Transient stability analysis method for network construction type wind power converter

Abstract

The invention relates to a transient stability analysis method for a network-building type wind power converter, and belongs to the technical field of network-building type wind power converter control, and the method comprises the following steps: S1, decomposing different linear solvable equations and nonlinear perturbation functions based on a standardized power angle swing equation, and providing a standardized model for the application of a parameter perturbation analysis solving method; s2, constructing a transient stability criterion for the power angle track of the network-forming converter based on a phase plane method; and S3, under the change of the power grid intensity, verifying the transient power angle track of the grid-forming converter. According to the method, accurate analysis of the transient stability of the network construction type wind power converter is achieved, and accurate judgment of the transient stability of the network construction type wind power converter is achieved through the constructed transient stability criterion.

Description

Technical Field

[0001] The invention belongs to the technical field of grid-type wind power converter control and relates to a transient stability analysis method for a grid-type wind power converter. Background Art

[0002] Grid voltage drop is the most frequent and most serious type of grid fault among grid faults. The traditional grid-following control method directly controls the current and has good transient stability. Although the grid-building control technology can respond to and support the voltage and frequency of the power system, it can show better adaptability in power grids with weak power system strength and low physical inertia. However, since the grid-building control is equivalent to a voltage source to the outside, during the grid fault, the grid-building control cannot directly control the current, resulting in the instantaneous current exceeding the limit. The addition of a virtual impedance limiting link to the current will also affect the transient characteristics of the system, resulting in the system's transient stability characteristics under the grid symmetrical fault being different from the grid-building control.

[0003] Considering the influence of saturated nonlinear system caused by current limiting control, the synchronous unit of grid-type wind power converter will become a more complex high-order system, which will inevitably pose a severe challenge to transient stability analysis. At present, the singular perturbation method is used to consider the high order and strong conservatism of the model, and cannot be directly applied to solve the transient power angle equation of grid-type wind power converter with complex perturbation function. The transient energy method can be used to study the influence of grid-type control parameters on transient stability, but the conservatism is lower. Therefore, it is urgent to carry out research on transient stability analysis of grid-type wind power converters to provide theoretical and technical support for transient stability analysis of grid-type power electronic equipment connected to large power grids. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide a transient stability analysis method for a grid-type wind power converter, aiming to achieve accurate analysis of the transient stability of the grid-type wind power converter, and to construct a transient stability criterion to achieve accurate judgment of the transient stability of the grid-type wind power converter.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A transient stability analysis method for a grid-connected wind power converter comprises the following steps:

[0007] S1: Based on the standardized power angle swing equation, different linear solvable equations and nonlinear perturbation functions are decomposed to provide a standardized model for the application of parameter perturbation analytical solution methods;

[0008] S2: Construct transient stability criterion for power angle trajectory of grid-connected converter based on phase plane method;

[0009] S3: Verify the transient power angle trajectory of the grid-connected converter under the change of grid strength.

[0010] Furthermore, the normalized power angle swing equation is:

[0011] Jδ"+Dδ'=P0-P e

[0012] Among them, J is the inertia parameter, D is the damping coefficient, ω g is the grid angular frequency, δ is the power angle, P0 is the active power command value, P e It is the actual output value of active power.

[0013] Further, step S1 specifically includes the following steps:

[0014] The nonlinear second-order differential equation after the second-order Taylor expansion fitting of sinδ at δ0 is:

[0015] δ"+Aδ'=Bδ 3 +Cδ 2 +Dδ+E

[0016] Where δ is the power angle and the coefficients are defined as follows:

[0017]

[0018] Among them, J is the inertia parameter, D is the damping coefficient, U g is the grid voltage, X g is the grid reactance, ω g is the grid angular frequency, δ is the power angle, P0 is the active power command value, P e is the actual output value of active power, and E is the potential inside the virtual synchronous control.

[0019] Furthermore, in step S2, the first oscillation amplitude of the power angle is compared with the position of the unstable equilibrium point δ2 as a condition for determining whether the system is transiently stable or not.

[0020] Further, step S2 specifically includes the following steps:

[0021] S21: When the grid-forming converter is working normally, the operation control parameters and the grid status are collected;

[0022] S22: Use multi-scale method to solve transient power angle trajectory;

[0023] S23: solving the first oscillation amplitude δ1 and the unstable equilibrium point δ2, and calculating the position difference Δ=δ1-δ2 between the first oscillation and the unstable equilibrium point;

[0024] S24: Determine whether the transient stability criterion Δ≤0 is satisfied. If so, it indicates that the system is stable. If not, it indicates that the system is transiently unstable.

[0025] Furthermore, the operation control parameters and the grid status are collected in step S21, including the inertia parameter J, the damping coefficient D, the grid voltage U g , grid reactance X g , grid angular frequency ω g , active power command value P0, active power actual output value P e , virtual synchronous control internal potential E, impedance angle

[0026] Further, the multi-scale method is used to solve the transient power angle trajectory in step S22, and the steps are as follows:

[0027] Take two time scales t0 and t1, and treat different time scales as independent quantities, expressed as:

[0028]

[0029] Where t is the running time and ε is the perturbation parameter.

[0030] So let the analytical solution be in the form of

[0031] δ(t,ε)=δ0(t)+εδ1(t)+o(ε 2 )

[0032] Among them, δ0(t) is the solution corresponding to time scale t0, δ1(t) is the solution corresponding to time scale t1, and o(ε 2 ) is a high-order term of ε. Substitute it into the quasi-linear differential equation and arrange the coefficients of the same power to be equal, and get the perturbation equation. After eliminating the long-term term and separating the real and imaginary parts, we get the mathematical analytical model of the power angle:

[0033]

[0034] Among them, J is the inertia parameter, D is the damping coefficient, U g is the grid voltage, X g is the grid reactance, ω g is the grid angular frequency, δ is the power angle, P0 is the active power command value, P e is the actual output value of active power, E is the potential inside the virtual synchronous control, is the impedance angle.

[0035] From the power angle trajectory analytical model, it can be obtained that the transient power angle trajectory is related to the drop amplitude, active power reference, power loop control parameters, etc. By changing the initial state, control parameters and external large disturbances, the transient operation trajectory of the grid-connected converter can be characterized.

[0036] The beneficial effects of the present invention are:

[0037] (1) The transient stability criterion proposed by the present invention based on the power angle trajectory can conveniently obtain the influence of the damping coefficient, reactive voltage coefficient, drop depth and active power instruction on the transient stability of the system.

[0038] (2) The present invention considers complex perturbation parameters to solve the power angle trajectory equation with good accuracy, and adopts the phase plane method to use the difference between the first oscillation amplitude and the unstable equilibrium point as the transient stability criterion, which can realize accurate analysis of the transient power angle trajectory under different working conditions of power grid strength. Compared with other methods, it can more accurately analyze the transient stability of grid-type wind power converters.

[0039] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0041] Figure 1 It is a flow chart for transient stability assessment of grid-type converter;

[0042] Figure 2 This is the transient stability analysis result diagram of damping coefficient change;

[0043] Figure 3 This is the transient stability analysis result diagram of the reactive voltage coefficient change;

[0044] Figure 4 This is the transient stability analysis result diagram under the change of voltage sag depth;

[0045] Figure 5 This is the transient stability analysis result diagram of the active power command value change;

[0046] Figure 6 This is a comparison diagram of transient stability boundary analysis between the method of the present invention and the other three methods; DETAILED DESCRIPTION

[0047] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0048] Among them, the drawings are only used for illustrative explanations, and they only represent schematic diagrams rather than actual pictures, and should not be understood as limitations on the present invention. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0049] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "front", "rear", etc. indicate the orientation or position relationship, they are based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the terms describing the position relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0050] like Figure 1 As shown, the present invention provides a transient stability analysis method of a grid-connected converter based on a power angle trajectory. The method comprises the following steps:

[0051] S1: The power angle swing equation can be described as a second-order nonlinear differential equation

[0052] Jδ"+Dδ'=P0-P e

[0053] Based on the above standardized swing equation, it can be decomposed into different linear solvable equations and nonlinear perturbation functions as needed, thereby providing a standardized model for the application of parameter perturbation analytical solution methods.

[0054] The nonlinear second-order differential equation after fitting the second-order Taylor expansion of sinδ at δ0 is:

[0055] δ"+Aδ'=Bδ 3 +Cδ 2 +Dδ+E

[0056] S2: Based on the phase plane method, the transient stability criterion is constructed for the power angle trajectory of the grid-connected converter. Figure 2 As shown in the figure, the first oscillation amplitude of the power angle is compared with the position of the unstable equilibrium point UEPδ2 as a condition for judging whether the system is transiently stable or not. The following steps are performed:

[0057] S21: When the grid-forming converter is working normally, collect the operation control parameters and grid status, including inertia parameter J, damping coefficient D, grid voltage U g , grid reactance X g , grid angular frequency ω g , active power command value P0, active power actual output value P e , virtual synchronous control internal potential E, impedance angle

[0058] S22: Use the multi-scale method to solve the transient power angle trajectory. The steps are as follows:

[0059] Take two time scales t0 and t1, and treat different time scales as independent quantities

[0060]

[0061] So let the analytical solution be in the form of

[0062] δ(t,ε)=δ0(t)+εδ1(t)+o(ε 2 )

[0063] Substitute it into the quasi-linear differential equation, and arrange the coefficients of the same power to be equal, and get the perturbation equation. After eliminating the long-term terms and separating the real and imaginary parts, we get the mathematical analytical model of the power angle.

[0064]

[0065] S23: solving the first oscillation amplitude δ1 and the unstable equilibrium point δ2, and calculating the position difference Δ=δ1-δ2 between the first oscillation and the unstable equilibrium point;

[0066] S24: Determine whether the transient stability criterion Δ≤0 is satisfied. If so, it indicates that the system is stable. If not, it indicates that the system is transiently unstable.

[0067] S3: In order to verify the correctness of the transient stability assessment process based on the power angle trajectory of the present invention, the transient power angle trajectory verification of the grid-connected converter under the change of grid strength is carried out.

[0068] like Figure 2-5As shown in the figure, when the damping coefficient increases, the damping coefficient in the grid-type power control loop affects the transient stability by affecting the equivalent damping area. The larger the equivalent damping area during the fault period, the better the transient stability of the system. When the reactive voltage coefficient increases, the reactive voltage coefficient in the grid-type power control loop affects the transient stability by changing the reactive power output of the system. When the drop depth is the same, the reactive power emitted by the system increases, the effect of supporting the grid voltage recovery is enhanced, and the transient stability of the system is improved. When the grid voltage drop depth increases, the imbalance degree of input and output power of the grid-type control system increases, and the transient stability of the system decreases. When the active power command value decreases, the imbalance degree of input and output power of the grid-type control system decreases, and the transient stability of the system is improved.

[0069] like Figure 6 As shown, compared with the traditional equal-area method and transient energy function method, the method proposed in the present invention is closer to the numerical simulation method on the stability boundary, has a lower degree of conservatism, has a larger stability boundary range, and has more advantages in transient stability analysis.

[0070] A person skilled in the art can understand that all or part of the steps in the above-mentioned embodiment method can be completed by instructing related hardware through a program. The program can be stored in a computer-readable storage medium. When the program is executed, the steps of the method can be implemented. The storage medium, such as ROM / RAM, a disk, an optical disk, etc.

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A transient stability analysis method for grid-connected wind power converters. Features: The following steps are involved: S1: Based on the standardized power angle swing equation, different linear solvable equations and nonlinear perturbation functions are decomposed to provide a standardized model for the application of parameter perturbation analytical solution methods; S2: Construct transient stability criterion for power angle trajectory of grid-connected converter based on phase plane method; S3: Verify the transient power angle trajectory of the grid-connected converter under the change of grid strength.

2. According to the transient stability analysis method of the grid-connected wind power converter of claim 1, Features: The normalized power angle swing equation is: Jδ”+Dδ'=P 0 -P e Among them, J is the inertia parameter, D is the damping coefficient, ω g is the grid angular frequency, δ is the power angle, P 0 is the active power command value, P e It is the actual output value of active power.

3. According to the transient stability analysis method of the grid-connected wind power converter of claim 2, Features: Step S1 specifically includes the following steps: For sinδ in δ 0 The nonlinear second-order differential equation after the second-order Taylor expansion fitting is: δ”+Aδ'=Bδ 3 +Cδ 2 +Dδ+E Where δ is the power angle and the coefficients are defined as follows: Among them, J is the inertia parameter, D is the damping coefficient, U g is the grid voltage, X g is the grid reactance, ω g is the grid angular frequency, δ is the power angle, P 0 is the active power command value, P e is the actual output value of active power, and E is the potential inside the virtual synchronous control.

4. According to the transient stability analysis method of the grid-connected wind power converter of claim 3, Features: In step S2, the first oscillation amplitude of the power angle is compared with the unstable equilibrium point δ 2 The position comparison is used as the condition for judging whether the system is transiently stable or not.

5. According to the transient stability analysis method of grid-connected wind power converter according to claim 4, Features: Step S2 specifically includes the following steps: S21: When the grid-forming converter is working normally, the operation control parameters and the grid status are collected; S22: Use multi-scale method to solve transient power angle trajectory; S23: Solve for the first oscillation amplitude δ 1 and the unstable equilibrium point δ 2 , calculate the position difference between the first oscillation and the unstable equilibrium point Δ=δ 1 -δ 2 ; S24: Determine whether the transient stability criterion Δ≤0 is satisfied. If so, it indicates that the system is stable. If not, it indicates that the system is transiently unstable.

6. According to the transient stability analysis method of grid-connected wind power converter according to claim 5, Features: Step S21 collects the operation control parameters and the grid status, including the inertia parameter J, the damping coefficient D, the grid voltage U g , grid reactance X g , grid angular frequency ω g , active power command value P 0 , the actual output value of active power P e , virtual synchronous control internal potential E, impedance angle 7. The transient stability analysis method for a grid-connected wind power converter according to claim 6, Features: Step S22 uses a multi-scale method to solve the transient power angle trajectory, and the steps are as follows: Take two time scales t 0 and t 1 , different time scales are regarded as independent quantities, expressed as: Where, t is the running time, ε is the perturbation parameter; So let the analytical solution be in the form of δ(t,ε)=δ 0 (t)+ed 1 (t)+o(e 2 ) Among them, δ 0 (t) is the time scale t 0 The corresponding solution, δ 1 (t) is the time scale t 1 The corresponding solution, o(ε 2 ) is the high-order term of ε; substitute it into the quasi-linear differential equation, and arrange the coefficients of the same power to be equal, and get the perturbation equation. After eliminating the long-term term and separating the real and imaginary parts, we get the mathematical analytical model of the power angle: Among them, J is the inertia parameter, D is the damping coefficient, U g is the grid voltage, X g is the grid reactance, ω g is the grid angular frequency, δ is the power angle, P 0 is the active power command value, P e is the actual output value of active power, E is the potential inside the virtual synchronous control, is the impedance angle; From the power angle trajectory analytical model, it can be obtained that the transient power angle trajectory is related to the drop amplitude, active power reference, power loop control parameters, etc. By changing the initial state, control parameters and external large disturbances, the transient operation trajectory of the grid-connected converter can be characterized.

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