Network configuration type converter fault transient modeling method and system based on frequency shift transformation

Abstract

The application is a network-based converter fault transient modeling method and system based on frequency shift transformation, which comprises the following steps: S1, establishing a transient model of positive and negative sequence separation control link in dq coordinate system; S2, calculating active and reactive power to obtain frequency and inner loop voltage reference value; S3, establishing a closed-loop control model of positive and negative sequence inner loop to primary loop; S4, establishing a transient equation of converter terminal and grid side, performing frequency shift transformation on negative sequence component with double frequency, and constructing a frequency shift matrix of input and output relationship of converter transient; S5, linearizing the time delay link in the frequency shift matrix; S6, solving the linearized matrix, decoupling the positive and negative sequence transient output components, and establishing a linear analytical model of positive and negative sequence transient output voltage and current of the converter. The application realizes analytical solution of the model while having higher modeling accuracy, and establishes a general fault transient model suitable for full fault types and grid-connected and island operation modes.

Description

Technical Field

[0001] This invention relates to the field of transient modeling technology for converter faults, and in particular to a method and system for transient modeling of grid-type converter faults based on frequency shift transform. Background Technology

[0002] With the increasing penetration of new energy power generation, represented by wind and solar power, into my country's power system, the power system exhibits characteristics such as low inertia and weak damping, resulting in a significant decrease in its synchronous support and anti-interference capabilities in the face of large disturbances and faults. Grid-type converters, by simulating the operating characteristics of synchronous generators, can autonomously construct voltage amplitude and phase, providing active inertia and frequency support for the system, and have become a key technological path to improve the stability of high-proportion new energy power systems.

[0003] However, when a power grid fault occurs, the various control components of the converter are deeply coupled during the fault transient process, causing the converter to exhibit highly nonlinear dynamic behavior in the tens of milliseconds following the fault. Current technology has not yet achieved the construction of a complete fault transient model, resulting in models that are not refined enough and difficult to use for transient characteristic analysis and engineering applications. The fault transient model is the core foundation for analyzing the extreme operating condition behavior of the converter, and its role spans multiple levels of equipment design, system analysis, and protection control.

[0004] In converter transient modeling, current methods treat the converter as a voltage source in series impedance, essentially maintaining a steady-state model externally. This neglects fast-dynamic elements such as the voltage-current inner loop and positive-negative sequence separation, failing to reflect the converter's dynamic characteristics during the transient period after a fault. Regarding fault transient modeling, existing research primarily employs electromagnetic transient simulation, state-space modeling, and time-domain analysis to attempt to reproduce the dynamic behavior of the entire fault process. Electromagnetic simulation analysis studies the fault current of grid-type converters based on simulation results. However, this method suffers from time-consuming numerical analysis and cannot obtain the analytical form of the converter's transient output components after a fault, only providing a qualitative analysis of its external output characteristics. State-space modeling, when analyzing power system stability, suffers from insufficient accuracy in characterizing the converter's transient characteristics in the early stages of a fault because some simplified models neglect the nonlinear switching and transient coupling effects of control loops and are unsuitable for asymmetric fault conditions. The analytical method of time-domain equations can obtain the analytical results of converter fault current. However, in order to avoid complex calculations, this method requires simplification of many internal control links during modeling. The analytical results are not accurate enough and the attenuated harmonic components are easily ignored.

[0005] For transient modeling of faults in grid-connected converters, existing technologies mainly suffer from low modeling accuracy, slow computation speed, and poor versatility. Firstly, the delay in the positive-sequence separation stage prevents complete decoupling of the positive and negative sequence components, leading to mutual coupling and the generation of second harmonic components. This results in errors between the inner-loop control feedforward and the actual values ​​in the primary circuit. Existing modeling methods cannot decouple this coupling effect mechanistically, resulting in insufficient accuracy of the transient waveform within tens of milliseconds after a fault. Secondly, the nonlinear characteristics of the delay stage prevent the transient model from obtaining analytical solutions, leading to high computational complexity and an inability to quantify the decaying harmonic components. Finally, existing models cannot simultaneously adapt to symmetrical and asymmetrical faults in grid-connected and islanded operation modes, failing to meet the requirements of high-proportion renewable energy power systems for fault-fast transient characteristic analysis and rapid protection setting.

[0006] Given the shortcomings of existing technologies, there is an urgent need to propose a modeling method applicable to fault transients in grid-type converters. This method should focus on the rapid transient process after a fault occurs, and by establishing a transient model with analytical capabilities, quantitatively express the transient characteristics of the fault. This will provide solid theoretical support and technical basis for constructing fast protection schemes, improving the operational reliability of grid-type converters, revealing harmonic mechanisms, and analyzing converter stability. Summary of the Invention

[0007] To address the technical problems existing in the prior art, this invention provides a method and system for transient fault modeling of grid-type converters based on frequency shift transform. This invention proposes a positive and negative sequence component decoupling technique based on second harmonic frequency shift transform, which fundamentally solves the coupling problem caused by delays in various control links during transient fault processes, enabling independent solutions for positive and negative sequence components. By using Pade approximation to linearize the delay element, the analytical solution of the model is achieved while maintaining higher modeling accuracy. Finally, a general transient fault model that simultaneously adapts to all fault types and grid-connected and islanded operation modes is established. This model not only provides a basis for fault analysis and protection setting, research on harmonic generation mechanisms, and converter parameter design, but also reduces the difficulty of converter model development and equipment manufacturing.

[0008] The method of this invention is implemented using the following technical solution: a fault transient modeling method for grid-type converters based on frequency shift transform, comprising the following steps:

[0009] S1. By receiving real-time sampling data of converter output voltage, output current and filter inductor current, the positive and negative sequence components of converter voltage and current are separated by the separation control method, and a transient model of the positive and negative sequence separation control loop in the dq coordinate system is established.

[0010] S2. Calculate active and reactive power based on the positive sequence voltage and current components of the separated converter, and obtain the frequency and inner loop voltage reference values ​​through power outer loop and virtual impedance control, respectively.

[0011] S3. Using the inner loop voltage reference value as the inner loop input, establish closed-loop control models from the positive sequence inner loop to the primary loop for both the positive sequence and negative sequence inner loops.

[0012] S4. Establish the transient control equations between the converter terminal and the grid side, combine them with the closed-loop control model in step S3, and perform a second harmonic frequency shift transformation on all negative sequence components to construct the frequency shift matrix of the transient input and output relationship of the converter.

[0013] S5. Linearize the delay element in the frequency shift matrix;

[0014] S6. Solve for the linearized matrix, decouple the positive and negative sequence transient output components, and establish a linear analytical model of the positive and negative sequence transient output voltage and current of the converter.

[0015] The present invention employs the following technical solution: a fault transient modeling system for a grid-type converter based on frequency shift transform, comprising:

[0016] The positive and negative sequence separation modeling module receives real-time sampling data of converter output voltage, output current and filter inductor current, and uses the separation control method to separate the positive and negative sequence components of converter voltage and current, and establishes a transient model of the positive and negative sequence separation control loop in the dq coordinate system.

[0017] The inner loop reference value calculation module calculates active and reactive power using the separated positive sequence voltage and current components of the converter, and obtains frequency and inner loop voltage reference values ​​through power outer loop and virtual impedance control, respectively.

[0018] The inner loop control modeling module establishes closed-loop control models from the positive-sequence and negative-sequence inner loops to the primary loop by using the inner loop voltage reference value as the inner loop input.

[0019] The frequency shift matrix construction module establishes the transient control equations between the converter terminal and the grid side, and performs a second harmonic frequency shift transformation on all negative sequence components to construct the frequency shift matrix of the transient input and output relationship of the converter.

[0020] The delay linearization module is used to linearize the delay elements in the frequency shift matrix;

[0021] The matrix solving module is used to solve the linearized matrix, decouple the positive and negative sequence transient output components, and establish a linear analytical model of the positive and negative sequence transient output voltage and current of the converter.

[0022] The present invention also proposes a storage medium on which a computer program is stored, wherein when the computer program is executed by a processor, the steps of the present invention’s method for transient fault modeling of grid-type converters are implemented.

[0023] The present invention also proposes a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the fault transient modeling method for grid-type converters of the present invention.

[0024] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0025] 1. This invention has high modeling accuracy and takes into account both transient and steady-state characteristics. It treats the grid-type converter during the fault transient period as a model of multiple input response superposition. The modeling process considers the transient characteristics of each control link and the coupling effect between each link, and can accurately characterize the dynamic behavior of the complete transient to steady-state process after the fault.

[0026] 2. This invention achieves complete decoupling of positive and negative sequence, and is applicable to asymmetrical faults. By using frequency shift transformation and positive and negative sequence decoupling technology, the double frequency coupling effect caused by the delay of the positive and negative sequence separation link is eliminated, and the transient output voltage and current of positive and negative sequence after the fault are completely decoupled. It is applicable not only to symmetrical faults, but also to asymmetrical fault conditions.

[0027] 3. This invention is applicable to both grid-connected and islanded operation modes, with a wide range of application scenarios. The fault transient model can be applied to both grid-connected and islanded operation modes of converters, and is not limited by fault type. It is applicable to both symmetrical and asymmetrical fault modeling, providing a unified theoretical basis for fault analysis and protection control in different application scenarios.

[0028] 4. This invention reduces the computational complexity of the model, facilitating engineering applications. By employing the Pade approximation to linearize the delay element, the complex nonlinear time-domain solution problem is transformed into a linear matrix solution problem, realizing an external equivalent transient model of the converter at the machine end. This significantly reduces the computational complexity of the model and improves the engineering practicality of fault transient analysis.

[0029] 5. This invention simultaneously constructs transient models of output voltage and current faults, supports multi-dimensional analysis, and can simultaneously obtain transient output voltage and transient output current models of the converter. The required transient model can be flexibly selected according to specific application scenarios, such as equipment design, protection setting, and harmonic mechanism revelation. Therefore, it can provide comprehensive technical support for equipment design, system analysis, and protection control of grid-type converters. Attached Figure Description

[0030] Figure 1 This is a flowchart of the method of the present invention;

[0031] Figure 2 This is a diagram of the main circuit topology and control structure of a grid-type converter;

[0032] Figure 3This is the positive sequence loop control structure diagram;

[0033] Figure 4 This is a diagram of a negative sequence loop control structure;

[0034] Figure 5 This is the equivalent model diagram of the positive sequence transient output voltage of the converter;

[0035] Figure 6 This is the equivalent model diagram of the negative-sequence transient output voltage of the converter;

[0036] Figure 7 This is a comparison chart of the model calculation results and PSCAD simulation results of the converter's d-axis positive sequence output voltage under a symmetrical fault (positive sequence voltage drop of 0.2 pu);

[0037] Figure 8 This is a comparison chart of the model calculation results and PSCAD simulation results of the converter's q-axis positive sequence output voltage under a symmetrical fault (positive sequence voltage drop of 0.2 pu);

[0038] Figure 9 This is a diagram showing the harmonic component distribution of the positive sequence output voltage of phase A of the converter under a symmetrical fault (positive sequence voltage drop of 0.2 pu).

[0039] Figure 10 This is a comparison chart of the model calculation results and PSCAD simulation results of the converter's d-axis positive sequence output voltage under asymmetrical faults (positive sequence voltage drops by 0.2 pu and negative sequence voltage rises by 0.1 pu).

[0040] Figure 11 This is a comparison chart of the model calculation results and PSCAD simulation results of the converter's q-axis positive sequence output voltage under asymmetrical faults (positive sequence voltage drops by 0.2 pu and negative sequence voltage rises by 0.1 pu). Detailed Implementation

[0041] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0042] Example

[0043] like Figure 1 As shown, this embodiment of the fault transient modeling method for grid-type converters based on frequency shift transform includes the following steps:

[0044] S1. By receiving real-time sampling data of converter output voltage, output current and filter inductor current, the positive and negative sequence components of converter voltage and current are separated by the separation control method, and a transient model of the positive and negative sequence separation control loop in the dq coordinate system is established.

[0045] S2. Calculate active and reactive power based on the positive sequence voltage and current components of the separated converter, and obtain the frequency and inner loop voltage reference values ​​through power outer loop and virtual impedance control, respectively.

[0046] S3. Using the inner loop voltage reference value as the inner loop input, establish closed-loop control models from the positive sequence inner loop to the primary loop for both the positive sequence and negative sequence inner loops.

[0047] S4. Establish the transient control equations between the converter terminal and the grid side, combine them with the closed-loop control model in step S3, and perform a second harmonic frequency shift transformation on all negative sequence components to construct the frequency shift matrix of the transient input and output relationship of the converter.

[0048] S5. Linearize the delay element in the frequency shift matrix;

[0049] S6. Solve for the linearized matrix, decouple the positive and negative sequence transient output components, and establish a linear analytical model of the positive and negative sequence transient output voltage and current of the converter.

[0050] In this embodiment, the main circuit topology and control structure of the grid converter are as follows: Figure 2 As shown. According to Figure 2 The converter main circuit topology and control structure diagram are shown below. After a fault, the positive and negative sequence separation must be performed first. Preferably, step S1 uses the T / 4 delay method to perform positive and negative sequence separation. However, the control process inside the converter is completed in the dq coordinate system. Therefore, it is necessary to establish a transient model in the dq coordinate system, as shown below:

[0051] (1)

[0052] in, Represents the composite vector along the dq axes, x d x represents the actual value of the d-axis voltage or current. q The actual value of the q-axis voltage or current, where j is the imaginary unit, and x... dq Including converter output voltage u dq Output current i dq and converter filter inductor current i Ldq ; For the corresponding x dq The measured values ​​after separation of positive and negative sequences are indicated by the superscript "+" for positive sequence components and "-" for negative sequence components. s is the Laplace operator, T is the grid period, and ω is the converter angular frequency.

[0053] Specifically, in this embodiment, the specific process of step S2 is as follows:

[0054] S21. Perform power calculations, using the positive and negative sequence separated voltage and current components to calculate active and reactive power:

[0055] (2)

[0056] Where P is active power and Q is reactive power; and These are the measured values ​​of the positive sequence voltage and current on the d-axis obtained from the positive and negative sequence separation process, respectively. and These are the measured values ​​of the positive-sequence voltage and current on the q-axis obtained from the positive-negative sequence separation process, respectively.

[0057] S22. Calculate frequency and voltage reference values ​​using the power outer loop:

[0058] By simulating the rotor motion equation of the synchronous generator using the active-frequency loop, the output frequency of the converter after a fault is obtained, and its expression is as follows:

[0059] (3)

[0060] Where J is the virtual moment of inertia; ω0 is the rated angular frequency of the converter; P ref This is the active power reference value; D p θ is the damping coefficient; θ is the output phase of the converter; t is time; ω is the angular frequency of the converter.

[0061] The voltage reference value is calculated using the reactive power-voltage loop, and its expression is as follows:

[0062] (4)

[0063] in, and These are the voltage reference values ​​for the d-axis and q-axis, respectively; K v Q is the reactive power-voltage droop factor. ref E0 is the reactive power reference value; E0 is the voltage command value.

[0064] S23. The inner loop input voltage is obtained through virtual impedance control as follows:

[0065] (5)

[0066] Among them, E dq This is the reference value for the inner loop input voltage; e dq R is the output voltage of the reactive power control circuit; v and L v These are virtual resistance and inductance, respectively; This is the positive sequence output current of the converter.

[0067] like Figure 3 and Figure 4 As shown, in this embodiment, the closed-loop control model established in step S3 for the positive and negative sequence inner loops to the primary loop is as follows:

[0068] (6)

[0069] in, , and These are the positive sequence output voltage, output current, and filter inductor current of the converter, respectively. , and The positive and negative order separation steps are obtained respectively. , and The corresponding measured value; , and These represent the negative sequence output voltage, output current, and filter inductor current of the converter, respectively; L f For the filter inductor, R f C is the parasitic resistance of the filter inductor. f For filtering capacitors; For the transfer function of the voltage loop PI controller, k pu k is the voltage loop proportionality coefficient. iu The voltage loop integral coefficient; For the transfer function of the current loop PI controller, k pi k is the proportionality coefficient of the current loop. ii E is the integral coefficient of the current loop; dq This is the reference value for the inner loop input voltage.

[0070] Specifically, in this embodiment, the specific process of step S4 is as follows:

[0071] S41. Combining the output variables from step S3 , , , , and To decouple the converter terminal and grid side by adding control equations for positive and negative order, the expression is as follows:

[0072] (7)

[0073] Among them, R g and L g These are the resistor and inductor connecting the converter to the grid side, respectively; and These represent the positive-sequence and negative-sequence voltage disturbances following a grid-side fault;

[0074] S42. By performing a double frequency shift on the negative sequence components in equations (6) and (7), the frequency shift matrix of the transient input and output relationship of the converter after the fault is obtained as follows:

[0075] (8)

[0076] Among them, M 11 ~M 66 M represents the coefficients obtained after negative sequence frequency shift transformation. ik The coefficient of the kth variable in the i-th equation in expression (8) is expressed as follows.

[0077] Specifically, in this embodiment, there is a nonlinear delay element in the coefficients of the frequency shift matrix in equation (8). Step S5 linearizes the delay element in the frequency shift matrix using a 3rd-order Pade approximation, and the formula is as follows:

[0078] (9)

[0079] in, This is the delay time.

[0080] Specifically, in this embodiment, the specific process of step S6 is as follows:

[0081] The output voltage and output current of the converter can be solved by matrix calculation based on equation (8). Taking the transient output voltage model of the converter as an example, the linearized matrix is ​​solved to obtain the linear analytical model of the positive sequence transient output voltage and current of the converter as follows:

[0082] (10)

[0083] Where B(s) is the characteristic polynomial of the positive-sequence closed-loop system. C(s) is the characteristic polynomial of the negative-order closed-loop system. ;

[0084] The linear analytical model of the negative-sequence transient output voltage and current of the converter is as follows:

[0085] (11)

[0086] From equations (10) and (11), it can be seen that the fault transient model established in this invention equates the grid-type converter to a linear system superimposed with responses from multiple independent input sources: the positive-sequence transient output voltage and current are jointly determined by three input components: the inner loop reference voltage, the grid positive-sequence voltage, and the grid negative-sequence voltage, while the negative-sequence transient output voltage and current are determined only by the grid negative-sequence voltage. For symmetrical faults, Setting the values ​​to 0 yields the transient model of the output voltage and current. For converters operating in grid-connected or islanded mode, the results are calculated using the control equations. The amount of disturbance change and the positive and negative sequence voltage on the grid side after monitoring the fault and After determining the disturbance, simply input it into the model to obtain the corresponding output component. To obtain the damped harmonic components during the transient period, simply convert the output into a time-domain expression. Taking voltage as an example, the final equivalent model of the converter's transient output voltage is as follows: Figure 5 and Figure 6 As shown.

[0087] To verify the accuracy of the fault transient modeling method for grid-connected converters of this invention, a grid-connected converter model was built in PSCAD, and symmetrical and asymmetrical fault disturbance tests were performed. Since the converter mainly outputs the positive-sequence component during a fault, the positive-sequence transient output voltage model of the converter was used as an example. During the symmetrical fault verification, a symmetrical fault was set to occur at t=1s, and the fault voltage dropped by 0.2 pu. The model calculation results of the dq-axis positive-sequence voltage were compared with the PSCAD simulation results as follows: Figure 7 and Figure 8 As shown. Furthermore, by linearizing the delay element using a third-order Pade, the attenuated harmonic components during the fault transient can be obtained, such as... Figure 9 As shown in the figure. The results show that the proposed fault transient modeling method can accurately reflect the transient process of positive voltage and current after a symmetrical fault, with high model accuracy. It can also quantify the attenuated harmonic components during the transient period. When verifying asymmetrical faults, the asymmetrical fault is set to occur at t=1s. The positive sequence voltage drops by 0.2pu and the negative sequence voltage rises by 0.1pu. The model calculation results of the positive sequence voltage of the dq axis are compared with the PSCAD simulation results as shown in the figure. Figure 10 and Figure 11 As shown, the transient output components of the proposed fault transient modeling method are consistent with the simulation results, indicating that the model built by this method can be accurately applied to asymmetric faults and can reflect the transient change process of the converter output after the fault.

[0088] Based on the same inventive concept, this invention proposes a fault transient modeling system for grid-type converters based on frequency shift transform, comprising:

[0089] The positive and negative sequence separation modeling module receives real-time sampling data of converter output voltage, output current and filter inductor current, and uses the separation control method to separate the positive and negative sequence components of converter voltage and current, and establishes a transient model of the positive and negative sequence separation control loop in the dq coordinate system.

[0090] The inner loop reference value calculation module calculates active and reactive power using the separated positive sequence voltage and current components of the converter, and obtains frequency and inner loop voltage reference values ​​through power outer loop and virtual impedance control, respectively.

[0091] The inner loop control modeling module establishes closed-loop control models from the positive-sequence and negative-sequence inner loops to the primary loop by using the inner loop voltage reference value as the inner loop input.

[0092] The frequency shift matrix construction module establishes the transient control equations between the converter terminal and the grid side, and performs a second harmonic frequency shift transformation on all negative sequence components to construct the frequency shift matrix of the transient input and output relationship of the converter.

[0093] The delay linearization module is used to linearize the delay elements in the frequency shift matrix;

[0094] The matrix solving module is used to solve the linearized matrix, decouple the positive and negative sequence transient output components, and establish a linear analytical model of the positive and negative sequence transient output voltage and current of the converter.

[0095] Furthermore, this invention also proposes a storage medium and a computer device. The storage medium stores a computer program, which, when executed by a processor, implements steps S1-S6 of the grid-type converter fault transient modeling method of this invention. The computer device includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements the grid-type converter fault transient modeling method of this invention, which includes the aforementioned steps S1-S6.

[0096] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for transient fault modeling of a network converter based on frequency shift transform, characterized in that, Includes the following steps: S1. By receiving real-time sampling data of converter output voltage, output current and filter inductor current, the positive and negative sequence components of converter voltage and current are separated by the separation control method, and a transient model of the positive and negative sequence separation control loop in the dq coordinate system is established. S2. Calculate active and reactive power based on the positive sequence voltage and current components of the separated converter, and obtain the frequency and inner loop voltage reference values ​​through power outer loop and virtual impedance control, respectively. S3. Using the inner loop voltage reference value as the inner loop input, establish closed-loop control models from the positive sequence inner loop to the primary loop for both the positive sequence and negative sequence inner loops. S4. Establish the transient control equations between the converter terminal and the grid side, combine them with the closed-loop control model in step S3, and perform a second harmonic frequency shift transformation on all negative sequence components to construct the frequency shift matrix of the transient input and output relationship of the converter. S5. Linearize the delay element in the frequency shift matrix; S6. Solve for the linearized matrix, decouple the positive and negative sequence transient output components, and establish a linear analytical model of the positive and negative sequence transient output voltage and current of the converter.

2. The method for transient fault modeling of a grid-type converter based on frequency shift transform according to claim 1, characterized in that, The separation control method used in step S1 includes the T / 4 delay method, and the established transient model is shown below: （1） in, Represents the composite vector along the dq axis, x d x represents the actual value of the d-axis voltage or current. q The actual value of the q-axis voltage or current, where j is the imaginary unit, and x... dq Including converter output voltage u dq Output current i dq and converter filter inductor current i Ldq ; For the corresponding x dq The measured values ​​after positive and negative sequence separation are indicated by the superscript "+" for positive sequence components and "-" for negative sequence components. s is the Laplace operator, T is the grid period, and ω is the converter angular frequency.

3. The method for transient fault modeling of a grid-type converter based on frequency shift transform according to claim 1, characterized in that, The specific process of step S2 is as follows: S21. Perform power calculations, using the positive and negative sequence separated voltage and current components to calculate active and reactive power: （2） Where P is active power and Q is reactive power; and These are the measured values ​​of the positive sequence voltage and current on the d-axis obtained from the positive and negative sequence separation process, respectively. and These are the measured values ​​of the positive-sequence voltage and current on the q-axis obtained from the positive-negative sequence separation process, respectively. S22. Calculate frequency and voltage reference values ​​using the power outer loop: By simulating the rotor motion equation of the synchronous generator using the active-frequency loop, the output frequency of the converter after a fault is obtained, and its expression is as follows: （3） Where J is the virtual moment of inertia; ω0 is the rated angular frequency of the converter; P ref This is the active power reference value; D p θ is the damping coefficient; θ is the output phase of the converter; t is time; ω is the angular frequency of the converter. The voltage reference value is calculated using the reactive power-voltage loop, and its expression is as follows: （4） in, and These are the voltage reference values ​​for the d-axis and q-axis, respectively; K v Q is the reactive power-voltage droop factor. ref E0 is the reactive power reference value; E0 is the voltage command value. S23. The inner loop input voltage is obtained through virtual impedance control as follows: （5） Among them, E dq This is the reference value for the inner loop input voltage; e dq R is the output voltage of the reactive power control circuit; v and L v These are virtual resistance and inductance, respectively; is the positive sequence output current of the converter; j is the imaginary unit.

4. The method for transient fault modeling of a grid-type converter based on frequency shift transform according to claim 1, characterized in that, In step S3, the closed-loop control models for the positive and negative sequence inner loops to the primary loop are established as follows: （6） in, , and These are the positive sequence output voltage, output current, and filter inductor current of the converter, respectively. , and The positive and negative order separation steps are obtained respectively. , and The corresponding measured value; , and These represent the negative sequence output voltage, output current, and filter inductor current of the converter, respectively; L f For filter inductance, R f C is the parasitic resistance of the filter inductor. f For filtering capacitors; For the transfer function of the voltage loop PI controller, k pu k is the voltage loop proportionality coefficient. iu The voltage loop integral coefficient; For the transfer function of the current loop PI controller, k pi k is the proportionality coefficient of the current loop. ii E is the integral coefficient of the current loop; dq ω is the reference value for the inner loop input voltage; s is the Laplace operator; ω is the converter angular frequency; j is the imaginary unit.

5. The method for transient fault modeling of a grid-type converter based on frequency shift transform according to claim 4, characterized in that, The specific process of step S4 is as follows: S41. Combining the output variables from step S3 , , , , and To decouple the converter terminal and grid side transient control equations, positive and negative sequence equations are added. The expression is as follows: （7） Among them, R g and L g These are the resistor and inductor connecting the converter to the grid side, respectively; and These represent the positive-sequence and negative-sequence voltage disturbances following a grid-side fault. S42. By performing a double frequency shift on the negative sequence components in equations (6) and (7), the frequency shift matrix of the transient input and output relationship of the converter after the fault is obtained as follows: （8） Among them, M 11 ~M 66 M represents the coefficients obtained after negative sequence frequency shift transformation. ik The coefficient of the kth variable in the i-th equation in expression (8) is expressed as follows.

6. The method for transient fault modeling of a grid-type converter based on frequency shift transform according to claim 1, characterized in that, In step S5, the delay element in the frequency shift matrix is ​​linearized using the 3rd order Pade approximation, and the formula is as follows: （9） in, is the delay time; s is the Laplace operator.

7. The method for transient fault modeling of a grid-type converter based on frequency shift transform according to claim 5, characterized in that, The specific process of step S6 is as follows: Solving the linearized matrix, we obtain the following linear analytical model of the positive-sequence transient output voltage and current of the converter: （10） Where B(s) is the characteristic polynomial of the positive-sequence closed-loop system. C(s) is the characteristic polynomial of the negative-order closed-loop system. E dq This is the reference value for the inner loop input voltage; For the transfer function of the voltage loop PI controller, k pu k is the voltage loop proportionality coefficient. iu The voltage loop integral coefficient; For the transfer function of the current loop PI controller, k pi k is the proportionality coefficient of the current loop. ii M is the integral coefficient of the current loop; 11 ~M 66 The coefficients are obtained after negative sequence frequency shift transformation; s is the Laplace operator; ω is the converter angular frequency; j is the imaginary unit; and These represent the positive-sequence and negative-sequence voltage disturbances following a grid-side fault. The linear analytical model of the negative-sequence transient output voltage and current of the converter is as follows: （11）。 8. A fault transient modeling system for a grid-type converter based on frequency shift transform, characterized in that, include: The positive and negative sequence separation modeling module receives real-time sampling data of converter output voltage, output current and filter inductor current, and uses the separation control method to separate the positive and negative sequence components of converter voltage and current, and establishes a transient model of the positive and negative sequence separation control loop in the dq coordinate system. The inner loop reference value calculation module calculates active and reactive power using the separated positive sequence voltage and current components of the converter, and obtains frequency and inner loop voltage reference values ​​through power outer loop and virtual impedance control, respectively. The inner loop control modeling module establishes closed-loop control models from the positive-sequence and negative-sequence inner loops to the primary loop by using the inner loop voltage reference value as the inner loop input. The frequency shift matrix construction module establishes the transient control equations between the converter terminal and the grid side, and performs a second harmonic frequency shift transformation on all negative sequence components to construct the frequency shift matrix of the transient input and output relationship of the converter. The delay linearization module is used to linearize the delay elements in the frequency shift matrix; The matrix solving module is used to solve the linearized matrix, decouple the positive and negative sequence transient output components, and establish a linear analytical model of the positive and negative sequence transient output voltage and current of the converter.

9. A storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the fault transient modeling method for grid-type converters as described in any one of claims 1-7.

10. A computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the fault transient modeling method for grid-type converters as described in any one of claims 1-7.

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