A multi-objective cooperative control method for network configuration type converter under asymmetric fault

Abstract

This invention discloses a multi-objective cooperative control method for grid-connected converters under asymmetrical faults. The method constructs a feasible region for the negative-sequence reactive current reference value of the grid-connected converter under asymmetrical faults and designs an adaptive adjustment equation for the negative-sequence reactive current reference value; it also constructs a feasible region for the positive-sequence terminal voltage reference value and designs an adaptive adjustment equation for the positive-sequence terminal voltage reference value; further, it constructs a feasible region for the active power reference value and designs an adaptive adjustment equation for the active power reference value. This invention achieves multi-objective cooperative control of the grid-connected converter under asymmetrical faults through the adaptive adjustment of the negative-sequence reactive current, positive-sequence terminal voltage, and active power reference values. During the fault period, it simultaneously satisfies multiple objective constraints such as transient synchronous stability, current stress constraints, and positive and negative-sequence reactive current requirements of the grid connection guidelines, thus achieving reliable asymmetrical fault ride-through for the grid-connected converter.

Description

Technical Field

[0001] This invention relates to the field of new energy power generation technology, and in particular to a multi-objective cooperative control method for grid-type converters under asymmetrical fault conditions. Background Technology

[0002] With the continuous increase in the proportion of new energy sources such as wind power and photovoltaics connected to the grid, the trend of power system electrification is becoming increasingly apparent, exhibiting characteristics such as low inertia, weak damping, and low disturbance rejection capability, posing severe challenges to the safe and stable operation of the system. Grid-connected converters, possessing the ability to autonomously construct voltage and frequency, can compensate for the inertia and damping deficiencies in power electronic power systems, and have become one of the mainstream solutions for grid-connected converters for new energy sources. However, under grid faults, grid-connected converters are highly susceptible to transient synchronization instability and current overruns. Furthermore, the probability of asymmetrical faults in actual power grids far exceeds that of symmetrical faults, and my country's new energy grid connection guidelines have made explicit requirements on the positive and negative sequence reactive currents generated by converters under asymmetrical faults, in order to achieve positive sequence voltage support and negative sequence voltage suppression under asymmetrical faults. Therefore, multi-objective cooperative control methods for grid-connected converters that meet constraints such as transient synchronization stability, current limitation, positive sequence voltage support, and negative sequence voltage suppression have become a difficult problem of common concern to both academia and industry.

[0003] Because grid-connected converters use voltage source control, they cannot directly control positive and negative sequence active and reactive currents by changing current commands. This makes it difficult for them to effectively limit overcurrent and respond to the positive and negative sequence reactive current requirements of grid connection guidelines under asymmetrical faults. Furthermore, complex coupling constraints exist between multiple objectives: transient synchronization constraints require the system to generate more positive sequence active current, grid connection guidelines require the system to inject positive sequence reactive current while absorbing negative sequence reactive current, and the converter's current stress constraints require its maximum phase current to not exceed the limit. Moreover, existing technologies mostly focus on coordinated control under symmetrical faults, lacking multi-objective coordinated control methods under asymmetrical faults. In summary, how to achieve multi-objective coordinated control of grid-connected converters under asymmetrical faults is a pressing technical challenge and a key to the safe and stable operation of new power systems. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a multi-objective coordinated control method for grid-connected converters under asymmetrical faults. This method enables grid-connected converters to simultaneously meet multiple objective constraints during asymmetrical faults, such as transient synchronous stability, current stress constraints, and positive and negative sequence reactive current requirements of grid connection guidelines, thereby ensuring reliable fault ride-through of the system.

[0005] To achieve the above objectives, the present invention provides a multi-objective cooperative control method for a grid-type converter under asymmetric fault conditions, comprising:

[0006] Step 1: Collect the grid connection point voltage of the grid-connected converter system and construct a nonlinear model of the grid-connected converter considering active and reactive power loops;

[0007] Step 2: Based on the requirements of the grid connection guidelines for negative sequence reactive current under asymmetrical faults and the current stress constraints of grid-connected converters, construct the feasible region of the negative sequence reactive current reference value of grid-connected converters, and construct the adaptive adjustment equation of the negative sequence reactive current reference value based on the feasible region to regulate the negative sequence reactive current reference value.

[0008] Step 3: Based on the requirements of the grid connection guidelines for positive and negative sequence reactive currents under asymmetrical faults and the current stress constraints of the grid-connected converter, construct the feasible region of the positive sequence terminal voltage reference value of the grid-connected converter, and construct the adaptive adjustment equation of the positive sequence terminal voltage reference value based on the feasible region to regulate the negative sequence and positive sequence terminal voltage reference values.

[0009] Step 4: Based on the requirements of the positive and negative sequence reactive currents under the grid connection guidelines for asymmetrical faults and the current stress constraints of the grid-connected converter, construct the feasible region of the active power reference value of the grid-connected converter, and construct the adaptive adjustment equation of the active power reference value based on the feasible region to regulate the active power reference value.

[0010] As a preferred embodiment, in step one, the nonlinear model of the grid-connected converter considering both active and reactive power loops is as follows:

[0011]

[0012]

[0013] in: and These are virtual inertia and damping, respectively; and These are the positive-sequence active power reference value and the active power output value, respectively. and These are the positive sequence components of the grid connection point voltage and the positive sequence components of the grid-connected converter terminal voltage, respectively. and These are the positive-sequence phase difference and angular frequency difference between the terminal voltage of the grid-connected converter and the voltage at the grid connection point, respectively. for The second derivative; This is the reactive power loop droop coefficient; and These are the positive-sequence reactive power reference value and the reactive power output value, respectively. The amplitude of the terminal voltage generated by the reactive power control loop; This is the reference value for the positive sequence terminal voltage.

[0014] Preferably, in step two, the reference value of the negative sequence reactive current of the grid-connected converter under asymmetrical fault conditions is... The upper and lower bound constraints of the feasible region are:

[0015]

[0016]

[0017] in: This is the rated current of the converter; The current safety threshold of the converter; It represents the negative sequence component of the grid connection point voltage.

[0018] As a preferred option, the adaptive adjustment equation for the negative sequence reactive current reference value of the grid-connected converter under asymmetrical fault conditions is: .

[0019] Preferably, in step three, the reference value of the positive sequence terminal voltage of the grid-connected converter under asymmetrical fault conditions is... The upper and lower bound constraints of the feasible region are:

[0020]

[0021]

[0022] in: and These are the upper and lower limits of the positive sequence terminal voltage of the grid-type converter under asymmetrical fault conditions.

[0023] As a preferred option and The corresponding expressions are respectively and .

[0024] As a preferred option, the adaptive adjustment equation for the positive sequence terminal voltage reference value of the grid-connected converter under asymmetrical fault conditions is:

[0025]

[0026] in: This indicates finding the minimum value; and These are the upper and lower limits of the reference voltage at the positive sequence terminal of the grid-type converter under asymmetrical fault conditions.

[0027] As a preferred option, in step four, the active power reference value of the grid-connected converter under asymmetrical fault conditions is... The upper and lower bound constraints of the feasible region are:

[0028]

[0029]

[0030] in: This represents the maximum value of the positive-sequence active current.

[0031] As a preferred option, the formula for calculating the maximum value of the positive-sequence active current is:

[0032]

[0033] in: for k Maximum positive sequence active current of phase; , representing the three phases A, B, and C.

[0034] As a preferred option k The expression for the maximum positive sequence active current of a phase is:

[0035]

[0036] in: ; and These are the positive-sequence reactive current and the negative-sequence reactive current, respectively.

[0037] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention provides a multi-objective cooperative control method for grid-connected converters under asymmetrical faults. By constructing a feasible domain of reference values ​​for negative-sequence reactive current, positive-sequence terminal voltage, and active power of the grid-connected converter under asymmetrical faults, it further designs adaptive adjustment equations for the reference values ​​of negative-sequence reactive current, positive-sequence terminal voltage, and active power. This invention achieves multi-objective adaptive cooperative control of grid-connected converters under asymmetrical faults, simultaneously satisfying multiple objective constraints such as transient synchronous stability, current stress constraints, and positive and negative-sequence reactive current requirements of grid connection guidelines. It requires no additional control loops or devices, is simple and easy to implement, and ensures reliable asymmetrical fault ride-through of the grid-connected converter. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 A flowchart illustrating the multi-objective cooperative control method for a grid converter under asymmetric fault conditions provided in this embodiment of the invention;

[0040] Figure 2The control structure diagram is shown for the multi-objective cooperative control method of the grid converter under asymmetric fault provided in the embodiment of the present invention.

[0041] Figure 3 The negative sequence reactive current reference value provided in the embodiments of the present invention The feasible region graph;

[0042] Figure 4 Voltage and current phasor diagram of a grid converter under asymmetrical fault conditions provided in this embodiment of the invention;

[0043] Figure 5 The positive sequence terminal voltage reference value provided in the embodiments of the present invention The feasible region graph;

[0044] Figure 6 Active power reference value provided for embodiments of the present invention The feasible region graph;

[0045] Figure 7 The simulation comparison diagrams of the multi-objective cooperative control method of the grid converter under asymmetrical fault provided in the embodiments of the present invention and the traditional control method under two-phase short-circuit fault are shown. Among them, (a) is the simulation result under the traditional control method, and (b) is the simulation result under the multi-objective cooperative control method of the grid converter under asymmetrical fault in this embodiment.

[0046] Figure 8 The simulation comparison diagrams of the multi-objective cooperative control method for a grid-type converter under asymmetrical faults provided in this embodiment of the invention and the traditional control method under two-phase-to-ground short-circuit faults are shown. Among them, (a) is the simulation result under the traditional control method, and (b) is the simulation result under the multi-objective cooperative control method for a grid-type converter under asymmetrical faults in this embodiment. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0048] Figure 1 The flowchart illustrates the multi-objective cooperative control method for a grid converter under asymmetric fault conditions provided in this embodiment of the invention.

[0049] like Figure 1 As shown in the figure, this embodiment discloses a multi-objective cooperative control method for a grid-type converter under asymmetric fault conditions, including:

[0050] Step 1: Collect the grid connection point voltage of the grid-connected converter system and construct a nonlinear model of the grid-connected converter considering active and reactive power loops;

[0051] Step 2: Based on the requirements of the grid connection guidelines for negative sequence reactive current under asymmetrical faults and the current stress constraints of grid-connected converters, construct the feasible region of the negative sequence reactive current reference value of grid-connected converters, and construct the adaptive adjustment equation of the negative sequence reactive current reference value based on the feasible region to regulate the negative sequence reactive current reference value.

[0052] Step 3: Based on the requirements of the grid connection guidelines for positive and negative sequence reactive currents under asymmetrical faults and the current stress constraints of the grid-connected converter, construct the feasible region of the positive sequence terminal voltage reference value of the grid-connected converter, and construct the adaptive adjustment equation of the positive sequence terminal voltage reference value based on the feasible region to regulate the negative sequence and positive sequence terminal voltage reference values.

[0053] Step 4: Based on the requirements of the positive and negative sequence reactive currents under the grid connection guidelines for asymmetrical faults and the current stress constraints of the grid-connected converter, construct the feasible region of the active power reference value of the grid-connected converter, and construct the adaptive adjustment equation of the active power reference value based on the feasible region to regulate the active power reference value.

[0054] Figure 2 This is a control structure diagram of a multi-objective cooperative grid-type converter fault ride-through method provided in an embodiment of the present invention.

[0055] like Figure 2 As shown, in the main circuit section, the converter, filter circuit, low-voltage transformer, line impedance and high-voltage transformer are connected in sequence; among them, the high-voltage transformer is connected to the power grid. The voltage at the grid connection point. This refers to the terminal voltage of a grid-type converter. For grid-connected current, For the output current of the grid-type converter; For filtering inductors, For filtering capacitors; This represents the equivalent grid impedance. The control section of a grid-connected converter mainly includes a power control loop, a negative-sequence phase-locked loop, a positive-sequence voltage and current control loop, a negative-sequence current control loop, a positive-negative-sequence separation circuit, and a PWM circuit.

[0056] The mathematical expression for active power control of a grid-type converter is:

[0057] (1)

[0058] in: This is virtual inertia; For damping; This is a reference value for positive sequence active power; This represents the active power output value. The positive sequence component of the grid connection point voltage; This refers to the positive sequence component of the terminal voltage of a grid-type converter. and These are the positive-sequence phase difference and angular frequency difference between the terminal voltage of the grid-connected converter and the voltage at the grid connection point, respectively. for The second derivative of .

[0059] The mathematical expression for reactive power control in a grid-type converter is:

[0060] (2)

[0061] in: This is the reactive power loop droop coefficient; This is the reference value for positive sequence reactive power; This is the reactive power output value; The amplitude of the terminal voltage generated by the reactive power control loop; This is the reference value for the positive-sequence terminal voltage. Since the voltage and current inner loop bandwidth of a grid-connected converter is much larger than the power outer loop bandwidth, the equivalent gain of the inner loop is considered to be 1, i.e. .

[0062] According to the national standards GB / T 19963.1-2021 "Technical Specifications for Wind Farm Connection to Power System Part 1: Onshore Wind Power" and GB / T 19964-2024 "Technical Specifications for Photovoltaic Power Station Connection to Power System", when an asymmetrical fault occurs in the system, the converter should inject positive-sequence reactive current to support the positive-sequence voltage, while absorbing negative-sequence reactive current to suppress the negative-sequence voltage. Specific requirements are as follows:

[0063] (3)

[0064] (4)

[0065] in: and These are the positive-sequence and negative-sequence components of the grid-connected current, respectively. This is the rated current of the converter; and These represent the positive and negative sequence components of the grid-connected voltage, respectively. Furthermore, since the current through the filter capacitor is essentially zero, it can be ignored. Therefore, the output current of the grid-connected converter is considered equal to the grid-connected current, i.e. Furthermore, the corresponding positive and negative order components are also equal.

[0066] According to equation (4), the lower limit constraint of the negative sequence reactive current reference value that meets the grid connection guidelines is:

[0067] (5)

[0068] Meanwhile, constrained by the current stress of the grid-type converter, the upper limit of the negative sequence current reference value is:

[0069] (6)

[0070] in: This is the current safety threshold for the converter. During a fault, constrained by the converter's current stress, the current should be limited within this safety threshold to prevent damage to the converter's semiconductor devices. Based on engineering experience, this embodiment uses... .

[0071] Figure 3 The negative sequence reactive current reference value provided in the embodiments of the present invention The feasible region diagram is obtained through equations (5) and (6). During asymmetrical faults, the grid-connected converter needs to generate additional positive-sequence active current to avoid the imbalance between active power input and output, and the grid-connected circuit requires it to inject positive-sequence reactive current to support the positive-sequence voltage. Therefore, the reference value of the negative-sequence reactive current is taken as the lower limit of its feasible region, so as to leave enough capacity to meet other constraints. During asymmetrical faults, the adaptive adjustment equation of the negative-sequence reactive current reference value of the grid-connected converter is:

[0072] (7)

[0073] Figure 4 The voltage and current phasor diagram of a grid-type converter under asymmetrical fault conditions provided in this embodiment of the invention. According to... Figure 4 It can be obtained that the positive sequence reactive current The expression is:

[0074] (8)

[0075] Substituting equation (3) into equation (8), we obtain the positive sequence terminal voltage range of the grid-connected converter that satisfies the positive sequence reactive current injection requirements of the grid connection guidelines:

[0076] (9)

[0077] Because the operating range of the stable equilibrium point of a grid-type converter is ,therefore Therefore, the lower limit constraint of the positive sequence terminal voltage of the grid-connected converter that satisfies the positive sequence reactive current injection requirements of the grid connection guideline is:

[0078] (10)

[0079] The upper limit of the positive sequence voltage of a grid-connected converter is simultaneously subject to the grid-connected negative sequence reactive current requirement and current stress constraint. According to Figure 4The phase current amplitude of the grid-type converter under asymmetrical fault conditions can be obtained as follows:

[0080] (11)

[0081] in: for Phase current, , .according to Figure 4 As shown in the phasor diagram, when the positive sequence work angle... That is, positive sequence active current At that time, positive sequence reactive current The maximum value is taken, and the positive sequence terminal voltage is also taken at its maximum value. Therefore, by combining equations (7) and (11), the upper limit constraint of the positive sequence reactive current is obtained as follows:

[0082] (12)

[0083] Substituting equation (12) into equation (8), we obtain the upper limit constraint of the positive sequence terminal voltage of the grid-type converter as follows:

[0084] (13)

[0085] Equations (10) and (13) together determine the feasible region of the positive sequence terminal voltage of the grid-connected converter. Substituting equations (10) and (13) into equation (2), the lower and upper limits of the reference value of the positive sequence terminal voltage of the grid-connected converter are obtained as follows:

[0086] (14)

[0087] (15)

[0088] Figure 5 The positive sequence terminal voltage reference value provided in the embodiments of the present invention The feasible region graph is obtained through equations (14) and (15). Figure 5 As shown, at the grid connection point where the positive-sequence voltage component is close to 0 and the negative-sequence voltage is close to 1 p.u., the upper and lower limits of the voltage reference value intersect, indicating a conflict between the current stress constraint and the positive-sequence voltage support constraint. During fault periods, the current stress constraint is the primary constraint that must be strictly satisfied, because once an overcurrent occurs, the system may collapse due to damage to semiconductor devices. Therefore, the adaptive adjustment equation for the positive-sequence terminal voltage reference value of the grid-connected converter is:

[0089] (16)

[0090] in: This indicates finding the minimum value; and These are the upper and lower limits of the reference value for the positive sequence terminal voltage of the grid-type converter under asymmetrical fault conditions. Figure 3 The expressions for the sine and cosine functions of the positive sequence work angle are obtained as follows:

[0091] (17)

[0092] Substituting equation (17) into equation (11), we obtain the equation that satisfies the current stress constraint. k The phase current equation is:

[0093] (18)

[0094] Solving equation (18), we obtain the expression for the positive-sequence active current that satisfies the current stress constraint:

[0095] (19)

[0096] During an asymmetrical fault, the current in each phase of the grid-type converter must satisfy the current stress constraint. Therefore, the maximum value of the positive-sequence active current is:

[0097] (20)

[0098] Therefore, the upper limit constraint of the active power reference value of the grid-connected converter that satisfies the current stress constraint and the positive and negative sequence reactive current requirements of the grid connection guide is as follows:

[0099] (twenty one)

[0100] The lower limit constraint for the active power reference value of grid-type converters is:

[0101] (twenty two)

[0102] Figure 6 Active power reference value provided for embodiments of the present invention The feasible region diagram is obtained through equations (21) and (22). To improve the utilization rate of new energy sources during asymmetric faults, the upper limit of the active power reference value of the grid-type converter is taken. Therefore, the adaptive adjustment equation of the active power reference value of the grid-type converter is:

[0103] (twenty three)

[0104] In summary, the multi-objective coordinated control of the grid-type converter under asymmetrical faults can be achieved through the adaptive adjustment of the negative-sequence reactive current reference value, voltage reference value, and active power reference value proposed in this example, as shown in equations (7), (16), and (23). The specific steps of the multi-objective coordinated control of the grid-type converter under asymmetrical faults are as follows:

[0105] Step 1: Collect the grid connection point voltage of the grid-connected converter system and construct a nonlinear model of the grid-connected converter considering the active and reactive power loops, as shown in Equations (1) and (2).

[0106] Step 2: Based on the requirements of the grid connection guidelines for negative sequence reactive current under asymmetrical faults and the current stress constraints of the grid-connected converter, construct the feasible domain of the reference value of negative sequence reactive current of the grid-connected converter under asymmetrical faults, as shown in Equations (5) and (6); and construct the adaptive adjustment equation of the reference value of negative sequence reactive current based on the feasible domain, as shown in Equation (7).

[0107] Step 3: Based on the requirements of the positive and negative sequence reactive currents in the grid connection guidelines under asymmetrical faults and the current stress constraints of the grid-connected converter, construct the feasible domain of the positive sequence terminal voltage reference value of the grid-connected converter under asymmetrical faults, as shown in Equations (14) and (15); and construct the adaptive adjustment equation of the positive sequence terminal voltage reference value based on the feasible domain, as shown in Equation (16).

[0108] Step 4: Based on the requirements of the positive and negative sequence reactive currents in the grid connection guidelines under asymmetrical faults and the current stress constraints of the grid-connected converter, construct the feasible domain of the active power reference value of the grid-connected converter under asymmetrical faults, as shown in Equations (21) and (22); and construct the adaptive adjustment equation of the active power reference value based on the feasible domain, as shown in Equation (23).

[0109] The parameters of the grid-connected converter system are shown in Table 1.

[0110] Table 1

[0111]

[0112] In specific implementation, such as Figure 7 As shown, Figure 7The simulation comparison diagrams of the multi-objective cooperative control method for grid-type converters under asymmetrical faults provided in this embodiment of the invention and the traditional control method under two-phase short-circuit faults are shown. (a) shows the simulation results under the traditional control method; (b) shows the simulation results under the multi-objective cooperative control method for grid-type converters under asymmetrical faults in this embodiment. To demonstrate the effectiveness of the multi-objective cooperative fault-crossing control method proposed in this embodiment, a two-phase short-circuit fault occurs in the grid at 3.5s in the simulation, and the fault is cleared after 1.5s. During the fault, both the positive and negative sequence components of the grid connection point voltage are 0.5pu. As shown in (a), the grid-type converter under the traditional control method experiences transient synchronous instability during the fault, with voltage, current, and power all oscillating. During the fault, the current exceeds the safety threshold and cannot stably output positive and negative sequence reactive currents. The oscillation problem persists even after the fault is cleared. The simulation results using the multi-objective cooperative control method proposed in this embodiment are shown in (b). During asymmetrical faults, the grid-type converter can maintain synchronous stability, the three-phase currents all meet the current stress constraints, and the output positive and negative sequence reactive currents meet the grid connection guidelines. Figure 7 The implementation case of the grid-type converter can fully verify the effectiveness of the multi-objective cooperative control method for the grid-type converter under asymmetrical faults proposed in this embodiment. It can meet the objectives such as transient synchronous stability, current stress constraint and positive and negative sequence reactive current requirements of grid connection guidelines during asymmetrical faults.

[0113] Figure 8 The simulation comparison diagrams of the multi-objective cooperative control method for grid-type converters under asymmetrical faults provided in this embodiment of the invention and the traditional control method under two-phase-to-ground short-circuit faults are shown. (a) shows the simulation results under the traditional control method; (b) shows the simulation results under the multi-objective cooperative control method for grid-type converters under asymmetrical faults in this embodiment. As shown in (a), the grid-type converter under the traditional control method experiences transient synchronous instability during the fault, with voltage, current, and power all oscillating. The maximum phase current exceeds the safety threshold during the fault and fails to meet the grid connection guidelines' requirements for positive and negative sequence reactive currents. The simulation results using the multi-objective cooperative control method proposed in this embodiment are shown in (b). During asymmetrical faults, the grid-type converter can simultaneously meet transient synchronous stability, current stress constraints, and the positive and negative sequence reactive current requirements of the grid connection guidelines. Figure 8 The implementation case of the grid converter further verifies the effectiveness of the multi-objective cooperative control method for the grid converter under asymmetric fault conditions proposed in this embodiment.

[0114] The above embodiments are only used to illustrate the design concept and features of the present invention, and their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. The protection scope of the present invention is not limited to the above embodiments. Therefore, all equivalent changes or modifications made based on the principles and design ideas disclosed in the present invention are within the protection scope of the present invention.

Claims

1. A multi-objective cooperative control method for a grid-type converter under asymmetric fault conditions, characterized in that, include: Step 1: Collect the grid connection point voltage of the grid-connected converter system and construct a nonlinear model of the grid-connected converter considering active and reactive power loops; Step 2: Based on the requirements of the grid connection guidelines for negative sequence reactive current under asymmetrical faults and the current stress constraints of grid-connected converters, construct the feasible region of the negative sequence reactive current reference value of grid-connected converters, and construct the adaptive adjustment equation of the negative sequence reactive current reference value based on the feasible region to regulate the negative sequence reactive current reference value. Step 3: Based on the requirements of the grid connection guidelines for positive and negative sequence reactive currents under asymmetrical faults and the current stress constraints of the grid-connected converter, construct the feasible region of the positive sequence terminal voltage reference value of the grid-connected converter, and construct the adaptive adjustment equation of the positive sequence terminal voltage reference value based on the feasible region to regulate the negative sequence and positive sequence terminal voltage reference values. Step 4: Based on the requirements of the positive and negative sequence reactive currents under the grid connection guidelines for asymmetrical faults and the current stress constraints of the grid-connected converter, construct the feasible region of the active power reference value of the grid-connected converter, and construct the adaptive adjustment equation of the active power reference value based on the feasible region to regulate the active power reference value. In step three, the reference value of the positive sequence terminal voltage of the grid-connected converter under asymmetrical fault conditions. The upper and lower bound constraints of the feasible region are: in: and These are the upper and lower limits of the positive sequence voltage of the grid-type converter under asymmetrical fault conditions. This is the reactive power loop droop coefficient; The positive sequence component of the grid connection point voltage; This is the reference value for positive sequence reactive power; Equivalent grid impedance; and The corresponding expressions are respectively and ;in, This is the negative sequence component of the grid connection point voltage. This is the rated current of the converter. The current safety threshold of the converter; The adaptive adjustment equation for the positive sequence terminal voltage reference value of a grid-connected converter under asymmetrical fault conditions is as follows: in: This indicates finding the minimum value; and These are the upper and lower limits of the reference voltage at the positive sequence terminal of the grid-type converter under asymmetrical fault conditions.

2. The multi-objective cooperative control method for a grid-type converter under asymmetric faults according to claim 1, characterized in that, In step one, the nonlinear model of the grid-connected converter considering active and reactive power loops is as follows: in: and These are virtual inertia and damping, respectively; and These are the positive-sequence active power reference value and the active power output value, respectively. and These are the positive sequence components of the grid connection point voltage and the positive sequence components of the grid-connected converter terminal voltage, respectively. and These are the positive-sequence phase difference and angular frequency difference between the terminal voltage of the grid-connected converter and the voltage at the grid connection point, respectively. for The second derivative; This is the reactive power loop droop coefficient; and These are the positive-sequence reactive power reference value and the reactive power output value, respectively. The amplitude of the terminal voltage generated by the reactive power control loop; This is the reference value for the positive sequence terminal voltage.

3. The multi-objective cooperative control method for a grid-type converter under asymmetric fault conditions according to claim 1, characterized in that, In step two, the reference value of negative sequence reactive current for grid-connected converters under asymmetrical fault conditions. The upper and lower bound constraints of the feasible region are: in: This is the rated current of the converter; The current safety threshold of the converter; It represents the negative sequence component of the grid connection point voltage.

4. The multi-objective cooperative control method for a grid-type converter under asymmetric fault conditions according to claim 3, characterized in that, The adaptive adjustment equation for the negative sequence reactive current reference value of a grid-connected converter under asymmetrical fault conditions is as follows: .

5. The multi-objective cooperative control method for a grid-type converter under asymmetric fault conditions according to claim 1, characterized in that, In step four, the active power reference value of the grid-connected converter under asymmetrical fault conditions. The upper and lower bound constraints of the feasible region are: in: This represents the maximum value of the positive-sequence active current.

6. The multi-objective cooperative control method for a grid-type converter under asymmetric fault conditions according to claim 5, characterized in that, The formula for calculating the maximum value of the positive-sequence active current is: in: for k Maximum positive sequence active current of phase; , representing the three phases A, B, and C.

7. The multi-objective cooperative control method for a grid-type converter under asymmetric faults according to claim 6, characterized in that, k The expression for the maximum positive sequence active current of a phase is: in: ; and These are the positive-sequence reactive current and the negative-sequence reactive current, respectively.

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