Power converter and control method of power converter

Abstract

The embodiment of the invention provides a power converter and a control method of the power converter, a direct current end of the power converter is used for connecting a direct current power supply, an alternating current end of the power converter is used for connecting a power grid, and the power converter is used for converting direct current into alternating current. And the power converter is also used for controlling the absolute value of the reactive current output by the AC end to change along with the change of the AC voltage output by the AC end when the power grid has a low-voltage fault or a high-voltage fault. The power converter is further used for controlling the absolute value of the reactive current output by the alternating current end to be reduced when the difference value between the amplitude of the target output signal and the average value of the multiple output signals in the multiple output signals output by the alternating current end within the set time interval is larger than a first threshold value or when the standard deviation of the multiple output signals is larger than a second threshold value. Or the change speed of the reactive current output by the alternating current end is controlled to be reduced, and the target output signal is the output signal with the maximum amplitude in the multiple output signals.

Description

Technical Field

[0001] This application relates to the field of power electronics, and more particularly to a power converter and a control method for the power converter. Background Technology

[0002] As the penetration rate of distributed energy sources such as photovoltaics, wind power, and energy storage devices gradually increases, the interaction between inverters or converters, as important components of energy exchange, and the power grid is becoming increasingly significant. During the process of power supply systems supplying power to the grid, factors such as short circuits in transmission lines between devices or equipment failures can lead to low-voltage or high-voltage faults in the AC power grid. Currently, grid connection standards in many countries require inverters or converters in the power supply system to have fault ride-through capability, meaning that when a low-voltage or high-voltage fault occurs in the grid, the relevant equipment at the power supply end continues to operate without disconnecting from the grid and injects reactive current to help the grid restore normal voltage. However, due to the increase in the installed capacity of new energy sources in the power supply system, or the increase in system line impedance due to aging transmission lines, the grid environment of the power supply system is weakening. When the grid connection environment of an inverter or converter changes from a strong grid to a weak grid, if a low-voltage or high-voltage fault occurs in the grid, the reactive current output by the inverter or converter under weak grid conditions will significantly change the voltage imbalance at the equipment ports. The changing port voltage will interfere with reactive current control, causing voltage, current, and power oscillations in inverters or converters that were originally operating stably under a strong grid. This greatly affects the fault ride-through capability of various devices in the power supply system, resulting in poor grid power supply reliability. Summary of the Invention

[0003] This application provides a power converter and a control method for the power converter, which can prevent the continuous oscillation of the AC output of the power converter during fault ride-through in a weak power grid environment from interfering with reactive current control, thereby improving the stability of the power converter operation.

[0004] In a first aspect, this application provides a power converter. The DC terminal of the power converter is connected to a DC power source, and the AC terminal is connected to a power grid or load. The power converter converts DC power into AC power for output to the power grid or load. The power converter is further configured to, in the event of a low-voltage or high-voltage fault in the power grid, control the absolute value of the reactive current output from the AC terminal to change with the AC voltage output from the AC terminal. The power converter is also configured to, when the difference between the amplitude of a target output signal and the average value of the multiple output signals is greater than a first threshold among multiple output signals output from the AC terminal within a set time interval, control the absolute value of the reactive current output from the AC terminal to decrease, and the target output signal is the output signal with the largest amplitude among the multiple output signals. Alternatively, the power converter is further configured to, when the difference between the amplitude of a target output signal and the average value of the multiple output signals is greater than a first threshold among multiple output signals output from the AC terminal within a set time interval, control the rate of change of the reactive current output from the AC terminal to decrease, and the target output signal is the output signal with the largest amplitude among the multiple output signals. Alternatively, the power converter is further configured to, when the standard deviation of multiple output signals output by the AC terminal within a set time interval is greater than a second threshold, control the absolute value of the reactive current output by the AC terminal to decrease. Alternatively, the power converter is further configured to, when the standard deviation of multiple output signals output by the AC terminal within a set time interval is greater than a second threshold, control the rate of change of the reactive current output by the AC terminal to decrease.

[0005] In this application, when a low-voltage or high-voltage fault occurs in the power grid, and the grid connection environment of the power converter is a weak grid, the reactive current output by the power converter under weak grid conditions can cause oscillation at its AC terminal output. By controlling the absolute value of the reactive current output at the AC terminal of the power converter to decrease, the absolute value of the reactive current is gradually reduced during AC terminal output oscillation to smooth out the oscillation. Alternatively, by controlling the rate of change of the reactive current output at the AC terminal to decrease, the absolute value of the reactive current output at the AC terminal is prevented from deviating too much from its value before the oscillation occurs. This prevents continuous oscillation at the AC terminal output during fault ride-through under weak grid conditions, which could interfere with reactive current control, and improves the stability of the power converter's operation.

[0006] In one possible implementation, the power converter is further configured to control the absolute value of the reactive current output at the AC terminal to change with the AC voltage output at the AC terminal during a low-voltage or high-voltage fault in the power grid. This includes controlling the absolute value of the reactive current output at the AC terminal to change with the AC voltage output at the AC terminal when the effective value or minimum peak-to-peak value of the three-phase phase voltage output at the AC terminal is lower than a low-voltage threshold, or when the effective value or minimum peak-to-peak value of the three-phase line voltage output at the AC terminal is lower than a low-voltage threshold, or when the positive-sequence voltage output at the AC terminal is lower than a low-voltage threshold. Conversely, when the effective value or maximum peak-to-peak value of the three-phase phase voltage output at the AC terminal is higher than a high-voltage threshold, or when the effective value or maximum peak-to-peak value of the three-phase line voltage output at the AC terminal is higher than a high-voltage threshold, or when the positive-sequence voltage output at the AC terminal is higher than a high-voltage threshold, the absolute value of the reactive current output at the AC terminal is controlled to change with the AC voltage output at the AC terminal, where the high-voltage threshold is greater than the low-voltage threshold. When the power converter detects a low-voltage or high-voltage fault in the power grid, it injects reactive current into the grid to support the grid voltage. The power equipment in the power supply system does not disconnect from the grid and continues to operate, and can smoothly transition to normal operation.

[0007] In one possible implementation, the power converter is further configured to control the absolute value of the reactive current output at the AC terminal to vary with the AC voltage output at the AC terminal. Specifically, during a low-voltage fault in the grid, the absolute value of the reactive current is controlled to change negatively with either the positive-sequence or negative-sequence voltage output at the AC terminal. During a high-voltage fault in the grid, the absolute value of the reactive current is controlled to change positively with either the positive-sequence or negative-sequence voltage output at the AC terminal. When the power converter detects a low-voltage or high-voltage fault in the grid, it injects reactive current into the grid to support the grid voltage, ensuring that the power equipment in the power supply system continues to operate without disconnecting from the grid and can smoothly transition to normal operation.

[0008] In one possible implementation, the power converter is also used to control the absolute value of the reactive current output at the AC terminal to decrease, and the magnitude of the decrease is positively correlated with the magnitude of the difference or the standard deviation. During voltage faults, the power converter adjusts the absolute value of the output reactive current according to the oscillation intensity of the output signal, so that the absolute value of the reactive current gradually decreases during oscillation to smooth the AC terminal oscillation, preventing continuous oscillation of the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control, and improving the stability of the power converter operation.

[0009] In one possible implementation, the power converter is also used to control the rate of change of the reactive current output at the AC terminal to decrease, including filtering out current components in the reactive current output at the AC terminal with frequencies higher than the grid voltage frequency. By directly filtering out current components in the reactive current output at the AC terminal with frequencies higher than the cutoff frequency, the power converter reduces the rate of change of the absolute value of the reactive current output at the AC terminal, ensuring that the absolute value of the reactive current output at the AC terminal does not deviate too much from its value before oscillation. This prevents continuous oscillation of the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control, thereby improving the stability of the power converter's operation.

[0010] In one possible implementation, the power converter is further configured to reduce the rate of change of the reactive current output at the AC terminal, including filtering out voltage components in the AC voltage with frequencies higher than the grid voltage frequency, and controlling the absolute value of the reactive current output at the AC terminal to change with the AC voltage after filtering out the voltage components. By controlling the absolute value of the reactive current output at the AC terminal to change with the positive-sequence voltage after filtering out the voltage components, the rate of change of the absolute value of the reactive current output at the AC terminal is reduced. This ensures that the absolute value of the reactive current output at the AC terminal does not deviate too much from its value before oscillation, preventing continuous oscillation of the AC terminal output during fault ride-through in weak grid environments that could interfere with reactive current control, and improving the stability of the power converter operation.

[0011] Secondly, this application provides a control method for a power converter. The method includes controlling the absolute value of the reactive current output by the AC terminal of the power converter to change with the AC voltage output by the AC terminal during a low-voltage or high-voltage fault in the power grid. When the difference between the amplitude of a target output signal and the average value of the multiple output signals is greater than a first threshold among multiple output signals output by the AC terminal within a set time interval, the absolute value of the reactive current output by the AC terminal is reduced, and the target output signal is the output signal with the largest amplitude among the multiple output signals. Alternatively, when the difference between the amplitude of the target output signal and the average value of the multiple output signals is greater than a first threshold among multiple output signals output by the AC terminal within a set time interval, the rate of change of the reactive current output by the AC terminal is reduced, and the target output signal is the output signal with the largest amplitude among the multiple output signals. Alternatively, when the standard deviation of the multiple output signals is greater than a second threshold among multiple output signals output by the AC terminal within a set time interval, the absolute value of the reactive current output by the AC terminal is reduced. Alternatively, when the standard deviation of multiple output signals from the AC terminal within a set time interval exceeds a second threshold, the rate of change of the reactive current output from the AC terminal is reduced. The DC terminal of the power converter is used to connect to a DC power source, and the AC terminal is used to connect to the power grid or load. The power converter converts DC power into AC power for output to the power grid or load.

[0012] In this application, when a low-voltage or high-voltage fault occurs in the power grid and the output signal of the power converter oscillates at the AC terminal, the absolute value of the reactive current output at the AC terminal is reduced. This gradually decreases the absolute value of the reactive current during AC terminal output oscillation to smooth out the oscillation. Alternatively, the rate of change of the reactive current output at the AC terminal is reduced to ensure that the absolute value of the reactive current output at the AC terminal does not deviate excessively from its value before oscillation. This prevents continuous oscillation of the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control, thereby improving the stability of the power converter operation.

[0013] In one possible implementation, when a low-voltage or high-voltage fault occurs in the power grid, the absolute value of the reactive current output at the control AC terminal changes with the AC voltage output at the control AC terminal. This includes situations where the effective value or minimum peak-to-peak value of the three-phase phase voltage output at the control AC terminal is below a low-voltage threshold, or where the effective value or minimum peak-to-peak value of the three-phase line voltage output at the control AC terminal is below a low-voltage threshold, or where the positive-sequence voltage output at the control AC terminal is below a low-voltage threshold. Conversely, when the effective value or maximum peak-to-peak value of the three-phase phase voltage output at the control AC terminal is above a high-voltage threshold, or when the effective value or maximum peak-to-peak value of the three-phase line voltage output at the control AC terminal is above a high-voltage threshold, or when the positive-sequence voltage output at the control AC terminal is above a high-voltage threshold, the absolute value of the reactive current output at the control AC terminal changes with the AC voltage output at the control AC terminal. By injecting reactive current into the power grid to support the grid voltage when a low-voltage or high-voltage fault is detected, the power equipment in the power supply system can continue to operate without disconnecting from the grid and can smoothly transition to normal operation.

[0014] In one possible implementation, the absolute value of the reactive current output at the control terminal varies with the AC voltage output at the control terminal. Specifically, during a low-voltage fault in the grid, the absolute value of the reactive current changes negatively with either the positive-sequence or negative-sequence voltage output at the AC terminal. During a high-voltage fault in the grid, the absolute value of the reactive current changes positively with either the positive-sequence or negative-sequence voltage output at the AC terminal. By injecting reactive current into the grid to support the grid voltage when a low-voltage or high-voltage fault is detected, the power equipment in the power supply system can operate continuously without disconnecting from the grid and can smoothly transition to normal operation.

[0015] In one possible implementation, reducing the absolute value of the reactive current output at the AC terminal includes controlling the absolute value of the reactive current output at the AC terminal to decrease, and the magnitude of the decrease is positively correlated with the magnitude of the difference or the standard deviation. During voltage faults, the absolute value of the output reactive current is adjusted according to the oscillation intensity of the output signal, so that the absolute value of the reactive current gradually decreases during oscillation to smooth the AC terminal oscillation. This prevents continuous oscillation of the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control, thereby improving the stability of the power converter operation.

[0016] In one possible implementation, the rate of change of the reactive current output at the AC terminal is reduced by filtering out current components in the reactive current output at the AC terminal with frequencies higher than the grid voltage frequency. By directly filtering out current components in the reactive current output at the AC terminal with frequencies higher than the cutoff frequency, the rate of change of the absolute value of the reactive current output at the AC terminal is reduced. This ensures that the absolute value of the reactive current output at the AC terminal does not deviate too much from its value before oscillation, preventing continuous oscillation of the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control and improving the stability of the power converter operation.

[0017] In one possible implementation, the rate of change of the reactive current output at the AC terminal is reduced by filtering out voltage components in the AC voltage with frequencies higher than the grid voltage frequency, and by controlling the absolute value of the reactive current output at the AC terminal to change with the AC voltage after filtering out the voltage components. By controlling the absolute value of the reactive current output at the AC terminal to change with the positive-sequence voltage after filtering out the voltage components, the rate of change of the absolute value of the reactive current output at the AC terminal is reduced. This ensures that the absolute value of the reactive current output at the AC terminal does not deviate too much from its value before oscillation, preventing continuous oscillation of the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control, and improving the stability of the power converter operation. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of an application scenario of the power supply system provided in this application;

[0019] Figure 2 This is a schematic diagram of another application scenario of the power supply system provided in this application;

[0020] Figure 3 This is a waveform diagram of the output voltage of the power converter provided in this application;

[0021] Figure 4 This is another waveform diagram of the output voltage of the power converter provided in this application;

[0022] Figure 5 This is a waveform diagram of the output current of the power converter provided in this application;

[0023] Figure 6 This is another waveform diagram of the output voltage of the power converter provided in this application;

[0024] Figure 7 This is a schematic diagram of the spectrum of the output signal provided in this application;

[0025] Figure 8 This is another waveform diagram of the output current of the power converter provided in this application;

[0026] Figure 9 This is a flowchart illustrating the control method for the power converter provided in this application. Detailed Implementation

[0027] See Figure 1 , Figure 1 This is a schematic diagram of an application scenario for the power supply system provided in this application. The power supply system provided in this application includes multiple power converters. The DC terminals of each power converter are used to connect to photovoltaic modules, and the AC terminals of the multiple power converters are connected in parallel to connect to a load or the power grid through a parallel connection point. Each power converter can invert and convert the DC power provided by the photovoltaic modules, and output the converted AC power to the load or the power grid. The load can be AC-powered equipment such as household appliances or communication base stations.

[0028] In some feasible implementations, the power supply system may include a transformer; please refer again to Figure 1 , Figure 1 The output terminals of each power converter can be connected to the load through a transformer. The transformer can change (either step up or step down) the AC power provided by the power converter and supply the transformed AC power to the load or the power grid.

[0029] In some feasible implementations, the DC terminals of each power converter can be used to connect to an energy storage battery; see [link to relevant documentation]. Figure 2 , Figure 2 This is a schematic diagram of another application scenario of the power supply system provided in this application. In the power supply system, among multiple power converters, the DC terminals of some power converters are used to connect to energy storage batteries, and the AC terminals of multiple power converters are connected in parallel to connect to the load or the power grid through a parallel connection point. Each power converter can invert and convert the DC power provided by the photovoltaic modules or energy storage batteries, and output the converted AC power to the load or the power grid. Here, the power converter connected to the photovoltaic modules in the power supply system can be an inverter, and the power converter connected to the energy storage batteries in the power supply system can be an energy storage converter.

[0030] exist Figure 1 or Figure 2In the application scenario of the power supply system shown, during the process of supplying power to the grid, factors such as short circuits in transmission lines between devices or equipment failures can lead to low-voltage or high-voltage faults in the grid. To ensure that power devices in the power supply system can continue operating without disconnecting from the grid and can smoothly transition to normal operation during grid faults, the power converters in the power supply system are required to have fault ride-through capability. Specifically, when a low-voltage or high-voltage fault is detected in the grid, the power converter injects reactive current into the grid to support the grid voltage. For example, the power converter determines whether a low-voltage or high-voltage fault has occurred by detecting the three-phase phase voltage or three-phase line voltage at the AC terminal output, and injects reactive current into the grid when a fault occurs to keep the power converter connected to the grid during the fault period. As the installed capacity of new energy sources in the power supply system increases, i.e., the number of power converters in the power supply system increases, the system's short-circuit capacity ratio decreases, and the grid environment weakens. Alternatively, when transmission lines in the power supply system age—that is, when wires and cables are used and affected by various factors such as environment, load, and voltage—they gradually lose their original electrical performance, exhibiting aging and corrosion. This leads to increased system line impedance, decreased system short-circuit capacity, and a weakening of the power grid environment. When the grid connection environment of the power converter changes from a strong grid to a weak grid, if a low-voltage or high-voltage fault occurs in the grid, the reactive current output by the power converter under weak grid conditions will significantly alter the imbalance of its port voltage. The changing port voltage will interfere with reactive current control, causing oscillations in the output signals of voltage, current, and power of the power converter that was originally operating stably under a strong grid. This greatly affects the fault ride-through capability of each power converter in the power supply system, resulting in poor power grid reliability.

[0031] In the power supply system provided in this application, each power converter is used to control the absolute value of its AC terminal output reactive current to change with the AC voltage output when a low-voltage or high-voltage fault occurs in the power grid. When the output signal of the power converter's AC terminal oscillates, the absolute value of the AC terminal output reactive current decreases, or the rate of change of the AC terminal output reactive current decreases. Specifically, when the power converter detects a low-voltage or high-voltage fault in the power grid, it controls the AC terminal to output reactive current to the power grid so that the power converter remains connected to the grid during the fault. For example, the power converter determines whether a low-voltage or high-voltage fault has occurred in the power grid by detecting the three-phase phase voltage, three-phase line voltage, or positive-sequence voltage output at the AC terminal. Specifically, a low-voltage fault is determined to have occurred in the current power grid when the effective value or the minimum peak-to-peak value of the three-phase phase voltage output at the AC terminal is lower than the low-voltage threshold, or when the effective value or the minimum peak-to-peak value of the three-phase line voltage output at the AC terminal is lower than the low-voltage threshold, or when the positive-sequence voltage output at the AC terminal is lower than the low-voltage threshold. When the effective value or peak-to-peak value of the three-phase phase voltage output from the AC terminal is higher than the high-voltage threshold, or when the effective value or peak-to-peak value of the three-phase line voltage output from the AC terminal is higher than the high-voltage threshold, or when the positive-sequence voltage output from the AC terminal is higher than the high-voltage threshold, it is determined that a high-voltage fault has occurred in the current power grid. The high-voltage threshold is greater than the low-voltage threshold. Next, the power converter controls the absolute value of the reactive current output from the AC terminal to change with the AC voltage output from the AC terminal. This AC voltage can be either the negative-sequence voltage or the positive-sequence voltage output from the AC terminal. For example, if a low-voltage fault or a high-voltage fault occurs in the power grid, and it is an asymmetrical fault, the absolute value of the reactive current output from the AC terminal is controlled to change with the positive-sequence voltage output from the AC terminal. Assuming the positive-sequence voltage before the fault is 1, the reactive current output from the AC terminal can be expressed as (1-Up)K, where Up is the positive-sequence voltage and K is a positive number. If a low-voltage fault occurs in the power grid, the value of Up is less than 1, and the absolute value of the control reactive current changes negatively with the positive-sequence voltage. That is, the smaller the positive-sequence voltage output at the AC terminal, the larger the absolute value of the reactive current output at the AC terminal controlled by the power converter. If a high-voltage fault occurs in the power grid, the value of Up is greater than 1, and the absolute value of the control reactive current changes positively with the positive-sequence voltage. That is, the larger the positive-sequence voltage output at the AC terminal, the larger the absolute value of the reactive current output at the AC terminal controlled by the power converter. Optionally, the absolute value of the control reactive current output at the AC terminal can also change with the negative-sequence voltage output at the AC terminal. Assuming the negative-sequence voltage is 0 before the fault, the reactive current output at the AC terminal can be expressed as (0-Un)K, where Un is the negative-sequence voltage and K is a positive number.If a low-voltage fault occurs in the power grid, the value of Un is less than 0, and the absolute value of the control reactive current changes negatively with the negative-sequence voltage. That is, the smaller the negative-sequence voltage output at the AC terminal, the larger the absolute value of the reactive current output by the power converter at the AC terminal. If a high-voltage fault occurs in the power grid, the value of Un is greater than 0, and the absolute value of the control reactive current changes positively with the negative-sequence voltage. That is, the larger the negative-sequence voltage output at the AC terminal, the larger the absolute value of the reactive current output by the power converter at the AC terminal. Furthermore, when the power converter is connected to a weak power grid, the reactive current output by the power converter can cause oscillations at its AC terminal output. For example, the voltage, current, or power output at the AC terminal may oscillate, thus interfering with the control of the reactive current. When a low-voltage or high-voltage fault occurs in the power grid, the power converter detects the output signals such as voltage, current, or power at the AC terminal. When the output signal oscillates, it determines that AC terminal output oscillation has occurred. The voltage output from the AC terminal can be a negative-sequence voltage or a positive-sequence voltage, the effective value or peak-to-peak value of the three-phase line voltage, or the effective value or peak-to-peak value of the three-phase phase voltage, etc. The specific value can be determined according to the actual application scenario and is not limited here. When the output signal of the power converter oscillates at the AC terminal, the absolute value of the reactive current output from the AC terminal decreases, and the amplitude of the decrease in the reactive current is positively correlated with the oscillation intensity of the output signal. That is, the power converter adjusts the absolute value of the output reactive current according to the oscillation intensity of the output signal, gradually reducing the absolute value of the reactive current during oscillation to smooth the AC terminal oscillation. Alternatively, the rate of change of the reactive current output from the AC terminal can be reduced to ensure that the absolute value of the reactive current output from the AC terminal does not deviate too much from its value before oscillation, preventing continuous oscillation of the AC terminal output during fault ride-through in weak grid environments that could interfere with reactive current control, thus improving the stability of the power converter operation.

[0032] In some feasible implementations, the power converter detects the positive-sequence voltage output at the AC terminal. When the positive-sequence voltage is below a low-voltage threshold, it determines that a low-voltage fault has occurred in the current power grid; when the positive-sequence voltage is above a high-voltage threshold, it determines that a high-voltage fault has occurred in the current power grid. The high-voltage threshold is greater than the low-voltage threshold. See also... Figure 3 , Figure 3 This is a waveform diagram of the output voltage of the power converter provided in this application. For example... Figure 3As shown, the power converter detects the positive-sequence voltage Up at the AC terminal output. When Up is lower than the low-voltage threshold UL, the power converter determines that a low-voltage fault has occurred in the current power grid. The power converter then controls the AC terminal to output reactive current to the power grid, and the absolute value of the reactive current changes with the positive-sequence voltage at the AC terminal output. When the power converter is connected to a strong power grid, the AC terminal output of the power converter will not be disturbed by the output reactive current, i.e., the AC terminal output remains stable. After time T0, the power converter is connected to a weak power grid. If a low-voltage fault occurs in the power grid, the power converter controls the AC terminal to output reactive current to the power grid. However, under weak power grid conditions, the reactive current output by the power converter will cause its AC terminal output to oscillate. Therefore, the positive-sequence voltage will oscillate during the low-voltage fault period, and the oscillating positive-sequence voltage will also interfere with the control of the aforementioned reactive current.

[0033] In some feasible implementations, the power converter is used to acquire multiple output signals from the AC terminal within a set time interval when a low-voltage or high-voltage fault occurs in the power grid. If the difference between the amplitude of a target output signal and the average value of the multiple output signals is greater than a first threshold, it determines that AC terminal output oscillation has occurred and controls the absolute value of the output reactive current to decrease, or controls the rate of change of the AC terminal output reactive current to decrease. The target output signal is the output signal with the largest amplitude among the multiple output signals. Specifically, when a low-voltage or high-voltage fault occurs in the power grid, taking the acquisition of multiple positive-sequence voltages from the AC terminal within a set time interval by the power converter as an example, see [reference needed]. Figure 4 , Figure 4 This is another waveform diagram of the output voltage of the power converter provided in this application. For example... Figure 4As shown, the power converter detects the positive-sequence voltage Up at the AC terminal output. When Up is lower than the low-voltage threshold UL, the power converter determines that a low-voltage fault has occurred in the current power grid. The power converter controls the AC terminal to output reactive current to the power grid, and the absolute value of the reactive current changes with the negative-sequence or positive-sequence voltage at the AC terminal output. When the power converter's grid connection environment is a strong power grid, the AC terminal output of the power converter will not be disturbed by the output reactive current, that is, the AC terminal output remains stable. After time T0, the power converter's grid connection environment is a weak power grid. When the positive-sequence voltage is lower than the low-voltage threshold UL, the power converter controls the AC terminal to output reactive current to the power grid and acquires multiple positive-sequence voltages at the AC terminal within a set time interval, for example, acquiring n positive-sequence voltages from U1 to Un in the time interval T1 to T2. Among the above multiple positive-sequence voltages, the positive-sequence voltage with the largest amplitude is Ux. When the difference between the amplitude of Ux and the average value of the multiple positive-sequence voltages is greater than the first threshold, it is determined that AC terminal output oscillation has occurred. Furthermore, the absolute value of the reactive current output by the power converter decreases, and the magnitude of this decrease is positively correlated with the aforementioned difference; that is, the larger the difference, the stronger the oscillation intensity and the greater the decrease; conversely, the smaller the difference, the weaker the oscillation intensity and the lower the decrease. For example, if the difference between the amplitude of Ux and the average value of multiple positive-sequence voltages is S1, then the total reduction in reactive current is C1*S1, where C1 is a positive number. Here, the power converter can also control the absolute value of the output reactive current by reducing the positive-sequence reactive compensation factor, reducing the negative-sequence reactive compensation factor, or other methods. The specific method can be determined based on the actual application scenario and is not limited here. During voltage faults, the power converter adjusts the absolute value of the output reactive current according to the oscillation intensity of the output positive-sequence voltage, gradually reducing the absolute value of the reactive current during oscillation to smooth the AC terminal oscillation. This prevents continuous oscillation at the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control, thus improving the stability of the power converter's operation.

[0034] In some feasible implementations, the power converter is used to acquire multiple output signals from the AC terminal within a set time interval when a low-voltage or high-voltage fault occurs in the power grid. When the standard deviation of these multiple output signals exceeds a second threshold, it determines that AC terminal output oscillation has occurred and controls the absolute value of the output reactive current to decrease, or controls the rate of change of the AC terminal output reactive current to decrease. Specifically, taking the acquisition of multiple positive-sequence voltages from the AC terminal within a set time interval as an example when a low-voltage or high-voltage fault occurs in the power grid, see again... Figure 4When the positive-sequence voltage is lower than the low-voltage threshold UL, the power converter determines that a low-voltage fault has occurred in the current power grid. The power converter controls the AC terminal to output reactive current to the power grid, and the absolute value of the reactive current changes with the negative-sequence or positive-sequence voltage output at the AC terminal. When the grid connection environment of the power converter is a weak power grid, and the positive-sequence voltage is lower than the low-voltage threshold UL, the power converter controls the AC terminal to output reactive current to the power grid and acquires multiple positive-sequence voltages at the AC terminal within a set time interval, for example, acquiring n positive-sequence voltages from U1 to Un in the time interval T1 to T2. When the standard deviation of the amplitude of the above n positive-sequence voltages is greater than a second threshold, it is determined that AC terminal output oscillation has occurred. Furthermore, the absolute value of the reactive current output by the power converter decreases, and the magnitude of the decrease is positively correlated with the above standard deviation; that is, the larger the standard deviation, the stronger the oscillation intensity and the greater the decrease; the smaller the standard deviation, the weaker the oscillation intensity and the lower the decrease. For example, if the standard deviation of the amplitude of Ux and the average of multiple positive-sequence voltages is S2, then the total reduction in reactive current is C2*S2, where C2 is a positive number. Here, the power converter can also control the reduction of the absolute value of the output reactive current by reducing the positive-sequence reactive compensation factor, reducing the negative-sequence reactive compensation factor, or other methods. The specific method can be determined according to the actual application scenario and is not limited here. During voltage faults, the power converter adjusts the absolute value of the output reactive current according to the oscillation intensity of the output positive-sequence voltage, gradually reducing the absolute value of the reactive current during oscillation to smooth the AC terminal oscillation. This prevents continuous oscillation of the AC terminal output during fault ride-through in weak grid environments, which could interfere with reactive current control, thus improving the stability of the power converter's operation.

[0035] See Figure 5 , Figure 5 This is a waveform diagram of the output current of the power converter provided in this application. Figure 5As shown, when a low-voltage fault occurs, the power converter controls the AC terminal to output reactive current to the grid, and the absolute value of the output reactive current is Iq0. When the grid environment of the power converter is a strong grid, the AC terminal output of the power converter will not be disturbed by the output reactive current, that is, the AC terminal output remains stable. When the grid environment of the power converter is a strong grid, the AC terminal output of the power converter will not be disturbed by the output reactive current, that is, the AC terminal output remains stable, and the absolute value of the reactive current remains Iq0. After time T0, the grid environment of the power converter becomes a weak grid, and a low-voltage fault occurs. The power converter controls the AC terminal to output reactive current to the grid. However, under weak grid conditions, the reactive current output by the power converter will cause its AC terminal output to oscillate. Therefore, during the low-voltage fault, the positive sequence voltage will oscillate, and the oscillating positive sequence voltage will also interfere with the control of the aforementioned reactive current, causing the absolute value of the reactive current to oscillate. The power converter adjusts the absolute value of the output reactive current according to the oscillation intensity of the positive sequence voltage, so that the absolute value of the reactive current gradually decreases to Iq1 during oscillation. The reduction of the absolute value of the reactive current can smooth the AC terminal oscillation and prevent the AC terminal output from continuously oscillating during fault ride in a weak grid environment, which would interfere with the reactive current control, thus improving the stability of the power converter operation.

[0036] In some feasible implementations, the power converter is used to control the rate of change of the reactive current output at the AC terminal to decrease when a low-voltage or high-voltage fault occurs in the power grid and the output signal at the AC terminal oscillates, so that the absolute value of the reactive current output at the AC terminal does not deviate too much from its value before the oscillation. For example, voltage components with frequencies higher than the grid voltage frequency in the AC voltage output at the AC terminal are filtered out, and the absolute value of the reactive current output at the AC terminal is controlled to change with the AC voltage after the voltage components are filtered out. Specifically, the aforementioned filtering stage can be a first-order low-pass filter, with a transfer function of H(s) = 1 / (Ts+1) and a corresponding filtering formula of Y(n) = aX(n) + (1-a)Y(n-1), where X(n) and Y(n) are the input and output signals, respectively, and a is the filter coefficient, which can be expressed as a = deltaT / (T+deltaT), where T is the filtering time constant. When T is 0, a is 1, indicating a more sensitive filtering system. When T is much greater than deltaT, the value of a approaches 0, indicating a more stable system. The filtering time constant T can be expressed as T = 1 / ωc, where ωc is the cutoff frequency, which filters out high-frequency components in the input signal with frequencies higher than the cutoff frequency. Here, we take the detection of the positive sequence voltage at the AC output of a power converter as an example. See [link to relevant documentation]. Figure 6 , Figure 6 This is another waveform diagram of the output voltage of the power converter provided in this application. For example... Figure 6As shown, the power converter detects the positive-sequence voltage Up at the AC terminal output. When Up is lower than the low-voltage threshold UL, the power converter determines that a low-voltage fault has occurred in the current power grid. The power converter then controls the AC terminal to output reactive current to the grid, and the absolute value of the reactive current changes with the positive-sequence voltage at the AC terminal output. When the power converter is connected to a strong grid, the AC terminal output is not affected by the output reactive current, meaning the AC terminal output remains stable. When the power converter is connected to a weak grid, and Up is lower than the low-voltage threshold UL, the power converter controls the AC terminal to output reactive current to the grid. However, in a weak grid environment, the reactive current output by the power converter causes the positive-sequence voltage at its AC terminal output to oscillate. When the power converter detects oscillation at the AC terminal output, it can obtain the time interval Ta between the two positive-sequence voltages with the largest amplitudes in the output positive-sequence voltage, and based on the time interval Ta, determine that the frequency range of the high-frequency component in the output positive-sequence voltage is approximately 1 / Ta. Furthermore, the cutoff frequency is set to be lower than 1 / Ta but higher than the grid voltage frequency (e.g., 50Hz). A first-order low-pass filter removes voltage components in the positive-sequence voltage with frequencies higher than the cutoff frequency, thus preventing the output positive-sequence voltage from oscillating. Optionally, during the oscillation of the positive-sequence voltage output from the power converter, the frequency of the high-frequency components in the positive-sequence voltage is obtained by acquiring the frequency spectrum, i.e., the correspondence between frequency and amplitude. See [link to relevant documentation]. Figure 7 , Figure 7 This is a schematic diagram of the spectrum of the output signal provided in this application. For example... Figure 7 As shown, Figure 7 This includes the relationship between frequency and corresponding positive-sequence voltage amplitude. Among multiple voltage component amplitudes, if the grid voltage frequency is 50Hz, then the frequency corresponding to the voltage amplitude with a frequency exceeding 50Hz is f0. Further, the cutoff frequency is set to be lower than f0 but greater than 50Hz. A first-order low-pass filter removes voltage components in the positive-sequence voltage with frequencies higher than the cutoff frequency, thus reducing the oscillation amplitude of the output positive-sequence voltage. The absolute value of the reactive current output at the AC terminal of the power converter changes with the positive-sequence voltage after filtering out voltage components. This reduces the rate of change of the absolute value of the reactive current output at the AC terminal, ensuring that the absolute value of the reactive current output at the AC terminal does not deviate too much from its value before oscillation. This prevents continuous oscillation of the AC terminal output during fault ride-through in weak grid environments, which could interfere with reactive current control, and improves the stability of the power converter operation.

[0037] In some feasible implementations, the power converter is used to control the rate of change of the reactive current output at the AC terminal to decrease when a low-voltage or high-voltage fault occurs in the power grid and the output signal at the AC terminal oscillates, so that the absolute value of the reactive current output at the AC terminal does not deviate too much from its value before the oscillation. For example, it can directly filter out the current component in the reactive current output at the AC terminal whose frequency is higher than the cutoff frequency. Specifically, the above filtering stage can be a first-order low-pass filter, with the filter transfer function being H(s) = 1 / (Ts+1), and the corresponding filtering formula being Y(n) = aX(n) + (1-a)Y(n-1), where X(n) and Y(n) are the input and output signals, respectively, and a is the filter coefficient, which can be expressed as a = deltaT / (T+deltaT), where T is the filtering time constant. When T is 0, a is 1, and the filtering system is more sensitive. When T is much greater than deltaT, the value of a is close to 0, and the system is more stable. The filtering time constant T can be expressed as T = 1 / ωc, where ωc is the cutoff frequency, which can filter out high-frequency components in the input signal with frequencies greater than the cutoff frequency. Here, the power converter can detect the reactive current output at the AC terminal. When AC terminal output oscillation is detected, the time interval Tb between the two reactive currents with the largest amplitudes in the output reactive current is obtained. Based on the time interval Tb, the frequency range of the high-frequency component in the positive sequence voltage of the output is found to be around 1 / Tb. Further, the cutoff frequency is set to be lower than 1 / Tb and higher than the grid voltage frequency (e.g., 50Hz). A first-order low-pass filter is used to filter out current components in the reactive current with frequencies higher than the cutoff frequency, so that the output reactive current no longer oscillates. Alternatively, during the reactive current oscillation period of the power converter output, the frequency spectrum of the reactive current, i.e., the correspondence between frequency and amplitude of the reactive current, is obtained to obtain the frequency f1 corresponding to the high-frequency component in the reactive current. The cutoff frequency is set to be lower than f0 and higher than 50Hz. The frequency spectrum of the reactive current can be referenced above. Figure 7 Description of the spectrum of positive sequence voltage. By directly filtering out current components with frequencies higher than the cutoff frequency in the reactive current output from the AC terminal, the power converter reduces the rate of change of the absolute value of the reactive current output from the AC terminal. This ensures that the absolute value of the reactive current output from the AC terminal does not deviate too much from its value before oscillation, preventing continuous oscillation of the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control, and improving the stability of the power converter operation.

[0038] See Figure 8 , Figure 8 This is another waveform diagram of the output current of the power converter provided in this application. For example... Figure 8As shown, when a low-voltage fault occurs, the power converter controls the AC terminal to output reactive current to the grid, and the absolute value of the output reactive current is Iq0. When the grid environment of the power converter is a strong grid, the AC terminal output of the power converter will not be disturbed by the output reactive current, that is, the AC terminal output remains stable. When the grid environment of the power converter is a strong grid, the AC terminal output of the power converter will not be disturbed by the output reactive current, that is, the AC terminal output remains stable, and the absolute value of the reactive current remains Iq0. After time T0, the grid environment of the power converter becomes a weak grid, and a low-voltage fault occurs. The power converter controls the AC terminal to output reactive current to the grid. However, under weak grid conditions, the reactive current output by the power converter will cause its AC terminal output to oscillate. Therefore, during the low-voltage fault, the positive sequence voltage will oscillate, and the oscillating positive sequence voltage will also interfere with the control of the aforementioned reactive current, causing the absolute value of the reactive current to oscillate. The power converter filters out voltage components in the AC output voltage with frequencies higher than the grid voltage frequency, controls the absolute value of the reactive current output at the AC terminal to change with the AC voltage after filtering out the voltage components, or directly filters out current components in the reactive current output at the AC terminal with frequencies higher than the cutoff frequency, thereby reducing the rate of change of the absolute value of the reactive current output at the AC terminal. This ensures that the absolute value of the reactive current output at the AC terminal does not deviate too much from Iq0, improving the stability of the power converter's operation.

[0039] See Figure 9 , Figure 9 This is a flowchart illustrating the control method for the power converter provided in this application. The control method for the power converter provided in this application is applicable to the above-mentioned... Figure 1 or Figure 2 The power converter in any of the power supply systems shown. For example... Figure 9 As shown, the control method for the power converter provided in this application includes the following steps:

[0040] S901 indicates that a low-voltage or high-voltage fault has occurred in the power grid.

[0041] S902 controls the absolute value of the reactive current output at the AC terminal to change with the AC voltage output at the AC terminal.

[0042] In some feasible implementations, the DC terminal of the power converter is used to connect to photovoltaic modules or energy storage units, and the AC terminal of the power converter is used to connect to the power grid or load. When a low-voltage or high-voltage fault is detected in the power grid, the power converter's AC terminal is controlled to output reactive current to the power grid so that the power converter remains connected to the grid during the fault. For example, the presence of a low-voltage or high-voltage fault in the power grid is determined by detecting the three-phase phase voltage, three-phase line voltage, or positive-sequence voltage output from the power converter's AC terminal. Taking the detection of the three-phase phase voltage output from the AC terminal as an example, a low-voltage fault is determined when the effective value or the minimum peak-to-peak value of the three-phase phase voltage is lower than a low-voltage threshold, and a high-voltage fault is determined when the effective value or the maximum peak-to-peak value of the three-phase phase voltage is higher than a high-voltage threshold, where the high-voltage threshold is greater than the low-voltage threshold. Then, the absolute value of the reactive current output from the AC terminal is controlled to change with the AC voltage output from the AC terminal, which can be either a negative-sequence voltage or a positive-sequence voltage output from the AC terminal. For example, if a low-voltage or high-voltage fault occurs in the power grid, and it is an asymmetrical fault, the absolute value of the reactive current output at the control terminal changes with the positive-sequence voltage output at the AC terminal. Assuming the positive-sequence voltage before the fault is 1, the reactive current output at the AC terminal can be expressed as (1-Up)K, where Up is the positive-sequence voltage and K is a positive number. If a low-voltage fault occurs, the value of Up is less than 1, and the absolute value of the reactive current changes negatively with the positive-sequence voltage; if a high-voltage fault occurs, the value of Up is greater than 1, and the absolute value of the reactive current changes positively with the positive-sequence voltage. Optionally, the absolute value of the reactive current output at the control terminal can also change with the negative-sequence voltage output at the AC terminal. Assuming the negative-sequence voltage before the fault is 0, the reactive current output at the AC terminal can be expressed as (0-Un)K, where Un is the negative-sequence voltage and K is a positive number. If a low-voltage fault occurs in the power grid, the value of Un is less than 0, and the absolute value of the control reactive current changes negatively with the negative sequence voltage; if a high-voltage fault occurs in the power grid, the value of Un is greater than 0, and the absolute value of the control reactive current changes positively with the negative sequence voltage.

[0043] S903, whether the output signal of the AC terminal oscillates. If the judgment result is yes, then proceed to step S904; if the judgment result is no, then proceed to step S905.

[0044] S904 controls the absolute value of the reactive current output from the AC terminal to decrease, or controls the rate of change of the reactive current output from the AC terminal to decrease.

[0045] S905, fault crossing complete.

[0046] In some feasible implementations, when the power converter is connected to a weak grid, the reactive current output by the power converter can cause oscillations at its AC terminal output. For example, the voltage, current, or power output at the AC terminal may oscillate, thus interfering with reactive current control. When a low-voltage or high-voltage fault occurs in the grid, the output signals (voltage, current, or power) at the AC terminal are detected. If the output signal oscillates, it is determined that AC terminal output oscillation has occurred. The aforementioned AC terminal output voltage can be a negative-sequence voltage or a positive-sequence voltage, the effective value or peak-to-peak value of the three-phase line voltage, or the effective value or peak-to-peak value of the three-phase phase voltage, etc. The specific value can be determined according to the actual application scenario and is not limited here.

[0047] In some feasible implementations, when a low-voltage or high-voltage fault occurs in the power grid, multiple output signals from the AC terminal of the power converter are acquired at set time intervals. If the difference between the amplitude of the target output signal and the average value of the multiple output signals exceeds a first threshold, it is determined that AC terminal output oscillation has occurred, and the absolute value of the reactive current output by the power converter is controlled to decrease. The target output signal is the output signal with the largest amplitude among the multiple output signals. Further, the absolute value of the reactive current output by the power converter is controlled to decrease, and the magnitude of the decrease is positively correlated with the aforementioned difference; that is, the larger the difference, the stronger the oscillation intensity and the higher the magnitude of the decrease; the smaller the difference, the weaker the oscillation intensity and the lower the magnitude of the decrease. Here, the absolute value of the output reactive current can also be controlled to decrease by reducing the positive-sequence reactive power compensation factor, reducing the negative-sequence reactive power compensation factor, or other methods. The specific method can be determined according to the actual application scenario and is not limited here. During voltage faults, the absolute value of the output reactive current is adjusted according to the oscillation intensity of the positive sequence voltage. This gradually reduces the absolute value of the reactive current during oscillation to smooth the AC terminal oscillation, preventing continuous oscillation of the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control and improving the stability of the power converter operation.

[0048] In some feasible implementations, when a low-voltage or high-voltage fault occurs in the power grid, multiple output signals from the AC terminal are acquired at a set time interval. If the standard deviation of these multiple output signals is greater than a second threshold, it is determined that AC terminal output oscillation has occurred, and the absolute value of the output reactive current is controlled to decrease. Furthermore, the absolute value of the reactive current output by the power converter is controlled to decrease, and the magnitude of the decrease is positively correlated with the aforementioned standard deviation; that is, the larger the standard deviation, the stronger the oscillation intensity and the greater the decrease; the smaller the standard deviation, the weaker the oscillation intensity and the lower the decrease. Here, the absolute value of the output reactive current can also be controlled by reducing the positive-sequence reactive power compensation factor, reducing the negative-sequence reactive power compensation factor, or other methods. The specific method can be determined according to the actual application scenario and is not limited here. During voltage faults, the absolute value of the output reactive current is adjusted according to the oscillation intensity of the positive-sequence voltage, so that the absolute value of the reactive current gradually decreases during oscillation to smooth AC terminal oscillation, preventing continuous AC terminal output oscillation during fault ride-through in weak grid environments from interfering with reactive current control, thus improving the stability of the power converter operation.

[0049] In some feasible implementations, when a low-voltage or high-voltage fault occurs in the power grid and the output signal at the AC terminal oscillates, the rate of change of the reactive current output at the AC terminal of the control power converter is reduced, ensuring that the absolute value of the reactive current output at the AC terminal does not deviate excessively from its value before the oscillation. For example, voltage components with frequencies higher than the grid voltage frequency in the AC voltage output at the AC terminal of the control power converter are filtered out, and the absolute value of the reactive current output at the control power converter changes with the AC voltage after the voltage components are filtered out. By controlling the absolute value of the reactive current output at the AC terminal of the control power converter to change with the positive-sequence voltage after the voltage components are filtered out, the rate of change of the absolute value of the reactive current output at the AC terminal is reduced, ensuring that the absolute value of the reactive current output at the AC terminal does not deviate excessively from its value before the oscillation. This prevents continuous oscillation of the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control, thereby improving the stability of the power converter operation.

[0050] In some feasible implementations, when a low-voltage or high-voltage fault occurs in the power grid and the output signal at the AC terminal oscillates, the rate of change of the reactive current output at the AC terminal of the control power converter is reduced, ensuring that the absolute value of the reactive current output at the AC terminal does not deviate excessively from its value before the oscillation. For example, current components with frequencies higher than the cutoff frequency in the reactive current output at the AC terminal can be directly filtered out. By directly filtering out these components, the rate of change of the absolute value of the reactive current output at the AC terminal is reduced, ensuring that its absolute value does not deviate excessively from its value before the oscillation. This prevents continuous oscillation of the AC terminal output during fault ride-through in weak grid environments from interfering with reactive current control, thus improving the stability of the power converter operation.

Claims

1. A power converter, characterized by, The direct current end of the power converter is used to connect a direct current power supply, and the alternating current end of the power converter is used to connect a power grid or a load, and the power converter is used to convert direct current input by the direct current power supply into alternating current and output to the power grid or the load; The power converter is further used to control the absolute value of the reactive current output by the alternating current end to change with the alternating current voltage output by the alternating current end when a low voltage fault or a high voltage fault occurs in the power grid; The power converter is further used to control the absolute value of the reactive current output by the alternating current end to decrease when the difference between the amplitude of a target output signal and the average value of a plurality of output signals output by the alternating current end within a set time interval is greater than a first threshold value, the target output signal being an output signal with the largest amplitude among the plurality of output signals; Alternatively, the power converter is further used to control the change speed of the reactive current output by the alternating current end to decrease when the difference between the amplitude of a target output signal and the average value of a plurality of output signals output by the alternating current end within a set time interval is greater than a first threshold value, the target output signal being an output signal with the largest amplitude among the plurality of output signals; Alternatively, the power converter is further used to control the absolute value of the reactive current output by the alternating current end to decrease when the standard deviation of a plurality of output signals output by the alternating current end within a set time interval is greater than a second threshold value; Alternatively, the power converter is further used to control the change speed of the reactive current output by the alternating current end to decrease when the standard deviation of a plurality of output signals output by the alternating current end within a set time interval is greater than a second threshold value.

2. The power converter of claim 1, wherein, The power converter is further used to control the absolute value of the reactive current output by the alternating current end to change with the alternating current voltage output by the alternating current end when a low voltage fault or a high voltage fault occurs in the power grid, and the method comprises: controlling the absolute value of the reactive current output by the alternating current end to change with the alternating current voltage output by the alternating current end when the minimum value of the effective value or the peak-to-peak value of the three-phase phase voltage output by the alternating current end is lower than a low voltage threshold value, or when the minimum value of the effective value or the peak-to-peak value of the three-phase line voltage output by the alternating current end is lower than the low voltage threshold value, or when the positive sequence voltage output by the alternating current end is lower than the low voltage threshold value; controlling the absolute value of the reactive current output by the alternating current end to change with the alternating current voltage output by the alternating current end when the maximum value of the effective value or the peak-to-peak value of the three-phase phase voltage output by the alternating current end is higher than a high voltage threshold value, or when the maximum value of the effective value or the peak-to-peak value of the three-phase line voltage output by the alternating current end is higher than the high voltage threshold value, or when the positive sequence voltage output by the alternating current end is higher than the high voltage threshold value, the high voltage threshold value being greater than the low voltage threshold value.

3. The power converter of claim 1 or 2, characterized in that, The power converter is further used to control the absolute value of the reactive current output by the alternating current end to change with the alternating current voltage output by the alternating current end, and the method comprises: In the event of a low voltage fault of the power grid, the absolute value of the reactive current output by the AC terminal is controlled to change in negative correlation with the positive sequence voltage or the negative sequence voltage output by the AC terminal. In the event of a high voltage fault of the power grid, the absolute value of the reactive current output by the AC terminal is controlled to change in positive correlation with the positive sequence voltage or the negative sequence voltage output by the AC terminal.

4. A power converter according to any one of claims 1-3, characterized in that, The power converter is further configured to control the absolute value of the reactive current output by the AC terminal to decrease, and the decreasing amplitude is positively correlated with the size of the difference value or the size of the standard deviation.

5. The power converter of any of claims 1-3, wherein, The power converter is further configured to control the change speed of the reactive current output by the AC terminal to decrease, including: filtering out a current component with a frequency greater than the frequency of the grid voltage from the reactive current output by the AC terminal.

6. The power converter of any of claims 1-3, wherein, The power converter is further configured to control the change speed of the reactive current output by the AC terminal to decrease, including: filtering out a voltage component with a frequency greater than the frequency of the grid voltage from the AC voltage, and controlling the absolute value of the reactive current output by the AC terminal to change with the change of the AC voltage after filtering out the voltage component.

7. A control method of a power converter, characterized by, The method comprises: In the event of a low voltage fault or a high voltage fault of the power grid, controlling the absolute value of the reactive current output by the AC terminal of the power converter to change with the change of the AC voltage output by the AC terminal; controlling the absolute value of the reactive current output by the AC terminal to decrease when the difference between the amplitude of a target output signal and the average value of a plurality of output signals output by the AC terminal within a set time interval is greater than a first threshold value, the target output signal being the output signal with the largest amplitude among the plurality of output signals; controlling the change speed of the reactive current output by the AC terminal to decrease when the difference between the amplitude of a target output signal and the average value of a plurality of output signals output by the AC terminal within a set time interval is greater than a first threshold value, the target output signal being the output signal with the largest amplitude among the plurality of output signals; controlling the absolute value of the reactive current output by the AC terminal to decrease when the standard deviation of a plurality of output signals output by the AC terminal within a set time interval is greater than a second threshold value; controlling the change speed of the reactive current output by the AC terminal to decrease when the standard deviation of a plurality of output signals output by the AC terminal within a set time interval is greater than a second threshold value; The DC terminal of the power converter is configured to be connected to a DC power source, the AC terminal of the power converter is configured to be connected to a power grid or a load, and the power converter is configured to convert DC power into AC power and output the AC power to the power grid or the load.

8. The method of claim 7, wherein, The controlling of the absolute value of the reactive current output by the AC terminal to change with the change of the AC voltage output by the AC terminal in the event of a low voltage fault or a high voltage fault of the power grid comprises: controlling the absolute value of the reactive current output by the AC terminal to vary with the variation of the AC voltage output by the AC terminal when the minimum of the effective value or the peak-to-peak value of the three-phase phase voltage output by the AC terminal is lower than a low-voltage threshold, or when the minimum of the effective value or the peak-to-peak value of the three-phase line voltage output by the AC terminal is lower than the low-voltage threshold, or when the positive-sequence voltage output by the AC terminal is lower than the low-voltage threshold; controlling the absolute value of the reactive current output by the AC terminal to vary with the variation of the AC voltage output by the AC terminal when the maximum of the effective value or the peak-to-peak value of the three-phase phase voltage output by the AC terminal is higher than a high-voltage threshold, or when the maximum of the effective value or the peak-to-peak value of the three-phase line voltage output by the AC terminal is higher than the high-voltage threshold, or when the positive-sequence voltage output by the AC terminal is higher than the high-voltage threshold.

9. The method according to claim 7 or 8, characterized in that, the controlling the absolute value of the reactive current output by the AC terminal to vary with the variation of the AC voltage output by the AC terminal comprises: controlling the absolute value of the reactive current to vary negatively with the positive-sequence voltage or the negative-sequence voltage output by the AC terminal when a low-voltage fault occurs in the power grid, and to vary positively with the positive-sequence voltage or the negative-sequence voltage output by the AC terminal when a high-voltage fault occurs in the power grid.

10. The method according to any one of claims 7-9, characterized in that, the controlling the absolute value of the reactive current output by the AC terminal to decrease comprises: controlling the absolute value of the reactive current output by the AC terminal to decrease, and the decreasing amplitude to vary positively with the size of the difference value or the size of the standard deviation.

11. The method according to any one of claims 7-9, characterized in that, the controlling the variation speed of the reactive current output by the AC terminal to decrease comprises: filtering out the current component with a frequency greater than the frequency of the power grid voltage from the reactive current output by the AC terminal.

12. The method according to any one of claims 7-9, characterized in that, the controlling the variation speed of the reactive current output by the AC terminal to decrease comprises: filtering out the voltage component with a frequency greater than the frequency of the power grid voltage from the AC voltage, and controlling the absolute value of the reactive current output by the AC terminal to vary with the variation of the AC voltage after filtering out the voltage component.

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