Grid-forming converter and active reference control method for fault ride-through thereof

Abstract

The application provides a grid-forming converter and an active reference control method for fault ride-through of the grid-forming converter. The method comprises: obtaining a voltage drop degree of a power grid and a virtual inductance state quantity of the grid-forming converter under the action of current limiting; in a fault duration stage, determining an adaptive adjustment coefficient according to the voltage drop degree, and reconstructing the active reference based on the adaptive adjustment coefficient and the virtual inductance state quantity, so that the reconstructed active reference is dynamically adjusted with the power angle change of the grid-forming converter; after the fault is cleared, decoupling the exit of the virtual inductance from the recovery of the active reference, so that the active reference is continuously recovered from the value in the fault duration stage to the given value of the active reference before the fault. The grid-forming converter can maintain transient synchronous stability in the fault duration stage, and the active reference can be smoothly recovered from the value in the fault duration stage to the given value before the fault after the fault is cleared, so as to reduce the secondary active impact after the fault is cleared.

Description

Technical Field

[0001] This application relates to the field of converter technology, specifically to a grid-type converter and its active power reference control method for fault ride-through. Background Technology

[0002] With the continuous increase in the grid connection rate of new energy sources such as wind power, photovoltaics, and energy storage, the power system is gradually shifting from being dominated by synchronous generators to developing towards a high proportion of power electronics. Grid-type converters can actively establish and support the voltage and frequency of the power grid in the form of voltage sources, and are used in new energy power plants, energy storage systems, and microgrids.

[0003] When a short-circuit fault or deep voltage drop occurs in the power grid, the grid-connected converter enters fault ride-through control mode. To prevent damage to power devices, the grid-connected converter typically limits its output current during fault ride-through, which alters the equivalent output impedance and the characteristics of the actual output active power as a function of the power angle during the fault period.

[0004] During the fault duration, the active power reference of the grid-type converter may not match the characteristics of the actual output active power during the fault period as the power angle changes, resulting in insufficient transient synchronization stability margin of the grid-type converter during the fault period and the risk of transient loss of synchronization.

[0005] After the fault is cleared, the grid-type converter switches from the fault operation state back to the normal operation state. The value of the active power reference changes accordingly. During the recovery process, the grid-type converter is prone to active power surges, frequency deviations and power angle oscillations, which affect its operational stability after the fault ride-through.

[0006] Therefore, how to maintain transient synchronous stability of grid-type converters during the fault duration phase, and how to achieve smooth recovery of active power after the fault is cleared, are technical problems that urgently need to be solved. Summary of the Invention

[0007] In view of this, the present application provides a grid-type converter and its active power reference control method for fault ride-through, which enables the grid-type converter to maintain transient synchronous stability during the fault duration phase and to smoothly restore the active power reference from the value during the fault duration phase to the given value before the fault after the fault is cleared, so as to reduce the secondary active power impact after the fault is cleared.

[0008] The first aspect of this application provides an active power reference control method for fault ride-through of a grid-type converter, comprising: S1. Obtain the voltage drop of the power grid and the virtual inductance state quantity formed by the grid-type converter under current limiting; S2. During the fault duration phase, an adaptive adjustment coefficient is determined based on the voltage drop level, and the active power reference is reconstructed based on the adaptive adjustment coefficient and the virtual inductance state quantity, so that the reconstructed active power reference is dynamically adjusted according to the power angle change of the grid-type converter. S3. After the fault is cleared, the exit of the virtual inductor is decoupled from the recovery of the active power reference, so that the active power reference is continuously restored from the value taken during the fault duration to the active power reference setpoint before the fault.

[0009] In one embodiment, prior to obtaining the voltage drop level of the power grid, the method further includes: The voltage amplitude of the power grid and the output current amplitude of the grid-connected converter are detected. ; When the voltage amplitude of the power grid is detected to be lower than a preset voltage threshold. or the output current amplitude Exceeding the preset rate limit threshold When at least one of the following conditions is met, the grid-type converter is determined to enter fault ride-through control; Otherwise, the grid-type converter maintains normal grid control, and the virtual inductance state quantity corresponding to the current limiting effect is zero.

[0010] In one embodiment, the active power reference during the fault persistence phase is... Reconstructed as: in, Characterizing the degree of voltage drop, The active power reference setpoint before the fault. The power angle of the grid-type converter is given. This refers to the virtual inductance state quantity that varies with the power angle during the fault persistence phase. This is a reference coefficient that is adaptively adjusted according to the degree of voltage drop. For stability margin coefficient, The adaptive adjustment coefficient is constituted; within the power angle range Within, the virtual inductance state quantity As the work angle monotonically increases, the reconstructed active reference... It exhibits a flexible downward tilting characteristic as the work angle decreases monotonically.

[0011] In one embodiment, the benchmark coefficient Take the first lower bound corresponding to the transient stability constraint. The second lower bound corresponding to the reactive current injection constraint The larger value in: First lower bound To: make the stability margin coefficient take The lower bound of the reference coefficient corresponding to the intersection of the baseline active power reference curve at the peak and the power angle curve during the fault duration phase, and satisfying the following: in, This represents the maximum value of the power angle curve during the fault duration phase. The peak power angle corresponding to the maximum value. The virtual inductance state quantity at the peak power angle; the second lower bound To ensure that the reactive current at the stable equilibrium point during the fault duration reaches the lower bound of the reference coefficient corresponding to the lower limit of reactive current; the stability margin coefficient The values ​​of satisfy: in, This is the virtual inductance state quantity when the power angle is zero.

[0012] In one embodiment, decoupling the virtual inductor from the active power reference includes: When the voltage amplitude of the power grid recovers to above a preset voltage threshold and the output current amplitude of the grid-connected converter falls back to within a preset current limiting threshold, the fault is determined to be cleared, and the time of fault clearing is recorded. Virtual inductance state quantity ; Delay hold time after fault clearing Within, the virtual inductance is maintained as ; After the delay period ends, the virtual inductor is removed from the output impedance channel, and an auxiliary recovery variable is introduced. The auxiliary recovery variable Decreasing according to a first-order exponential law, and with an initial value at the end of the delay hold time. : And the active power reference during the fault recovery phase Determined as: in, This is the flexible recovery time constant.

[0013] In one embodiment, the rate of change of the active power reference with respect to time during the fault recovery phase is: The rate of change reaches its maximum value at the end of the delay hold time. To ensure that the rate of change does not exceed the maximum permissible recovery slope of the active power reference. The flexible recovery time constant satisfy: The delay holding time The peak time of the first power angle swing and the main adjustment time during the initial recovery of the grid-type converter are determined based on the grid-type converter.

[0014] A second aspect of this application provides a grid-type converter, including a power circuit and a controller; the power circuit includes a converter bridge, a filter inductor, and a filter capacitor connected in sequence, and is connected to the power grid via a line inductor; the controller includes a power calculation unit, a power synchronization control unit, a reactive voltage control unit, a three-phase voltage synthesis unit, a virtual inductor unit, a voltage and current dual closed-loop control unit, a modulation unit, a current limiting unit, an active power reference reconstruction unit, and an active power reference flexible recovery unit; The input terminal of the power calculation unit is connected to the voltage sampling terminal of the filter capacitor and the output current sampling terminal of the grid converter, respectively, and the output terminal outputs active power and reactive power, respectively. The input terminal of the power synchronization control unit is connected to the active power output terminal and the active power reference input terminal of the power calculation unit, respectively, and the output terminal outputs the phase angle; the input terminal of the reactive voltage control unit is connected to the reactive power output terminal of the power calculation unit, and the output terminal outputs the voltage amplitude. The input terminals of the three-phase voltage synthesis unit are respectively connected to the phase angle and the voltage amplitude, and its output terminal is connected to the voltage and current dual closed-loop control unit via the virtual inductor unit. The voltage and current dual closed-loop control unit is connected to the converter bridge via the modulation unit. The input terminal of the current limiting unit receives the deviation between the amplitude of the limiting current and the amplitude of the output current, and its output terminal outputs a virtual inductor state quantity. The virtual inductor state quantity is connected to the controlled terminal of the virtual inductor unit and the input terminal of the active reference reconfiguration unit, respectively. The input terminal of the active power reference reconstruction unit is connected to the output terminal of the virtual inductance state quantity of the current limiting unit, and is used to output the active power reference during the fault duration based on the virtual inductance state quantity. The input of the active power reference flexible recovery unit is connected to the current limiting unit via an auxiliary recovery variable channel, and its output outputs the active power reference during the fault recovery phase. The active power reference during the fault duration phase and the active power reference during the fault recovery phase are selected and then connected to the active power reference input terminal of the power synchronization control unit.

[0015] In one embodiment, the current limiting unit includes a subtractor and a PI regulator connected in sequence; the two input terminals of the subtractor are respectively connected to the amplitude of the limiting current and the amplitude of the output current, and the output terminal of the PI regulator outputs the virtual inductor state quantity and is respectively connected to the controlled terminal of the virtual inductor unit and the input terminal of the active power reference reconfiguration unit.

[0016] In one embodiment, the active power reference reconfiguration unit includes a multiplier and a subtractor; the input of the multiplier receives an adjustment coefficient adjusted according to the degree of voltage drop in the grid and the virtual inductor state quantity, respectively, and its output outputs an adjustment quantity; the input of the subtractor is connected to the active power reference setpoint before the fault and the adjustment quantity, respectively, and its output outputs the active power reference during the fault duration.

[0017] In one embodiment, the active power reference flexible recovery unit includes a delay holding module and an auxiliary recovery variable generation module; the delay holding module is used to maintain the virtual inductor state variable at the value at the time of fault clearance during the delay holding period after fault clearance; the output terminal of the auxiliary recovery variable generation module is connected to the output terminal of the active power reference flexible recovery unit through the auxiliary recovery variable channel, and the auxiliary recovery variable channel is independent of the channel where the virtual inductor unit is located, so that the exit of the virtual inductor unit is decoupled from the recovery of the active power reference in the fault recovery stage.

[0018] The first aspect of this application provides a method for reconstructing the active power reference during the fault duration phase and flexibly restoring it after the fault is cleared. This enables grid-connected converters to balance transient synchronous stability during faults and smooth recovery after faults, even under grid voltage dips or short-circuit fault conditions. Building upon existing grid-connected converter current-limiting control, the active power reference during the fault period is dynamically adjusted based on the grid voltage dip depth and the virtual inductance state, allowing it to adapt to changes in the power angle during the fault, thus ensuring the existence of a stable equilibrium point and expanding the transient synchronous stability margin. To address potential active power surges and oscillations after fault clearing, a flexible active power reference recovery method is proposed. This method decouples the virtual inductance exit from the active power reference recovery, reducing the adverse effects of sudden active power reference changes on the recovery process. By combining the active power reference reconstruction during the fault duration phase and the active power reference recovery process after fault clearing, the grid-connected converter exhibits a clear active power reference adjustment pattern during both the fault duration and recovery phases, improving the coordination of the entire fault-crossing control process. By introducing auxiliary recovery variables, the active power reference can be flexibly restored to the pre-fault value after fault clearance, which can reduce the secondary impact of active power, frequency deviation and power angle oscillation, thereby improving the operational stability and reliability of grid-type converters after fault ride-through.

[0019] It is understandable that the beneficial effects of the second aspect mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a flowchart illustrating the active power reference control method for fault ride-through of a grid-type converter provided in an embodiment of the present invention. Figure 2 This is a schematic diagram of the fault ride-through control process for a grid-type converter provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the active power reference curves under different stability margin coefficients provided in the embodiments of the present invention; Figure 4 This is a comparative schematic diagram of the active power reference step recovery method and the flexible recovery method provided in the embodiments of the present invention; Figure 5 This is a general block diagram of the grid-type converter and its control provided in the embodiments of the present invention; Figure 6 This is a simulation waveform diagram of the fault duration stage under the condition of voltage drop factor of 0.7 using the traditional strategy; Figure 7 This is a simulation waveform diagram of the fault duration stage under the condition that the voltage drop factor is 0.7, provided by the embodiment of the present invention. Figure 8 is a comparison of the frequency deviation and active power recovery process under different fault recovery methods; Figure 8(a) is a schematic diagram of the virtual inductor immediately exiting and the active power reference step recovering; Figure 8(b) is a schematic diagram of the virtual inductor exiting with a delay and the active power reference still step recovering; and Figure 8(c) is a schematic diagram of the virtual inductor exiting with a delay and the active power reference flexibly recovering using the present invention. Detailed Implementation

[0022] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of this application. However, those skilled in the art will understand that this application may also be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods have been omitted so as not to obscure the description of this application with unnecessary detail.

[0023] It should be understood that, when used in this application specification and the appended claims, the term "comprising" indicates the presence of the described features, integrals, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or a collection thereof.

[0024] It should also be understood that the term “and / or” as used in this application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0025] As used in this application specification and the appended claims, the term "if" may be interpreted, depending on the context, as "when," "once," "in response to determination," or "in response to detection." Similarly, the phrase "if determined" or "if detected [the described condition or event]" may be interpreted, depending on the context, as meaning "once determined," "in response to determination," "once detected [the described condition or event]," or "in response to detection [the described condition or event]."

[0026] Furthermore, in the description of this application and the appended claims, the terms "first," "second," "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0027] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0028] The active power reference control method for fault ride-through of grid-connected converters provided in this invention is applicable to the transient synchronization stability maintenance and active power smooth recovery control of grid-connected converters under grid voltage dips or short-circuit fault conditions. It can be applied to grid-connected converters that actively establish and support grid voltage and frequency in the form of voltage sources in new energy power plants, energy storage systems, and microgrids, such as wind power, photovoltaic, and energy storage facilities. Specifically, it can be executed by the controller of the grid-connected converter when running a computer program with corresponding functions. When a short-circuit fault or deep voltage dip occurs in the grid, the grid-connected converter must enter the fault ride-through control state. To prevent device damage, a current limiting circuit is usually activated, thereby changing the converter's equivalent output impedance, current distribution relationship, and the balance characteristics of the power synchronization circuit, causing a significant change in the actual output active power curve of the converter during the fault. This invention improves upon existing fault current limiting control for grid-connected converters by reconstructing the active power reference during the fault duration phase and restoring the active power reference during the fault recovery phase.

[0029] like Figure 1 As shown, the active power reference control method for fault ride-through of a grid-type converter provided in this embodiment of the invention includes the following steps S1 to S3 executed by the controller of the grid-type converter: Step S1: Obtain the voltage drop of the power grid and the virtual inductance state quantity formed by the grid-type converter under current limiting.

[0030] In application, after entering fault ride-through control mode, the grid voltage amplitude and the virtual inductance state quantity formed by the current limiting circuit are obtained. Based on the voltage amplitude of the power grid during the duration of the power grid fault. With the rated voltage amplitude of the power grid Calculate voltage drop factor : in, The voltage drop coefficient Used to characterize the severity of the fault, i.e., the degree of voltage drop; at the same time, the virtual inductance state quantity is used as the input quantity for subsequent active power reference reconstruction and active power reference flexible recovery control.

[0031] In application, the virtual inductance state quantity This is formed by the existing virtual inductor current limiting control loop of the grid-type converter, specifically by limiting the current. The amplitude of the output current of the grid-type converter The deviation between them is adjusted by proportional-integral method to obtain the virtual inductance state quantity. and the virtual inductance state quantity The output impedance channel of the grid-connected converter is used to achieve current limiting; the output impedance channel refers to the signal channel that effectively increases the output impedance of the grid-connected converter by the virtual inductance acting on the three-phase reference voltage synthesis stage, and the virtual inductance state quantity... The larger the value, the greater the equivalent output impedance of the grid-type converter, and the stronger the restriction on the output current; the virtual inductance state quantity varies with the power angle. It changes with the changes, hence it is denoted as .

[0032] Step S2: During the fault persistence phase, an adaptive adjustment coefficient is determined based on the voltage drop level, and the active power reference is reconstructed based on the adaptive adjustment coefficient and the virtual inductance state quantity, so that the reconstructed active power reference is dynamically adjusted according to the power angle change of the grid-type converter.

[0033] In applications, during the fault persistence phase, based on the voltage drop coefficient... Introducing an adaptive adjustment coefficient And combined with the virtual inductance state quantity The active power reference during the fault is reconstructed so that the reconstructed active power reference is adjusted according to the change of power angle state, thereby ensuring the existence of a stable equilibrium point during the fault and improving the transient synchronization stability margin.

[0034] In application, the power angle The phase difference between the internal potential of the grid converter and the grid voltage is determined by the power synchronization control circuit of the grid converter. The control loop is obtained based on the deviation between the active power and the active power reference; the power synchronization control loop, while outputting the phase angle for three-phase voltage synthesis, also provides the power angle characterizing the operating state of the grid-type converter. The virtual inductor state quantities upon which the active power reference reconstruction is based That is, according to the angle of work The active power reference changes in real time, therefore the reconstructed active power reference can change with the stated power angle. It can be dynamically adjusted according to changes, without the need to introduce a separate power angle detection branch for the active power reference reconstruction.

[0035] Step S3: After the fault is cleared, the exit of the virtual inductor is decoupled from the recovery of the active power reference, so that the active power reference is continuously restored from the value taken during the fault duration to the active power reference setpoint before the fault.

[0036] In application, when the grid voltage amplitude is detected to have recovered to above a preset voltage threshold and the output current amplitude of the grid-connected converter has fallen back to within a preset current limiting threshold, the grid fault is determined to be cleared; after the fault is cleared, the virtual inductance state quantity at the time of fault clearing is recorded. and maintain for the preset delay time. The virtual inductor state is maintained unchanged during the delay period; after the delay period ends, the virtual inductor is removed from the output impedance path, and an auxiliary recovery variable is introduced. By decoupling the virtual inductor from the active power reference, the active power reference is flexibly restored to the active power reference setpoint before the fault, thereby reducing the secondary active power surge after the fault is cleared.

[0037] In one embodiment, prior to step S1, the method includes: detecting the voltage amplitude of the power grid and the output current amplitude of the grid-connected converter. When the voltage amplitude of the power grid is detected to be lower than a preset voltage threshold... or the output current amplitude Exceeding the preset rate limit threshold If at least one of the following conditions is met, the grid-type converter is determined to enter fault ride-through control; otherwise, the grid-type converter maintains normal grid control, and the virtual inductance state quantity corresponding to the current limiting effect is zero.

[0038] In applications, the magnitude of the grid voltage and the magnitude of the output current of the grid-connected converter are detected. When the grid voltage amplitude is lower than the preset voltage threshold ,or Exceeding the preset rate limit threshold When a short-circuit fault or deep voltage drop is detected in the power grid, the grid-type controller enters the fault ride-through control mode; otherwise, the grid-type converter maintains normal grid-type control mode operation. In this case, the virtual inductor link is not activated, and the virtual inductor state quantity... .

[0039] like Figure 2 As shown, the active power reference control method for fault ride-through of a grid-type converter provided in this embodiment of the invention includes the following steps S301 to S309 executed by the controller of the grid-type converter: Step S301: Measure and obtain the grid voltage amplitude. and the output current amplitude of the grid-type converter Proceed to step S302; Step S302: Determine whether a drop in grid voltage or an overcurrent is detected; if no, that is, the grid voltage amplitude is not lower than a preset voltage threshold and the output current amplitude does not exceed a preset current limiting threshold, then proceed to step S303; if yes, that is, the grid voltage amplitude is lower than a preset voltage threshold or the output current amplitude exceeds a preset current limiting threshold, then proceed to step S304. Step S303: The virtual inductor circuit is not activated, so that the virtual inductor state quantity is... The grid-type converter operates under conventional grid control, and the process returns to step S301; Step S304: During the fault persistence phase, obtain the operating status during the fault period and calculate the voltage drop coefficient. And obtain the virtual inductor state quantity Proceed to step S305; Step S305: Reconstruct the active power reference during the fault persistence phase, i.e. Proceed to step S306; Step S306: Determine whether the fault has been cleared; if not, return to step S304; if yes, proceed to step S307. Step S307: Start the delay timer and maintain the virtual inductance at the virtual inductance state at the fault clearing time. Proceed to step S308; Step S308: Determine whether the delay hold time has ended; if not, continue to hold the virtual inductance. If so, proceed to step S309; Step S309: Exit the virtual inductor state quantity And update the useful reference, that is .

[0040] Figure 2 The overall flow of the fault ride-through control of the grid-type converter is illustrated in the figure. Starting from the measurement and acquisition of the grid voltage amplitude and the output current amplitude, the circuit enters the normal operation branch or the fault duration branch after the fault ride-through judgment. In the normal operation branch, the virtual inductor link is not activated and the circuit operates according to the conventional grid control. In the fault duration branch, the operation status quantity is acquired, the active power reference is reconstructed, and the fault clearing judgment is executed in sequence. After the fault is cleared, the active power reference is flexibly restored by starting the delay timer, holding the virtual inductor, judging the end of the delay, exiting the virtual inductor status quantity, and updating the active power reference.

[0041] In application, steps S301 to S309 correspond to the four stages of fault ride-through control in this invention: judgment, acquisition, reconstruction, and recovery. In steps S301 to S302, the grid voltage amplitude and the output current amplitude of the grid-connected converter are detected. When the grid voltage amplitude is lower than the preset voltage threshold or the output current amplitude Exceeding the preset rate limit threshold When a short-circuit fault or deep voltage sag is detected in the power grid, the grid-connected converter enters the fault ride-through control mode; otherwise, it maintains normal grid-connected control mode operation. In step S304, the voltage sag coefficient is calculated based on the grid voltage amplitude and the rated voltage amplitude. To characterize the severity of the fault, and to convert the virtual inductance state quantity formed by the current limiting circuit into a quantity that represents the current. As input for subsequent active power reference reconfiguration and active power reference flexible recovery control; in step S305, based on the voltage sag coefficient Introducing an adaptive adjustment coefficient And combined with the virtual inductance state quantity The active power reference during the fault period is reconstructed so that the reconstructed active power reference adjusts according to the change in power angle state; in steps S307 to S309, the virtual inductance state quantity at the time of fault clearance is recorded. And maintain for the preset delay time The virtual inductance is maintained as The delay period remains unchanged. After the delay period ends, the virtual inductor is removed from the output impedance channel, and an auxiliary recovery variable is introduced. The virtual inductor is decoupled from the active power reference, allowing the active power reference to flexibly recover to the active power reference setpoint before the fault.

[0042] In one embodiment, step S2 includes: using the active power reference during the fault persistence phase. Reconstructed as: in, Characterizing the degree of voltage drop, The active power reference setpoint before the fault. The power angle of the grid-type converter is given. This refers to the virtual inductance state quantity that varies with the power angle during the fault persistence phase. This is a reference coefficient that is adaptively adjusted according to the degree of voltage drop. For stability margin coefficient, The adaptive adjustment coefficient is constituted; within the power angle range Within, the virtual inductance state quantity As the work angle monotonically increases, the reconstructed active reference... It exhibits a flexible downward tilting characteristic as the work angle decreases monotonically.

[0043] In applications, under purely inductive line conditions, the electrical output active power of a grid-type converter during a fault can be expressed as: in, This represents the amplitude of the converter capacitor voltage. It is the power frequency angular frequency. The line inductance is used; under current-limiting conditions, the output current amplitude is limited to [value missing]. Therefore: From the above formula, we can obtain the relationship between the virtual inductance and the power angle: Regarding the above formula Differentiation yields: Among them, Inside, And the denominator is always positive, therefore ,Right now Monotonically increasing; while and All are positive numbers, therefore Follow It decreases monotonically, forming a flexible downward tilting characteristic.

[0044] In one embodiment, the benchmark coefficient Take the first lower bound corresponding to the transient stability constraint. The second lower bound corresponding to the reactive current injection constraint The larger value in: First lower bound To: make the stability margin coefficient take The lower bound of the reference coefficient corresponding to the intersection of the baseline active power reference curve at the peak and the power angle curve during the fault duration phase, and satisfying the following: in, This represents the maximum value of the power angle curve during the fault duration phase. The peak power angle corresponding to the maximum value. The virtual inductance state quantity at the peak power angle; the second lower bound To ensure that the reactive current at the stable equilibrium point during the fault duration reaches the lower bound of the reference coefficient corresponding to the lower limit of reactive current; the stability margin coefficient The values ​​of satisfy: in, This is the virtual inductance state quantity when the power angle is zero.

[0045] In application, the benchmark coefficient It is determined jointly by transient stability constraints and reactive current injection constraints. Based on the transient stability constraints, the power angle curve during the fault duration phase is explicitly expressed as: Regarding the above formula Differentiate and set it equal to zero, in Under the condition of peak power angle for: Substituting it back, we can obtain the maximum value of the fault power angle curve: At this point And make the active power reference curve intersect the fault power angle curve at the peak point, that is: Therefore, the lower bound of the benchmark coefficients for determining the transient stability constraints can be obtained. Based on reactive current injection constraints, grid-type converters must meet the reactive current support requirements during low-voltage ride-through during fault periods. According to national standards, the lower limit of reactive current during fault periods can be expressed as: in, This is the rated current; due to Increased active power reference during faults reduces With the reduction of shaft active current, more can be released under the constraint of total current limiting. The shaft reactive current capability, therefore when The reactive current at the corresponding stable equilibrium point is still less than At that time, it is necessary to continue to increase. This continues until the reactive power injection requirement is met; the lower bound of the baseline coefficient at this point is denoted as... .because When the peak values ​​of the active power reference curve and the power angle curve intersect or just meet the reactive current requirement, the stability margin tends to zero. Therefore, a value greater than [missing information] is introduced. stability margin coefficient By increasing the downtilt rate, the unstable equilibrium point on the right side is pushed further towards a larger work angle, providing the system with the necessary stability margin; on the other hand... If the value is too large, the active power transmission capacity will be excessively sacrificed during the fault period. Its upper limit is determined by the active power reference curve. The nonnegativity determination at the location, let Get The feasible range of values.

[0046] like Figure 3 As shown, Figure 3 Different stability margin coefficients are shown as examples. The active power reference curve is shown below, with the horizontal axis representing the work angle. The unit is radians and is marked with... , , , The vertical axis represents active power in per-unit values; the black solid line represents the power angle curve during the fault duration phase. The blue dashed line, red dashed line, and green dashed line represent the stability margin coefficients taken as follows: , , Reconstruction active power reference curve , , ,and The left intersection of the reconstructed active power reference curve and the power angle curve is the stable equilibrium point, and the right intersection is the unstable equilibrium point. As... The increase in the active power reference curve leads to an overall downward shift, the unstable equilibrium point on the right side continues to shift towards a larger power angle, and the critical clearance angle margin increases accordingly. However, the overall downward shift of the active power reference curve means a reduction in active power output capability.

[0047] In application, the stable equilibrium point and the unstable equilibrium point refer to the intersection of the reconstructed active power reference curve and the power angle curve during the fault duration phase. The intersection point located to the left of the peak value of the power angle curve is the stable equilibrium point, and the intersection point located to the right of the peak value is the unstable equilibrium point. The existence of the stable equilibrium point during the fault is a prerequisite for the grid-type converter to maintain transient synchronization and stability. If the reconstructed active power reference curve and the power angle curve during the fault duration phase do not intersect during the fault, then there is no stable equilibrium point, and the grid-type converter will lose synchronization. The critical clearing angle margin refers to the difference between the power angle corresponding to the unstable equilibrium point and the operating power angle during the fault. The larger the critical clearing angle margin, the larger the allowable power angle swing range of the grid-type converter during the fault, and the higher the transient synchronization and stability margin.

[0048] In one embodiment, step S3 includes: determining that the fault has been cleared when the voltage amplitude of the power grid recovers to above a preset voltage threshold and the output current amplitude of the grid-connected converter falls back to within a preset current limiting threshold, and recording the time of fault clearing. Virtual inductance state quantity ; Delay hold time after fault clearing Within, the virtual inductance is maintained as After the delay period ends, the virtual inductor is removed from the output impedance channel, and an auxiliary recovery variable is introduced. The auxiliary recovery variable Decreasing according to a first-order exponential law, and with an initial value at the end of the delay hold time. : And the active power reference during the fault recovery phase Determined as: in, This is the flexible recovery time constant.

[0049] In applications, if the virtual inductor is immediately deactivated and the active power reference is directly restored to the pre-fault value upon fault clearance, the grid-type converter will be simultaneously affected by impedance path abrupt changes and active power reference abrupt changes, easily causing active power surges, increased frequency deviations, and power angle oscillations. To avoid these problems, this embodiment of the invention sets a delay holding time after fault clearance. ,exist The virtual inductance will be maintained at a certain value during the time period. This delay hold process is used to retain a certain equivalent line inductance during the initial recovery phase, enhance system damping, and suppress power angle overshoot during fault clearing; when the delay hold time ends, i.e. At this time, the virtual inductor is removed from the impedance path to restore the normal output impedance characteristics of the grid-type converter. At the same time, to avoid a step change in the active power reference as the virtual inductor is removed, an auxiliary recovery variable is introduced. Used for receiving The impact on the active reference channel, the auxiliary recovery variable Satisfies the first-order exponential decay equation and By delaying the holding time With flexible recovery time constant They act on different control channels and cooperate with each other. After the delay, the virtual inductor exits the impedance channel, allowing the reactive power regulation capability to return to normal, while the active power reference still recovers through the auxiliary recovery variable. Flexible recovery suppresses the secondary active power surge caused by the superposition of virtual inductor exit and active power reference step recovery in traditional strategies.

[0050] like Figure 4 As shown, Figure 4 An illustrative comparison is shown between the active power reference step recovery method and the flexible recovery method of this invention. In the figure, the vertical axis represents the active power reference in per-unit values, and the horizontal axis represents time in seconds, with labels indicating... , , , At each moment; the curves corresponding to the active power reference step recovery method are in The curve corresponding to the flexible recovery method of this invention recovers from the step value taken during the fault to the given value before the fault. From time point A, the system continuously and gradually recovers to the pre-fault given value along the exponential curve, with a slope of [missing information]. The corresponding dashed line represents the maximum allowable recovery slope in the initial stage of recovery; it can be seen intuitively that the active power reference flexible recovery method transforms the step change of the active power reference into a continuous gradual process.

[0051] In one embodiment, the rate of change of the active power reference with respect to time during the fault recovery phase is: The rate of change reaches its maximum value at the end of the delay hold time. To ensure that the rate of change does not exceed the maximum permissible recovery slope of the active power reference. The flexible recovery time constant satisfy: The delay holding time The peak time of the first power angle swing and the main adjustment time during the initial recovery of the grid-type converter are determined based on the grid-type converter.

[0052] In application, the delay hold time It can cover the most critical transient process in the early stage of recovery, and avoid the virtual inductance from staying for a long time and affecting normal reactive power regulation; by limiting the recovery rate of active power reference in the fault recovery stage, it avoids frequency deviation caused by too fast recovery.

[0053] In application, the maximum permissible recovery slope The active power reference recovery rate is a pre-set upper limit based on engineering requirements, used to limit the rise rate of the active power reference during the fault recovery phase and avoid frequency deviation caused by excessively rapid active power reference recovery; since the rate of change of the active power reference with respect to time during the fault recovery phase reaches its maximum value at the end of the delay hold time. Therefore, it is only necessary to ensure that the maximum rate of change does not exceed the maximum permissible recovery slope. This ensures that the rate of change of the active power reference does not exceed [a certain value] throughout the entire recovery process. Thus, the flexible recovery time constant is determined. The lower limit.

[0054] like Figure 5As shown, this embodiment of the invention also provides a grid-type converter, including a power circuit and a controller; the power circuit includes a converter bridge, a filter inductor, and a filter capacitor connected in sequence, and connected to the power grid via a line inductor; the controller includes a power calculation unit, a power synchronization control unit, a reactive power voltage control unit, a three-phase voltage synthesis unit, a virtual inductor unit, a voltage and current dual closed-loop control unit, a modulation unit, a current limiting unit, an active power reference reconstruction unit, and an active power reference flexible recovery unit; the input terminal of the power calculation unit is respectively connected to the voltage sampling terminal of the filter capacitor and the output current sampling terminal of the grid-type converter, and the output terminal outputs active power and reactive power respectively; the input terminal of the power synchronization control unit is respectively connected to the active power output terminal and the active power reference input terminal of the power calculation unit, and the output terminal outputs the phase angle; the input terminal of the reactive power voltage control unit is connected to the reactive power output terminal of the power calculation unit, and the output terminal outputs the voltage amplitude; the three-phase voltage synthesis unit... The input terminals are connected to the phase angle and the voltage amplitude, respectively. The output terminal is connected to the voltage-current dual-loop control unit via the virtual inductor unit. The voltage-current dual-loop control unit is connected to the converter bridge via the modulation unit. The input terminal of the current limiting unit receives the deviation between the amplitude of the limiting current and the output current. Its output terminal outputs a virtual inductor state quantity. This virtual inductor state quantity is connected to the controlled terminal of the virtual inductor unit and the input terminal of the active power reference reconfiguration unit. The input terminal of the active power reference reconfiguration unit is connected to the output terminal of the virtual inductor state quantity of the current limiting unit, used to output the active power reference during the fault duration phase based on the virtual inductor state quantity. The input terminal of the active power reference flexible recovery unit is connected to the current limiting unit via an auxiliary recovery variable channel. Its output terminal outputs the active power reference during the fault recovery phase. The active power reference during the fault duration phase and the active power reference during the fault recovery phase are selected and connected to the active power reference input terminal of the power synchronization control unit.

[0055] In applications, such as Figure 5 As shown, the power circuit includes a DC side. Converter bridge, filter inductor and filter capacitor and through the line inductance Connected to the power grid, as shown in the diagram , , To filter the three-phase current of the inductor, , , The three-phase voltage of the filter capacitor. , , The three-phase output current flowing through the line inductance, , , The three-phase voltage of the power grid is used; the power calculation unit calculates and outputs active power based on the filter capacitor voltage and the output current. and reactive power The power synchronization control unit is - Control unit, based on active power Phase angle with active reference output The reactive power control unit is - Control unit, based on reactive power Output voltage amplitude The - Control unit and the - The control unit is in a frozen control loop after fault detection; the three-phase voltage synthesis unit is based on the phase angle. With voltage amplitude Synthetic three-phase reference voltage The current is sent to the voltage and current dual closed-loop control unit via the virtual inductor unit, and then drives the converter bridge via the modulation unit (PWM unit); the current limiting unit limits the current. With output current amplitude After difference, the virtual inductor state quantity is output through the PI regulator. The virtual inductance state quantity One path is sent to the virtual inductor unit, and the other path is sent to the active power reference reconfiguration unit. The process, the aforementioned The output of the component and the active power reference setpoint before the fault Subtracting the summation steps yields the active power reference for the fault persistence phase. The diagram uses active reference. Follow the angle The changing downward sloping curve illustrates the reconfiguration characteristics; the virtual inductance state quantity Also, auxiliary recovery variables The data is sent to the active power reference flexible recovery unit, which outputs the active power reference for the fault recovery phase. The diagram uses active reference. The gradual rise curve over time illustrates the flexible recovery characteristics; the active power reference during the fault duration phase. With the active power reference of the fault recovery phase After selection, it is sent to the active reference. - Control unit.

[0056] In application, the current limiting unit can be composed of a subtractor and a proportional-integral (PI) regulator. The subtractor calculates the difference between the amplitude of the limiting current and the output current, and the PI regulator adjusts this difference to output the virtual inductor state quantity. The active power reference reconstruction unit can be composed of a multiplier and a subtractor. The multiplier multiplies the virtual inductor state quantity by an adjustment coefficient adjusted according to the degree of grid voltage drop to obtain an adjustment amount. The subtractor subtracts the adjustment amount from the active power reference setpoint before the fault to obtain the active power reference during the fault duration. The active power reference flexible recovery unit can be composed of a delay holding module and an auxiliary recovery variable generation module. The delay holding module maintains the virtual inductor state quantity at the value at the fault clearing time during the delay holding period. The auxiliary recovery variable generation module generates an auxiliary recovery variable that decays according to a first-order exponential law and acts on the active power reference through an auxiliary recovery variable channel independent of the channel where the virtual inductor unit is located, thereby structurally decoupling the exit of the virtual inductor unit from the recovery of the active power reference during the fault recovery stage.

[0057] In one embodiment, the current limiting unit includes a subtractor and a PI regulator connected in sequence; the two input terminals of the subtractor are respectively connected to the amplitude of the limiting current and the amplitude of the output current, and the output terminal of the PI regulator outputs the virtual inductor state quantity and is respectively connected to the controlled terminal of the virtual inductor unit and the input terminal of the active power reference reconfiguration unit.

[0058] In one embodiment, the active power reference reconfiguration unit includes a multiplier and a subtractor; the input of the multiplier receives an adjustment coefficient adjusted according to the degree of voltage drop in the grid and the virtual inductor state quantity, respectively, and its output outputs an adjustment quantity; the input of the subtractor is connected to the active power reference setpoint before the fault and the adjustment quantity, respectively, and its output outputs the active power reference during the fault duration.

[0059] In one embodiment, the active power reference flexible recovery unit includes a delay holding module and an auxiliary recovery variable generation module; the delay holding module is used to maintain the virtual inductor state variable at the value at the time of fault clearance during the delay holding period after fault clearance; the output terminal of the auxiliary recovery variable generation module is connected to the output terminal of the active power reference flexible recovery unit through the auxiliary recovery variable channel, and the auxiliary recovery variable channel is independent of the channel where the virtual inductor unit is located, so that the exit of the virtual inductor unit is decoupled from the recovery of the active power reference in the fault recovery stage.

[0060] In applications, the reconfiguration unit, current limiting unit, active power reference flexible recovery unit, and other units can be software program units, or they can be implemented through different logic circuits integrated in the controller, or they can be implemented through multiple distributed processors.

[0061] The active power reference control method and grid-type converter fault ride-through provided in this invention, based on current limiting control, reconstructs the active power reference during the fault duration phase according to the voltage drop and virtual inductor state variables, and decouples the virtual inductor from the active power reference recovery after the fault is cleared. This enables the grid-type converter to balance transient synchronous stability during the fault and smooth recovery after the fault under grid voltage drop or short-circuit fault conditions. Because the reconstructed active power reference... Virtual inductor state quantity in In the range of work angle The internal active power reference increases monotonically with the power angle, while the reconstructed active power reference exhibits a flexible downward tilt characteristic as the power angle decreases monotonically. This ensures that the active power reference during the fault matches the power angle characteristics under current limiting, thereby guaranteeing the existence of a stable equilibrium point during the fault duration. This avoids the problem of the absence of a stable equilibrium point or a narrow stable range under traditional horizontal references, thus expanding the transient synchronization stability margin. Due to the reference coefficient... Take the first lower bound corresponding to the transient stability constraint. The second lower bound corresponding to the reactive current injection constraint The larger value in the formula ensures that the reconstructed active power reference curve intersects the fault power angle curve at its peak to guarantee transient stability, while also meeting the reactive current support requirements during low-voltage ride-through. The reference coefficient determined by these two factors balances both transient stability and reactive power support requirements. Due to the introduction of values ​​greater than [a certain value], the reference curve is designed to ensure that the reconstructed active power reference curve intersects the fault power angle curve at its peak to guarantee transient stability, while also meeting the reactive current support requirements during low-voltage ride-through. stability margin coefficient By increasing the downslope of the active power reference curve, the unstable equilibrium point on the right side is pushed further towards a larger power angle, providing the system with the necessary stability margin. The upper bound is determined by the non-negativity constraint of the active power reference curve at the zero work angle, avoiding... Excessive power consumption during a fault can lead to excessive sacrifice of active power transmission capacity, necessitating a balance between stability margin and active power output capacity; this is achieved by setting a delay hold-up time after the fault is cleared. Keep the virtual inductance as In the initial recovery phase, a certain equivalent line inductance is retained to enhance system damping and suppress power angle overshoot during fault clearing; after the delay hold time ends, an auxiliary recovery variable that satisfies the first-order exponential decay law is used. Accept The impact on the active power reference channel allows the virtual inductor to exit the impedance channel after the delay and restore normal reactive power regulation capability, while the active power reference continues to recover continuously along the exponential curve. This decouples the virtual inductor exit and the active power reference recovery on two different control channels, suppressing the secondary active power surge caused by the superposition of virtual inductor exit and active power reference step recovery in traditional strategies. Due to the flexible recovery time constant... satisfy To ensure that the recovery rate of the active reference does not exceed the maximum permissible recovery slope. This avoids frequency deviation caused by excessively rapid recovery. The above features work together to ensure that the grid-type converter has a clear active power reference regulation law in both the fault persistence stage and the fault recovery stage, improves the coordination of the whole process of fault ride-through control, reduces the secondary impact of active power, frequency deviation and power angle oscillation, and improves the operational stability and reliability of the grid-type converter after fault ride-through.

[0062] To verify the above technical effects, a simulation model of a grid-type converter connected to an infinite power grid via line impedance was built in MATLAB / Simulink. In the simulation model, the grid voltage is... A three-way symmetrical drop occurred at that time. Timely fault recovery. For example... Figure 6 As shown, in the voltage drop factor Under operating conditions, traditional strategies suffer from a small stability margin because the active power reference during a fault is horizontal and difficult to adjust to changes in fault conditions and virtual inductance state variables. and reactive power Significant oscillations occurred, and the system exhibited out-of-step characteristics. The horizontal axis in the graph represents time in seconds, and the vertical axis represents power in per-unit values. The active power dropped to a minimum of approximately [value missing]. per-unit value; such as Figure 7 As shown, after adopting the method described in this invention, the active power reference is reconstructed into a flexible curve that monotonically decreases with the power angle, and the system maintains synchronous operation during faults. Figure 7 The upper and middle figures show the power waveform, representing active power. After the fault, it dropped to approximately Stable reactive power after per-unit value Rise to approximately Per-unit value, the middle figure shows the output current amplitude. The waveform has a peak value of approximately The per-unit value stabilizes at approximately Within the current limiting boundary of the per-unit value, as shown in the figure below. shaft current and shaft current Waveform, By The per-unit value rose to approximately per unit value By Change to approximately Per-unit value, the output current is kept within the current limiting boundary; as shown in Figure 8, the voltage sag factor The fault recovery phase was verified under operating conditions. Figure 8(a) shows the case where the virtual inductor immediately exits and the active power reference recovers stepwise, with frequency deviation... peak approximately The system exhibits prolonged oscillations. Figure 8(b) shows the scenario where the virtual inductor exits after a delay, but the active power reference still recovers stepwise. At the end of the delay, approximately [missing information - likely referring to a oscillation pattern] still exists. The peak value of the secondary frequency deviation, Figure 8(c) shows the case where the virtual inductor of the present invention is delayed and the active power reference is flexibly restored. The active power reference is continuously restored along the exponential curve, and the actual active power is... Smoothly follow active reference The frequency deviation peak decreased to approximately [missing information - likely a number]. Secondary active power surges were effectively suppressed; in Figure 8, the horizontal axis of each sub-graph represents time in seconds, and the vertical axis of the left sub-graph represents frequency deviation. The unit is Hertz. The vertical axis of the right subgraph represents active power, with the unit being per unit.

[0063] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. An active power reference control method for fault ride-through in a grid-type converter, characterized in that, include: S1. Obtain the voltage drop of the power grid and the virtual inductance state quantity formed by the grid-type converter under current limiting; S2. During the fault duration phase, an adaptive adjustment coefficient is determined based on the voltage drop level, and the active power reference is reconstructed based on the adaptive adjustment coefficient and the virtual inductance state quantity, so that the reconstructed active power reference is dynamically adjusted according to the power angle change of the grid-type converter. S3. After the fault is cleared, the exit of the virtual inductor is decoupled from the recovery of the active power reference, so that the active power reference is continuously restored from the value taken during the fault duration to the active power reference setpoint before the fault.

2. The method according to claim 1, characterized in that, Before obtaining the voltage drop level of the power grid, the method further includes: The voltage amplitude of the power grid and the output current amplitude of the grid-connected converter are detected. ; When the voltage amplitude of the power grid is detected to be lower than a preset voltage threshold. or the output current amplitude Exceeding the preset rate limit threshold When at least one of the following conditions is met, the grid-type converter is determined to enter fault ride-through control; Otherwise, the grid-type converter maintains normal grid control, and the virtual inductance state quantity corresponding to the current limiting effect is zero.

3. The method according to claim 1, characterized in that, The reconstruction of the active power reference includes: reconstructing the active power reference during the fault duration phase. Reconstructed as: in, Characterizing the degree of voltage drop, The active power reference setpoint before the fault. The power angle of the grid-type converter is given. This refers to the virtual inductance state quantity that varies with the power angle during the fault persistence phase. This is a reference coefficient that is adaptively adjusted according to the degree of voltage drop. For stability margin coefficient, The adaptive adjustment coefficient is constituted; within the power angle range Within, the virtual inductance state quantity As the work angle monotonically increases, the reconstructed active reference... It exhibits a flexible downward tilting characteristic as the work angle decreases monotonically.

4. The method according to claim 3, characterized in that, The benchmark coefficient Take the first lower bound corresponding to the transient stability constraint. The second lower bound corresponding to the reactive current injection constraint The larger value in: First lower bound To: make the stability margin coefficient take The lower bound of the reference coefficient corresponding to the intersection of the baseline active power reference curve at the peak and the power angle curve during the fault duration phase, and satisfying the following: in, This represents the maximum value of the power angle curve during the fault duration phase. The peak power angle corresponding to the maximum value. The virtual inductance state quantity at the peak power angle; the second lower bound To ensure that the reactive current at the stable equilibrium point during the fault duration reaches the lower bound of the reference coefficient corresponding to the lower limit of reactive current; the stability margin coefficient The values ​​of satisfy: in, This is the virtual inductance state quantity when the power angle is zero.

5. The method according to claim 3, characterized in that, The step of decoupling the virtual inductor from the active power reference includes: When the voltage amplitude of the power grid recovers to above a preset voltage threshold and the output current amplitude of the grid-connected converter falls back to within a preset current limiting threshold, the fault is determined to be cleared, and the time of fault clearing is recorded. Virtual inductance state quantity ; Delay hold time after fault clearing Within, the virtual inductance is maintained as ; After the delay period ends, the virtual inductor is removed from the output impedance channel, and an auxiliary recovery variable is introduced. The auxiliary recovery variable Decreasing according to a first-order exponential law, and with an initial value at the end of the delay hold time. : And the active power reference during the fault recovery phase Determined as: in, This is the flexible recovery time constant.

6. The method according to claim 5, characterized in that, The rate of change of the active power reference with respect to time during the fault recovery phase is: The rate of change reaches its maximum value at the end of the delay hold time. To ensure that the rate of change does not exceed the maximum permissible recovery slope of the active power reference. The flexible recovery time constant satisfy: The delay holding time The peak time of the first power angle swing and the main adjustment time during the initial recovery of the grid-type converter are determined based on the grid-type converter.

7. A grid-type converter, characterized in that, It includes a power circuit and a controller; the power circuit includes a converter bridge, a filter inductor and a filter capacitor connected in sequence, and connected to the power grid via a line inductor; the controller includes a power calculation unit, a power synchronization control unit, a reactive voltage control unit, a three-phase voltage synthesis unit, a virtual inductor unit, a voltage and current dual closed-loop control unit, a modulation unit, a current limiting unit, an active power reference reconstruction unit and an active power reference flexible recovery unit; The input terminal of the power calculation unit is connected to the voltage sampling terminal of the filter capacitor and the output current sampling terminal of the grid converter, respectively, and the output terminal outputs active power and reactive power, respectively. The input terminal of the power synchronization control unit is connected to the active power output terminal and the active power reference input terminal of the power calculation unit, respectively, and the output terminal outputs the phase angle; the input terminal of the reactive voltage control unit is connected to the reactive power output terminal of the power calculation unit, and the output terminal outputs the voltage amplitude. The input terminals of the three-phase voltage synthesis unit are respectively connected to the phase angle and the voltage amplitude, and its output terminal is connected to the voltage and current dual closed-loop control unit via the virtual inductor unit. The voltage and current dual closed-loop control unit is connected to the converter bridge via the modulation unit. The input terminal of the current limiting unit receives the deviation between the amplitude of the limiting current and the amplitude of the output current, and its output terminal outputs a virtual inductor state quantity. The virtual inductor state quantity is connected to the controlled terminal of the virtual inductor unit and the input terminal of the active reference reconfiguration unit, respectively. The input terminal of the active power reference reconstruction unit is connected to the output terminal of the virtual inductance state quantity of the current limiting unit, and is used to output the active power reference during the fault duration based on the virtual inductance state quantity. The input of the active power reference flexible recovery unit is connected to the current limiting unit via an auxiliary recovery variable channel, and its output outputs the active power reference during the fault recovery phase. The active power reference during the fault duration phase and the active power reference during the fault recovery phase are selected and then connected to the active power reference input terminal of the power synchronization control unit.

8. The grid-type converter according to claim 7, characterized in that, The current limiting unit includes a subtractor and a PI regulator connected in sequence; the two input terminals of the subtractor are respectively connected to the amplitude of the limiting current and the amplitude of the output current, and the output terminal of the PI regulator outputs the virtual inductor state quantity and is respectively connected to the controlled terminal of the virtual inductor unit and the input terminal of the active reference reconfiguration unit.

9. The grid-type converter according to claim 7, characterized in that, The active power reference reconfiguration unit includes a multiplier and a subtractor; the input of the multiplier receives the adjustment coefficient adjusted according to the degree of voltage drop in the power grid and the virtual inductor state quantity, respectively, and its output outputs the adjustment quantity; the input of the subtractor is connected to the active power reference setpoint before the fault and the adjustment quantity, respectively, and its output outputs the active power reference during the fault duration.

10. The grid-type converter according to claim 7, characterized in that, The active power reference flexible recovery unit includes a delay holding module and an auxiliary recovery variable generation module. The delay holding module is used to maintain the virtual inductor state quantity at the value at the time of fault clearance during the delay holding period after fault clearance. The output terminal of the auxiliary recovery variable generation module is connected to the output terminal of the active power reference flexible recovery unit through the auxiliary recovery variable channel, and the auxiliary recovery variable channel is independent of the channel where the virtual inductor unit is located, so that the exit of the virtual inductor unit is decoupled from the recovery of the active power reference in the fault recovery stage.

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