A grid-connected and off-grid switching control method, a converter device and a power supply system

Abstract

The application relates to the field of power systems, in particular to a control switching method and device of a converter device in a power supply system, which can be applied to energy storage converters in new energy micro-grid, electrochemical energy storage power station and the like. In the method, the voltage of the converter device at the control switching moment is saved, and the voltage is taken as an initial value after switching, then the rated voltage is transitioned in a first time period, so that voltage and current distortion and regulation time in the control switching process can be reduced. Meanwhile, through given phase initial values, circulation between converter devices in a power supply system with multiple converter devices under asynchronous switching is avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power systems, and more particularly, to a parallel and off-grid switching control method applied to a power conversion device, and a corresponding power conversion device and power supply system. BACKGROUND

[0002] At present, with the rapid development of new energy, power conversion systems (PCSs) in new energy microgrids and electrochemical energy storage power stations are widely concerned. According to the connection relationship with the alternating current grid, the PCS has two operating modes of grid connection and off-grid, and in different operating modes, the PCS needs to work in different control modes. When the PCS performs control switching, the switching of the control loop and the instruction will cause the output voltage of the alternating current port of the PCS to change suddenly, thereby causing serious fluctuations in system voltage and current, and further possibly causing serious voltage distortion, too long dynamic regulation time, or even system instability.

[0003] Therefore, how to perform control switching of the PCS, reduce control switching time, and reduce voltage and current fluctuations during switching has become a problem to be solved. SUMMARY

[0004] The present application provides a parallel and off-grid switching control method applied to a power conversion device, which can reduce control switching time and voltage and current fluctuations during switching.

[0005] In a first aspect, a parallel and off-grid switching control method is provided, which is applied to switching between two control modes of a power conversion device, the two control modes being a grid-connected control mode and an off-grid control mode. The grid-connected control mode is to control the power conversion device to convert direct current output by an energy storage system into alternating current and connect the alternating current to an alternating current grid. The off-grid control mode is to control the power conversion device to convert direct current output by the energy storage system into alternating current and supply power to a local load, and the alternating current output by the power conversion device is not connected to the alternating current grid. The method comprises: obtaining grid scheduling information, the grid scheduling information being used to indicate that the power conversion device is switched from the grid-connected control mode to the off-grid control mode; obtaining an output voltage of the power conversion device at a first time, the first time being a time of switching processing of the power conversion device from the grid-connected control mode to the off-grid control mode, or the first time being a receiving time of the grid scheduling information; determining an output voltage of the power conversion device in the off-grid control mode based on the output voltage of the power conversion device at the first time; and a difference between the output voltage of the power conversion device at the first time and the output voltage of the power conversion device in the off-grid control mode within a preset time being less than a set threshold.

[0006] In a possible implementation, the converter device is located in a power supply system formed by a new energy micro-grid or an electrochemical energy storage power station and an alternating current power grid, the converter device is an energy storage converter, and the energy storage system can include a multi-battery system or a coupling of the multi-battery system and a photovoltaic power generation unit.

[0007] It should be understood that a control voltage is generated in the converter device in the embodiment of the application, the control voltage controls the corresponding voltage output by the converter device after being modulated and demodulated, and the output voltage of the converter device at the first moment in the above scheme can include the control voltage for controlling the output voltage of the converter device at the first moment.

[0008] Further, when the converter device connects the energy storage system to the alternating current power grid, the converter device is in a grid-connected operation mode, to improve the active and reactive power instruction following performance of the energy storage system and better participate in the adjustment of the alternating current power grid, the converter device needs to work in a current control mode based on the control voltage output by the controller, that is, the grid-connected control mode in the above scheme is the current control mode. Specifically, in the current control mode, the controller includes a PQ control loop and a current loop, the PQ control loop implements closed-loop following control by executing the active and reactive power instructions output by the converter device, the output current of the PQ control loop is the reference current input to the current loop, the current loop executes closed-loop control of the output current of the converter device, and the control voltage output by the controller is the output voltage of the current loop superimposed on the output voltage of the alternating current port of the converter device.

[0009] When the system is disconnected from the large power grid and only supplies power to the local load, the converter device is in an off-grid operation mode, to improve the voltage and frequency stability and transient support capability of the island system and improve the reliability of the local load power supply, the converter device needs to work in a voltage control mode based on the control voltage output by the controller, that is, the off-grid control mode is the voltage control mode. Specifically, in the voltage control mode, the controller includes a virtual synchronous machine control loop, a voltage reference generator, a voltage loop, and a current loop, the virtual synchronous machine control loop executes virtual synchronous machine control, the voltage reference generator calculates the reference voltage input to the voltage loop according to the voltage amplitude, frequency, and latch value of the latch output by the virtual synchronous machine control loop, and the voltage and current loops respectively execute closed-loop control of the output voltage and current of the converter device, and the output current of the current loop is the reference current input to the voltage loop.

[0010] Based on the above scheme, the difference between the output voltage of the converter device at the first moment and the output voltage of the converter device in the off-grid control mode within a preset time is less than a set threshold, which can reduce the control switching time and the fluctuation of voltage and current in the switching process

[0011] In some implementations of the first aspect, the determining the output voltage of the power conversion device in the off-grid control mode based on the output voltage of the power conversion device at the first time point comprises: setting an output reference voltage of the power conversion device according to a first control voltage, the first control voltage being a control voltage of the power conversion device at the first time point, the control voltage being used to control the output voltage of the power conversion device; obtaining a feedback parameter, the feedback parameter comprising a voltage and a current of alternating current output by the power conversion device; determining the second control voltage based on the output reference voltage of the power conversion device and the feedback parameter; and determining the output voltage of the power conversion device in the off-grid control mode based on the second control voltage.

[0012] It should be noted that the first control voltage as the output reference voltage of the power conversion device means that the amplitude of the first control voltage is used as the initial amplitude of the output reference voltage of the power conversion device, and the phase of the first control voltage is used as the initial phase of the output reference voltage of the power conversion device.

[0013] It should be understood that the output reference voltage of the power conversion device refers to the output voltage of a reference voltage generator in a voltage controller and is the reference voltage of an input voltage loop.

[0014] Based on the above scheme, the first control voltage is saved at the moment when the control switching starts, and is used as the initial value of the amplitude and the phase of the output reference voltage of the power conversion device in the off-grid control mode, so that the control voltage for converting direct current into alternating current by the power conversion device at the moment of control switching does not suddenly change, and the output voltage of the alternating current port of the power conversion device does not suddenly change, thereby reducing the voltage and current distortion and the regulation time in the control switching process. By giving the initial phase value, the circulating current between the power conversion devices in the non-synchronous switching in the power supply system with multiple power conversion devices is avoided.

[0015] In addition, the output reference voltage of the power conversion device is transitioned from the first control voltage to the rated voltage in the steady state operation in the off-grid control mode, so that the control quantity does not suddenly change in the switching transient state, and the voltage and current distortion and the regulation time in the switching process are reduced.

[0016] In some implementations of the first aspect, the determining the output voltage of the power conversion device in the off-grid control mode based on the output reference voltage of the power conversion device and the feedback parameter comprises: generating an intermediate voltage after the output reference voltage of the power conversion device and the feedback parameter pass through a current loop and a voltage loop; and generating the second control voltage by adding the output reference voltage of the power conversion device and the intermediate voltage.

[0017] Specifically, the output reference voltage of the converter and the voltage of the AC power output by the converter are taken as inputs of a voltage loop, the output current of the voltage loop and the current of the AC power output by the converter are taken as inputs of a current loop, the output voltage of the current loop is the intermediate voltage in the above solution, and the output reference voltage of the converter is superimposed on the intermediate voltage as a feedforward quantity to determine the control voltage.

[0018] In the above solution, the output reference voltage of the converter is superimposed on the output voltage of the current loop as a feedforward quantity, so that the hysteresis effect of the PI controller used after the VCM control loop is enabled is avoided, the control voltage cannot be smoothly transitioned, and it is ensured that the voltage of the AC power output by the converter does not suddenly change during the control switching process.

[0019] With reference to the first aspect, in some implementations of the first aspect, the method further includes gradually setting the output reference voltage of the converter to a rated voltage at a set step, the rated voltage being an output voltage of the converter when the converter is stably operated in the off-grid control state.

[0020] By setting the step, the voltage change amplitude during the switching process can be controlled, and voltage mutation is avoided.

[0021] With reference to the first aspect, in some implementations of the first aspect, the second control voltage satisfies:

[0022]

[0023] V out，m = V out，Inner + V out，ref

[0024] wherein I out represents the current of the AC power output by the converter, V out represents the voltage of the AC power output by the converter, V out，ref represents the output reference voltage of the converter, V out，Inner represents the intermediate voltage, G out (s) represents a transfer function of the voltage loop in the off-grid control mode, G Inner (s) represents a transfer function of the current loop in the off-grid control mode, V out，m represents the second control voltage.

[0025] In the above solution, the output reference voltage of the converter is superimposed on the output voltage of the current loop as a feedforward quantity, so that the hysteresis effect of the PI controller used after the VCM control loop is enabled is avoided, the control voltage cannot be smoothly transitioned, and it is ensured that the voltage of the AC power output by the converter does not suddenly change during the control switching process.

[0026] In combination with the first aspect, in some implementations of the first aspect, the output reference voltage of the current conversion device satisfies:

[0027]

[0028] where Δt represents a time length from the first time point to a current time point, t0 represents a time length of the first time period, E out，CCM represents a voltage amplitude of the first control voltage, θ out，CCM represents a phase of the first control voltage, E out，VSG represents a voltage amplitude of the rated voltage, ω out，VSG represents an angular frequency of the rated voltage, and f(Δt) is a voltage amplitude switching function.

[0029] In a possible implementation, the voltage amplitude of the rated voltage and the angular frequency of the rated voltage are obtained through simulation of a virtual synchronous machine control loop.

[0030] Based on the above scheme, the amplitude of the output reference voltage of the current conversion device is smoothly transitioned from a given initial value to a rated voltage amplitude in a selected transition curve form in stable operation; the phase of the output reference voltage of the current conversion device is obtained by integrating the angular frequency of the rated voltage as a proportional coefficient with respect to time to obtain the phase of the rated voltage. Further, the above scheme ensures smooth transition of the current conversion device from the grid-connected control mode to the off-grid control mode after control switching, avoids large fluctuations of control quantities in the switching transient state, and reduces voltage, current distortion and regulation time in the switching process.

[0031] In combination with the first aspect, in some implementations of the first aspect, the voltage amplitude switching function is:

[0032] f(Δt)=e -τ·Δt

[0033] where τ is a constant for adjusting the time length of the first time period.

[0034] In combination with the first aspect, in some implementations of the first aspect, the voltage amplitude switching function is:

[0035] f(Δt)=k·Δt

[0036] where k is a constant for adjusting the time length of the first time period.

[0037] In combination with the first aspect, in some implementations of the first aspect, the method further comprises: obtaining grid fault information, the grid fault information being used to indicate that the AC power grid loses power and to indicate that the current conversion device is switched from the grid-connected control mode to the off-grid control mode.

[0038] In a possible implementation, the power grid fault information is generated by a detection module in the power conversion device after detecting that the AC power grid is out of power.

[0039] In a possible implementation, the power grid fault information is sent by a central controller in communication with the power conversion device. Further, the central controller is in communication with a grid connection switch, one end of which is connected to the power conversion device and the other end of which is connected to the AC power grid, and the state of the grid connection switch is used to determine whether the AC power grid connected to the power conversion device is out of power; or the central controller receives a signal from another power conversion device connected to the AC power grid, and the signal is used to determine whether the AC power grid connected to the power conversion device is out of power.

[0040] Based on the above scheme, in a power supply system with multiple power conversion devices, island state determination and control switching triggering are performed by the detection module and the central processor, so that the single power conversion device can immediately enter the switching state when detecting the island state, and at the same time, the failure of the overall grid-connected / off-grid switching of the cluster due to the fact that some power conversion devices are converted to off-grid VCM operation first and the remaining power conversion devices cannot accurately determine the island state while the system reaches power balance is effectively avoided.

[0041] In a second aspect, a power conversion device is provided. The power conversion device has at least two control modes, i.e., a grid-connected control mode and an off-grid control mode. In the grid-connected control mode, the power conversion device is controlled to convert direct current output by an energy storage system into alternating current and connect the alternating current to an AC power grid. In the off-grid control mode, the power conversion device is controlled to convert direct current output by the energy storage system into alternating current and supply the alternating current to a local load, and the alternating current output by the power conversion device is not connected to the AC power grid. The power conversion device includes a controller and a power conversion circuit. A first end of the power conversion circuit is connected to an output end of the controller, a second end of the power conversion circuit is connected to the energy storage system, and a third end of the power conversion circuit is connected to the AC power grid. The controller is configured to acquire power grid scheduling information, which is used to instruct the power conversion device to switch from the grid-connected control mode to the off-grid control mode. The controller is further configured to acquire an output voltage of the power conversion device at a first time, which is a time of switching processing of the power conversion device from the grid-connected control mode to the off-grid control mode, or a time of receiving the power grid scheduling information. The controller is further configured to determine an output voltage of the power conversion device in the off-grid control mode based on the output voltage of the power conversion device at the first time, and the difference between the output voltage of the power conversion device at the first time and the output voltage of the power conversion device in the off-grid control mode within a preset time is less than a set threshold. The power conversion circuit is configured to convert the direct current output by the energy storage system into alternating current and supply the alternating current to the local load based on the output voltage of the power conversion device in the off-grid control mode.

[0042] With reference to the second aspect, in some implementations of the second aspect, the determining the output voltage of the power conversion device in the off-grid control mode based on the output voltage of the power conversion device at the first time point comprises: setting, by the controller, an output reference voltage of the power conversion device according to a first control voltage, the first control voltage being a control voltage of the power conversion device at the first time point, the control voltage being used to control the output voltage of the power conversion device; obtaining, by the controller, feedback parameters, the feedback parameters comprising a voltage and a current of the alternating current output by the power conversion device; determining, by the controller, the second control voltage based on the output reference voltage of the power conversion device and the feedback parameters; and determining, by the controller, the output voltage of the power conversion device in the off-grid control mode based on the second control voltage.

[0043] With reference to the second aspect, in some implementations of the second aspect, the generating the second control voltage based on the output reference voltage of the power conversion device and the feedback parameters comprises: inputting, by the controller, the output reference voltage of the power conversion device and the feedback parameters into a current loop and a voltage loop to generate an intermediate voltage; and adding, by the controller, the output reference voltage of the power conversion device and the intermediate voltage to generate the second control voltage.

[0044] With reference to the second aspect, in some implementations of the second aspect, the controller is further configured to gradually set the output reference voltage of the power conversion device to a rated voltage at a set step length, the rated voltage being an output voltage of the power conversion device when the power conversion device is stably operated in the off-grid control state.

[0045] With reference to the second aspect, in some implementations of the second aspect, the second control voltage satisfies:

[0046] V out，Inner = G Inner (s) × [G out (s) · (V out，ref - V out ) - I out ]

[0047] V out，m = V out，Inner + V out，ref

[0048] wherein I out represents the current of the alternating current output by the power conversion device, V out represents the voltage of the alternating current output by the power conversion device, V out，ref represents the output reference voltage of the power conversion device, V out，Inner represents the intermediate voltage, G out (s) represents a transfer function of a voltage loop in the off-grid control mode, and G Inner(s) represents a transfer function of a current loop in the off-grid control mode, V out，m represents the second control voltage.

[0049] In combination with the second aspect, in some implementations of the second aspect, the output reference voltage of the current conversion device satisfies:

[0050]

[0051] where Δt represents a time length from the first time to the current time, t0 represents a time length of the first time period, E out，CCM represents a voltage amplitude of the first control voltage, θ out，CCM represents a phase of the first control voltage, E out，VSG represents a voltage amplitude of the rated voltage, ω out，VSG represents an angular frequency of the rated voltage, and f(Δt) is a voltage amplitude switching function.

[0052] In combination with the second aspect, in some implementations of the second aspect, the voltage amplitude switching function is:

[0053] f(Δt) = e -τ·Δt

[0054] where τ is a constant, used to adjust the time length of the first time period.

[0055] In combination with the second aspect, in some implementations of the second aspect, the voltage amplitude switching function is:

[0056] f(Δt) = k·Δt

[0057] where k is a constant, used to adjust the time length of the first time period.

[0058] In combination with the second aspect, in some implementations of the second aspect, the current conversion device further comprises a detection module, the detection module being configured to detect whether the AC power grid loses power, and send grid failure information to the controller if the detection module detects that the AC power grid loses power, the grid failure information being used to indicate that the AC power grid loses power and instruct the current conversion device to switch from the grid-connected control mode to the off-grid control mode.

[0059] In combination with the second aspect, in some implementations of the second aspect, if the detection module detects that the AC power grid loses power, the grid failure information is sent to a central processor; and the central processor instructs other current conversion devices connected to the AC power grid to switch control modes based on the grid failure information.

[0060] Exemplarily, the central controller can establish communication with the current conversion device.

[0061] In a third aspect, a power supply system is provided, comprising: a plurality of converter devices, an energy storage system, an AC grid, a grid-tie switch, a central processor, and a point of common coupling; the central processor is configured to instruct the grid-tie switch to open or close; AC terminals of the plurality of converter devices are connected in parallel at the point of common coupling, and DC terminals of the plurality of converter devices are connected to the energy storage system; the plurality of converter devices have at least two control modes, which are a grid-connected control mode and an off-grid control mode; in the grid-connected control mode, the converter devices are controlled to convert DC power output by the energy storage system into AC power and to feed the AC power into the AC grid; in the off-grid control mode, the converter devices are controlled to convert DC power output by the energy storage system into AC power and to supply the AC power to local loads, and the AC power output by the converter devices is not fed into the AC grid; the point of common coupling is connected to the AC grid through the grid-tie switch; and the central processor is communicatively connected to the plurality of converter devices and the grid-tie switch.

[0062] With reference to the third aspect, in some implementations of the third aspect, the energy storage system comprises: a plurality of battery systems and / or photovoltaic power generation units.

[0063] With reference to the third aspect, in some implementations of the third aspect, if the central processor instructs the grid-tie switch to open, the central processor is further configured to instruct the plurality of converter devices to switch control modes.

[0064] With reference to the third aspect, in some implementations of the third aspect, the converter devices are further configured to detect whether the AC grid loses power.

[0065] With reference to the third aspect, in some implementations of the third aspect, if the AC grid loses power, the converter devices switch control modes.

[0066] With reference to the third aspect, in some implementations of the third aspect, if the AC grid loses power, the converter devices are further configured to instruct the central controller to open the grid-tie switch.

[0067] In a fourth aspect, a processor is provided, comprising: an input circuit, an output circuit, and a processing circuit. The processing circuit is configured to receive a signal through the input circuit and to transmit a signal through the output circuit, so that the processor performs the method in any possible implementation manner of the first aspect.

[0068] In the implementation process, the processor can be one or more chips, the input circuit can be an input pin, the output circuit can be an output pin, and the processing circuit can be a transistor, a gate circuit, a flip-flop, various logic circuits, etc. The input signal received by the input circuit can be received and input by, for example but not limited to, a transceiver, the output signal output by the output circuit can be output to and transmitted by, for example but not limited to, a transmitter, and the input circuit and the output circuit can be the same circuit which is used as the input circuit and the output circuit at different times. The embodiments of the present application do not limit the specific implementation of the processor and various circuits.

[0069] For the sending and obtaining / receiving operations related to the processor, if there is no special description, or if it does not contradict the actual role or internal logic in the related description, it can be understood as the processor output and receive, input, etc. operation, and also can be understood as the sending and receiving operation performed by the radio frequency circuit and the antenna, and the present application does not limit this.

[0070] In a fifth aspect, a processing device is provided, including a processor and a memory. The processor is configured to read instructions stored in the memory, and can receive signals through a transceiver and transmit signals through a transmitter to perform the method in any possible implementation manner of the first aspect.

[0071] Optionally, the processor is one or more, and the memory is one or more.

[0072] Optionally, the memory can be integrated with the processor, or the memory and the processor can be separately arranged.

[0073] In the implementation process, the memory can be a non-transitory memory, such as a read only memory (ROM), which can be integrated on the same chip with the processor, or can be arranged on different chips respectively, and the embodiments of the present application do not limit the type of memory and the arrangement manner of the memory and the processor.

[0074] It should be understood that the related data interaction process, such as sending indication information, can be the process of outputting indication information from the processor, and receiving capability information can be the process of receiving input capability information by the processor. Specifically, the data output by the processor can be output to the transmitter, and the input data received by the processor can come from the transceiver. Among them, the transmitter and the transceiver can be collectively referred to as a transceiver.

[0075] The processing device in the fifth aspect can be one or more chips. The processor in the processing device can be implemented by hardware or software. When implemented by hardware, the processor can be a logic circuit, an integrated circuit, or the like; when implemented by software, the processor can be a general-purpose processor, which implements the above method by reading software codes stored in a memory. The memory can be integrated in the processor or exist independently outside the processor.

[0076] In a sixth aspect, a chip is provided, which obtains instructions and executes the instructions to implement the method in the first aspect and any implementation manner of the first aspect.

[0077] Optionally, as an implementation manner, the chip includes a processor and a data interface, the processor reads instructions stored in a memory through the data interface, and executes the method in the first aspect and any implementation manner of the first aspect.

[0078] Optionally, as an implementation manner, the chip can further include a memory, the memory stores instructions, and the processor is configured to execute the instructions stored in the memory, and when the instructions are executed, the processor is configured to execute the method in the first aspect and any implementation manner of the first aspect.

[0079] In a seventh aspect, a computer program product is provided, which includes computer program codes, when the computer program codes run on a computer, the computer program codes make the computer execute the method in the first aspect and any implementation manner of the first aspect.

[0080] In an eighth aspect, a computer readable storage medium is provided, which includes instructions; the instructions are used to implement the method in the first aspect and any implementation manner of the first aspect.

[0081] As an example, the computer readable storage includes but is not limited to one or more of the following: read-only memory (ROM), programmable ROM (PROM), erasable PROM (EPROM), Flash memory, electrically EPROM (EEPROM), and hard drive.

[0082] Optionally, as an implementation manner, the storage medium can be a non-volatile storage medium.

[0083] In a ninth aspect, there is provided a computing device comprising a processor and a memory, the processor of the one computing device being configured to execute instructions stored in the memory to cause the computing device to perform any of the methods of the first aspect.

[0084] In a tenth aspect, there is provided a cluster of computing nodes comprising at least one computing node, each computing node comprising a processor and a memory, the processor of the at least one computing node being configured to execute instructions stored in the memory of the at least one computing node to cause the cluster of computing nodes to perform any of the methods of the first aspect. BRIEF DESCRIPTION OF DRAWINGS

[0085] Figure 1 is a schematic diagram of an application scenario of an embodiment of the present application.

[0086] Figure 2 is a flowchart of a control switching method 200 of an embodiment of the present application.

[0087] Figure 3 is a schematic block diagram of a controller algorithm 300 of an embodiment of the present application.

[0088] Figure 4 is a flowchart of a controller algorithm 400 of an embodiment of the present application.

[0089] Figure 5 is a schematic diagram of a power conversion device 500 of an embodiment of the present application.

[0090] Figure 6 is a schematic diagram of a power supply system 600 of an embodiment of the present application.

[0091] Figure 7 is a flowchart of a control algorithm 700 of a central controller of an embodiment of the present application.

[0092] Figure 8 is a schematic diagram of a controller 10 of an embodiment of the present application.

[0093] Figure 9 is a schematic diagram of a central controller 20 of an embodiment of the present application. DETAILED DESCRIPTION

[0094] The technical solutions in the present application will be described below with reference to the drawings.

[0095] For ease of understanding, a number of terms involved in the embodiments of the present application will first be described.

[0096] Power Conversion System (PCS): A power conversion system in an electrochemical energy storage system, which is connected between the battery system and the grid and / or load, and can directly supply AC load in the absence of the grid. The PCS includes a converter and a control unit. The PCS can control the charging and discharging process of the battery system. Specifically, the control unit of the PCS receives the background control instructions through communication, controls the charging or discharging of the battery system according to the sign and size of the power instruction, so as to realize the adjustment of the active power and the reactive power of the grid. The control unit of the PCS communicates with the battery management system (BMS) through the CAN interface to obtain the battery pack state information, and can realize the protective charging and discharging of the battery to ensure the safe operation of the battery.

[0097] Microgrid: A small power network that can achieve internal power balance, including distributed power generation equipment, power load, monitoring / protection and automation devices, and energy storage equipment when necessary. It can operate in parallel with the external grid or operate independently.

[0098] Electrochemical energy storage power station: An electric power station that uses electrochemical cells as energy storage components and can store, convert and release electric energy, including a plurality of electrochemical energy storage systems of different or same types.

[0099] Grid-connected operation mode: The PCS is synchronized with the AC grid and operates as a current source to store grid power in the battery pack or feed back the battery pack energy to the grid.

[0100] Off-grid operation mode: The PCS operates as a voltage source to provide power for the AC side of the energy storage system or the connected grid.

[0101] Current control mode (CCM): Simultaneously monitor voltage and current, adjust PWM duty cycle, ensure stable output voltage and output current within normal range, and prevent overcurrent. The principle of current control mode is to send the voltage on the sampling resistor to the input of the current comparator and compare it with the output of the voltage error amplifier, and then control the size of PWM to realize stable voltage and appropriate current range.

[0102] Voltage control mode (VCM): monitor the size of the output voltage, adjust the PWM duty cycle, ensure the stability of the output voltage. The principle of voltage control mode is to take the voltage at the load end as the input of the error amplifier, and the other input of the error amplifier is the reference voltage. After comparison, it is sent to the input of the PWM controller and processed with the fixed sawtooth wave signal to generate the PWM signal to control the conduction and turn-off of the switch tube, thereby realizing the regulation of voltage.

[0103] Virtual synchronous generator (VSG): VSG is to embed the mathematical model of synchronous generator into the control algorithm of inverter, simulate the operation technology of static power electronic device as rotating machine, and has the functions of damping voltage and frequency rapid fluctuation, automatic power distribution, synchronous grid operation by simulating the primary frequency regulation and voltage regulation of synchronous generator. By embedding the synchronous generator equation in the control system of the converter, the VSG can realize the power exchange between the DC power supply and the system according to the characteristics of the synchronous generator. From the system point of view, if the high frequency component generated by the switching action of power electronic device is ignored, the VSG is equivalent to the synchronous generator.

[0104] It should be understood that the voltage and current in this application are vector descriptions, including the amplitude, phase and angular frequency of the voltage and current. The voltage value refers to the voltage amplitude, which is a scalar, and is not the same as the voltage. The current value is the same.

[0105] Figure 1 is the application scenario of an embodiment of the application. As shown in Figure 1 , the application scenario includes an energy storage system 110, a converter device 120, an AC power grid 130, a local load 140, a central controller 150 and a grid-connected switch 160. Among them, the DC end of the converter device is connected with the energy storage system, the AC end of the converter device is connected with the AC power grid through the grid-connected switch, and the AC end of the converter device is also connected with the local load.

[0106] In some examples, the above-mentioned energy storage system (ESS) can include a multi-battery system, and can also include a coupling of a multi-battery system and a photovoltaic power generation unit.

[0107] In the embodiment of the application, the central controller can communicate with the converter device, and can also communicate with the grid-connected switch. Exemplarily, if the above-mentioned application scenario is a micro-grid scenario, the central controller is a micro-grid central controller; if the above-mentioned application scenario is an energy storage power station scenario, the central controller is an energy storage power station controller.

[0108] It should be understood that the converter device in the present application can be applied to a new energy micro-grid or an electrochemical energy storage power station, and can also be applied to other fields.

[0109] In the embodiments of the present application, when the grid connection switch is closed, the ESS is connected to the AC power grid for operation, and the converter device is in a grid-connected operation mode. In order to improve the active and reactive power instruction following performance of the system and better participate in the grid regulation, the converter device needs to work in a current control mode. When the grid connection switch is opened, the ESS is disconnected from the AC power grid, and only supplies power to the local load, and the converter device is in an off-grid operation mode. In order to improve the voltage and frequency stability and transient support capability, and improve the reliability of local load power supply, the converter device needs to work in a voltage control mode.

[0110] The application scenarios of the embodiments of the present application can include multiple energy storage systems, and correspondingly, multiple converter devices. The AC sides of the multiple converter devices are connected in parallel at a point of common coupling (PCC) to form a converter device cluster. Further, the AC sides of the multiple converter devices are connected in parallel at the PCC through filters to form a converter device cluster. Each converter device is connected to an ESS, the PCC is connected to a local load, the PCC is also connected to an AC power grid through a grid connection switch, and a central controller is also connected to the grid connection switch and each converter device in communication.

[0111] Figure 2 FIG. 2 is a flowchart of a control switching method 200 according to an embodiment of the present application. The method 200 can be applied to a converter device which can be placed in a power supply system. The converter device has at least two control modes. In a grid-connected control mode, the converter device can convert DC power output by an energy storage system into AC power based on a control voltage and connect the AC power to an AC power grid. In an off-grid control mode, the converter device can convert DC power output by an energy storage system into AC power based on a control voltage and supply the AC power to a local load. In the off-grid control mode, the AC power output by the converter device is not connected to the AC power grid.

[0112] In a possible implementation, further, when the converter device connects the energy storage system to the AC power grid, the converter device is in a grid-connected operation mode. In order to improve the active and reactive power instruction following performance of the energy storage system and better participate in the AC power grid regulation, the converter device needs to work in a current control mode, that is, the grid-connected control mode in the above scheme is a current control mode. Specifically, in the current control mode, the controller includes a PQ control loop and a current loop. The PQ control loop implements closed-loop following control by executing active and reactive power instructions output by the converter device. The output current of the PQ control loop is a reference current input to the current loop. The current loop executes closed-loop control of the output current of the current device. The control voltage output by the controller is the output voltage of the current loop superimposed with the output voltage of the AC port of the converter device.

[0113] When the system is disconnected from the large power grid and only supplies power to the local load, the converter device is in an off-grid operation mode. In order to improve the voltage and frequency stability and transient support capability of the island system, and improve the reliability of the local load power supply, the converter device needs to work in a voltage control mode based on the control voltage output by the controller, that is, the off-grid control mode is a voltage control mode. Specifically, in the voltage control mode, the controller includes a virtual synchronous machine control loop, a voltage reference generator, a voltage loop and a current loop. The virtual synchronous machine control loop executes virtual synchronous machine control. The voltage reference generator calculates the reference voltage of the voltage loop according to the voltage amplitude, frequency and latch value of the latch output by the virtual synchronous machine control loop. The voltage loop and the current loop respectively execute closed-loop control of the output voltage and current of the converter device. The output current of the current loop is the reference current of the voltage loop.

[0114] It should be understood that in the embodiments of the present application, the converter device outputs voltage according to the control voltage output by the controller in the converter device, therefore, obtaining the output voltage of the converter device can include obtaining the corresponding control voltage, and correspondingly, determining the control voltage can determine the corresponding output voltage of the converter device.

[0115] It should be understood that in the embodiments of the present application, when the energy storage system is connected to the AC power grid through the converter device, the energy storage system can supply power to the local load together with the AC power grid, and the energy storage system can also store the power of the AC power grid. When the energy storage system is connected to the AC power grid through the converter device, the current device is working in the grid-connected control mode.

[0116] The converter device includes a controller and a power conversion circuit, wherein the controller is used to control the converter device in different modes according to the output control voltage, and the execution subject of the method 200 is the controller in the converter device or the whole converter device.

[0117] In some examples, the direct-current power supply in the power supply system can be a photovoltaic grid-connected power generation system, the alternating-current power grid in the power supply system can be a commercial power network, and the converter device can be a PCS.

[0118] As shown in Figure 2 , the method includes:

[0119] S210, obtaining first indication information, the first indication information being used to indicate that the converter device is switched from the grid-connected control mode to the off-grid control mode.

[0120] In some examples, the converter device can be a PCS or a converter circuit in the PCS.

[0121] In a possible implementation, the first indication information includes grid scheduling information.

[0122] In a possible implementation, the first indication information comprises power grid failure information, and the power grid failure information is from a detection module. Specifically, the detection module determines that the AC power grid connected with the power conversion device is out of power, and sends the power grid failure information. The detection module can be a detection module in the power conversion device, or can be an external detection module, and the application does not limit this.

[0123] Exemplarily, the detection module is an island detection module. When the detection module detects an island state, it can be determined that the AC power grid connected with the power conversion device is out of power, and thus the power grid failure information is sent.

[0124] For ease of understanding, the island state and island detection are described below.

[0125] The island state refers to a self-provided power island phenomenon formed by a photovoltaic grid-connected power generation system installed at each user end supplying power to surrounding loads when the photovoltaic grid-connected power generation system fails to detect a power-off state in time and cannot quickly cut off the power supply from the power grid due to reasons such as failure accidents or power-off maintenance.

[0126] The island detection method can be divided into three categories: passive detection method, active detection method and switch state monitoring method.

[0127] The passive method uses the change of the voltage, frequency, phase or harmonic at the output end of the inverter when the AC power grid is out of power for island detection.

[0128] The active island detection method refers to controlling the inverter to make the output power, frequency or phase have a certain disturbance. When the AC power grid is working normally, the disturbance cannot be detected due to the balance of the power grid. When the AC power grid fails, the disturbance output by the inverter will quickly accumulate and exceed the allowed range, thereby triggering the island detection circuit.

[0129] The switch state monitoring method mainly uses the communication mode to judge the island state by monitoring the state of the grid-connected switch.

[0130] In a possible implementation, the power grid failure information is from a central controller. Specifically, the central processor determines that the AC power grid connected with the power conversion device is out of power, and sends the power grid failure information.

[0131] Exemplarily, the central controller receives information from the grid-connected switch. If the grid-connected switch indicates that the grid-connected switch has been disconnected, the central controller determines that the AC power grid is out of power and sends the power grid failure information.

[0132] Exemplarily, the central controller receives the signal from the power conversion device, judges the signal from the power conversion device, and if it is judged that the AC power grid is out of power, sends the power grid failure information to the power conversion device which establishes communication with the central controller. Specifically, the central controller establishes communication with multiple PCSs, all the PCSs upload their respective Switch_PCS signals to the central controller, if the central controller monitors that there is Switch_PCS=1, it is determined that the AC power grid is out of power, and the central controller sends the power grid failure information to other PCSs which establish communication, so as to ensure that when the AC power grid is out of power, all the PCSs which establish communication with the central controller obtain the power grid failure information.

[0133] S220, obtaining the output voltage of the power conversion device at the first time, the first time being the time of the switching process in which the power conversion device switches the grid-connected control mode to the off-grid control mode, or the first time being the receiving time of the power grid scheduling information.

[0134] It should be noted that obtaining the output voltage of the power conversion device at the first time includes obtaining a first control voltage used for controlling the output voltage of the power conversion device at the first time.

[0135] Specifically, the obtained first control voltage includes saving the amplitude and phase of the first control voltage.

[0136] S230, determining the output voltage of the power conversion device in the off-grid control mode based on the output voltage of the power conversion device at the first time; the difference between the output voltage of the power conversion device at the first time and the output voltage of the power conversion device in the off-grid control mode within a preset time is less than a set threshold.

[0137] Wherein, determining the output voltage of the power conversion device in the off-grid control mode based on the output voltage of the power conversion device at the first time means determining the control voltage of the power conversion device in the off-grid control mode based on the first control voltage of the power conversion device at the first time, and then determining the output voltage of the power conversion device in the off-grid control mode according to the control voltage of the power conversion device in the off-grid control mode.

[0138] Further, in the off-grid control mode, the control voltage is generated by the power conversion device based on the output reference voltage of the power conversion device, and the output reference voltage of the power conversion device meets the rated voltage when the power conversion device is in stable operation.

[0139] Specifically, the output reference voltage of the converter is set according to the first control voltage, and the first control voltage is used as the initial voltage of the output reference voltage of the converter. In some examples, the output reference voltage of the converter is the output voltage of the voltage reference generator in the voltage control mode, which is used to generate the control voltage for controlling the converter to convert the direct current into alternating current in the voltage control mode. The initial voltage amplitude of the output reference voltage of the converter is the voltage amplitude of the first control voltage, and the initial phase of the output reference voltage of the converter is the phase of the first control voltage. Further, the frequency of the first control voltage can also be saved in S220, and the initial frequency of the output reference voltage of the converter is the frequency of the first control voltage.

[0140] In the above process, the first control voltage is saved at the control switching moment, and is used as the initial value of the output reference voltage amplitude and phase of the converter in the off-grid control mode. This ensures that the voltage for controlling the converter to convert the direct current into alternating current does not suddenly change at the control switching moment, and further ensures that the alternating current port output voltage of the converter does not suddenly change, thereby reducing the voltage and current distortion and the regulation time in the control switching process. By giving the initial phase, the circulating current between the converters in the non-synchronous switching of the power supply system with multiple converters is avoided.

[0141] Further, the output reference voltage of the converter is transitioned from the initial voltage to the rated voltage in the first time period.

[0142] The first time period can refer to the time period for completing the control switching process, and the start time of the first time period is the first time, which refers to the time when the converter starts the switching process of switching from the grid-connected control mode to the off-grid control mode, or the receiving time of the first indication information. The first time period can also refer to the time period for completing the entire grid-off switching process of the power supply system, and the start time of the first time period is the time when the grid-connected switch is turned off. It should be understood that the length of the first time period can be set according to the actual application, and the specific setting method of the length of the first time period is not limited in the present application.

[0143] The rated voltage is the output reference voltage of the converter after the control switching is completed and the converter is stably operated in the voltage control mode.

[0144] For example, the specific value of the rated voltage can be determined by an algorithm simulation VSG control loop calculation, and the rated voltage includes the amplitude and angular frequency of the rated voltage.

[0145] In one possible implementation, the output reference voltage of the converter is gradually set to the rated voltage at a set step.

[0146] Specifically, in some examples, when the output reference voltage of the current conversion device is transited from the first control voltage to the rated voltage in the first time period, the output reference voltage of the current conversion device satisfies:

[0147]

[0148] wherein Δt represents a time length from the first time to the current time, t0 represents a time length of the first time period, E out，CCM represents a voltage amplitude of the first control voltage, θ out，CCM represents a phase of the first control voltage, E out，VSG represents a voltage amplitude of the rated voltage, ω out，VSG represents an angular frequency of the rated voltage, and f(Δt) is a voltage amplitude switching function.

[0149] In a possible implementation, the voltage amplitude adopts an exponential transition curve, and the voltage amplitude switching function is:

[0150] f(Δt)=1-e -τ·Δt

[0151] wherein τ is a constant, and is used to adjust the time length of the first time period. Specifically, if it is needed to increase the time length of the first time period, the value of τ can be reduced to achieve; if it is needed to reduce the time length of the first time period, the value of τ can be increased to achieve.

[0152] In a possible implementation, the voltage amplitude adopts a linear transition curve, and the voltage amplitude switching function is:

[0153] f(Δt)=k·Δt

[0154] wherein k is a constant, and is used to adjust the time length of the first time period. Specifically, if it is needed to increase the time length of the first time period, the value of k can be reduced to achieve; if it is needed to reduce the time length of the first time period, the value of k can be increased to achieve.

[0155] In the above process, the amplitude of the output reference voltage of the current conversion device is smoothly transited from a given initial value to the rated voltage amplitude output by the VSG control loop in the form of a selected transition curve; and the phase of the output reference voltage of the current conversion device is obtained by integrating the time with the angular frequency of the VSG control output as a proportional coefficient from a given initial value. The scheme ensures that the current conversion device is smoothly transited from CCM to VCM state based on VSG control after control switching, avoids large fluctuations of control quantity in switching transient state, reduces voltage and current distortion and regulation time in the switching process.

[0156] After the output reference voltage of the current conversion device is determined, a feedback parameter is obtained, a second control voltage is generated according to the output reference voltage of the current conversion device and the feedback parameter, the second control voltage is used to control the current conversion device to convert the direct current into the alternating current in the voltage control mode, and then the output voltage of the current conversion device in the off-grid control mode is determined based on the second control voltage. The feedback parameter includes the voltage and the current output by the alternating current port of the current conversion device, that is, the voltage and the current of the alternating current grid to which the current conversion device is connected.

[0157] Specifically, an intermediate voltage is generated according to the output reference voltage of the current conversion device and the feedback parameter, and the second control voltage is determined by adding the output reference voltage of the current conversion device and the intermediate voltage.

[0158] In a possible implementation, the second control voltage satisfies:

[0159] V out，Inner = G Inner (s) × [G out (s) · (V out，ref -V out )-I out ]

[0160] V out，m = V out，Inner + V out，ref

[0161] wherein I out represents the current of the alternating current output by the current conversion device, V out represents the voltage of the alternating current output by the current conversion device, V out，ref represents the output reference voltage of the current conversion device, V out，Inner represents the intermediate voltage, G out (s) represents a transfer function of a voltage loop in the voltage control mode, G Inner (s) represents a transfer function of a current loop in the voltage control mode, V out，m represents the second control voltage.

[0162] As can be seen from the formula satisfied by the above second control voltage, the output reference voltage of the current conversion device is superimposed on the output voltage of the current loop as a feedforward amount, which avoids the hysteresis effect of the PI controller used after the second control loop is enabled, so that the control voltage cannot be smoothly transitioned, and ensures that the output voltage of the alternating current port of the current conversion device does not suddenly change during the control switching process.

[0163] In the above scheme, the first control voltage is saved at the control switching moment, the output reference voltage of the inverter device in the off-grid control mode is transitioned from the first control voltage to the rated voltage, and the output reference voltage of the inverter device is used to generate the control voltage as a feedforward quantity, so that the voltage of the inverter device for converting the direct current into the alternating current does not suddenly change at the control switching moment, and further, the voltage of the alternating current port of the inverter device does not suddenly change, thereby reducing the voltage, current distortion and regulation time in the control switching process. Meanwhile, in the power supply system with multiple inverter devices, through the given phase initial value, the circulating current between the inverter devices in the non-synchronous switching is avoided.

[0164] In addition, in the power supply system with multiple inverter devices, the island state judgment and the control switching triggering are performed through the detection module and the central processing unit, so that while ensuring that a single inverter device enters the switching state immediately when the island state is detected, the failure of the overall grid-off-grid switching of the cluster due to the fact that the remaining inverter devices cannot accurately judge the island state when the system reaches the power balance after part of the inverter devices are first converted into the off-grid VCM operation is effectively avoided.

[0165] It should be understood that the transfer function of the voltage loop and the transfer function of the current loop in the above formula can be obtained by simulating the voltage loop and the current loop through an algorithm. The following will be described in combination with Figure 3 and Figure 4 The algorithm implementation of the method 200 will be specifically described.

[0166] Figure 3 is a schematic block diagram of the controller algorithm 300 of an embodiment of the present application, and the controller algorithm 300 is a controller algorithm for implementing the above method 200. For the convenience of understanding, the grid-connected control mode in the method 200 is represented by a current control mode, and the off-grid control mode in the method 200 is represented by a voltage control mode. As shown in Figure 3 The current controller, the voltage controller, the switching module and the information extraction module are simulated in the controller algorithm 300, wherein the current controller is used to control the inverter device or the power conversion circuit in the inverter device in the CCM mode, and the voltage controller is used to control the inverter device or the power conversion circuit in the inverter device in the VCM mode.

[0167] When the power supply system in which the inverter device is located is in the grid-connected state, the switching module selects the current controller, the current controller is started, the voltage controller is frozen, and the inverter device is controlled in the CCM mode. The controller algorithm 300 simulates the PQ control loop and the current loop in the current control mode. Specifically, the reference current I out，ref’ is calculated by simulating the PQ control loop, and the reference current is subtracted from the output current value I out, and then the output voltage of the current loop is subtracted from the output voltage value V out of the AC port of the converter device, and the difference is taken as the input current of the current loop, and then the output voltage of the current loop is added to the output voltage value V out，CCM of the AC port of the converter device, and the difference is taken as the input current of the current loop, and then the output voltage of the current loop is added to the reference voltage V out，m as the control voltage V out，m in the CCM mode. Therefore, in the CCM mode, the control voltage V out，CCM satisfies:

[0168] V cur = V out，ref′ + G out (s) * (I out -I cur ) + V out

[0169] wherein G out，CCM (s) represents a current loop transfer function.

[0170] PQ control, also known as active / reactive power control, is a constant power control, which controls the output power to be a given value by controlling the current, and is a current control. PQ control is applied to a grid-connected distributed power generation or microgrid system, in which an AC power grid provides voltage support, and a distributed power generation system is treated as a constant power output. PQ control can ensure the balance between power generation and power consumption on the AC power grid by controlling active power and reactive power.

[0171] When the power supply system in which the converter device is located is in an off-grid state, the switching module selects the voltage controller, the voltage controller is started, the current controller is frozen, and the converter device is controlled in the VCM mode. The controller algorithm 300 simulates a VSG control loop, a reference voltage generator, a voltage loop and a current loop in the voltage control mode.

[0172] Specifically, the reference voltage generator is used to generate a reference voltage V out，ref , and the output voltage value V out of the AC port of the converter device is subtracted from the reference voltage, and the difference is taken as the input of the voltage loop, and the output current I out，ref of the voltage loop is subtracted from the output current value I out of the AC port of the converter device, and the difference is taken as the input of the current loop, and then the reference voltage V out，ref is added to the output of the current loop as the control voltage V out，VCM in the VCM mode, wherein the output voltage of the current loop is the intermediate voltage in the method 200. Therefore, in the VCM mode, the control voltage V out，m satisfies:

[0173] V out，m = G Inner (s) * [G out (s) * V out，ref -Vout )-I out ]+V out，ref

[0174] Among them, G out (s) represents the transfer function of the voltage loop in voltage control mode, G Inner (s) represents the transfer function of the current loop in voltage control mode.

[0175] At the moment when the switching module selects the voltage controller, the information extraction module collects and saves the V value at that moment. out，CCM and extract V out，CCM amplitude E out，CCM and phase θ out，CCM Send it to the reference voltage generator, the reference voltage generator will send E out，CCM and θ out，CCM As a reference voltage V out，ref The initial value. The VSG control loop calculates the rated voltage V of the reference voltage during steady-state operation in VCM mode. out，VSG and the rated voltage V out，CCM The corresponding amplitude E out，VSG and angular frequency ω out，VSG Sending it to the reference voltage generator. The voltage reference generator, after switching modules to select the voltage controller, will send the reference voltage from V for a period of time. out，CCM Transition to V out，VSG This time period is referred to as the first time period. The duration of the first time period can be set according to actual needs. During the first time period, the reference voltage V... out，ref satisfy:

[0176]

[0177] Where Δt represents the duration from the start time of the first time period to the current time, t0 represents the duration of the first time period, and E out，ref The voltage amplitude representing the reference voltage, θ out，ref The phase of the reference voltage is represented by f(Δt), which is the voltage amplitude switching function.

[0178] The voltage amplitude switching function can gradually set the output reference voltage of the converter to the rated voltage in a set step size.

[0179] In one possible implementation, the voltage amplitude uses an exponential transition curve, and the voltage amplitude switching function is:

[0180] f(Δt)=1-e -τ·Δt

[0181] Wherein, τ is a constant, used to adjust the length of the first time period. Specifically, if the length of the first time period needs to be increased, the value of τ can be reduced to achieve; if the length of the first time period needs to be reduced, the value of τ can be increased to achieve.

[0182] In a possible implementation, the voltage amplitude adopts a linear transition curve, and the voltage amplitude switching function is:

[0183] f (Δt) = k · Δt

[0184] Wherein, k is a constant, used to adjust the length of the first time period. Specifically, if the length of the first time period needs to be increased, the value of k can be reduced to achieve; if the length of the first time period needs to be reduced, the value of k can be increased to achieve.

[0185] When the reference voltage transitions to V out，VSG , the control switching process is completed, and the converter operates in the VCM mode. After the control switching process is completed, the amplitude of the reference voltage is equal to the voltage amplitude of the output of the VSG control loop, and the angular frequency of the reference voltage is equal to the angular frequency of the output of the VSG control loop.

[0186] It should be understood that, corresponding to the method 200, the reference voltage in the algorithm 300 refers to the output reference voltage of the converter in the method 200, and the time when the switching module selects the V out，CCM Corresponding to the first control voltage in the method 200, the control voltage V out，m Corresponding to the second control voltage in the method 200.

[0187] In the embodiments of the present application, the switching module can establish communication with the detection module and the central controller. Specifically, if the switching module receives a switching instruction signal from the detection module and / or the central controller, the switching module switches from the current controller to the controller.

[0188] In a possible implementation, if the detection module detects that the AC power grid is out of power, the switching module sends a switching instruction signal to the switching module, and further, the detection module sends fault indication information to the central controller.

[0189] In a possible implementation, the central controller sends switching instruction information to the switching module if it receives an off-grid indication signal from the grid-connected switch and / or receives fault indication information from the detection module. If the power supply system in which the central controller is located includes multiple converter devices connected to the same AC power grid, the central controller sends switching instruction information to the switching modules of all the converter devices if it receives an off-grid indication signal from the grid-connected switch and / or receives fault indication information from the detection module corresponding to any of the converter devices. It should be understood that the connection of the converter device to the AC power grid can be direct or indirect, that is, the connection of the converter device to the AC power grid means that the AC power grid fault will affect the converter device.

[0190] In this application, the switching module can select V out，CCM or V out，VCM as V out，m , and then input V out，m to the modulation module, and send the modulated signal to the converter after modulation, to control the converter to convert the DC power into AC power that can be connected to the AC power grid, wherein V out，m may also be referred to as the modulation wave of the converter device.

[0191] In some examples, the converter device can be a PCS, and the corresponding DC power comes from an energy storage system connected to the PCS.

[0192] It should be understood that the algorithm 300 is an algorithmic implementation of the method 200, and in fact, those skilled in the art can realize that each module in the algorithm 300 can also be implemented by electronic hardware.

[0193] In the above scheme, the output voltage of the current controller at the control switching moment is saved as the initial value of the reference voltage in the voltage controller, and then the reference voltage is transitioned to the rated voltage, and the reference voltage is used as the feedforward amount to generate the output voltage of the voltage controller, which ensures that the voltage for converting the DC power to AC power by the converter device at the control switching moment will not suddenly change, thereby ensuring that the AC port output voltage of the converter device will not suddenly change, thereby reducing the voltage and current distortion and regulation time during the control switching process. At the same time, in a power supply system including multiple converter devices, by giving the phase initial value, the circulating current between the converter devices under asynchronous switching is avoided.

[0194] In addition, in a power supply system including multiple converter devices, island state judgment and control switching triggering are performed by the detection module and the central processor, which ensures that a single converter device enters the switching state immediately when it detects the island state, and effectively avoids the failure of the overall grid-connected / off-grid switching of the cluster due to the fact that some converter devices are the first to run in off-grid VCM, and the remaining converter devices cannot accurately judge the island state, resulting in the failure of the overall grid-connected / off-grid switching of the cluster.

[0195] Figure 4 is a flow chart of the controller algorithm 400 of an embodiment of the present application, which is a controller algorithm for implementing the above-mentioned method 200. For ease of description, the power conversion device in the embodiments of the present application takes the PCS as an example, as shown in the following figure: Figure 4

[0196] S410: the CCM controller is in an enabled state, the VCM controller is in a frozen state, and the PCS modulation wave V out，m is an output variable of the CCM controller out，CCM . This step is a control mode before the PCS turns to off-grid, and is used to generate the modulation wave V out，m for the PWM pulse. The calculation method can refer to the method for calculating the control voltage V out，m in the CCM mode in the algorithm 300, which will not be described herein. This step implements the power scheduling instruction response function of the PCS in the grid-connected mode.

[0197] S420: whether the detection module detects that the AC power grid loses power. This step is used to determine whether the power supply system enters an island state, and the method for detecting the island state can refer to step S210. If the power supply system enters an island state, step S430 is executed; if the power supply system does not enter an island state, step S440 is executed.

[0198] S430: Switch_PCS=1, and Switch_PCS is uploaded to the central controller. Switch_PCS is a PCS island detection flag signal, Switch_PCS=1 in an off-grid state, and Switch_PCS=0 in a grid-connected state. In the case where there are multiple PCSs in the system, if a PCS detects that the main grid loses power, the PCS island detection flag signal Switch_PCS is set to 1, and the value of Switch_PCS is uploaded to the central controller.

[0199] S440: whether Switch_CC is 1. Switch_CC represents an island detection flag signal issued by the central controller to each PCS, Switch_CC=1 in an off-grid state, and Switch_CC=0 in a grid-connected state. If Switch_CC is 1, step S450 is executed; if Switch_CC is 0, step S410 is executed.

[0200] ​S450: Set Switch_off to 1. Switch_off is the on / off switching command. In off-grid mode, Switch_off = 1; in on-grid mode, Switch_off = 0. If the PCS islanding detection flag signal Switch_PCS is 1, or if the system central controller sends a Switch_CC signal of 1, then this PCS immediately enters the on / off switching process and sets the on / off switching command Switch_off to 1. Switch_off is used to drive the PCC controller to freeze the on-grid CCM controller, start the VCM controller, and modulate the V-wave. out，m Value switching and VCM voltage reference amplitude and initial phase value latching.

[0201] S460: Latch this moment V out，CCM The amplitude and phase of the voltage are used as the initial amplitude and initial phase of the off-grid VCM voltage reference value.

[0202] Specifically, during the CCM operation, the PCS extraction module extracts V. out，CCM Fundamental amplitude E out，CCM and phase θo ut，CCM When Switch_off = 1, that is, when the PCS enters the parallel-to-offline handover process, the latch records V at this moment. out，CCM Fundamental amplitude and phase information; V at this moment out，CCM The fundamental amplitude and phase information are input into the voltage reference generator module in the VCM as the initial values ​​of the reference voltage amplitude and phase.

[0203] S470: Freeze the grid-connected CCM controller, start the off-grid VCM controller, and set the voltage control reference V. out，ref The feedforward quantity is added to the current loop output variable to generate the VCM controller output variable V. out，VCM PCS modulated wave V out，m Switch to V out，VCM Switch the PCS control mode to VCM. In this state, the modulation wave V used to generate the PWM pulse is... out，m For the calculation method, please refer to Algorithm 300 for calculating the control voltage V in VCM mode. out，m The method for calculating the second control voltage in step S205 will not be elaborated here.

[0204] S480: Based on the E output of the VSG control loop out，VSG ω out，VSG Reference voltage V out，ref The amplitude and phase transition to their rated values ​​respectively. This step is performed during... Figure 3 The reference voltage generator in the VCM controller shown calculates the reference voltage V. out，ref amplitude E out，refand phase θ out，ref The method for calculating the reference voltage V out，ref The method for transitioning the output reference voltage of the current conversion device from the first control voltage to the rated voltage in the first time period in step S240 is not described herein.

[0205] Figure 5 is a schematic diagram of a current conversion device 500 according to an embodiment of the present application. As shown in Figure 5 The current conversion device includes a controller and a power conversion circuit. One end of the power conversion circuit is connected to a direct current power supply (not shown in the figure), and the other end is connected to an alternating current power supply (not shown in the figure). The power conversion circuit converts direct current into alternating current under the action of the voltage output by the controller. The controller is used to implement the functions of the controller in algorithms 300 and 400 or to implement method 200. The controller outputs a voltage V out，CCM to the power conversion circuit in a current control mode if the current conversion device is in a grid-connected state. The controller outputs a voltage V out，CCM to the power conversion circuit in a voltage control mode if the current conversion device is in an off-grid state.

[0206] For example, the controller stores a computer program that can implement the functions of the controller in algorithms 300 and 400 or implement method 200.

[0207] In a possible implementation, the controller receives indication information from a detection module to obtain the state of the current conversion device.

[0208] In a possible implementation, the controller receives indication information from a central controller to obtain the state of the current conversion device.

[0209] In the embodiments of the present application, the current conversion device can further include a detection module. Specifically, if the detection module detects that the alternating current power grid has lost power, the detection module sends first indication information to the controller. The controller determines that the alternating current power grid has lost power according to the first indication information, and that the current conversion device is in an off-grid state. If there are multiple current conversion devices in the power supply system, the detection module further sends second indication information to the central controller. The central controller sends the first indication information to the controller corresponding to each current conversion device, so that the controller of each current conversion device in the power supply system is switched from the current control mode to the voltage control mode.

[0210] Figure 6 is a schematic diagram of a power supply system 600 according to an embodiment of the present application. As shown in Figure 6As shown, the power supply system 600 includes an energy storage system 610, a plurality of converter devices 620, an AC grid 630, a grid-connected switch 640, a central processor 650, a point of common coupling (PCC) 660, and a local load 670. The AC ends of the plurality of converter devices are connected in parallel at the PCC, and the DC ends are connected to the energy storage system. The PCC is connected to the local load, and the PCC is connected to the AC grid through the grid-connected switch. Further, the power supply system 600 can further include a filter, and each converter device corresponds to a filter, and the plurality of converter devices are connected in parallel at the PCC 160 through the filters to form a converter device cluster.

[0211] It should be noted that in the embodiments of the present application, the converter device 620 is a converter device in the converter device cluster 620, and can realize the functions in 200-400. Figure 5

[0212] In some examples, the energy storage system includes a multi-battery system and / or a photovoltaic power generation unit.

[0213] In the embodiments of the present application, the central controller can communicate with the converter device, and can also communicate with the grid-connected switch. For example, if the above application scenario is a microgrid scenario, the central controller is a microgrid central controller; if the above application scenario is an energy storage power station scenario, the central controller is an energy storage power station controller.

[0214] The central processor can communicate with the grid-connected switch, and specifically, the grid-connected switch can accept the instructions of the system central controller to perform closing and opening actions. When the grid-connected switch identifies that the power grid loses power, it will actively open and notify the central controller.

[0215] The central processor can communicate with the plurality of converter devices, and specifically, the central controller can receive the island detection flag signal sent by each converter device. If the island detection flag signal of a certain converter device or certain converter devices shows that the converter device has determined that the power supply system is in an island state, the central controller rechecks the off-grid switch state and the island detection result. If the off-grid switch state is open or the island detection result shows that the power supply system is in an island state, the central controller sends a grid-connected / off-grid switching instruction to each converter device in the power supply system.

[0216] If the detection module in the converter device determines that the AC grid loses power, i.e., the power supply system is in an island state, or receives a grid-connected / off-grid switching instruction from the central controller, the converter device performs control switching. For specific switching methods, refer to the method 200, which will not be described here.

[0217] It should be understood that the converter device in the present application can be applied to a new energy microgrid or an electrochemical energy storage power station, and can also be applied to other fields. ​

[0218] Figure 7 is a flow chart of a control algorithm 700 of a central controller of an embodiment of the present application. For ease of description, the power conversion device in communication with the central controller is a PCS, such as Figure 7

[0219] S710: Determine whether there is a PCS uploading Switch_PCS=1. All PCSs upload their respective Switch_PCS signals to the central controller, which monitors and determines them in real time. If there is a PCS uploading Switch_PCS=1, step S720 is performed; if there is no PCS uploading Switch_PCS=1, step S710 is performed.

[0220] S720: Determine whether the grid switch state is open. If the Switch_PCS signal of a certain PCS is received as 1, island rechecking is performed, and whether the grid switch is in an open state is determined according to the state feedback of the grid switch. If the grid switch state is open, step S740 is performed; if the grid switch state is closed, step S730 is performed.

[0221] S730: Determine whether the central controller detects that the AC power grid is lost. If the Switch_PCS signal of a certain PCS is received as 1 and the grid switch is not open, it is determined whether the central controller detects that the AC power grid is lost. If the AC power grid is detected to be lost, step S740 is performed; if the AC power grid is not detected to be lost, step S710 is performed.

[0222] S740: Set the value of Switch_CC to 1 and send Switch_CC to each PCS in the power supply system. The central controller determines that the grid switch state is open or detects that the main grid is lost, and then determines that the power supply system needs to perform grid-off-grid switching, sets the value of Switch_CC to 1, and sends it to each PCS in the system. After each PCS detects that the value of Switch_CC jumps to 1, it will immediately perform grid-off-grid switching.

[0223] The above scheme avoids the failure of the overall cluster grid-off-grid switching due to the fact that the remaining PCSs cannot accurately determine the island state after some PCSs are converted to off-grid VCM operation first and the system reaches power balance.

[0224] Figure 8 is a schematic diagram of a controller 10 of an embodiment of the present application, as shown, the controller 10 can be a device participating in control switching, or a chip or circuit, such as a chip or circuit that can be provided in the above-mentioned device participating in control switching. Figure 8

[0225] ​​The apparatus 10 can include a processor 11 (i.e., an example of a processing unit) and a memory 12. The memory 12 is configured to store instructions, and the processor 11 is configured to execute the instructions stored in the memory 12 to cause the apparatus 10 to implement the method as described in Figure 2 the corresponding method in the specification.

[0226] Further, the apparatus 10 can further include an input port 13 and an output port 14. Further, the processor 11, the memory 12, the input port 13 and the output port 14 can communicate with each other through internal connection paths to transfer control and / or data signals. The memory 12 is configured to store a computer program, which can be used to implement the algorithm in Figure 3 and Figure 4 The processor 11 can be configured to call and run the computer program from the memory 12 to control the input port 13 to receive signals and control the output port 14 to send signals, so as to complete the steps of the controller in the above method. The memory 12 can be integrated in the processor 11 or can be separately arranged from the processor 11.

[0227] In one possible implementation, the controller 10 is a device, the input port 13 is a receiver, and the output port 14 is a transmitter. The receiver and the transmitter can be the same or different physical entities. When they are the same physical entity, they can be collectively referred to as a transceiver.

[0228] In one possible implementation, the controller 10 is a chip or a circuit, the input port 13 is an input interface, and the output port 14 is an output interface.

[0229] As an implementation, the functions of the input port 13 and the output port 14 can be implemented by a transceiver circuit or a dedicated transceiver chip. The processor 11 can be implemented by a dedicated processing chip, a processing circuit, a processor or a general-purpose chip.

[0230] As another implementation, a general-purpose computer can be used to implement the device provided in the embodiments of the present application. That is, program codes for implementing the functions of the processor 11, the input port 13 and the output port 14 are stored in the memory 12, and the general-purpose processor implements the functions of the processor 11, the input port 13 and the output port 14 by executing the codes in the memory 12.

[0231] It should be understood that the processor 11 in the embodiments of the present application can be a central processing unit (CPU), and can also be other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), ready programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or can also be any conventional processor.

[0232] The memory 12 can include read-only memory and random access memory, and provide instructions and data for the processor 11. Part of the memory 12 can also include non-volatile random access memory. For example, the memory 12 can also store device type information.

[0233] The bus 15 can include not only a data bus, but also a power bus, a control bus, a status signal bus, etc. However, for the purpose of clear illustration, all kinds of buses are marked as bus 15 in the figure.

[0234] In the implementation process, each step of the above method can be completed by integrated logic circuits of hardware in the processor 11 or instructions in the form of software. The steps of the method disclosed in combination with the embodiments of the present application can be directly embodied as hardware processor execution completion, or executed by a combination of hardware and software modules in the processor. The software module can be located in a random access memory, a flash memory, a read-only memory, a programmable read-only memory, an electrically erasable programmable memory, a register, or other mature storage media in the art. The storage medium is located in the memory 12, and the processor 11 reads the information in the memory 12, and combines the hardware to complete each action or processing process executed by the device for control switching in the above 200-400. To avoid repetition, it will not be described in detail here.

[0235] The concepts, explanations, detailed descriptions and other steps related to the technical solutions provided by the embodiments of the present application involved in the device 10 are described in the foregoing method or other embodiments, and will not be described here.

[0236] Figure 9 The schematic diagram of the central controller 20 provided by the embodiments of the present application is shown in Figure 9 The central controller 20 can be a device, a chip or a circuit, such as a chip or a circuit of the above-mentioned central processor device.

[0237] The device 20 can include a processor 21 and a memory 22. The memory 22 is used to store instructions, and the processor 21 is used to execute the instructions stored in the memory 22, so that the device 20 realizes the steps executed by the central controller as Figures 1-7 in the above.

[0238] Further, the apparatus 20 can further include an input port 23 and an output port 24. Further, the processor 21, the memory 22, the input port 23 and the output port 24 can communicate with each other through internal connection paths to transfer control and / or data signals. The memory 22 is configured to store a computer program, which can implement the algorithm in the above-mentioned scheme, and the processor 21 can be configured to call and run the computer program from the memory 22 to control the input port 23 to receive signals and control the output port 24 to send signals, so as to complete the steps of the central controller in the above-mentioned scheme. The memory 22 can be integrated in the processor 21 or can be separately arranged from the processor 21. Figure 7

[0239] In a possible implementation, the central controller 20 is a device, the input port 23 is a receiver, and the output port 24 is a transmitter. The receiver and the transmitter can be the same or different physical entities. When they are the same physical entity, they can be collectively referred to as a transceiver.

[0240] In a possible implementation, the central controller 20 is a chip or a circuit, the input port 23 is an input interface, and the output port 24 is an output interface.

[0241] As an implementation, the functions of the input port 23 and the output port 24 can be implemented by a transceiver circuit or a transceiver dedicated chip. The processor 21 can be implemented by a dedicated processing chip, a processing circuit, a processor or a general-purpose chip.

[0242] As another implementation, a general-purpose computer can be used to implement the communication device provided in the embodiments of the present application. That is, program codes for implementing the functions of the processor 21, the input port 23 and the output port 24 are stored in the memory 22, and the general-purpose processor implements the functions of the processor 21, the input port 23 and the output port 24 by executing the codes in the memory 22.

[0243] It should be understood that, in the embodiments of the present application, the processor 21 can be a central processing unit (CPU), and the processor 21 can also be other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), ready programmable gate arrays (FPGAs) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or the processor can also be any conventional processor.

[0244] ​The memory 22 can include read-only memory and random access memory, and provide instructions and data to the processor 21. A portion of the memory 22 can also include non-volatile random access memory. For example, the memory 22 can also store device type information.

[0245] The bus 25 can include, in addition to the data bus, a power supply bus, a control bus, and a status signal bus, etc. However, for the sake of clarity, all the buses are marked as bus 25 in the figure.

[0246] In the implementation process, the steps of the above method can be completed by the integrated logic circuit of the hardware in the processor 21 or the instructions in the form of software. The steps of the method disclosed in combination with the embodiments of the application can be directly embodied as hardware processor execution completion, or executed by a combination of hardware and software modules in the processor. The software module can be located in a random access memory, a flash memory, a read-only memory, a programmable read-only memory, or an electrically erasable programmable memory, a register, and other mature storage media in the art. The storage medium is located in the memory 22, and the processor 21 reads the information in the memory 22, and combines the hardware to complete the actions or processing processes of each action or processing process executed by the central controller in the above method. To avoid repetition, it will not be described in detail here.

[0247] The concepts, explanations, detailed descriptions and other steps related to the technical solutions provided by the embodiments of the application involved in the device 20 are described in the foregoing method or other embodiments. Here, no further description is made.

[0248] Those of ordinary skill in the art can realize that the modules and algorithm steps of each example described in combination with the embodiments disclosed herein can be realized in electronic hardware or a combination of computer software and electronic hardware. Whether the functions are executed in hardware or software depends on the specific application and design constraints of the technical solutions. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of the application.

[0249] Those skilled in the art can clearly understand that, for the convenience and brevity of the description, the specific working process of the above-described system, device and module can refer to the corresponding process in the foregoing method embodiments, which will not be described here.

[0250] In several embodiments provided in the present application, it should be understood that the disclosed system, device and method can be implemented in other manners. For example, the described device embodiments are merely schematic. For example, the division of the modules is merely logical function division. There can be another division manner for the actual implementation, for example, multiple modules or components can be combined or integrated into another system, or some features can be ignored or not executed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections can be indirect couplings or communication connections through some interfaces, devices or modules, and can be in electrical, mechanical or other forms.

[0251] The modules illustrated as separated components can or can not be physically separated, and the components illustrated as modules can or can not be physical modules, i.e., can be located in one place, or can be distributed to multiple modules. Some or all of the modules can be selected according to actual needs to achieve the purpose of the embodiments.

[0252] In addition, each functional module in each embodiment of the present application can be integrated in one processing unit, or each module can be physically present alone, or two or more modules can be integrated in one unit.

[0253] If the functions are realized in the form of software function modules and sold or used as independent products, they can be stored in a computer readable storage medium. Based on this understanding, the technical solutions of the present application can be embodied in the form of a software product, and the computer software product is stored in a storage medium, and includes several instructions for causing a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present application. The aforementioned storage medium includes: a U disk, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk or an optical disk, and various media that can store program codes.

[0254] The above description is merely specific embodiments of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art can easily think of changes or replacements within the technical scope disclosed in the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A current conversion device, characterized by, The converter comprises a controller, a power conversion circuit, a first end of the power conversion circuit is connected with an output end of the controller, a second end of the power conversion circuit is used for being connected with an energy storage system, and a third end of the power conversion circuit is used for being connected with an AC power grid; The controller is used for receiving grid scheduling information, the grid scheduling information is used for instructing the converter to switch from a grid-connected control mode to an off-grid control mode, wherein the grid-connected control mode is to control the converter to convert DC power output by the energy storage system into AC power and connect the AC power into the AC power grid; the off-grid control mode is to control the converter to convert the DC power output by the energy storage system into AC power and supply power to a local load, and AC power output by the converter is not connected into the AC power grid; The controller is further used for obtaining an output voltage of the converter at a first time, the first time is a time of switching processing of the converter from the grid-connected control mode to the off-grid control mode, or the first time is a receiving time of the grid scheduling information, and a difference between the output voltage of the converter at the first time and an output voltage of the converter in the off-grid control mode within a preset time is less than a set threshold value; The controller is further used for determining the output voltage of the converter in the off-grid control mode based on the output voltage of the converter at the first time; The power conversion circuit is used for converting the DC power output by the energy storage system into AC power and supplying power to the local load based on the output voltage of the converter in the off-grid control mode; The obtaining of the output voltage of the converter at the first time comprises obtaining a first control voltage, the first control voltage is a control voltage of the converter at the first time, and the control voltage is used for controlling the output voltage of the converter; The determination of the output voltage of the converter in the off-grid control mode based on the output voltage of the converter at the first time comprises: determining a second control voltage of the converter in the off-grid control mode based on the first control voltage; determining the output voltage of the converter in the off-grid control mode based on the second control voltage.

2. The apparatus of claim 1, wherein, The determination of the second control voltage of the converter in the off-grid control mode based on the first control voltage comprises: the controller sets an output reference voltage of the converter according to the first control voltage; the controller obtains feedback parameters, the feedback parameters comprise a voltage and a current of AC power output by the converter; the controller determines the second control voltage based on the output reference voltage of the converter and the feedback parameters.

3. The apparatus of claim 2, wherein, The generation of the second control voltage based on the output reference voltage of the converter and the feedback parameters comprises: the controller inputs the output reference voltage of the converter and the feedback parameters into a current loop and a voltage loop to generate an intermediate voltage; the controller adds the output reference voltage of the converter and the intermediate voltage to generate the second control voltage.

4. The apparatus of claim 2 or 3, wherein, The controller is further configured to gradually set an output reference voltage of the power conversion device to a rated voltage in a set step, the rated voltage being an output voltage of the power conversion device when the power conversion device is stably operated in the off-grid control mode.

5. The apparatus of any one of claims 1 to 3, wherein, The power conversion device further comprises: a detection module configured to detect whether the AC power grid is out of power; if the detection module detects that the AC power grid is out of power, the detection module sends grid failure information to the controller, the grid failure information being used to indicate that the AC power grid is out of power and to instruct the power conversion device to switch from the grid-connected control mode to the off-grid control mode.

6. The apparatus of claim 5, wherein, if the detection module detects that the AC power grid is out of power, the detection module sends the grid failure information to the central controller; the central controller instructs other power conversion devices connected to the AC power grid to switch control modes based on the grid failure information.

7. A parallel and off-grid switching control method applied to a variable flow device, characterized in that, The method comprises: receiving grid scheduling information, the grid scheduling information being used to instruct the power conversion device to switch from a grid-connected control mode to an off-grid control mode, wherein the grid-connected control mode is to control the power conversion device to convert direct current output by an energy storage system into alternating current and connect the alternating current to an AC power grid; the off-grid control mode is to control the power conversion device to convert direct current output by the energy storage system into alternating current and supply power to a local load, and the alternating current output by the power conversion device is not connected to the AC power grid; obtaining an output voltage of the power conversion device at a first time, the first time being a time when the power conversion device switches from the grid-connected control mode to the off-grid control mode, or the first time being a time when the grid scheduling information is received, and a difference between the output voltage of the power conversion device at the first time and an output voltage of the power conversion device in the off-grid control mode within a preset time is less than a set threshold; adjusting the output voltage of the power conversion device in the off-grid control mode based on the output voltage of the power conversion device at the first time; wherein the obtaining of the output voltage of the power conversion device at the first time comprises obtaining a first control voltage, the first control voltage being a control voltage of the power conversion device at the first time, the control voltage being used to control the output voltage of the power conversion device; the determining of the output voltage of the power conversion device in the off-grid control mode based on the output voltage of the power conversion device at the first time comprises: determining a second control voltage of the power conversion device in the off-grid control mode based on the first control voltage; determining the output voltage of the power conversion device in the off-grid control mode based on the second control voltage.

8. The method of claim 7, wherein, The determining of the second control voltage of the power conversion device in the off-grid control mode based on the first control voltage comprises: setting an output reference voltage of the power conversion device according to the first control voltage; obtaining feedback parameters, the feedback parameters comprising a voltage and a current of alternating current output by the power conversion device; determining the second control voltage based on the output reference voltage of the power conversion device and the feedback parameters.

9. The method of claim 8, wherein, The generating of the second control voltage based on the output reference voltage of the power conversion device and the feedback parameters comprises: The output reference voltage of the converter and the feedback parameter generate an intermediate voltage after passing through the current loop and the voltage loop; The output reference voltage of the converter and the intermediate voltage generate a second control voltage.

10. The method according to claim 8 or 9, characterized in that, The method further comprises: The output reference voltage of the converter is gradually set to a rated voltage at a set step, and the rated voltage is the output voltage of the converter when the converter is stably operated in the off-grid control mode.

11. The method according to any one of claims 7 to 9, characterized in that, The method further comprises: Grid failure information is obtained, and the grid failure information is used to indicate that the AC power grid is out of power and to indicate that the converter is switched from the grid-connected control mode to the off-grid control mode.

12. A power supply system characterized by comprising: Comprise: A plurality of converters, an energy storage system, an AC power grid, a grid-connected switch, a central controller and a point of common coupling, the plurality of converters comprising the converter of any one of claims 1 to 11; The AC ends of the plurality of converters are connected in parallel at the point of common coupling, and the DC ends are connected with the energy storage system, the plurality of converters have at least two control modes, the two control modes being a grid-connected control mode and an off-grid control mode; the grid-connected control mode is to control the converter to convert the DC power output by the energy storage system into AC power and connect the AC power to the AC power grid; the off-grid control mode is to control the converter to convert the DC power output by the energy storage system into AC power and supply the AC power to a local load, and the AC power output by the converter is not connected to the AC power grid; The point of common coupling is connected with the AC power grid through the grid-connected switch; The central controller is in communication connection with the plurality of converters and the grid-connected switch.

13. The system of claim 12, wherein, If the central controller instructs the grid-connected switch to be disconnected, the central controller is further used to instruct the plurality of converters to switch the control mode.

14. The system of claim 12 or 13, wherein, The plurality of converters are further used to detect whether the AC power grid is out of power.

15. The system of claim 14, wherein, If the AC power grid is out of power, the plurality of converters switch the control mode.

16. The system of claim 14 or 15, wherein, If the AC power grid is out of power, the plurality of converters are further used to instruct the central controller to disconnect the grid-connected switch.

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