A method and system for suppressing unbalanced voltage at wind turbine grid connection point under asymmetric fault

Abstract

The present invention discloses a method and system for suppressing unbalanced voltage at a wind turbine grid-connected point under an asymmetric fault, and relates to the technical field of new energy grid-connected control. The method comprises: obtaining the current limiting condition of the wind turbine inverter and the grid-connected condition of the wind turbine, and constructing a wind turbine inverter grid-connected voltage calculation model; based on the wind turbine inverter grid-connected voltage calculation model, constructing a wind turbine grid-connected point unbalanced voltage suppression optimization model under an asymmetric fault; optimizing the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model through KKT conditions to minimize the objective function, and calculating the final current instruction when the objective function is minimized; the objective function is the grid-connected voltage imbalance, and the grid-connected voltage imbalance refers to the ratio of the grid-connected negative sequence voltage to the grid-connected positive sequence voltage; the final current instruction is input into the inner current loop of the wind turbine inverter, and instruction tracking is realized through PI control. The present invention makes the suppression effect of voltage imbalance better.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy grid-connected control, and in particular to a method and system for suppressing unbalanced voltage at a wind turbine grid-connected point under an asymmetric fault. Background Art

[0002] Grid voltage imbalance is one of the most common power quality problems caused by asymmetric faults or loads. Grid voltage imbalance can lead to economic and safety issues, such as increased fan heating and reduced operating efficiency of three-phase symmetrical equipment. Therefore, mitigating grid voltage imbalance is crucial to improving power quality. Inverter-based power generation equipment such as wind turbines have flexible negative sequence regulation capabilities and have been considered to be an effective tool for alleviating voltage imbalance, which can achieve both positive sequence voltage support and negative sequence voltage suppression. However, since the positive and negative sequence currents are coupled with each other and jointly occupy the current capacity of the wind turbine inverter, there are still several issues to be resolved in this research: 1. What evaluation indicators are used to evaluate the imbalance; 2. How to balance the positive sequence voltage support and negative sequence voltage suppression to achieve the best suppression effect of voltage imbalance.

[0003] However, in the existing method for suppressing unbalanced voltage at the wind turbine grid-connected point, only the negative sequence voltage is used as the evaluation index of voltage imbalance, and the role of positive sequence voltage support in reducing the imbalance degree is also ignored, resulting in poor suppression effect of voltage imbalance. Summary of the invention

[0004] The technical problem to be solved by the present invention is that the existing method for suppressing the unbalanced voltage at the wind turbine grid connection point only uses the negative sequence voltage as the evaluation index of the voltage imbalance, and also ignores the role of the positive sequence voltage support in reducing the imbalance, resulting in poor suppression effect of the voltage imbalance. The purpose of the present invention is to provide a method and system for suppressing the unbalanced voltage at the wind turbine grid connection point under an asymmetric fault. The present invention optimizes the grid connection point imbalance based on the ratio of the positive sequence voltage to the negative sequence voltage, and obtains the current instruction of the wind turbine inverter when the grid connection point imbalance is minimized. The present invention eliminates the influence of the negative sequence voltage, and even if the negative sequence voltage still exists, the negative phase voltage can be reduced to a minimum, so that the voltage imbalance suppression effect is better.

[0005] The present invention is achieved through the following technical solutions:

[0006] In a first aspect, the present invention provides a method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault, the method comprising:

[0007] Obtain the wind turbine inverter current limit and wind turbine grid connection status, and build a wind turbine inverter grid connection voltage calculation model;

[0008] Based on the wind turbine inverter grid-connected voltage calculation model, an optimization model for suppressing unbalanced voltage at the wind turbine grid-connected point under asymmetric faults is constructed;

[0009] The objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model is optimized through the KKT (Karush–Kuhn–Tucker) condition to minimize the objective function, and the final current command when the objective function is minimized is calculated; the objective function is the grid voltage unbalance, which refers to the ratio of the grid negative sequence voltage to the grid positive sequence voltage;

[0010] The final current command is input into the inner current loop of the wind turbine inverter, and command tracking is achieved through PI control.

[0011] Further, the wind turbine inverter current limiting condition includes the maximum current allowed by the wind turbine inverter;

[0012] The wind turbine grid-connected conditions include the remote positive sequence voltage at the wind turbine grid-connected point, the remote negative sequence voltage at the wind turbine grid-connected point, the line impedance and the minimum allowable positive sequence voltage.

[0013] Furthermore, the wind turbine inverter grid-connected voltage calculation model refers to a calculation model of grid-connected positive-sequence voltage and negative-sequence voltage formed by the influence of line current, remote voltage and line impedance on the grid-connected voltage.

[0014] Furthermore, the objective function of the optimization model for suppressing unbalanced voltage at the wind turbine grid connection point is:

[0015] ;

[0016] In the formula, VU is the grid voltage unbalance; V + is the grid positive sequence voltage, V − is the grid-connected negative sequence voltage; V g+ and V g− They are the positive sequence voltage of the remote grid and the negative sequence voltage of the remote grid respectively; R and X They are the equivalent resistance and inductance of the line connecting the wind turbine to the remote power grid; I d+ is the positive sequence d-axis current, I d− is the negative-sequence d-axis current; I q+ is the positive sequence q-axis current, I q− is the negative sequence q-axis current.

[0017] Furthermore, when optimizing the objective function of the unbalanced voltage suppression optimization model at the wind turbine grid connection point, the constraints adopted include wind turbine inverter current constraint, line positive sequence power flow constraint, line negative sequence power flow constraint and minimum positive sequence voltage constraint.

[0018] Furthermore, the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model is optimized through the KKT condition to minimize the objective function, and the final current command when the objective function is minimized is calculated, including:

[0019] The objective function of the optimization model for suppressing unbalanced voltage at wind turbine grid connection point is optimized through KKT conditions, and the positive-sequence current command allocation method and the negative-sequence current command allocation method are obtained when the positive-sequence voltage and the negative-sequence voltage have the optimal value.

[0020] Based on the positive sequence current command allocation method and the negative sequence current command allocation method, the current allocation of the wind turbine inverter is analyzed to reduce the positive sequence current variable and the negative sequence current variable to one wind turbine inverter current variable.

[0021] According to the wind turbine inverter current variable, the positive and negative sequence current instructions are solved by the maximum value of the single variable function, and the solved positive and negative sequence current instructions are used as the final current instructions.

[0022] Furthermore, a positive-sequence current instruction allocation method and a negative-sequence current instruction allocation method are obtained when the positive-sequence voltage and the negative-sequence voltage have optimal values, including:

[0023] Through the KKT condition, the positive sequence current command allocation method and the negative sequence current command allocation method are determined; wherein the positive sequence current command allocation method is the relationship between the positive sequence d axis current and the positive sequence q axis current when the positive sequence voltage reaches the maximum value; the negative sequence current command allocation method is the relationship between the negative sequence d axis current and the negative sequence q axis current when the negative sequence voltage reaches the minimum value;

[0024] In the above determination process, the four current variables of the positive-sequence d-axis current, the positive-sequence q-axis current, the negative-sequence d-axis current and the negative-sequence q-axis current are reduced to a positive-sequence current variable and a negative-sequence current variable.

[0025] In a second aspect, the present invention further provides a system for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault, the system comprising:

[0026] The first construction unit is used to obtain the current limit status of the wind turbine inverter and the wind turbine grid-connected status, and to construct a wind turbine inverter grid-connected voltage calculation model;

[0027] The second construction unit is used to construct an unbalanced voltage suppression optimization model for the wind turbine grid connection point under an asymmetric fault based on the wind turbine inverter grid connection voltage calculation model;

[0028] The optimization solving unit is used to optimize the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model through the KKT condition to minimize the objective function and calculate the final current command when the objective function is minimized; the objective function is the grid-connected voltage unbalance, and the grid-connected voltage unbalance refers to the ratio of the grid-connected negative sequence voltage to the grid-connected positive sequence voltage;

[0029] The control tracking unit is used to input the final current command into the current inner loop of the wind turbine inverter and realize the command tracking through PI control.

[0030] Furthermore, the objective function of the optimization model for suppressing unbalanced voltage at the wind turbine grid connection point is:

[0031] ;

[0032] In the formula, VU is the grid voltage unbalance; V + is the grid positive sequence voltage, V − is the grid-connected negative sequence voltage; V g+ and V g− They are the positive sequence voltage of the remote grid and the negative sequence voltage of the remote grid respectively; R and X They are the equivalent resistance and inductance of the line connecting the wind turbine to the remote power grid; I d+ is the positive sequence d-axis current, I d− is the negative-sequence d-axis current; I q+ is the positive sequence q-axis current, I q− is the negative sequence q-axis current.

[0033] Furthermore, the optimization solving unit includes:

[0034] The first optimization subunit is used to optimize the objective function of the unbalanced voltage suppression optimization model of the wind turbine grid connection point through the KKT condition, and obtain the positive sequence current command allocation method and the negative sequence current command allocation method when the positive sequence voltage and the negative sequence voltage achieve the optimal value;

[0035] The second optimization subunit is used to analyze the distribution of positive-sequence current and negative-sequence current of the wind turbine inverter current based on the positive-sequence current command distribution method and the negative-sequence current command distribution method, and reduce the positive-sequence current variable and the negative-sequence current variable to one wind turbine inverter current variable;

[0036] The third optimization subunit is used to solve the positive and negative sequence current instructions through the maximum value of the single variable function according to the wind turbine inverter current variable, and use the solved positive and negative sequence current instructions as the final current instructions.

[0037] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0038] 1. The present invention provides a method and system for suppressing unbalanced voltage at the wind turbine grid connection point under asymmetric faults. The present invention optimizes the grid connection point imbalance based on the ratio of positive sequence voltage to negative sequence voltage, and obtains the wind turbine inverter current instruction when the grid connection point imbalance is minimum. Under the condition of satisfying the wind turbine inverter current constraint, the wind turbine imbalance suppression capability is maximized, and the wind turbine grid connection voltage imbalance is minimized, providing a theoretical basis and control instructions for wind turbine imbalance management. The present invention eliminates the influence of negative sequence voltage, and even when negative sequence voltage still exists, it can reduce the negative phase voltage to a minimum, so that the voltage imbalance suppression effect is better.

[0039] 2. The present invention provides a method and system for suppressing unbalanced voltage at a wind turbine grid-connected point under an asymmetric fault, which realizes a closed current inner loop through PI control, so that the regulation of the wind turbine inverter can automatically follow the changes in the unbalanced degree of the grid-connected voltage, thereby enhancing the real-time and automatic nature of the control. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0041] Figure 1 A flow chart of a method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault according to the present invention;

[0042] Figure 2 This is a structural diagram of the wind turbine grid-connected control of the present invention;

[0043] Figure 3 The present invention is a structural block diagram of a system for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault. DETAILED DESCRIPTION

[0044] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0045] The present invention can construct a wind turbine inverter grid-connected voltage calculation model based on the acquired wind turbine inverter current limiting situation and wind turbine grid-connected situation, and on this basis construct a wind turbine grid-connected point unbalanced voltage suppression optimization model, optimize the minimum unbalanced voltage of the wind turbine grid-connected point through the KKT (Karush–Kuhn–Tucker) condition, and calculate the final current command of the wind turbine inverter when the grid-connected point unbalance is the smallest; input the final current command into the wind turbine inverter current inner loop, implement command tracking through PI control, and achieve the minimum grid-connected voltage unbalance, thereby providing an effective method for solving the grid-connected voltage unbalance for the wind turbine.

[0046] Example 1

[0047] like Figure 1 As shown, Figure 1 A flow chart of a method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault; Figure 2 This is the wind turbine grid-connected control structure diagram of the present invention. Figure 2 The PCC is the common connection point between the wind turbine and the remote power grid. V g is the voltage of the remote grid, including the positive sequence component V g+ and negative sequence component V g- , Z is the line impedance, R is the real part of the line impedance, X is the imaginary part of the line impedance, V is the grid connection point voltage, including positive sequence components and negative sequence components, I is the line current.

[0048] The present invention provides a method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault, the method comprising:

[0049] Step 1, obtaining the current limit status of the wind turbine inverter and the wind turbine grid-connected status, and constructing a wind turbine inverter grid-connected voltage calculation model;

[0050] In this embodiment, the wind turbine inverter current limiting condition includes the maximum current allowed by the wind turbine inverter;

[0051] The wind turbine grid-connected conditions include the remote positive sequence voltage at the wind turbine grid-connected point, the remote negative sequence voltage at the wind turbine grid-connected point, the line impedance and the minimum allowable positive sequence voltage.

[0052] In this embodiment, the wind turbine inverter grid-connected voltage calculation model refers to a calculation model of the grid-connected positive-sequence voltage and negative-sequence voltage formed by the influence of the line current, the remote voltage and the line impedance on the grid-connected voltage.

[0053] Specifically, the wind turbine inverter grid-connected voltage calculation model is shown in the following formula (2) and formula (3).

[0054] Step 2: Based on the wind turbine inverter grid-connected voltage calculation model, an unbalanced voltage suppression optimization model for the wind turbine grid-connected point is constructed under asymmetric faults;

[0055] Specifically, the optimization model for suppressing unbalanced voltage at the wind turbine grid connection point is shown in the following formulas (1) to (5).

[0056] Step 3, optimizing the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model through the KKT (Karush–Kuhn–Tucker) condition to minimize the objective function, and calculating the final current command when the objective function is minimized;

[0057] In this embodiment, the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model is the grid-connected voltage unbalance degree; the constraints adopted include wind turbine inverter current constraint, line positive sequence power flow constraint, line negative sequence power flow constraint and minimum positive sequence voltage constraint.

[0058] The details are as follows:

[0059] (I) Optimization goal: Minimize grid voltage imbalance

[0060] The grid voltage unbalance is the ratio of the grid negative sequence voltage to the grid positive sequence voltage;

[0061] ; (1)

[0062] (II) Constraints:

[0063] Wind turbine inverter current constraints:

[0064] ; (2)

[0065] Line positive sequence power flow constraints:

[0066] ; (3)

[0067] Negative sequence power flow constraints of the line:

[0068] ; (4)

[0069] Minimum positive sequence voltage constraint:

[0070] ; (5)

[0071] In the formula, VU is the grid voltage unbalance; V + is the grid positive sequence voltage, V− is the grid-connected negative sequence voltage; I + is the grid-connected positive sequence current, I − is the grid-connected negative sequence current; I max It is the maximum current allowed by the wind turbine inverter; V g+ and V g− They are the positive sequence voltage of the remote grid and the negative sequence voltage of the remote grid respectively; R and X They are the equivalent resistance and inductance of the line connecting the wind turbine to the remote power grid; I d+ is the positive sequence d-axis current, I d− is the negative-sequence d-axis current; I q+ is the positive sequence q-axis current, I q− is the negative sequence q-axis current; V +min is the minimum allowed positive sequence grid voltage. At the same time, formula (3) and formula (4) also provide the grid positive sequence voltage and grid negative sequence voltage used in formula (1) to calculate the grid voltage imbalance.

[0072] Therefore, considering the objective function and constraints, if the positive sequence d-axis and q-axis currents, and the negative sequence d-axis and q-axis currents are column vectors X , then the optimization model for suppressing unbalanced voltage at wind turbine grid connection point is F ( X ) can be expressed as:

[0073] min F ( X )

[0074] st: e i ( X ) = 0, i =1,…, M 1 (6)

[0075] h j ( X ) ≤ 0, j =1,…, M 2

[0076] in, e i ( X )and h j ( X) respectively represent the i The equality constraints and j Inequality constraints, M 1 and M 2 are the number of equality constraints and inequality constraints respectively.

[0077] In this embodiment, the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model is optimized by the KKT condition to minimize the objective function, and the final current instruction when the objective function is minimized is calculated, including:

[0078] Step A, optimizing the objective function of the unbalanced voltage suppression optimization model at the wind turbine grid connection point through the KKT condition, and obtaining the positive sequence current command allocation method and the negative sequence current command allocation method when the positive sequence voltage and the negative sequence voltage achieve the optimal value;

[0079] Step A specifically includes:

[0080] Through the KKT condition, the positive sequence current command allocation method and the negative sequence current command allocation method are determined; wherein the positive sequence current command allocation method is the relationship between the positive sequence d axis current and the positive sequence q axis current when the positive sequence voltage reaches the maximum value; the negative sequence current command allocation method is the relationship between the negative sequence d axis current and the negative sequence q axis current when the negative sequence voltage reaches the minimum value;

[0081] In the above determination process, four current variables, namely, a positive-sequence d-axis current, a positive-sequence q-axis current, a negative-sequence d-axis current and a negative-sequence q-axis current, are reduced to two current variables (a positive-sequence current variable and a negative-sequence current variable).

[0082] Step B, based on the positive sequence current command allocation method and the negative sequence current command allocation method, analyzing the current allocation of the wind turbine inverter to the positive sequence current and the negative sequence current, and reducing the positive sequence current variable and the negative sequence current variable to one wind turbine inverter current variable;

[0083] Step C, according to the wind turbine inverter current variable, solve the positive and negative sequence current instructions through the single variable function maximum value, and use the solved positive and negative sequence current instructions as the final current instructions.

[0084] Specifically, the process of solving this problem through KKT conditions is as follows:

[0085] 1) Consider the positive sequence support alone:

[0086] ; (7)

[0087] in, λ is the dual multiplier in the KKT condition, I + is the total positive sequence current. The corresponding KKT condition is:

[0088] ; (8)

[0089] ; (9)

[0090] ; (10)

[0091] ; (11)

[0092] The positive sequence d-axis and q-axis currents that satisfy the above KKT conditions are:

[0093] ; (12)

[0094] The best positive sequence voltage support effect can be obtained by distributing the positive sequence current on the d-axis and q-axis according to formula (12). At this time, the grid-connected positive sequence voltage is:

[0095] ; (13)

[0096] In the formula, Z is the line impedance;

[0097] 2) Consider negative sequence voltage suppression separately:

[0098] ; (14)

[0099] in, γ is the dual multiplier in the KKT condition, I − is the total negative sequence current. The corresponding KKT condition is:

[0100] ; (15)

[0101] ; (16)

[0102] ; (17)

[0103] ; (18)

[0104] The negative sequence d-axis and q-axis currents that satisfy the above KKT conditions are:

[0105] ; (19)

[0106] The best negative sequence voltage suppression effect can be obtained by distributing the negative sequence current on the d-axis and q-axis according to formula (19). At this time, the grid-connected negative sequence voltage is:

[0107] ; (20)

[0108] 3) Considering both positive sequence voltage support and negative sequence voltage suppression:

[0109] Substituting equations (13) and (20) into the optimization model for suppressing unbalanced voltage at the wind turbine grid connection point, we can obtain:

[0110] ;(twenty one)

[0111] Since the positive sequence current and the negative sequence current together constitute the wind turbine inverter current, there is a direct relationship between the two, which can be further obtained:

[0112] ;(twenty two)

[0113] Formula (22) is a single variable function. By analyzing formula (22), the final current command can be obtained as:

[0114] ;(twenty three)

[0115] This embodiment is based on the KKT condition. By obtaining the current limitation of the wind turbine inverter and the wind turbine grid-connected condition, the grid-connected voltage imbalance is minimized. The wind turbine inverter current constraint, the line positive-sequence power flow constraint, the negative-sequence power flow constraint and the minimum positive-sequence voltage constraint are considered. An optimization model for suppressing the unbalanced voltage at the wind turbine grid-connected point is established and the optimized solution is obtained through the KKT condition as the current instruction, taking into account both positive-sequence voltage support and negative-sequence voltage suppression.

[0116] Step 4: Input the final current command into the inner current loop of the wind turbine inverter and implement command tracking through PI control.

[0117] In this embodiment, based on the actual measured current and the above-mentioned final current command, the wind turbine inverter current inner loop control is performed to realize unbalanced current command tracking, which provides a feasible solution for the wind turbine to solve the grid-connected unbalanced voltage problem.

[0118] Example 2

[0119] like Figure 3 As shown, Figure 3 The present embodiment is a block diagram of a system for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault. The difference between the present embodiment and the first embodiment is that the present embodiment provides a system for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault, and the system corresponds to the method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault in a one-to-one manner in the first embodiment; the system includes:

[0120] The first construction unit is used to obtain the current limit status of the wind turbine inverter and the wind turbine grid-connected status, and to construct a wind turbine inverter grid-connected voltage calculation model;

[0121] A second construction unit is used to construct an unbalanced voltage suppression optimization model for a wind turbine grid connection point under an asymmetric fault based on the wind turbine inverter grid connection voltage calculation model;

[0122] An optimization solving unit is used to optimize the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model through KKT conditions to minimize the objective function and calculate the final current instruction when the objective function is minimized; the objective function is the grid-connected voltage unbalance degree;

[0123] The control tracking unit is used to input the final current command into the wind turbine inverter current inner loop and realize command tracking through PI control.

[0124] As a further implementation, the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model is formula (1), formula (3) and formula (4) of Example 1.

[0125] As a further implementation, the optimization solution unit includes:

[0126] The first optimization subunit is used to optimize the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model through the KKT condition to obtain a positive-sequence current command allocation method and a negative-sequence current command allocation method when the positive-sequence voltage and the negative-sequence voltage achieve optimal values;

[0127] The second optimization subunit is used to analyze the distribution of positive-sequence current and negative-sequence current of the wind turbine inverter current based on the positive-sequence current command distribution method and the negative-sequence current command distribution method, and reduce the positive-sequence current variable and the negative-sequence current variable to one wind turbine inverter current variable;

[0128] The third optimization subunit is used to solve the positive and negative sequence current instructions through the maximum value of the single variable function according to the wind turbine inverter current variable, and use the solved positive and negative sequence current instructions as the final current instructions.

[0129] The execution process of each unit can be performed according to the process steps of the method for suppressing unbalanced voltage at the wind turbine grid connection point under an asymmetric fault in Example 1, and will not be described in detail in this embodiment.

[0130] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present application may adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes.

[0131] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0132] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0133] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0134] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault, characterized in that: The method includes: Obtain the wind turbine inverter current limit and wind turbine grid connection status, and build a wind turbine inverter grid connection voltage calculation model; Based on the wind turbine inverter grid-connected voltage calculation model, an unbalanced voltage suppression optimization model for wind turbine grid-connected points is constructed under asymmetric faults; The objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model is optimized by the KKT condition to minimize the objective function, and the final current command when the objective function is minimized is calculated; the objective function is the grid-connected voltage unbalance, and the grid-connected voltage unbalance refers to the ratio of the grid-connected negative sequence voltage to the grid-connected positive sequence voltage; The final current command is input into the inner current loop of the wind turbine inverter, and command tracking is achieved through PI control; The final current instruction is: In the formula, I d+ is the positive sequence d-axis current, I d- is the negative sequence d-axis current; I q+ is the positive sequence q-axis current, I q- is the negative-sequence q-axis current, R and X are the equivalent resistance and inductance of the line connecting the wind turbine to the remote power grid, respectively; V + is the grid-connected positive sequence voltage; V +min is the minimum permissible positive sequence grid-connected voltage; Z is the line impedance; I max It is the maximum current allowed by the wind turbine inverter.

2. The method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault according to claim 1 is characterized in that: The wind turbine inverter current limiting condition includes the maximum current allowed by the wind turbine inverter; The wind turbine grid-connected condition includes a positive sequence voltage at a remote end of the wind turbine grid-connected point, a negative sequence voltage at a remote end of the wind turbine grid-connected point, line impedance, and a minimum allowable positive sequence voltage.

3. The method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault according to claim 1, characterized in that: The wind turbine inverter grid-connected voltage calculation model refers to a calculation model of grid-connected positive-sequence voltage and negative-sequence voltage formed by the influence of line current, remote voltage and line impedance on the grid-connected voltage.

4. The method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault according to claim 1, characterized in that: The objective function of the optimization model for suppressing unbalanced voltage at wind turbine grid connection point is: Where VUF is the grid voltage unbalance degree; V + is the grid positive sequence voltage, V - is the grid-connected negative sequence voltage; V g+ and V g- are the positive sequence voltage and negative sequence voltage of the remote power grid respectively; R and X are the equivalent resistance and inductance of the line connecting the wind turbine to the remote power grid respectively; I d+ is the positive sequence d-axis current, I d- is the negative sequence d-axis current; I q+ is the positive sequence q-axis current, I q- is the negative sequence q-axis current.

5. The method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault according to claim 1, characterized in that: When optimizing the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model, the constraints adopted include wind turbine inverter current constraints, line positive sequence power flow constraints, line negative sequence power flow constraints and minimum positive sequence voltage constraints.

6. The method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault according to claim 1, characterized in that: The objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model is optimized by the KKT condition to minimize the objective function, and the final current instruction when the objective function is minimized is calculated, including: The objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model is optimized through the KKT condition to obtain a positive-sequence current command allocation method and a negative-sequence current command allocation method when the positive-sequence voltage and the negative-sequence voltage have optimal values; Based on the positive sequence current command allocation method and the negative sequence current command allocation method, the current allocation of the wind turbine inverter is analyzed to reduce the positive sequence current variable and the negative sequence current variable to one wind turbine inverter current variable. According to the wind turbine inverter current variable, the positive and negative sequence current instructions are solved by the maximum value of the single variable function, and the solved positive and negative sequence current instructions are used as the final current instructions.

7. A method for suppressing unbalanced voltage at a wind turbine grid connection point under an asymmetric fault according to claim 6, characterized in that: A positive sequence current command allocation method and a negative sequence current command allocation method are obtained when the positive sequence voltage and the negative sequence voltage have optimal values, including: Through the KKT condition, the positive sequence current command allocation method and the negative sequence current command allocation method are determined; wherein the positive sequence current command allocation method is the relationship between the positive sequence d-axis current and the positive sequence q-axis current when the positive sequence voltage reaches the maximum value; the negative sequence current command allocation method is the relationship between the negative sequence d-axis current and the negative sequence q-axis current when the negative sequence voltage reaches the minimum value; In the above determination process, the four current variables of the positive-sequence d-axis current, the positive-sequence q-axis current, the negative-sequence d-axis current and the negative-sequence q-axis current are reduced to the positive-sequence current variable and the negative-sequence current variable; The relationship between the positive-sequence d-axis current and the positive-sequence q-axis current when the positive-sequence voltage reaches its maximum value is: Where R and X are the equivalent resistance and inductance of the line connecting the wind turbine to the remote power grid; I d+ is the positive sequence d-axis current, I q+ is the positive sequence q-axis current, Z is the line impedance; I + is the grid-connected positive sequence current; The relationship between the negative-sequence d-axis current and the negative-sequence q-axis current when the negative-sequence voltage reaches the minimum value is: In the formula, I d- is the negative-sequence d-axis current, I q- is the negative sequence q-axis current, I - is the grid-connected negative sequence current.

8. A system for suppressing unbalanced voltage at wind turbine grid connection points under asymmetric faults, characterized in that: The system includes: The first construction unit is used to obtain the current limit status of the wind turbine inverter and the wind turbine grid-connected status, and to construct a wind turbine inverter grid-connected voltage calculation model; A second construction unit is used to construct an unbalanced voltage suppression optimization model for a wind turbine grid connection point under an asymmetric fault based on the wind turbine inverter grid connection voltage calculation model; An optimization solving unit is used to optimize the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model through KKT conditions to minimize the objective function and calculate the final current instruction when the objective function is minimized; the objective function is the grid-connected voltage unbalance, and the grid-connected voltage unbalance refers to the ratio of the grid-connected negative sequence voltage to the grid-connected positive sequence voltage; A control tracking unit, used for inputting the final current command into the inner current loop of the wind turbine inverter, and realizing command tracking through PI control; The final current instruction is: In the formula, I d+ is the positive sequence d-axis current, I d- is the negative sequence d-axis current; I q+ is the positive sequence q-axis current, I q- is the negative-sequence q-axis current, R and X are the equivalent resistance and inductance of the line connecting the wind turbine to the remote power grid, respectively; V + is the grid-connected positive sequence voltage; V +min is the minimum permissible positive sequence grid-connected voltage; Z is the line impedance; I max It is the maximum current allowed by the wind turbine inverter.

9. The unbalanced voltage suppression system for wind turbine grid connection points under asymmetric faults according to claim 8 is characterized in that: The objective function of the optimization model for suppressing unbalanced voltage at wind turbine grid connection point is: Where VUF is the grid voltage unbalance degree; V + is the grid positive sequence voltage, V - is the grid-connected negative sequence voltage; V g+ and V g- are the positive sequence voltage and negative sequence voltage of the remote power grid respectively; R and X are the equivalent resistance and inductance of the line connecting the wind turbine to the remote power grid respectively; I d+ is the positive sequence d-axis current, I d- is the negative sequence d-axis current; I q+ is the positive sequence q-axis current, I q- is the negative sequence q-axis current.

10. The system for suppressing unbalanced voltage at wind turbine grid connection points under asymmetric faults according to claim 8, characterized in that: The optimization solution unit comprises: The first optimization subunit is used to optimize the objective function of the wind turbine grid-connected point unbalanced voltage suppression optimization model through the KKT condition to obtain a positive-sequence current command allocation method and a negative-sequence current command allocation method when the positive-sequence voltage and the negative-sequence voltage achieve optimal values; The second optimization subunit is used to analyze the distribution of positive-sequence current and negative-sequence current of the wind turbine inverter current based on the positive-sequence current command distribution method and the negative-sequence current command distribution method, and reduce the positive-sequence current variable and the negative-sequence current variable to one wind turbine inverter current variable; The third optimization subunit is used to solve the positive and negative sequence current instructions through the maximum value of the single variable function according to the wind turbine inverter current variable, and use the solved positive and negative sequence current instructions as the final current instructions.

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