Method, system and equipment for calculating ultimate resection angle of network construction type converter system and storage medium

Abstract

The invention discloses a method, a system and equipment for calculating a limit cut-off angle of a network-building type converter system and a storage medium, and the method comprises the steps: building a network-building type converter active ring mathematical model, and building a network-building type converter system limit cut-off angle solving equation considering a damping term based on an equal-area rule; solving the active loop mathematical model by adopting a numerical method; according to a solving result, obtaining a function expression of a damping item of the network-building type converter during a fault period through a fitting function, and further obtaining an acceleration area of the network-building type converter system; adopting a geometric approximation method to obtain the deceleration area of the network-forming converter system after the fault is removed; substituting the expressions of the acceleration area and the deceleration area of the network construction type converter system into the limit cut-off angle solving equation, and calculating to obtain a limit cut-off angle of the network construction type converter system considering the damping item; compared with a traditional method, the method considers the influence of the damping term of the grid-forming converter, reduces the conservative property of a solving result, and is more in line with an actual engineering application scene.

Description

Technical Field

[0001] The present invention relates to a method, system, device and storage medium for calculating a limit resection angle, and in particular to a method, system, device and storage medium for calculating a limit resection angle of a grid-connected converter system, and belongs to the technical field of safe and stable operation control of power grids. Background Art

[0002] In recent years, a large number of new energy sources such as wind and solar power have been connected to the power grid in my country, and the proportion of power electronic equipment in the power system has continued to increase, which has fundamentally changed the structure, physical form, and dynamic characteristics of the traditional power system dominated by synchronous generators. The synchronous stability of the traditional power system requires that the power angle difference between the synchronous generator sets running in parallel is bounded and the frequency is maintained within the allowable range []. It is an important basis for ensuring the safe and stable operation of the power system. With the access of new energy power generation equipment, the composition of the power generation equipment in the system has become more complex and there is strong interactive coupling between the equipment. The synchronous stability problem of the high-proportion new energy power system has taken on new forms and new connotations.

[0003] The grid-connected converter achieves autonomous synchronization with the power grid based on active power or DC voltage, and has certain active support and networking operation capabilities. Although the grid-connected converter using virtual synchronous control simulates the synchronization characteristics of the synchronous generator, the control synchronization mechanism of the converter has the characteristics of control strategy dominance, fast response speed, significant switching and discreteness, which is essentially different from the physical synchronization mechanism of the traditional synchronous generator. The original analysis method applicable to the synchronization stability problem of synchronous generator grid connection is difficult to be directly applied to the grid-connected converter. With the increase in the proportion of grid-connected converters in new energy grid-connected equipment, its disconnection from the grid will have a greater impact on the safe and stable operation of the system. Therefore, the grid-connected synchronization stability of the grid-connected converter during transients has become the focus of many studies. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a method, system, device and storage medium for calculating the limit cut-off angle of a grid-connected converter system, which can reduce the conservatism of the solution results.

[0005] Technical solution: A method for calculating the limit cut-off angle of a grid-connected converter system according to the present invention comprises:

[0006] S1: Construct the mathematical model of the active loop of the grid-type converter, and establish the solution equation of the limit cut-off angle of the grid-type converter system taking into account the damping term based on the equal area rule;

[0007] S2: Use numerical methods to solve the mathematical model of the active loop;

[0008] S3: According to the solution result, the function expression of the damping term of the grid-type converter during the fault period is obtained by fitting the function, and then the acceleration area of ​​the grid-type converter system is obtained;

[0009] S4: The deceleration area of ​​the grid-type converter system after fault removal is obtained by using the geometric approximation method;

[0010] S5: Substitute the expressions of the acceleration area and deceleration area of ​​the grid-type converter system into the limit cut-off angle solution equation, and calculate the limit cut-off angle of the grid-type converter system taking into account the damping term.

[0011] Furthermore, the step S1 of constructing the active loop mathematical model of the grid-connected converter is specifically as follows:

[0012] The grid-connected converter adopts virtual synchronous control and simulates the rotor motion equation of the synchronous generator through the active loop:

[0013]

[0014] In the formula, J and D p are virtual inertia and virtual damping respectively; ω and ω ref are the virtual angular frequency generated by the active loop of the grid-connected converter and the rated angular frequency of the grid; P and P ref are the actual value and command value of the converter output active power respectively;

[0015] D in formula (1) p (ω-ω ref ) is the damping term of the grid-type converter, which can be further transformed into:

[0016]

[0017] In the formula, δ v is the virtual power angle of the grid-connected converter, which is defined as the virtual angular frequency ω generated by the active loop of the grid-connected converter and the rated angular frequency ω of the grid ref Difference:

[0018] δ v =ω(ω-ω ref )dt (3)

[0019] Substituting equation (2) and equation (3) into equation (1) and further transforming it, we get the mathematical model of the active loop of the grid-connected converter:

[0020]

[0021] Furthermore, in step S1, the limiting cut-off angle solution equation of the grid-type converter system taking into account the damping term is established based on the equal area rule, specifically:

[0022] According to the equal area rule, the condition for the virtual power angle of the grid-type converter system to satisfy the first pendulum stability after a fault occurs is that the deceleration area formed by the first pendulum is less than or equal to the deceleration area after the fault is removed. Taking into account the damping term of the grid-type converter, the condition for the grid-type converter system to satisfy the first pendulum stability is obtained:

[0023]

[0024] Where: a is the virtual power angle corresponding to the starting time of the grid-connected converter; δ c is the virtual power angle corresponding to the fault removal moment of the grid-connected converter; δ e is the maximum swing angle of the grid-type converter; P f is the active power output by the converter during the fault period;

[0025] Active power P output by the converter during the fault f It can be obtained by the following formula:

[0026]

[0027] Where: E is the internal potential of the grid-connected converter; U g is the voltage vector amplitude on the grid side; m is the fault; X g It is the line reactance from the grid-connected converter port to the grid connection point;

[0028] After the fault is removed, if the rate of change of the virtual power angle is equal to 0 when the grid-type converter runs to the limit swing angle, the first swing of the converter virtual power angle meets the limit condition of stability. At this time, the corresponding removal angle is the limit removal angle CCA, and the acceleration area of ​​the first swing is equal to the deceleration area. The critical stability condition of the first swing of the grid-type converter system after considering the damping term is obtained:

[0029]

[0030] Where: CCA is the limit cut-off angle of the grid-type converter; δ f is the limit swing angle of the grid-type converter;

[0031] The limit cut-off angle of the grid-type converter is obtained by solving the δ CCA get.

[0032] Furthermore, the step S2 specifically includes:

[0033] The mathematical model of the active power loop is solved by numerical method:

[0034]

[0035] Where: the solution time is t = ih, h is the step length and i is a positive integer; yi is the virtual power angle δ v A discrete sequence of time t; y i ' is the virtual power angle δ v The rate of change of the virtual angular frequency ω with respect to the virtual power angle δ v discrete sequence; y0 and y′0 are y i and i 'The initial value;

[0036] Solve equation (8) by iteration to get dδ v / dt about δ v The discrete sequence [dδ v / dt,δ v ].

[0037] Furthermore, the step S3 specifically includes:

[0038] The polynomial function fitting method was used to v / dt about δ v The fitting function approximates the discrete sequence [dδ v / dt,δ v ]; using the polynomial function g(δ v ) is used to fit the discrete sequence. The expression of the fitting function is as follows:

[0039]

[0040] Where: k0 to k7 are fitting functions g(δ v )’s various order coefficients;

[0041] Substituting the obtained fitting function formula (9) into formula (2), we can get the damping term with respect to δ v The function expression is:

[0042]

[0043] Substituting formula (10) into the solution equation of the limit cut-off angle of the grid-type converter, that is, the left side of formula (7), the acceleration area of ​​the grid-type converter system is obtained as shown in the following formula:

[0044]

[0045] Furthermore, the step S4 specifically includes:

[0046] The geometric approximate substitution method is used to solve the deceleration area of ​​the grid-type converter system after fault removal. First, the grid-type converter damping term is included in the reference value P of the converter output active power. ref And record it as P refD , the expression is as follows:

[0047] P refD =P ref -D p (ω-ω ref ) (2)

[0048] According to formula (7), the existence of the damping term of the grid-type converter increases the deceleration area of ​​the converter after the fault is removed. Even if the value of the right half of the formula (7) increases, the increase in the deceleration area is P refD and P ref The area enclosed by the limit resection angle CCA is assumed to be refD The intersection point is g, and P ref The intersection point is h, then the area of ​​the fan fgh is the deceleration area increased after the damping term is taken into account; the area of ​​the triangle Δfgh formed by points f, g and h is used to approximate the area of ​​the fan fgh. According to the fitting function g(δ v ) and the symmetry of the sine curve, the area enclosed by the triangle Δfgh is expressed as:

[0049]

[0050] Where: D p g(CCA) and (π-δ a -CCA) are the lengths of the two sides of the triangle Δfgh, gh and fh;

[0051] Then, the damping term on the right side of equation (7) is expressed as v The definite integral of is approximately expressed as:

[0052]

[0053] Substituting equation (14) into the right half of equation (7) for solving the limit cut-off angle of the grid-type converter, the deceleration area of ​​the grid-type converter system is obtained as:

[0054]

[0055] Furthermore, the step S5 specifically includes:

[0056] Substitute the expressions of the acceleration area and deceleration area of ​​the grid-type converter system obtained in steps S3 and S4 into the grid-type converter limit cut-off angle to solve the left and right sides of equation (7) to obtain:

[0057]

[0058] Simplifying formula (16) we can obtain:

[0059]

[0060] Where: G(x) is the primitive function of g(x), that is, G′(x)=g(x); P N =3EU g / 2X g ;

[0061] According to the equal-area rule, if the grid-type converter satisfies the first pendulum stability condition, the value range of the limit cut-off angle is in the interval [0,π]. The exact root of the equation is solved by the numerical iteration method to obtain the limit cut-off angle of the grid-type converter system taking into account the damping term.

[0062] Based on the same inventive concept, the present invention also provides a grid-type converter system limit cut-off angle calculation system, comprising:

[0063] A construction module is used to construct a mathematical model of the active loop of a grid-type converter, and to establish a solution equation for the limit cut-off angle of the grid-type converter system taking into account the damping term based on the equal-area rule;

[0064] Model solving module, using numerical method to solve the mathematical model of active power loop;

[0065] An acceleration area solving module is used to obtain a function expression of a damping term of a grid-type converter during a fault period by fitting a function according to a solving result, and then obtain an acceleration area of ​​the grid-type converter system;

[0066] A deceleration area solving module is used to obtain the deceleration area of ​​the grid-type converter system after the fault is removed by using a geometric approximation method;

[0067] The calculation module is used to substitute the expressions of the acceleration area and the deceleration area of ​​the grid-type converter system into the limit cut-off angle solution equation, and calculate the limit cut-off angle of the grid-type converter system taking the damping term into account.

[0068] Based on the same inventive concept, the present invention also provides a computing device, comprising: one or more processors, one or more memories and one or more programs, wherein the programs are stored in the memories and configured to be executed by the processors, and when the programs are loaded into the processors, the steps of calculating the limiting cut-off angle of the grid-type converter system according to any one of the above items are implemented.

[0069] Based on the same inventive concept, the present invention also provides a storage medium, which stores a computer program, and the computer program includes program instructions. When the program instructions are executed by a processor, the processor executes the steps of the method for calculating the limiting cut-off angle of a grid-type converter system according to any one of the above items.

[0070] Beneficial effects: Compared with the prior art, the present invention provides an indicator for judging the synchronous stability margin of a grid-type converter system by solving the limit cut-off angle, which can provide guidance for the research on methods for improving the synchronous stability of grid-type converter systems in the future; compared with traditional methods, the present invention takes into account the influence of the damping term of the grid-type converter, reduces the conservatism of the solution, and is more in line with actual engineering application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 is a flow chart of a method according to an embodiment of the present invention;

[0072] Figure 2 A schematic diagram of a geometric method for solving a deceleration area of ​​a grid-type converter system according to an embodiment of the present invention;

[0073] Figure 3 It is an equivalent circuit diagram of a single-machine infinite system of a grid-type converter according to an embodiment of the present invention;

[0074] Figure 4 This is a simulation result diagram when the limit resection angle is 2.0969 rad in an example of an embodiment of the present invention;

[0075] Figure 5 This is a simulation result diagram when the limit resection angle is 2.1047 rad in an example of an embodiment of the present invention;

[0076] Figure 6 This is a simulation result diagram when the limit resection angle is 2.109 rad in an example of an embodiment of the present invention. DETAILED DESCRIPTION

[0077] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application.

[0078] As attached Figure 1 As shown, in view of the deficiencies of the existing research, in order to obtain the index of the synchronous stability margin of the grid-type converter system, and take into account the influence of the damping term of the grid-type converter to reduce the conservatism of the solution result, the calculation method of the limit cut-off angle of the grid-type converter system of this embodiment includes:

[0079] Step 1: Construct a mathematical model of the active loop of the grid-type converter, and establish a solution equation for the limit cut-off angle of the grid-type converter system taking into account the damping term based on the equal area rule;

[0080] Step 2: Using numerical methods to solve the established active loop mathematical model;

[0081] Step 3: According to the solution results, the function expression of the damping term of the grid-type converter during the fault period is obtained by fitting the function, and then the acceleration area of ​​the grid-type converter system is obtained;

[0082] Step 4: Use geometric approximation method to obtain the deceleration area of ​​the grid-type converter system after the fault is removed;

[0083] Step 5: Substitute the expressions of the acceleration area and deceleration area of ​​the grid-type converter system into the limit cut-off angle solution equation, and calculate the limit cut-off angle of the grid-type converter system taking into account the damping term.

[0084] The step 1 specifically includes:

[0085] The grid-connected converter adopts virtual synchronous control, and its active loop simulates the rotor motion equation of the synchronous generator. The control equation is shown in equation (1).

[0086]

[0087] Where: J and D p are virtual inertia and virtual damping respectively; ω and ω ref are the virtual angular frequency generated by the active loop of the grid-connected converter and the rated angular frequency of the grid; P and P ref They are the actual value and command value of the converter output active power respectively.

[0088] D in formula (1) p (ω-ω ref ) is the damping term of the grid-type converter, which can be further transformed as shown in formula (2).

[0089]

[0090] In formula (2), δ v is the virtual power angle of the grid-connected converter, which is defined as the virtual angular frequency ω generated by the active loop of the grid-connected converter and the rated angular frequency ω of the grid ref The difference is shown in formula (3).

[0091] δ v =∫(ω-ω ref )dt (3)

[0092] Substituting equation (2) and equation (3) into equation (1) and further transforming them, we can get the mathematical model of the active loop of the grid-connected converter as shown in equation (4).

[0093]

[0094] According to the equal area rule, the condition for the virtual power angle of the grid-type converter system to satisfy the first pendulum stability after a fault occurs is that the deceleration area formed by the first pendulum is less than or equal to the deceleration area after the fault is removed. If the damping term of the grid-type converter is taken into account, the condition for the grid-type converter system to satisfy the first pendulum stability can be expressed as shown in formula (5).

[0095]

[0096] Where: a is the virtual power angle corresponding to the starting time of the grid-connected converter; δ c is the virtual power angle corresponding to the fault removal moment of the grid-connected converter; δ e is the maximum swing angle of the grid-type converter; P f is the active power output by the converter during the fault period.

[0097] Active power P output by the converter during the fault f It can be obtained by formula (6).

[0098]

[0099] Where: E is the internal potential of the grid-connected converter; U g is the voltage vector amplitude on the grid side; m is the fault; X g It is the line reactance from the grid-connected converter port to the grid connection point.

[0100] After the fault is removed, if the rate of change of the virtual power angle is equal to 0 when the grid-type converter runs to the limit swing angle, the first swing of the converter virtual power angle just meets the limit condition of stability. The corresponding clearance angle in this case is called the critical clearing angle (CCA), and the acceleration area of ​​the first swing is equal to the deceleration area. Therefore, the critical stability condition of the first swing of the grid-type converter system after considering the damping term is shown in formula (7). Solving the δ in the formula CCA The limit cut-off angle of the grid-type converter can be obtained.

[0101]

[0102] Where: CCA is the limit cut-off angle of the grid-type converter; δ f is the limit swing angle of the grid-type converter.

[0103] The step 2 specifically includes:

[0104] In step 1, the mathematical model of the active loop of the grid-connected converter is established as shown in equation (4), and the mathematical model of the active loop is solved by a numerical method as shown in equation (8).

[0105]

[0106] Where: the solution time is t = ih, h is the step length and i is a positive integer; y i is the virtual power angle δ v A discrete sequence of time t; y i' is the virtual power angle δ v The rate of change of the virtual angular frequency ω with respect to the virtual power angle δ v discrete sequence; y0 and y′0 are y i and i 'The initial value of .

[0107] The step three specifically includes:

[0108] By solving equation (8) in step 2 by iterative method, we can get dδ v / dt about δ v The discrete sequence [dδ v / dt,δ v ], in order to obtain the damping term in formula (2) with respect to δ v The function expression of dδ is further expressed by using the polynomial function fitting method. v / dt about δ v The fitting function approximates the discrete sequence [dδ v / dt,δ v ]. Using the polynomial function g(δ v ) is used to fit the discrete sequence, and the expression of the fitting function is shown in formula (9).

[0109]

[0110] Where: k0 to k7 are fitting functions g(δ v )'s various order coefficients.

[0111] Substituting the obtained fitting function formula (9) into formula (2), we can get the damping term with respect to δ v The function expression of is shown in formula (10).

[0112]

[0113] Substituting equation (10) into the left half of equation (7) for solving the limiting cut-off angle of the grid-type converter, we can obtain the acceleration area of ​​the grid-type converter system as shown in equation (11).

[0114]

[0115] The step 4 specifically includes:

[0116] The geometric approximate substitution method is used to solve the deceleration area of ​​the grid-type converter system after fault removal. First, the grid-type converter damping term is included in the reference value P of the converter output active power. ref And record it as P refD , the expression is shown in formula (1).

[0117] P refD =Pref -D p (ω-ω ref ) (3)

[0118] According to formula (7), the existence of the damping term of the grid-type converter increases the deceleration area of ​​the converter after the fault is removed. Even if the value of the right half of the formula (7) increases, the increase in the deceleration area is P refD and P ref The area enclosed is as follows Figure 2 As shown. Figure 2 It can be seen that the perpendicular line passing through the limit resection angle CCA and P refD Intersecting at point g and P ref The area of ​​the sector fgh is the deceleration area increased after the damping term is taken into account. In this paper, the area of ​​the triangle Δfgh formed by points f, g, and h is used to approximate the area of ​​the sector fgh, as shown in the attached figure. Figure 2 As shown in the green triangle in the figure. According to the fitting function g(δ v ) and the symmetry of the sine curve, the area enclosed by the triangle Δfgh can be expressed as shown in formula (13).

[0119]

[0120] Where: D p g(CCA) and (π-δ a -CCA) are the lengths of the two sides of the triangle Δfgh, gh and fh respectively.

[0121] Therefore, the definite integral of the damping term on the right side of equation (7) with respect to can be approximately expressed as shown in equation (14).

[0122]

[0123] Substituting equation (14) into the right half of equation (7) for solving the limiting cut-off angle of the grid-type converter, we can obtain the deceleration area of ​​the grid-type converter system as shown in equation (15).

[0124]

[0125] The step five specifically includes:

[0126] Substituting the expressions of the acceleration area and deceleration area of ​​the grid-type converter system obtained in steps 3 and 4 (Equations (11) and (15)) into the grid-type converter limit cut-off angle to solve the left and right sides of equation (7) yields:

[0127]

[0128] Further simplifying formula (16) yields:

[0129]

[0130] Where: G(x) is the primitive function of g(x), that is, G′(x)=g(x); P N =3EU g / 2X g .

[0131] According to the equal area rule, if the grid-type converter satisfies the first pendulum stability condition, the value range of the limit cut-off angle must be in the interval [0,π]. Therefore, the root of equation (17) can be guessed initially, and then the exact root of the equation is solved by the numerical iteration method. The result is the limit cut-off angle of the grid-type converter system taking into account the damping term.

[0132] In order to illustrate that the present invention can effectively calculate the limit cut-off angle of the grid-type converter system and provide an indicator for judging the synchronization stability margin of the grid-type converter system, the effectiveness of the method and system for calculating the limit cut-off angle of the grid-type converter system based on the equal area rule and taking the damping term into account proposed by the present invention are illustrated through the following specific examples.

[0133] In this example, a grid-type converter single-machine infinite control system is built in the PSCAD / EMTDC simulation platform. Figure 3 As shown. System parameters Main parameters Virtual damping is 15000N·m·s / rad, virtual inertia is 314kg / m 2 , assuming that the grid voltage drops to 0.1pu during the fault, the limit resection angle calculation method and system of the grid-type converter system based on the equal area law and taking into account the damping term proposed in the present invention is used to obtain the limit resection angle of 2.0984rad in this example. In order to verify the accuracy of the calculation results of the present invention, this paper simulates three cases in which the limit resection angle is less than and greater than the limit resection angle calculated by the present invention in the PSCCAD / EMTDC simulation platform. The simulation curve is as follows: Figure 4 , Figure 5 and Figure 6As shown. It can be seen from the simulation curve that when the limit cut-off angle used in the simulation is smaller than the limit cut-off angle calculated by the present invention, the grid-type converter system can maintain synchronous stability after the fault is removed; when the limit cut-off angle used in the simulation exceeds a certain range of the limit cut-off angle calculated by the present invention, the grid-type converter system becomes unstable after the fault is removed. Combined with the simulation curve, it can be seen that the limit cut-off angle obtained by simulation is slightly larger than the result obtained by quantitative calculation of the present invention between 2.1047-2.109rad, indicating that the result obtained by the present invention is relatively accurate and has a certain degree of conservatism. Therefore, according to this example, it can be seen that the method and system for calculating the limit cut-off angle of the grid-type converter system based on the equal area rule and taking into account the damping term proposed by the present invention are effective and accurate, can guide the parameter adjustment and control optimization of the grid-type converter system, and are beneficial to improving the synchronous stability of the grid-type converter system.

[0134] Based on the same inventive concept, this embodiment also provides a grid-type converter system limit cut-off angle calculation system, including:

[0135] A construction module is used to construct a mathematical model of the active loop of a grid-type converter, and to establish a solution equation for the limit cut-off angle of the grid-type converter system taking into account the damping term based on the equal-area rule;

[0136] Model solving module, using numerical method to solve the mathematical model of active power loop;

[0137] An acceleration area solving module is used to obtain a function expression of a damping term of a grid-type converter during a fault period by fitting a function according to a solving result, and then obtain an acceleration area of ​​the grid-type converter system;

[0138] A deceleration area solving module is used to obtain the deceleration area of ​​the grid-type converter system after the fault is removed by using a geometric approximation method;

[0139] The calculation module is used to substitute the expressions of the acceleration area and the deceleration area of ​​the grid-type converter system into the limit cut-off angle solution equation, and calculate the limit cut-off angle of the grid-type converter system taking the damping term into account.

[0140] Based on the same inventive concept, this embodiment also provides a computing device, including: one or more processors, one or more memories and one or more programs, wherein the programs are stored in the memories and configured to be executed by the processors, and when the programs are loaded into the processors, the steps of calculating the limiting cut-off angle of the grid-type converter system according to any one of the above items are implemented.

[0141] Based on the same inventive concept, this embodiment also provides a storage medium, which stores a computer program, and the computer program includes program instructions. When the program instructions are executed by a processor, the processor executes the steps of the method for calculating the limiting cut-off angle of a grid-type converter system according to any one of the above items.

Claims

1. A method for calculating the limit cut-off angle of a grid-connected converter system, characterized in that: include: S1: Construct the mathematical model of the active loop of the grid-type converter, and establish the solution equation of the limit cut-off angle of the grid-type converter system taking into account the damping term based on the equal area rule; S2: Use numerical methods to solve the mathematical model of the active loop; S3: According to the solution result, the function expression of the damping term of the grid-type converter during the fault period is obtained by fitting the function, and then the acceleration area of ​​the grid-type converter system is obtained; S4: The deceleration area of ​​the grid-type converter system after fault removal is obtained by using the geometric approximation method; S5: Substitute the expressions of the acceleration area and deceleration area of ​​the grid-type converter system into the limit cut-off angle solution equation, and calculate the limit cut-off angle of the grid-type converter system taking into account the damping term.

2. The method for calculating the limit cut-off angle of a grid-connected converter system according to claim 1, characterized in that: The step S1 of constructing a mathematical model of the active loop of the grid-connected converter is specifically as follows: The grid-connected converter adopts virtual synchronous control and simulates the rotor motion equation of the synchronous generator through the active loop: In the formula, J and D p are virtual inertia and virtual damping respectively; ω and ω ref are the virtual angular frequency generated by the active loop of the grid-connected converter and the rated angular frequency of the grid; P and P ref are the actual value and command value of the converter output active power respectively; D in formula (1) p (ω-ω ref ) is the damping term of the grid-type converter, which can be further transformed into: In the formula, δ v is the virtual power angle of the grid-connected converter, which is defined as the virtual angular frequency ω generated by the active loop of the grid-connected converter and the rated angular frequency ω of the grid ref Difference: d v =∫(ω-ω ref )dt (3) Substituting equation (2) and equation (3) into equation (1) and further transforming it, we get the mathematical model of the active loop of the grid-connected converter:

3. The method for calculating the limit cut-off angle of a grid-connected converter system according to claim 2, characterized in that: The equation for solving the limit cut-off angle of the grid-type converter system taking into account the damping term is established based on the equal area rule in step S1, specifically: According to the equal area rule, the condition for the virtual power angle of the grid-type converter system to satisfy the first pendulum stability after a fault occurs is that the deceleration area formed by the first pendulum is less than or equal to the deceleration area after the fault is removed. Taking into account the damping term of the grid-type converter, the condition for the grid-type converter system to satisfy the first pendulum stability is obtained: Where: a is the virtual power angle corresponding to the starting time of the grid-connected converter; δ c is the virtual power angle corresponding to the fault removal moment of the grid-connected converter; δ e is the maximum swing angle of the grid-type converter; P f is the active power output by the converter during the fault period; Active power P output by the converter during the fault f It can be obtained by the following formula: Where: E is the internal potential of the grid-connected converter; U g is the voltage vector amplitude on the grid side; m is the fault; X g It is the line reactance from the grid-connected converter port to the grid connection point; After the fault is removed, if the rate of change of the virtual power angle is equal to 0 when the grid-type converter runs to the limit swing angle, the first swing of the converter virtual power angle meets the limit condition of stability. At this time, the corresponding removal angle is the limit removal angle CCA, and the acceleration area of ​​the first swing is equal to the deceleration area. The critical stability condition of the first swing of the grid-type converter system after considering the damping term is obtained: Where: CCA is the limit cut-off angle of the grid-type converter; δ f is the limit swing angle of the grid-type converter; The limit cut-off angle of the grid-type converter is obtained by solving the δ CCA get.

4. The method for calculating the limit cut-off angle of a grid-connected converter system according to claim 3, characterized in that: The step S2 specifically includes: The mathematical model of the active power loop is solved by numerical method: Where: the solution time is t = ih, h is the step length, i is a positive integer; y i is the virtual power angle δ v A discrete sequence of time t; y i ' is the virtual power angle δ v The rate of change of the virtual angular frequency ω with respect to the virtual power angle δ v discrete sequence; y0 and y′0 are y i and i 'The initial value; Solve equation (8) by iteration to get dδ v / dt about δ v The discrete sequence [dδ v / dt,δ v ].

5. The method for calculating the limit cut-off angle of a grid-connected converter system according to claim 4, characterized in that: The step S3 specifically includes: The polynomial function fitting method was used to v / dt about δ v The fitting function approximates the discrete sequence [dδ v / dt,δ v ]; using the polynomial function g(δ v ) is used to fit the discrete sequence. The expression of the fitting function is as follows: Where: k0 to k7 are fitting functions g(δ v )’s various order coefficients; Substituting the obtained fitting function formula (9) into formula (2), we can get the damping term with respect to δ v The function expression is: Substituting formula (10) into the solution equation of the limit cut-off angle of the grid-type converter, that is, the left side of formula (7), the acceleration area of ​​the grid-type converter system is obtained as shown in the following formula:

6. The method for calculating the limit cut-off angle of a grid-connected converter system according to claim 5, characterized in that: The step S4 specifically includes: The geometric approximate substitution method is used to solve the deceleration area of ​​the grid-type converter system after fault removal. First, the grid-type converter damping term is included in the reference value P of the converter output active power. ref And record it as P refD , the expression is as follows: P refD =P ref -D p (oh-oh ref ) (1) According to formula (7), the existence of the damping term of the grid-type converter increases the deceleration area of ​​the converter after the fault is removed. Even if the value of the right half of the formula (7) increases, the increase in the deceleration area is P refD and P ref The area enclosed by the limit resection angle CCA is assumed to be refD The intersection point is g, and P ref The intersection point is h, then the area of ​​the fan fgh is the deceleration area increased after the damping term is taken into account; the area of ​​the triangle Δfgh formed by points f, g and h is used to approximate the area of ​​the fan fgh. According to the fitting function g(δ v ) and the symmetry of the sine curve, the area enclosed by the triangle Δfgh is expressed as: Where: D p g(CCA) and (π-δ a -CCA) are the lengths of the two sides of the triangle Δfgh, gh and fh; Then, the damping term on the right side of equation (7) is expressed as v The definite integral of is approximately expressed as: Substituting equation (14) into the right half of equation (7) for solving the limit cut-off angle of the grid-type converter, the deceleration area of ​​the grid-type converter system is obtained as:

7. The method for calculating the limit cut-off angle of a grid-connected converter system according to claim 6, characterized in that: The step S5 specifically includes: Substitute the expressions of the acceleration area and deceleration area of ​​the grid-type converter system obtained in steps S3 and S4 into the grid-type converter limit cut-off angle to solve the left and right sides of equation (7) to obtain: Simplifying formula (16) we can obtain: Where: G(x) is the primitive function of g(x), that is, G′(x)=g(x); P N =3EU g / 2X g ; According to the equal-area rule, if the grid-type converter satisfies the first pendulum stability condition, the value range of the limit cut-off angle is in the interval [0,π]. The exact root of the equation is solved by the numerical iteration method to obtain the limit cut-off angle of the grid-type converter system taking into account the damping term.

8. A grid-type converter system limit cut-off angle calculation system, characterized in that: include: A construction module is used to construct a mathematical model of the active loop of a grid-type converter, and to establish a solution equation for the limit cut-off angle of the grid-type converter system taking into account the damping term based on the equal-area rule; Model solving module, using numerical method to solve the mathematical model of active power loop; An acceleration area solving module is used to obtain a function expression of a damping term of a grid-type converter during a fault period by fitting a function according to a solving result, and then obtain an acceleration area of ​​the grid-type converter system; A deceleration area solving module is used to obtain the deceleration area of ​​the grid-type converter system after the fault is removed by using a geometric approximation method; The calculation module is used to substitute the expressions of the acceleration area and the deceleration area of ​​the grid-type converter system into the limit cut-off angle solution equation, and calculate the limit cut-off angle of the grid-type converter system taking the damping term into account.

9. A computing device, characterized in that include: One or more processors, one or more memories and one or more programs, wherein the programs are stored in the memories and are configured to be executed by the processors, and when the programs are loaded into the processors, the steps of calculating the limiting cut-off angle of the grid-type converter system according to any one of claims 1 to 7 are implemented.

10. A storage medium, characterized in that: The storage medium stores a computer program, which includes program instructions. When the program instructions are executed by a processor, the processor executes the steps of the method for calculating the limit cut-off angle of a grid-type converter system according to any one of claims 1 to 7.

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