it can also include a photovoltaic inverter). Based on this, even if the communication performance between the power converter and the system controller (such as a microgrid controller) is poor, causing the power converter to fail to exit the zero-difference tracking of the dispatch power in time based on the on-off state of the grid-connected switch S issued by the system controller, the power converter can still establish a power grid for the local load. It can be seen from this that the solution provided in the embodiment of the present application does not rely on the speed, reliability and synchronization of the communication between the power converter and the system controller, and even in the case of a communication disconnection, it is still possible to achieve seamless off-grid switching.

[0075] Optionally, the controller 12 in the power converter may also receive the switch status information of the grid-connected switch S sent by the relay protection device.

[0076] Optionally, continue to refer to Figure 3 The controller 12 includes a sampling circuit 121, a control circuit 122, and a drive circuit 123. The sampling circuit 121 is used to sample the output power of the AC end of the DC / AC conversion circuit 11. The control circuit 122 is used to obtain the power sampling value (also called the power feedback value) obtained by the sampling circuit 121, and to control the drive circuit 123 to output a drive signal to the DC / AC conversion circuit 11 to control the output power of the AC end of the DC / AC conversion circuit 11 to maintain it at a power reference value. The drive signal is a PWM signal, and the signal parameters of the PWM signal are determined based on the difference between the power reference value and the power sampling value. The signal parameters include at least one of pulse width (i.e., duty cycle), frequency, and phase.

[0077] For example, Figure 4 As shown, the sampling circuit 121 can sample the voltage and current at the AC end of the DC / AC conversion circuit 11 to obtain a voltage sampling value U and a current sampling value I. The sampling circuit 121 can also calculate the voltage sampling value U and the current sampling value I to obtain a power sampling value, and input the power sampling value to the control circuit 122.

[0078] like Figure 3 and Figure 4 As shown, the control circuit 122 may include a power closed-loop control unit 122a, also known as a power scheduling and tracking unit. The power closed-loop control unit 122a can adjust the modulation signal (such as the modulation voltage) output to the driving circuit 123 based on the difference between the power reference value and the power sampling value.

[0079] The drive circuit 123, also known as a modulation circuit or a wave generating module, can output a PWM signal to the switching transistors in the DC / AC conversion circuit 11 under the control of the modulation signal output by the control circuit 122, thereby controlling the on / off state of the switching transistors in the DC / AC conversion circuit 11. This allows the output power of the AC end of the DC / AC conversion circuit 11 to be maintained at the power reference value, thereby achieving control over the output power of the AC end of the DC / AC conversion circuit 11.

[0080] The power closed-loop control unit 122a may be implemented using a closed-loop control algorithm such as a proportional-integral (PI) algorithm, a proportional-integral-derivative (PID) algorithm, or a fuzzy control algorithm.

[0081] Alternatively, as Figure 3 and Figure 4 As shown, the control circuit 122 also includes a virtual synchronous generator (VSG) control loop 122b. The power closed-loop control unit 122a can output an adjusted power reference value to the VSG control loop 122b based on the difference between the power reference value and the power sample value. The VSG control loop 122b can then adjust the modulation signal (e.g., modulation voltage) output to the drive circuit 123 based on the adjusted power reference value and the power sample value.

[0082] refer to Figure 4 It can be seen that the above power reference value includes the reference value of active power P ref and the reference value of reactive power Q ref The power sampling value includes the active power sampling value P and the reactive power sampling value Q. Correspondingly, the difference between the power reference value and the power sampling value includes: the active power reference value P ref The difference from the sampled value P, and the reference value of reactive power Q ref The power closed-loop control unit 122a outputs the adjusted power reference value to the VSG control loop 122b, including the adjusted active power reference value P. ref0 , and the adjusted reactive power reference value Q ref0 .

[0083] Continue to refer Figure 4 The control circuit 122 can adjust the power reference value (ie, P ref and Q ref) is input into the control loop. When the grid-connected switch S is turned on, that is, in the grid-connected state, the control circuit 122 can input the power reference value to the power closed-loop control unit 122a, so that the power closed-loop control unit 122a outputs the adjusted power reference value (i.e., P) to the VSG control loop 122b based on the difference between the power reference value and the power sampling value (i.e., P and Q). ref0 and Q ref0 ). The VSG control loop 122b can then adjust the modulation signal output to the drive circuit 123 based on the adjusted power reference value. When the grid-connected switch S is turned off, that is, in the off-grid state, the control circuit 122 can adjust the power reference value (i.e., P ref and Q ref ) is directly input to the VSG control loop 122b. The VSG control loop 122b can then adjust the modulation signal output to the drive circuit 123 directly based on the power reference value.

[0084] Alternatively, as Figure 3 and Figure 4 As shown, the control circuit 122 further includes a voltage loop 122c and a current loop 122d. The VSG control loop 122b is used to output a voltage reference value U according to the power reference value and the power sampling value. ref0 and frequency reference value f vsg The voltage reference value U ref0 Also known as the amplitude of the grid voltage, frequency reference value f vsg Also called network frequency. Voltage loop 122c is used to ref0 , frequency reference value f vsg And the voltage sampling value U, the output current reference value I ref The current loop 122d is used to calculate the current reference value I ref , frequency reference value f vsg and the current sampling value I, and outputs a modulation signal (such as a modulation voltage) to the driving circuit 123, so that the driving circuit 123 outputs a PWM signal to the DC / AC conversion circuit 11 based on the modulation signal.

[0085] Alternatively, as Figure 4 As shown, the control circuit 122 further includes a virtual impedance loop 122e, which can adjust the voltage reference value U output by the VSG control loop 122b based on the current sampling value I. ref0 Adjust the voltage and output the adjusted voltage reference value U to the voltage loop 122c. ref Understandably, Figure 3 and Figure 4 The control loop of the control circuit 122 shown is only a schematic diagram, and the control loop can be flexibly adjusted according to the needs of the application scenario. For example, Figure 3 and Figure 4The control loop shown may also not include the voltage loop 122c and / or the current loop 122d.

[0086] Optionally, the controller 12 is further configured to receive a power reference value issued by a system controller (such as a microgrid controller 30), namely, P ref and Q ref That is, the system controller can uniformly dispatch the output power of the power converter to ensure that the output power of the power converter can accurately meet the requirements of the power grid.

[0087] Optionally, the controller 12 in the power converter may be a micro-controller unit (MCU), a complex programming logic device (CPLD), or a field programmable gate array (FPGA).

[0088] Figure 5 Schematic diagram of the voltage and current output by a power converter before and after unplanned on-grid and off-grid switching according to an embodiment of the present application. Figure 6 Schematic diagram of the voltage and current output by another power converter before and after unplanned on-grid and off-grid switching according to an embodiment of the present application. Figure 5 This is a schematic diagram of voltage and current in a scenario where the power reference value in the grid-connected state (i.e., the grid-connected dispatching power) is consistent with the power required by the local load. Figure 6 Schematic diagram of voltage and current in a scenario where the power reference value in the grid-connected state (i.e., grid-connected dispatching power) is inconsistent with the power required by the local load. Figure 5 and Figure 6 The horizontal axis is time, and the vertical axis is the amplitude of voltage and current. Figure 5 and Figure 6 At time t0, the power converter switches from a grid-connected state to an off-grid state. Comparing the voltage and current waveforms before and after time t0 shows that the power converter provided by the embodiment of the present application does not experience significant changes in its external characteristics (such as voltage) during unplanned on-grid and off-grid switching, thus meeting the requirements for seamless on-grid and off-grid switching. Seamless on-grid and off-grid switching typically requires that the power converter output voltage recover to within ±25% of the rated voltage within 30ms.

[0089] Figure 7 Schematic diagram of the voltage and current output by a power converter when it is converted from discharging to charging, according to an embodiment of the present application. Figure 8 This is a schematic diagram of the voltage and current output by a power converter proposed in an embodiment of the present application when converting from charging to discharging. Figure 7 and Figure 8Taking the power converter as PCS as an example, the power converter discharge refers to the discharge of the energy storage battery connected to it in the grid-connected state, and charging refers to the charging of the energy storage battery connected to it in the grid-connected state. Among them, when the power converter is discharging, the power reference value (i.e., the dispatching power) issued by the system controller can be expressed as 1.0pu, where pu is the per unit value. When the power converter is charging, the power reference value (i.e., the dispatching power) issued by the system controller can be expressed as -1.0pu. Reference Figure 7 and Figure 8 It can be seen that the solution provided in the embodiment of the present application can not only achieve differential power tracking during charging or discharging (i.e., steady state), but also achieve differential power tracking in the transient state of charging to discharging, or discharging to charging.

[0090] In summary, an embodiment of the present application provides a power converter. Among them, the controller can control the DC / AC conversion circuit to operate in the grid-building mode in both the grid-connected state and the off-grid state, that is, the DC / AC conversion circuit is controlled to operate in the grid-building mode throughout the process after startup. As a result, it can be ensured that the local load connected to the power converter will not be powered off during the on-grid and off-grid switching process. In other words, it can be ensured that the power converter can achieve seamless on-grid and off-grid switching. In addition, since in the grid-connected state, the controller can perform closed-loop control on the output power of the AC end of the DC / AC conversion circuit based on the power reference value, it is possible to ensure that the power output of the DC / AC conversion circuit accurately meets the grid requirements. Since in the off-grid state, the controller can exit the power closed-loop control, it is possible to ensure that the DC / AC conversion circuit operates at the rated operating point and provides a stable voltage for the local load.

[0091] Based on the above analysis, it can be seen that the power converter provided in the embodiment of the present application can operate in the grid-building mode throughout the entire process, and has the function of power zero-difference tracking in the grid-connected scenario. In addition, after the grid fails and is cut off (that is, after the grid-connected switch is disconnected), the power converter can automatically and seamlessly establish the microgrid voltage in the grid-building mode, thereby seamlessly achieving grid-connected and off-grid switching. The above-mentioned solution provided in the embodiment of the present application does not require additional equipment in the system and does not rely on communication performance. It has low cost and high reliability.

[0092] The present application also provides a method for controlling a power converter, which can be applied to the power converter provided in the above embodiment and can be executed by a controller in the power converter. Figure 9 As shown, the method includes:

[0093] Step 101: When the power converter is in a grid-connected state, control the DC / AC conversion circuit to operate in a grid-connected mode, and control the output power of the AC end of the DC / AC conversion circuit to be maintained at a power reference value.

[0094] Step 102: When the power converter is in an off-grid state, control the DC / AC conversion circuit to operate in a grid-connected mode.

[0095] Optionally, when the power converter is in an off-grid state, the output power of the AC end of the DC / AC conversion circuit is not controlled by the power reference value. That is, in the off-grid state, the output power of the AC end of the DC / AC conversion circuit is no longer closed-loop controlled.

[0096] Optionally, the AC end of the DC / AC conversion circuit in the power converter is used to connect to the grid via a grid-connected switch. When the grid-connected switch is on, the power converter is in a grid-connected state; when the grid-connected switch is off, the power converter is in an off-grid state.

[0097] Accordingly, the above step 101 may include: when the grid-connected switch is turned on, performing closed-loop control on the output power of the AC end of the DC / AC conversion circuit based on the power reference value, so as to maintain the output power at the power reference value.

[0098] The above step 102 may further include: when the grid-connected switch is disconnected, stopping the closed-loop control of the output power of the AC end of the DC / AC conversion circuit.

[0099] Optionally, continue to refer to Figure 9 , the control method further includes:

[0100] Step 103: Receive switch status information sent by the system controller, where the switch status information is used to indicate the on / off status of the grid-connected switch.

[0101] Optionally, the process of controlling the output power of the AC end of the DC / AC conversion circuit to maintain at the power reference value in step 102 includes:

[0102] Step 1021: Acquire a power sampling value obtained by sampling the output power of the AC end of the DC / AC conversion circuit.

[0103] Step 1022: Output a PWM signal to the DC / AC converter circuit to control the output power of the AC end of the DC / AC converter circuit to maintain the output power at the power reference value. The PWM signal has a signal parameter determined based on the difference between the power reference value and the power sample value, and the signal parameter includes at least one of pulse width, frequency, and phase.

[0104] Optionally, continue to refer to Figure 9 , the control method further includes:

[0105] Step 104: Receive the power reference value sent by the system controller.

[0106] It is understood that the order of the steps of the power converter control method provided in the embodiments of the present application can be adjusted appropriately, and the steps can be increased or decreased accordingly. For example, step 103 and / or step 104 can be deleted according to the circumstances; or step 102 can be performed before step 101.

[0107] It can also be understood that the control method of the power converter provided in the embodiment of the present application has basically the same implementation method and technical effects as the aforementioned power converter. Therefore, for the purpose of brevity, the implementation method and technical effects of the control method will not be repeated here.

[0108] The present application also provides a power supply system, such as 1 and Figure 2 As shown, the power supply system includes: an energy storage battery, and a power converter 10 as provided in the above embodiment. The power converter 10 is a PCS. The DC end of the DC / AC conversion circuit 12 in the PCS is connected to the energy storage battery, for example, one or more energy storage batteries. The AC end of the DC / AC conversion circuit 12 is used to connect to the power grid, for example, the AC end is connected to the power grid via a transformer T and a grid-connected switch S. The DC / AC conversion circuit 12 is used to convert the DC power of the energy storage battery into AC power and output it to the power grid. The DC / AC conversion circuit 12 is also used to convert the AC power of the power grid into DC power for charging the energy storage battery.

[0109] Optionally, the power supply system may be an energy storage system (ESS), also known as a battery energy storage system (BESS). Alternatively, the power supply system may be a microgrid, such as a power station microgrid or an industrial and commercial microgrid. Figure 2 As shown, the microgrid also includes photovoltaic panels (PV), a photovoltaic inverter (INV), and external loads 50. The PV inverter's DC terminal is connected to the PV panels, and its AC terminal is connected to the grid via a transformer T and a grid-connected switch S. The PV inverter converts the DC power provided by the PV panels into AC power for output to the grid. External loads 50 can be powered by the grid.

[0110] Optionally, for scenarios where the power supply system is a microgrid, the photovoltaic inverter can always operate in grid-following mode. Alternatively, the photovoltaic inverter can also have the functions of the power converter provided in the above embodiments, that is, the photovoltaic inverter can always operate in grid-following mode and can perform closed-loop control of its output power in the grid-connected state. This can effectively improve the photovoltaic-to-storage ratio of the microgrid, which refers to the ratio of photovoltaic and energy storage capacity in the microgrid.

[0111] It is understandable that if the PV inverter in the microgrid operates in grid-following mode and the PCS operates in grid-building mode in an off-grid state, the PV capacity in the microgrid cannot be too high to avoid instability caused by an excessively high ratio of grid-following capacity to grid-building capacity. If the PV inverter in the microgrid can also operate in grid-building mode, the PV capacity in the microgrid can be effectively increased (i.e., the photovoltaic-to-storage ratio can be increased), thereby effectively reducing power supply costs.

[0112] Alternatively, as Figure 2 and Figure 10 As shown, the power supply system may include a plurality of power converters 10 (e.g., a plurality of PCSs), and the AC ends of the plurality of power converters 10 are connected in parallel to the transformer T. In the grid-connected state, the plurality of power converters 10 all operate in a grid-forming mode, and all are capable of performing closed-loop control of the output power based on the power reference value issued by the system controller. In the off-grid state, the plurality of power converters 10 all operate in a grid-forming mode, and no closed-loop control is performed on the output power, and the plurality of power converters 10 can operate in parallel in a non-communication (also known as non-communication line, i.e., wireless) manner.

[0113] contrast Figure 2 and Figure 10 It can be seen that a plurality of power converters 10 (eg, a plurality of PCSs) can be connected to different energy storage batteries, or can be connected to the same group of energy storage batteries, such as the same battery cluster.

[0114] Optionally, the power supply system provided in the embodiment of the present application further includes a system controller, also called an upper controller. The system controller is used to send a power reference value to the power converter 10 and send switch status information of the grid-connected switch S. For example, referring to Figure 1 、 Figure 2 and Figure 10 For a scenario where the power supply system is a microgrid, the system controller may be a microgrid controller 30. For a scenario where the power supply system is an energy storage system or a photovoltaic system, the system controller may be a power station controller.

[0115] It can be understood that the power supply system provided in the embodiment of the present application has basically the same technical effects as the aforementioned power converter, so for the purpose of brevity, the technical effects of the power supply system will not be repeatedly described here.

[0116] In the embodiments of the present application, the terms "first", "second" and "third" are used for descriptive purposes only and should not be understood as indicating or implying relative importance. The term "at least one" means one or more, and "a plurality" means two or more.

[0117] In the embodiments of this application, the term "and / or" is simply a description of the association relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this document generally indicates that the related objects are in an "or" relationship.

[0118] The above are merely optional embodiments of the present application, but the scope of protection of the present application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and such modifications or substitutions should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. A power converter, characterized in that: The power converter includes: a controller and a direct current / alternating current (DC / AC) conversion circuit; The DC end of the DC / AC conversion circuit is used to connect to a DC source, and the AC end of the DC / AC conversion circuit is used to connect to a power grid. The DC / AC conversion circuit is used to convert the DC power of the DC source into AC power and then output it to the power grid. The controller is used to: When the power converter is in a grid-connected state, controlling the DC / AC conversion circuit to operate in a grid-connected mode, and controlling the output power of the AC end of the DC / AC conversion circuit to be maintained at a power reference value; When the power converter is in an off-grid state, the DC / AC conversion circuit is controlled to operate in the grid-connected mode.

2. The power converter according to claim 1, wherein: When the power converter is in the off-grid state, the output power of the AC end of the DC / AC conversion circuit is not controlled by the power reference value.

3. The power converter according to claim 2, wherein: The AC end of the DC / AC conversion circuit is used to connect to the power grid through a grid-connected switch; When the grid-connected switch is turned on, the power converter is in the grid-connected state; When the grid-connected switch is disconnected, the power converter is in the off-grid state.

4. The power converter according to claim 3, wherein: The controller is further configured to receive switch status information sent by the system controller, wherein the switch status information is configured to indicate the on / off status of the grid-connected switch.

5. The power converter according to any one of claims 1 to 4, characterized in that: The controller includes: a sampling circuit, a control circuit and a driving circuit; The sampling circuit is used to sample the output power of the AC end of the DC / AC conversion circuit; The control circuit is used to obtain the power sampling value obtained by the sampling circuit, and to control the drive circuit to output a pulse width modulation (PWM) signal to the DC / AC conversion circuit, so as to control the output power of the AC end of the DC / AC conversion circuit to be maintained at a power reference value; The signal parameter of the PWM signal is determined based on the difference between the power reference value and the power sampling value, and the signal parameter includes at least one of pulse width, frequency and phase.

6. The power converter according to any one of claims 1 to 5, characterized in that: The controller is further configured to receive the power reference value sent by the system controller.

7. A method for controlling a power converter, characterized in that: The power converter includes a DC / AC conversion circuit; a DC terminal of the DC / AC conversion circuit is used to connect to a DC source, and an AC terminal of the DC / AC conversion circuit is used to connect to a power grid; the DC / AC conversion circuit is used to convert the DC power of the DC source into AC power and output it to the power grid; the method includes: When the power converter is in a grid-connected state, controlling the DC / AC conversion circuit to operate in a grid-connected mode, and controlling the output power of the AC end of the DC / AC conversion circuit to be maintained at a power reference value; When the power converter is in an off-grid state, the DC / AC conversion circuit is controlled to operate in the grid-connected mode.

8. The method according to claim 7, characterized in that When the power converter is in the off-grid state, the output power of the AC end of the DC / AC conversion circuit is not controlled by the power reference value.

9. The method according to claim 8, characterized in that The AC end of the DC / AC conversion circuit is used to connect to the power grid through a grid-connected switch; when the grid-connected switch is turned on, the power converter is in the grid-connected state; when the grid-connected switch is turned off, the power converter is in the off-grid state.

10. The method according to claim 9, characterized in that The method further includes: receiving switch status information sent by a system controller, where the switch status information is used to indicate an on / off status of the grid-connected switch.

11. The method according to any one of claims 7 to 10, characterized in that: The controlling the output power of the AC end of the DC / AC conversion circuit to maintain at a power reference value includes: Acquiring a power sampling value obtained by sampling the output power of the AC end of the DC / AC conversion circuit; Outputting a PWM signal to the DC / AC conversion circuit to control the output power of the AC end of the DC / AC conversion circuit to be maintained at a power reference value; The signal parameter of the PWM signal is determined based on the difference between the power reference value and the power sampling value, and the signal parameter includes at least one of pulse width, frequency and phase.

12. A power supply system, characterized in that: The power supply system comprises: an energy storage battery, and a power converter according to any one of claims 1 to 6; The DC end of the DC / AC conversion circuit in the power converter is connected to the energy storage battery, and the AC end of the DC / AC conversion circuit is used to connect to the power grid. The DC / AC conversion circuit is used to convert the DC power of the energy storage battery into AC power and then output it to the power grid.

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