A method, system, device, medium, and product for judging the stability of a grid-connected converter.

Abstract

This application discloses a method, system, device, medium, and product for determining the stability of a grid-connected converter, relating to the field of power grid monitoring technology. The method includes: after a power grid fault occurs, determining the initial values ​​of each state variable based on various operating data; based on the initial values ​​of each state variable, solving a fourth-order nonlinear transient model within a preset time period after the initial prediction time, according to a preset time step, to obtain the predicted values ​​of each state variable at each prediction time within the preset time period; plotting a time-varying curve of the grid-connected converter's power angle based on the predicted values ​​of the power angle at each prediction time; the horizontal axis of the time-varying curve of the grid-connected converter's power angle represents the prediction time, and the vertical axis represents the predicted value of the power angle of the grid-connected converter; and determining the stability of the grid-connected converter within the preset time period based on the time-varying curve of the power angle. This application realizes the determination of the stability of a grid-connected converter.

Description

Technical Field

[0001] This application relates to the field of power grid detection technology, and in particular to a method, system, device, medium and product for judging the stability of a grid-connected converter. Background Technology

[0002] As the earliest, most extensively researched, and most widely used new energy equipment, grid-connected converters have attracted widespread attention due to their transient stability. Because grid-connected converters isolate the wind turbine from the grid, wind turbines exhibit almost no inertia within the grid. Therefore, wind turbines rely on phase-locked loops (PLLs) for grid synchronization. When a grid fault is severe, the PLL output phase becomes unstable, causing the wind turbine to lose synchronization with the grid. Most existing research considers the current control loop time constant to be much smaller than the PLL time constant, thus neglecting the current dynamics after a fault. However, for high-power converters used in large wind turbines, the bandwidth of the current control loop, with its low switching frequency, cannot be designed too high. Therefore, the coupling effect between the current control loop and the PLL must be considered. Furthermore, the voltage clamping effect of pulse width modulation (PWM) during a fault can also cause sudden current changes; therefore, the initial fault conditions must be corrected. In severe faults, significant current surges leading to frequency surges further complicate the setting of initial fault conditions, making it difficult to determine the stability of the grid converter. Summary of the Invention

[0003] The purpose of this application is to provide a method, system, device, medium, and product for judging the stability of a grid-connected converter, so as to solve the problem that it is difficult to judge the stability of a grid-connected converter.

[0004] To achieve the above objectives, this application provides the following solution:

[0005] Firstly, this application provides a method for determining the stability of a grid-connected converter, including:

[0006] During power grid operation, operational data at each sampling moment is acquired in real time. The operational data includes: grid voltage, grid line inductance, filter inductance, d-axis component of output current measurement, d-axis component of output current reference value, q-axis component of output current reference value, grid frequency, phase-locked loop (PLL) PI parameters, and current control loop (DCL) PI parameters. The PLL PI parameters include the proportional and integral parameters of the PLL, and the DCL PI parameters include the proportional and integral parameters of the DCL.

[0007] After a grid fault occurs, the initial values ​​of each state variable are determined based on the operating data of the grid in steady state before the fault and the operating data at the first sampling time after the fault. The state variables include: the power angle of the grid-connected converter, the first derivative of the power angle of the grid-connected converter, the d-axis component difference, and the first derivative of the d-axis component difference. The d-axis component difference is the difference between the calculated output current value and the reference output current value of the grid-connected converter.

[0008] Using the time of the fault occurrence as the initial prediction time, based on the initial values ​​of each state variable, and according to a preset time step, the fourth-order nonlinear transient model is solved within a preset time period after the initial prediction time to obtain the predicted values ​​of each state variable at each prediction time within the preset time period; the fourth-order nonlinear transient model is an ordinary differential equation about the state variables and the operating data.

[0009] Based on the predicted values ​​of the power angle of the grid converter at each prediction time, a time-varying curve of the power angle of the grid converter is plotted; the horizontal axis of the time-varying curve of the power angle of the grid converter is the prediction time, and the vertical axis is the predicted value of the power angle of the grid converter.

[0010] Based on the time-varying power angle curve of the grid-connected converter, the stability of the grid-connected converter within the preset time period is determined; the grid-connected converter is connected to the power grid, and the stability is either stable or unstable.

[0011] Optionally, the fourth-order nonlinear transient model includes:

[0012]

[0013] Where x1 is the first state variable; x2 is the second state variable; x3 is the third state variable; x4 is the fourth state variable; and δ is the power angle of the grid converter. The first derivative of the power angle of the grid converter; ΔI d This represents the difference between the d-axis components. It is the first derivative of the difference between the d-axis components; The first derivative of x1; The first derivative of x²; The first derivative of x³; K is the first derivative of x⁴; p U is the proportional parameter of the phase-locked loop; f ω is the grid voltage after the fault occurs; n For grid frequency; L g For power grid line inductance; I dref The d-axis component of the output current reference value; K i K represents the integral parameters of the phase-locked loop.cp K is the proportional parameter of the current control loop. ci Here are the integral parameters of the current control loop; L is the total inductance, L = L g +L f L f This is a filter inductor.

[0014] Optionally, after a power grid fault occurs, the initial values ​​of each state variable are determined based on the operating data of the power grid in steady state before the fault and the operating data at the first sampling time after the fault, including:

[0015] Based on the grid frequency and the grid impedance, d-axis component of the output current measurement, and grid voltage when the grid is in steady state before the fault occurs, the initial value of the power angle of the grid-connected converter is determined.

[0016] Based on the phase-locked loop PI parameters, grid frequency, q-axis and d-axis components of the total voltage difference before and after the fault, grid line inductance and filter inductance at the first sampling moment after the fault, grid voltage after the fault, grid voltage before the fault, and the d-axis component and grid line inductance of the output current measurement at the last sampling moment before the fault, determine the initial value of the first derivative of the power angle of the grid-type converter.

[0017] The initial value of the d-axis component difference is determined based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling time after the fault.

[0018] Based on the proportional parameters of the current control loop and the initial values ​​of the d-axis component differences, the initial values ​​of the first derivatives of the d-axis component differences are determined.

[0019] Optionally, based on the grid frequency and the grid impedance when the grid is in steady state before the fault, the d-axis component of the output current measurement, and the grid voltage, the initial value of the power angle of the grid-connected converter is determined, including:

[0020] Using the power angle calculation formula for a grid-connected converter, the initial value of the power angle of the grid-connected converter is calculated based on the grid frequency, the grid impedance when the grid is in steady state before the fault, the d-axis component of the output current measurement, and the grid voltage. The power angle calculation formula for the grid-connected converter includes:

[0021]

[0022] Where δ0 is the initial value of the power angle of the grid converter; ω n L' is the power grid frequency; gI' is the grid impedance when the grid is in steady state before the fault occurs. d The d-axis component of the output current measurement value of the power grid in steady state before it reaches zero; U' g This is the grid voltage when the grid is in a steady state before the fault occurs.

[0023] Optionally, based on the phase-locked loop (PLL) PI parameters, grid frequency, the q-axis and d-axis components of the total voltage difference before and after the fault, the grid line inductance and filter inductance at the first sampling moment after the fault, the grid voltage after the fault, the grid voltage before the fault, and the d-axis component and grid line inductance of the output current measurement at the last sampling moment before the fault, the initial value of the first derivative of the power angle of the grid-type converter is determined, including:

[0024] Using the formula for calculating the first derivative of the power angle of a grid-connected converter, based on the phase-locked loop (PLL) PI parameters, grid frequency, the q-axis and d-axis components of the total voltage difference before and after a fault, the grid line inductance and filter inductance at the first sampling moment after the fault, the grid voltage after the fault, the grid voltage before the fault, and the d-axis component of the output current measurement and the grid line inductance at the last sampling moment before the fault, the initial value of the first derivative of the power angle of the grid-connected converter is calculated. The formula for calculating the first derivative of the power angle of the grid-connected converter includes:

[0025]

[0026] in, The initial value of the first derivative of the power angle of the grid converter; A and B are intermediate variables; ΔU q_0 L is the q-axis component of the total voltage difference before and after the fault. g+ L represents the inductance of the power grid line at the first sampling moment after the fault occurs. + L represents the total inductance at the first sampling moment after the fault occurs. + =L g+ +L f+ L f+ The filter inductance at the first sampling moment after the fault occurs; U g The grid voltage before the fault occurred; I d- The d-axis component of the output current measurement at the last sampling moment before the fault occurred; L g- The inductance of the power grid line at the last sampling moment before the fault occurred; ΔU d_0 This represents the d-axis component of the total voltage difference before and after the fault occurred.

[0027] Optionally, based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid-type converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling time after the fault, the initial value of the d-axis component difference is determined, including:

[0028] Using the d-axis component difference calculation formula, the initial value of the d-axis component difference is calculated based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid-connected converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling moment after the fault. The d-axis component difference calculation formula includes:

[0029] ΔI d_0 =I d_0 -I dref+ ;

[0030]

[0031] Where, ΔI d_0 The initial value of the d-axis component difference; I d_0 The d-axis component of the calculated output current at the first sampling moment after the fault occurs; I dref+ The d-axis component of the output current reference value at the first sampling moment after the fault occurs.

[0032] Optionally, based on the proportional parameters of the current control loop and the initial values ​​of the d-axis component differences, the initial values ​​of the first derivatives of the d-axis component differences are determined, including:

[0033] Using the formula for calculating the first derivative of the d-axis component difference, the initial value of the first derivative of the d-axis component difference is calculated based on the proportional parameters of the current control loop and the initial value of the d-axis component difference. The formula for calculating the first derivative of the d-axis component difference includes:

[0034]

[0035] in, is the initial value of the first derivative of the difference between the d-axis components.

[0036] Secondly, this application provides a stability assessment system for a grid-connected converter, comprising:

[0037] The data acquisition module is used to acquire operational data in real time at each sampling moment during power grid operation. The operational data includes: power grid voltage, power grid line inductance, filter inductance, d-axis component of output current measurement value, d-axis component of output current reference value, q-axis component of output current reference value, power grid frequency, phase-locked loop (PLL) PI parameters, and current control loop (DCL) PI parameters. The PLL PI parameters include the proportional and integral parameters of the PLL, and the DCL PI parameters include the proportional and integral parameters of the DCL.

[0038] An initialization module is used to determine the initial values ​​of each state variable after a grid fault occurs, based on the operating data of the grid when it was in steady state before the fault and the operating data at the first sampling time after the fault. The state variables include: the power angle of the grid-connected converter, the first derivative of the power angle of the grid-connected converter, the d-axis component difference, and the first derivative of the d-axis component difference. The d-axis component difference is the difference between the calculated output current value and the reference output current value of the grid-connected converter.

[0039] The solution module is used to solve the fourth-order nonlinear transient model with the fault occurrence time as the initial prediction time, based on the initial values ​​of each state variable, and according to a preset time step, within a preset time period after the initial prediction time, to obtain the predicted values ​​of each state variable at each prediction time within the preset time period; the fourth-order nonlinear transient model is an ordinary differential equation about the state variables and the operating data.

[0040] The curve plotting module is used to plot the time-varying curve of the power angle of the grid-connected converter based on the predicted values ​​of the power angle of the grid-connected converter at each prediction time; the horizontal axis of the time-varying curve of the power angle of the grid-connected converter is the prediction time, and the vertical axis is the predicted value of the power angle of the grid-connected converter.

[0041] The stability determination module is used to determine the stability of the grid-connected converter within a preset time period based on the power angle time-varying curve of the grid-connected converter; the grid-connected converter is connected to the power grid, and the stability is either stable or unstable.

[0042] Optionally, the fourth-order nonlinear transient model includes:

[0043]

[0044] Where x1 is the first state variable; x2 is the second state variable; x3 is the third state variable; x4 is the fourth state variable; and δ is the power angle of the grid converter. The first derivative of the power angle of the grid converter; ΔI d This represents the difference between the d-axis components. It is the first derivative of the difference between the d-axis components; The first derivative of x1; The first derivative of x²; The first derivative of x³; K is the first derivative of x⁴; p U is the proportional parameter of the phase-locked loop; f ω is the grid voltage after the fault occurs; n For grid frequency; L g For power grid line inductance; I dref The d-axis component of the output current reference value; K i K represents the integral parameters of the phase-locked loop. cp K is the proportional parameter of the current control loop. ci Here are the integral parameters of the current control loop; L is the total inductance, L = L g +L f L f This is a filter inductor.

[0045] Optionally, the initialization module includes:

[0046] The first initialization unit is used to determine the initial value of the power angle of the grid-connected converter based on the grid frequency, the grid impedance when the grid is in steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage.

[0047] The second initialization unit is used to determine the initial value of the first derivative of the power angle of the grid-type converter based on the phase-locked loop PI parameters, grid frequency, q-axis and d-axis components of the total voltage difference before and after the fault, grid line inductance and filter inductance at the first sampling time after the fault, grid voltage after the fault, grid voltage before the fault, and the d-axis component and grid line inductance of the output current measurement at the last sampling time before the fault.

[0048] The third initialization unit is used to determine the initial value of the d-axis component difference based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid converter, and the d-axis component of the grid line inductance, filter inductance and output current reference value at the first sampling time after the fault.

[0049] The fourth initialization unit is used to determine the initial value of the first derivative of the d-axis component difference based on the initial value of the proportional parameter of the current control loop and the initial value of the d-axis component difference.

[0050] Optionally, based on the grid frequency and the grid impedance when the grid is in steady state before the fault, the d-axis component of the output current measurement, and the grid voltage, the initial value of the power angle of the grid-connected converter is determined, including:

[0051] Using the power angle calculation formula for a grid-connected converter, the initial value of the power angle of the grid-connected converter is calculated based on the grid frequency, the grid impedance when the grid is in steady state before the fault, the d-axis component of the output current measurement, and the grid voltage. The power angle calculation formula for the grid-connected converter includes:

[0052]

[0053] Where δ0 is the initial value of the power angle of the grid converter; ω n L' is the power grid frequency; g I' is the grid impedance when the grid is in steady state before the fault occurs. d The d-axis component of the output current measurement value of the power grid in steady state before it reaches zero; U' g This is the grid voltage when the grid is in a steady state before the fault occurs.

[0054] Optionally, based on the phase-locked loop (PLL) PI parameters, grid frequency, the q-axis and d-axis components of the total voltage difference before and after the fault, the grid line inductance and filter inductance at the first sampling moment after the fault, the grid voltage after the fault, the grid voltage before the fault, and the d-axis component and grid line inductance of the output current measurement at the last sampling moment before the fault, the initial value of the first derivative of the power angle of the grid-type converter is determined, including:

[0055] Using the formula for calculating the first derivative of the power angle of a grid-connected converter, based on the phase-locked loop (PLL) PI parameters, grid frequency, the q-axis and d-axis components of the total voltage difference before and after a fault, the grid line inductance and filter inductance at the first sampling moment after the fault, the grid voltage after the fault, the grid voltage before the fault, and the d-axis component of the output current measurement and the grid line inductance at the last sampling moment before the fault, the initial value of the first derivative of the power angle of the grid-connected converter is calculated. The formula for calculating the first derivative of the power angle of the grid-connected converter includes:

[0056]

[0057] in, The initial value of the first derivative of the power angle of the grid converter; A and B are intermediate variables; ΔU q_0 L is the q-axis component of the total voltage difference before and after the fault. g+ L represents the inductance of the power grid line at the first sampling moment after the fault occurs. + L represents the total inductance at the first sampling moment after the fault occurs. + =L g+ +L f+ L f+ The filter inductance at the first sampling moment after the fault occurs; U gThe grid voltage before the fault occurred; I d- The d-axis component of the output current measurement at the last sampling moment before the fault occurred; L g- The inductance of the power grid line at the last sampling moment before the fault occurred; ΔU d_0 This represents the d-axis component of the total voltage difference before and after the fault occurred.

[0058] Optionally, based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid-type converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling time after the fault, the initial value of the d-axis component difference is determined, including:

[0059] Using the d-axis component difference calculation formula, the initial value of the d-axis component difference is calculated based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid-connected converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling moment after the fault. The d-axis component difference calculation formula includes:

[0060] ΔI d_0 =I d_0 -I dref+ ;

[0061]

[0062] Where, ΔI d_0 The initial value of the d-axis component difference; I d_0 The d-axis component of the calculated output current at the first sampling moment after the fault occurs; I dref+ The d-axis component of the output current reference value at the first sampling moment after the fault occurs.

[0063] Optionally, based on the proportional parameters of the current control loop and the initial values ​​of the d-axis component differences, the initial values ​​of the first derivatives of the d-axis component differences are determined, including:

[0064] Using the formula for calculating the first derivative of the d-axis component difference, the initial value of the first derivative of the d-axis component difference is calculated based on the proportional parameters of the current control loop and the initial value of the d-axis component difference. The formula for calculating the first derivative of the d-axis component difference includes:

[0065]

[0066] in, is the initial value of the first derivative of the difference between the d-axis components.

[0067] Thirdly, this application provides a computer device, including: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the stability judgment method for a grid-connected converter as described in any of the above claims.

[0068] Fourthly, this application provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the stability determination method for a grid-connected converter as described in any of the preceding claims.

[0069] Fifthly, this application provides a computer program product, including a computer program that, when executed by a processor, implements the stability judgment method for grid-connected converters as described in any of the preceding claims.

[0070] According to the specific embodiments provided in this application, the following technical effects are disclosed:

[0071] This application discloses a method, system, device, medium, and product for judging the stability of a grid-connected converter. First, a fourth-order nonlinear transient model is constructed. The fourth-order nonlinear transient model is an ordinary differential equation concerning state variables and operating data. The state variables include: the power angle of the grid-connected converter, the first derivative of the power angle, the d-axis component difference, and the first derivative of the d-axis component difference. The d-axis component difference is the difference between the calculated output current and the reference output current. The operating data includes: grid voltage, grid line inductance, filter inductance, the d-axis component of the measured output current, the d-axis component of the reference output current, the q-axis component of the reference output current, grid frequency, phase-locked loop (PLL) PI parameters, and current control loop (DCL) PI parameters. The PLL PI parameters include the proportional and integral parameters of the PLL, and the DCL PI parameters include the proportional and integral parameters of the DCL. Then, during grid operation... During the process, operational data at each sampling moment is acquired in real time. Secondly, after a grid fault occurs, the initial values ​​of each state variable are determined based on the operational data of the grid in steady state before the fault and the operational data at the first sampling moment after the fault. Subsequently, using the fault occurrence moment as the initial prediction moment, the fourth-order nonlinear transient model is solved within a preset time period after the initial prediction moment, according to a preset time step, based on the initial values ​​of each state variable, to obtain the predicted values ​​of each state variable at each prediction moment within the preset time period. Thirdly, based on the predicted values ​​of the power angle of the grid-connected converter at each prediction moment, a time-varying curve of the power angle of the grid-connected converter is plotted. The horizontal axis of the time-varying curve of the power angle of the grid-connected converter represents the prediction moment, and the vertical axis represents the predicted value of the power angle of the grid-connected converter. Finally, based on the time-varying curve of the power angle of the grid-connected converter, the stability of the grid-connected converter within the preset time period is determined. The grid-connected converter is connected to the grid, and its stability is either stable or unstable. This application utilizes the solver ode45 to iteratively solve a fourth-order nonlinear transient model, performs simulation predictions on state variables, and realizes the determination of the stability of the grid-type converter. Attached Figure Description

[0072] To more clearly illustrate the technical solutions in the embodiments of this application or related technologies, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0073] Figure 1 A schematic flowchart of a method for determining the stability of a grid-connected converter provided in an embodiment of this application;

[0074] Figure 2 This is a schematic diagram illustrating the connection between a grid-connected converter and the power grid.

[0075] Figure 3 The equivalent model diagram of VSC considering the coupling effect between SRF-PLL and current control loop;

[0076] Figure 4 This is a quasi-static large-signal model diagram of a PLL;

[0077] Figure 5 A schematic diagram of the equal area criterion without considering sudden changes in current.

[0078] Figure 6 A schematic diagram of the equal area criterion considering sudden changes in current;

[0079] Figure 7 This is a schematic diagram of the current control loop structure;

[0080] Figure 8 A schematic diagram of a synchronization model that takes current dynamics into account;

[0081] Figure 9 The simulation comparison diagram shows the fourth-order nonlinear transient model and the traditional second-order model.

[0082] Figure 10 This is a schematic diagram of the structure of a computer device provided in an embodiment of this application. Detailed Implementation

[0083] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0084] The purpose of this application is to provide a method, system, device, medium, and product for judging the stability of a grid-connected converter, aiming to achieve the judgment of the stability of the grid-connected converter.

[0085] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0086] Before assessing the stability of the grid-connected converter, the following analysis was conducted.

[0087] 1. Analyze the synchronizing and stabilizing mechanism of the grid converter.

[0088] Figure 2 To match the control structure of the grid-type converter. Figure 2In this system, a Synchronous Reference System Phase-Locked Loop (SRF-PLL) is used to detect the phase of the Point of Common Coupling (PCC). dc U is the DC side voltage. c L is the terminal voltage. f For filter inductance, U pcc I is the voltage at PCC. pcc Z is the current injected into the grid by the grid-connected converter at the PCC. g U is the power grid impedance. g For grid voltage, δ pll ω is the power angle of the PLL. pll ω is the angular frequency of the phase-locked loop (PLL). A grid-connected converter designed for stable current control can be viewed as a controlled current source oriented by the PLL.

[0089] The purpose of a grid converter is to generate a terminal voltage U c d-axis component U cq * and q-axis component U cd * Change the d-axis component U of the original terminal voltage cq and q-axis component U cd To control the d-axis component I of the output current d and q-axis component I q Tracking reference value I dref I qref Typically, assessing PLL stability only requires considering the PCC fault voltage. However, when assessing synchronization stability, the grid condition and transmission line impedance must also be considered. This is because line impedance negatively impacts synchronization stability, and neglecting it may lead to erroneous results. In most high-voltage systems, grid impedance is dominated by inductance. Therefore, for simplicity, this application primarily conducts stability analysis under purely inductive grid impedance. Furthermore, a purely inductive grid impedance system represents the worst-case scenario affecting converter transient synchronization stability because the current exhibits purely active current. In summary, to ensure the transient synchronization stability of grid-connected converters under various conditions, this application studies the worst-case scenario, neglecting resistance when considering line impedance.

[0090] To more intuitively describe and express the impact of the current control loop and PLL on the transient synchronization process of VSC, the following will be presented: Figure 2 The configuration in is equivalent to Figure 3 The Thevenin circuit is described, and the block diagrams of the current control loop and PLL are given in detail. The controlled current source corresponding to the grid converter is I... pcc Composed of the work angle δ: I pcc The power angle δ is determined by the current control loop and generated by the phase-locked loop.

[0091] Figure 3 middle, I dq_ref The dq-axis component of the output current reference value; K cp s is the proportional parameter of the current control loop; s is the complex variable in the complex plane; K ci For the integral parameters of the current control loop; U gdq The dq component of the grid voltage; L is the total inductance; I dq The dq-axis component represents the calculated output current value; (x, y) represents the current components in the dq coordinate system, where x is d and y is q; (r, θ) represents the current in polar coordinates, where r is the amplitude and θ is the phase angle. I Let θ be the current vector angle in the dq coordinate system. I =tan -1 (I q / I d );I q The d-axis component of the calculated output current; I d The output current is represented by the q-axis component; e is the natural exponent; j is the imaginary unit; Z g U is the line impedance; pccd For U pcc d-axis component; U pccq For U pcc The q-axis component; δ is the power angle of the grid converter; ω is the first derivative of the power angle of the grid converter; n This refers to the power grid frequency.

[0092] It can be seen that the phase-locked loop passes through U pccq Calculate the output frequency and power angle to ensure that the frequency of the grid-connected converter matches the grid frequency and that the power angle remains stable. During steady-state operation, to achieve stable current control, U... pccq It should equal zero. And when U passes pccq When the phase-locked loop is at 0, it decouples from the active and reactive currents of the grid converter. It is worth noting that due to the presence of harmonics, U... pccq In practice, it is impossible for it to be exactly zero. However, the focus of this application is on baseband synchronization, therefore it is assumed that due to the effect of the PLL, U pccq It is zero in steady state. For example... Figure 4 As shown, the synchronous transient model can be divided into a grid synchronization loop and a self-synchronization loop (assuming the power angle of the PCC is the reference angle, then the phase of the grid voltage is -δ). pll The grid synchronization loop is a negative feedback mechanism used to counteract the positive feedback effect of the self-synchronization loop. For example, when a disturbance occurs in the system causing an over-U... pccq ≠ 0 and not equal to 0, and the change in angular frequency Δω of the phase-locked loop is such that... pll When δ is positive, pllIt will also increase accordingly. If the phase-locked loop can gradually converge to a steady state, it indicates that the Δω in the self-synchronization loop... pll L g I d The incremental energy and the reduction in the grid synchronization loop offset each other, thus reducing the total U. pccq Adjust to zero. However, if δ pll The self-synchronization loop effect, which is already greater than 90°, has not yet been absorbed, Δω pll Still positive, δ pll This will further increase the risk of system instability due to the inability to converge.

[0093] In addition, according to Figure 3 It can also be observed that the synchronization process is related not only to the PLL but also to current dynamics, meaning there is a coupling effect between the phase-locked loop (PLL) and the current control loop (VSC). When the bandwidth of the current control loop is much larger than that of the PLL, i.e., the dynamic response of the current control loop is much faster than that of the PLL, it can indeed be considered that the current can always accurately track the current reference value within the PLL timescale. However, under some special operating conditions, the premise that the bandwidth of the current control loop is much larger than that of the PLL cannot be met, and the current cannot quickly track the reference value after a fault occurs. In this case, the grid-connected converter cannot be regarded as a controlled current source oriented by the PLL, and the existing current abrupt changes will affect the VSC synchronization process.

[0094] 2. Construct an improved fourth-order transient synchronous model (fourth-order nonlinear transient model) that considers the dynamics of the current control loop and frequency jumps.

[0095] Depend on Figure 2 It can be seen that the voltage of PCC in steady state is expressed as:

[0096]

[0097] Where, φ c θ is the line impedance angle; g θ is the phase angle of the grid voltage. c For I pcc The phase angle, θ c =θ I +θ pll .

[0098] Before the failure occurred, the system was stable at this time. pcc =0 and the reactive current reference in steady state is 0, i.e., I pcc =I d =I dref_ According to equation (1), U at the steady-state equilibrium point pcc with U g The angle difference, i.e., the power angle δ0 of the grid converter, is:

[0099]

[0100] Furthermore, the power angle cannot change abruptly before and after the fault, i.e.: δ0 = δ + =δ _ .

[0101] Where, δ + The power angle of the grid-type converter at the instant after the fault occurs (i.e., the first sampling time); δ _ The power angle of the grid-type converter is the instant before the fault occurs (i.e., the last sampling moment).

[0102] U pccq The q-axis component is:

[0103]

[0104] Among them, U f L represents the grid voltage after the fault occurred. g This refers to the inductance of the power grid line.

[0105] In fact, there is a coupling effect between the current control loop and the PLL in the transient synchronization process of VSC. When the bandwidth of the current control loop is much larger than the bandwidth of the PLL, the synchronization process can be considered to be dominated by the PLL alone, and the traditional second-order model can describe the transient synchronization process well. When the bandwidth of the current control loop is not large enough, the transient synchronization process of VSC is also affected by the dynamic ΔI of the d-axis current. d The impact of this makes the second-order model no longer applicable, so it is upgraded to a fourth-order model.

[0106]

[0107] Among them, U pccq_4 I represents the q-axis component of the PCC voltage in the 4th-order model. dref The d-axis component represents the reference value of the output current.

[0108] When performing large-signal analysis on the nonlinear model of a grid-connected converter during transient synchronization, the transient synchronization process is analogous to the motion of a synchronous generator (SG) rotor. By analogy with the equivalent inertia coefficient H and equivalent damping coefficient D in the oscillation equation of the SG, a large-signal model similar to the SG rotor oscillation equation is established. The synchronization process can be described as follows:

[0109]

[0110] in, K is the second derivative of the power angle of the grid converter; p K represents the proportional parameter of the phase-locked loop. i These are the integral parameters of the phase-locked loop.

[0111] Similar to the equal-area critique (EAC) analysis of synchronous generators, ω n L g (ΔI d +I dref ) is the input for VSC, U f sinδ is the output of VSC. Combining the input and output curves, the improved EAC is as follows: Figure 5 and Figure 6 As shown. Figure 5 and Figure 6 In the diagram, point A is the stable operating point before the fault, and point C is the stable operating point after the fault. After the fault occurs, the equivalent input ω n L g (ΔI d +I dref ) and equivalent output U f The deviation between sinδ and sinδ drives the PLL output angular frequency to accelerate or decelerate. The region where the equivalent input is greater than the equivalent output is the acceleration region, and the region where the equivalent input is less than the equivalent output is the deceleration region.

[0112] Without considering sudden current changes: if S1 > S2 (acceleration area is larger than deceleration area), the excess energy that the system cannot absorb will be converted into kinetic energy, leading to instability; if S1 < S2 (acceleration area is smaller than deceleration area), the system can stabilize at a new equilibrium point C after the fault occurs. Considering sudden current changes, the instantaneous I at the fault... d It will increase rapidly and then gradually decay to the reference value. The sudden change in current will cause the input of VSC to increase within a certain period of time, resulting in a larger acceleration area (S3) and a smaller deceleration area (S4). This means that the phase-locked loop is more prone to instability after considering the dynamic effects of the current control loop, and the system can only stabilize after a fault if S3 < S4. Compared with the EAC analysis corresponding to the traditional second-order model, it is clear that considering the difference ΔI of the d-axis components is more effective. d The equivalent model is less conservative and more accurate. To address this issue, a fourth-order nonlinear transient model considering the current dynamics caused by the current control loop is derived and established.

[0113] For U pccq_4 Differentiation yields:

[0114]

[0115] in, For U pccq_4 The first derivative; It is the first derivative of the difference between the d-axis components.

[0116] According to the working principle of a phase-locked loop, the formula for calculating the power angle is:

[0117] δ=∫(K p Upccq +K i U pccq dt+ω n )dt(8)

[0118] Taking the derivative of formula (7) twice consecutively, we get:

[0119]

[0120] in, For U pccq The first derivative.

[0121] When the bandwidth of the current control loop (CLO) is insufficient to negligibly compensate for the bandwidth of the phase-locked loop (PLL), the coupling effect between the CLO and the PLL must be considered. Compared to the fixed current assumption in previous methods, this transient current causes a change in the self-synchronization loop Δω in the quasi-static PLL model. pll L g I d Introducing larger positive feedback leads to δ during the transient period. pll Increasing the negative impact on δ pll Convergence before reaching 90° reduces the stability of the phase-locked loop (PLL). In other words, transient currents can lead to synchronization instability, a phenomenon that the second-order model cannot describe or reflect. Since the current surge is caused by the grid voltage drop, the grid voltage disturbance is used as the input to solve for the current control loop response. The block diagram of the current control loop is shown below. Figure 7 As shown, the transfer function between the output current and the input voltage disturbance of the current control loop is as follows:

[0122]

[0123] This application primarily studies the case where line resistance is neglected; therefore, after considering the coupling effect between the current control loop and the PLL, a state variable ΔI is added. d and ΔI d =I d -I dref .

[0124] Equation (9) can be transformed to obtain equations (10) and (11).

[0125]

[0126] in, The second derivative of the difference between the d-axis components; For U f The first derivative of the q-axis component.

[0127] In one exemplary embodiment, such as Figure 1As shown, a method for judging the stability of a grid-type converter is provided, including:

[0128] Step 1: During the operation of the power grid, acquire the operating data at each sampling time in real time.

[0129] The operating data includes: grid voltage, grid line inductance, filter inductance, d-axis component of output current measurement, d-axis component of output current reference value, q-axis component of output current reference value, grid frequency, phase-locked loop (PLL) PI parameters, and current control loop (DCL) PI parameters. The PLL PI parameters include the proportional and integral parameters of the PLL, and the DCL PI parameters include the proportional and integral parameters of the DCL.

[0130] Step 2: After a power grid failure occurs, determine the initial values ​​of each state variable based on the operating data of the power grid in steady state before the failure and the operating data at the first sampling time after the power grid failure.

[0131] Among them, the state variables include: the power angle of the grid-connected converter, the first derivative of the power angle of the grid-connected converter, the d-axis component difference, and the first derivative of the d-axis component difference. The d-axis component difference is the difference between the calculated output current value and the reference output current value of the grid-connected converter.

[0132] As an optional implementation, step 2 includes:

[0133] Step 21: Based on the grid frequency and the grid impedance, d-axis component of the output current measurement, and grid voltage when the grid is in steady state before the fault occurs, determine the initial value of the power angle of the grid-connected converter.

[0134] As an optional implementation, step 21 includes:

[0135] Using the power angle calculation formula for a grid-connected converter, the initial value of the power angle of the grid-connected converter is calculated based on the grid frequency, the grid impedance when the grid is in steady state before the fault, the d-axis component of the output current measurement, and the grid voltage. The power angle calculation formula for the grid-connected converter includes:

[0136]

[0137] Where δ0 is the initial value of the power angle of the grid converter; ω n L' is the power grid frequency; g I' is the grid impedance when the grid is in steady state before the fault occurs. d The d-axis component of the output current measurement value of the power grid in steady state before it reaches zero; U' g This is the grid voltage when the grid is in a steady state before the fault occurs.

[0138] Step 22: Based on the phase-locked loop PI parameters, grid frequency, q-axis and d-axis components of the total voltage difference before and after the fault, grid line inductance and filter inductance at the first sampling moment after the fault, grid voltage after the fault, grid voltage before the fault, and the d-axis component and grid line inductance of the output current measurement at the last sampling moment before the fault, determine the initial value of the first derivative of the power angle of the grid-type converter.

[0139] As an optional implementation, step 22 includes:

[0140] Using the formula for calculating the first derivative of the power angle of a grid-connected converter, and based on the phase-locked loop (PLL) PI parameters, grid frequency, the q-axis and d-axis components of the total voltage difference before and after a fault, the grid line inductance and filter inductance at the first sampling moment after the fault, the grid voltage after the fault, the grid voltage before the fault, and the d-axis component of the output current measurement and the grid line inductance at the last sampling moment before the fault, the initial value of the first derivative of the power angle of the grid-connected converter is calculated. The formula for calculating the first derivative of the power angle of the grid-connected converter includes:

[0141]

[0142]

[0143] in, The initial value of the first derivative of the power angle of the grid converter; A and B are intermediate variables; ΔU q_0 L is the q-axis component of the total voltage difference before and after the fault. g+ L represents the inductance of the power grid line at the first sampling moment after the fault occurs. + L represents the total inductance at the first sampling moment after the fault occurs. + =L g+ +L f+ L f+ The filter inductance at the first sampling moment after the fault occurs; U g The grid voltage before the fault occurred; I d- The d-axis component of the output current measurement at the last sampling moment before the fault occurred; L g- The inductance of the power grid line at the last sampling moment before the fault occurred; ΔU d_0 This represents the d-axis component of the total voltage difference before and after the fault occurred.

[0144] Step 23: Based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling time after the fault, determine the initial value of the d-axis component difference.

[0145] As an optional implementation, step 23 includes:

[0146] Using the d-axis component difference calculation formula, the initial value of the d-axis component difference is calculated based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid-type converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling moment after the fault. The d-axis component difference calculation formula includes:

[0147] ΔI d_0 =I d_0 -I dref+ (16)

[0148]

[0149] Where, ΔI d_0 The initial value of the d-axis component difference; I d_0 The d-axis component of the calculated output current at the first sampling moment after the fault occurs; I dref+ The d-axis component of the output current reference value at the first sampling moment after the fault occurs.

[0150] Step 24: Based on the proportional parameters of the current control loop and the initial values ​​of the d-axis component differences, determine the initial values ​​of the first derivatives of the d-axis component differences.

[0151] As an optional implementation, step 24 includes:

[0152] Using the formula for calculating the first derivative of the d-axis component difference, the initial value of the first derivative of the d-axis component difference is calculated based on the proportional parameters of the current control loop and the initial value of the d-axis component difference. The formula for calculating the first derivative of the d-axis component difference includes:

[0153]

[0154] in, is the initial value of the first derivative of the difference between the d-axis components.

[0155] Specifically, the derivation process of the calculation formulas for each state variable includes:

[0156] During a severe fault, the terminal voltage of the grid-connected converter cannot change abruptly but remains constant until the next PWM modulation (i.e., PWM voltage clamping). A sudden drop in the grid voltage will cause a significant current jump due to the voltage difference between the two. Under the action of the PLL proportional element, this current jump will also manifest as a frequency jump. In actual circuits, the inductance of the line causes a coupling relationship between current and frequency. Therefore, this part is analyzed in detail theoretically, and the current and frequency jump variables during a fault are given to correct for the system state changes during a severe fault. The frequency jump variable during a fault is expressed as... It can be decomposed into a proportional part and an integral part as follows:

[0157]

[0158] Among them, U pccq_4_0 For U pccq_4 The initial value.

[0159] At the moment of a fault, the VSC terminal voltage cannot change abruptly but remains fixed until the next PWM modulation. The combined effect of the non-abrupt VSC terminal voltage, the fault grid voltage, and the line impedance after the fault causes a change in current.

[0160]

[0161] ΔU d_0 +ΔU q_0 j = U cd+ +U cq+ j-(U f sinδ + +U f cosδ + j)(23)

[0162] The initial value of the first derivative of the power angle of the grid converter; A and B are intermediate variables; ΔU q_0 L is the q-axis component of the total voltage difference before and after the fault. g+ L represents the inductance of the power grid line at the first sampling moment after the fault occurs. + L represents the total inductance at the first sampling moment after the fault occurs. + =L g+ +L f+ L f+ The filter inductance at the first sampling moment after the fault occurs; U g The grid voltage before the fault occurred; I d- The d-axis component of the output current measurement at the last sampling moment before the fault occurred; L g- The inductance of the power grid line at the last sampling moment before the fault occurred; ΔU d_0This represents the d-axis component of the total voltage difference before and after the fault occurred.

[0163] After considering frequency jumps, the fault transient current I 4ord_0 The expression is:

[0164]

[0165] The integral action of the phase-locked loop requires time, and the influence of the last two terms of the integral part of the frequency jump variable at the instant of the fault is ignored. However, the first term has a derivative, and the q-axis component of the current in the current control loop before the fault is known to be 0. The integrated value... It can be represented as:

[0166]

[0167] After considering the PWM clamping effect, the q-axis component of the PCC voltage at the instant of the fault can be written as:

[0168]

[0169] By combining equations (19) and (27), a fourth-order nonlinear transient model is obtained.

[0170] Step 3: Using the time of the fault occurrence as the initial prediction time, based on the initial values ​​of each state variable, solve the fourth-order nonlinear transient model within a preset time period after the initial prediction time according to the preset time step, and obtain the predicted values ​​of each state variable at each prediction time within the preset time period.

[0171] The fourth-order nonlinear transient model is an ordinary differential equation concerning the state variables and the running data.

[0172] Specifically, the fourth-order nonlinear transient model was solved using the MATLAB solver ode45.

[0173] As an optional implementation method, such as Figure 8 As shown, the fourth-order nonlinear transient model includes:

[0174]

[0175]

[0176] Where x1 is the first state variable; x2 is the second state variable; x3 is the third state variable; x4 is the fourth state variable; and δ is the power angle of the grid converter. The first derivative of the power angle of the grid converter; ΔI d This represents the difference between the d-axis components. It is the first derivative of the difference between the d-axis components; The first derivative of x1; The first derivative of x²; The first derivative of x³; K is the first derivative of x⁴; p U is the proportional parameter of the phase-locked loop; f ω is the grid voltage after the fault occurs; n For grid frequency; L g For power grid line inductance; I dref The d-axis component of the output current reference value; K i K represents the integral parameters of the phase-locked loop. cp K is the proportional parameter of the current control loop. ci Here are the integral parameters of the current control loop; L is the total inductance, L = L g +L f L f This is a filter inductor.

[0177] Step 4: Based on the predicted values ​​of the power angle of the grid converter at each prediction time, plot the time-varying curve of the power angle of the grid converter; the horizontal axis of the time-varying curve of the power angle of the grid converter is the prediction time, and the vertical axis is the predicted value of the power angle of the grid converter.

[0178] Step 5: Based on the time-varying power angle curve of the grid-connected converter, determine the stability of the grid-connected converter within a preset time period; the grid-connected converter is connected to the grid, and the stability is either stable or unstable.

[0179] Specifically, when the power angle time-varying curve of a grid-connected converter fluctuates around its steady-state value, it is considered convergent and stable; otherwise, it is considered unstable or not stable. If the power angle fluctuates after a fault but eventually reaches stability, it indicates that the grid-connected converter can maintain transient synchronization stability under this operating condition; conversely, if the power angle fluctuation continues to intensify, it indicates that the grid-connected converter cannot maintain stability under this operating condition and has lost synchronization with the grid. Therefore, the power angle variation parameters obtained above can be used to determine the synchronization stability of the power grid system, thereby solving the problem of transient synchronization stability of the power grid.

[0180] Furthermore, to verify the effectiveness of the strategy proposed in this application, a simulation platform such as MATLAB / Simulink was built. Figure 2 The simplified model of the wind turbine grid connection system shown is illustrated in Table 1.

[0181] Table 1 Simplified Model of Wind Turbine Grid Connection System

[0182]

[0183] Set the grid voltage U of the wind turbine grid connection system gThe voltage drops sharply to 100V, and the waveforms of the power angle δ before and after Simulink simulation are compared with those of the improved transient synchronous fourth-order model considering the dynamics of the current control loop and frequency jumps, and the traditional second-order model. (See the image below.) Figure 9 As shown in the figure, the green curve represents the Simulink simulation results, while the yellow and red curves represent the simulation results of the improved fourth-order transient synchronization model considering the dynamics of the current control loop and the frequency jump, and the traditional second-order model, respectively. It can be seen that the improved fourth-order transient synchronization model considering the dynamics of the current control loop and the frequency jump can more accurately fit the transient synchronization process at the moment of the fault, verifying the correctness of the theoretical results of this application.

[0184] In one exemplary embodiment, a stability assessment system for a grid-connected converter is provided, comprising:

[0185] The data acquisition module is used to acquire operational data in real time at each sampling moment during power grid operation. The operational data includes: grid voltage, grid line inductance, filter inductance, d-axis component of output current measurement, d-axis component of output current reference value, q-axis component of output current reference value, grid frequency, phase-locked loop (PLL) PI parameters, and current control loop (DCL) PI parameters. The PLL PI parameters include the proportional and integral parameters of the PLL, and the DCL PI parameters include the proportional and integral parameters of the DCL.

[0186] The initialization module is used to determine the initial values ​​of each state variable after a grid fault occurs, based on the operating data of the grid when it was in steady state before the fault and the operating data at the first sampling time after the fault. The state variables include: the power angle of the grid-connected converter, the first derivative of the power angle of the grid-connected converter, the d-axis component difference, and the first derivative of the d-axis component difference. The d-axis component difference is the difference between the calculated output current value and the reference output current value of the grid-connected converter.

[0187] The solution module is used to solve the fourth-order nonlinear transient model with the fault occurrence time as the initial prediction time, based on the initial values ​​of each state variable, and according to the preset time step, within a preset time period after the initial prediction time, to obtain the predicted values ​​of each state variable at each prediction time within the preset time period; the fourth-order nonlinear transient model is an ordinary differential equation about the state variables and the operating data.

[0188] The curve plotting module is used to plot the time-varying curve of the power angle of the grid-connected converter based on the predicted values ​​of the power angle of the grid-connected converter at each prediction time. The horizontal axis of the time-varying curve of the power angle of the grid-connected converter is the prediction time, and the vertical axis is the predicted value of the power angle of the grid-connected converter.

[0189] The stability determination module is used to determine the stability of the grid-connected converter within a preset time period based on the power angle time-varying curve of the grid-connected converter; the grid-connected converter is connected to the grid, and the stability is either stable or unstable.

[0190] As an optional implementation, the initialization module includes:

[0191] The first initialization unit is used to determine the initial value of the power angle of the grid-connected converter based on the grid frequency, the grid impedance when the grid is in steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage.

[0192] The second initialization unit is used to determine the initial value of the first derivative of the power angle of the grid-type converter based on the phase-locked loop PI parameters, grid frequency, q-axis and d-axis components of the total voltage difference before and after the fault, grid line inductance and filter inductance at the first sampling moment after the fault, grid voltage after the fault, grid voltage before the fault, and the d-axis component and grid line inductance of the output current measurement at the last sampling moment before the fault.

[0193] The third initialization unit is used to determine the initial value of the d-axis component difference based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid converter, and the d-axis component of the grid line inductance, filter inductance, and output current reference value at the first sampling time after the fault.

[0194] The fourth initialization unit is used to determine the initial value of the first derivative of the d-axis component difference based on the initial value of the proportional parameter of the current control loop and the initial value of the d-axis component difference.

[0195] In one exemplary embodiment, a computer device is provided, including: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement a method for determining the stability of a grid-connected converter.

[0196] In one exemplary embodiment, a computer-readable storage medium is provided having a computer program stored thereon that, when executed by a processor, implements a method for determining the stability of a grid-connected converter.

[0197] In one exemplary embodiment, a computer program product is provided, including a computer program that, when executed by a processor, implements a method for determining the stability of a grid-connected converter.

[0198] In one exemplary embodiment, a computer device is provided, which may be a server or a terminal, and its internal structure diagram may be as follows. Figure 10As shown, this computer device includes a processor, memory, input / output (I / O) interfaces, and a communication interface. The processor, memory, and I / O interfaces are connected via a system bus, and the communication interface is also connected to the system bus via the I / O interfaces. The processor provides computational and control capabilities. The memory includes non-volatile storage media and internal memory. The non-volatile storage media stores the operating system, computer programs, and a database. The internal memory provides the environment for the operating system and computer programs stored in the non-volatile storage media to run. The I / O interfaces are used for exchanging information between the processor and external devices. The communication interface is used for communicating with external terminals via a network connection. When the computer program is executed by the processor, it implements a method for determining the stability of a grid-connected converter.

[0199] Those skilled in the art will understand that Figure 10 The structure shown is merely a block diagram of a portion of the structure related to the present application and does not constitute a limitation on the computer device to which the present application is applied. Specific computer devices may include more or fewer components than those shown in the figure, or combine certain components, or have different component arrangements.

[0200] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, data stored, data displayed, etc.) involved in this application are all information and data authorized by the user or fully authorized by all parties, and the collection, use and processing of the relevant data must comply with relevant regulations.

[0201] Those skilled in the art will understand that all or part of the processes in the methods of the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium, and when executed, it can include the processes of the embodiments of the above methods. Any references to memory, databases, or other media used in the embodiments provided in this application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM).

[0202] The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, etc., and are not limited to these.

[0203] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0204] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. Furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for judging the stability of a grid-connected converter, characterized in that, include: During the operation of the power grid, operational data at each sampling time is acquired in real time; The operating data includes: grid voltage, grid line inductance, filter inductance, d-axis component of output current measurement, d-axis component of output current reference value, q-axis component of output current reference value, grid frequency, phase-locked loop (PLL) PI parameters, and current control loop (DCL) PI parameters; the PLL PI parameters include: proportional and integral parameters of the PLL, and the DCL PI parameters include: proportional and integral parameters of the DCL. After a grid fault occurs, the initial values ​​of each state variable are determined based on the operating data of the grid in steady state before the fault and the operating data at the first sampling time after the fault. The state variables include: the power angle of the grid-connected converter, the first derivative of the power angle of the grid-connected converter, the d-axis component difference, and the first derivative of the d-axis component difference. The d-axis component difference is the difference between the calculated output current value and the reference output current value of the grid-connected converter. Using the time of the fault occurrence as the initial prediction time, based on the initial values ​​of each state variable, and according to a preset time step, the fourth-order nonlinear transient model is solved within a preset time period after the initial prediction time to obtain the predicted values ​​of each state variable at each prediction time within the preset time period; the fourth-order nonlinear transient model is an ordinary differential equation about the state variables and the operating data. Based on the predicted values ​​of the power angle of the grid converter at each prediction time, a time-varying curve of the power angle of the grid converter is plotted; the horizontal axis of the time-varying curve of the power angle of the grid converter is the prediction time, and the vertical axis is the predicted value of the power angle of the grid converter. Based on the time-varying power angle curve of the grid-connected converter, the stability of the grid-connected converter within the preset time period is determined; the grid-connected converter is connected to the power grid, and the stability is either stable or unstable.

2. The method for judging the stability of a grid-connected converter according to claim 1, characterized in that, The fourth-order nonlinear transient model includes: Where x1 is the first state variable; x2 is the second state variable; x3 is the third state variable; x4 is the fourth state variable; and δ is the power angle of the grid converter. The first derivative of the power angle of the grid converter; ΔI d This represents the difference between the d-axis components. It is the first derivative of the difference between the d-axis components; The first derivative of x1; The first derivative of x²; The first derivative of x³; K is the first derivative of x⁴; p U is the proportional parameter of the phase-locked loop; f ω represents the grid voltage after the fault occurs. n For grid frequency; L g For power grid line inductance; I dref The d-axis component of the output current reference value; K i K represents the integral parameters of the phase-locked loop. cp K is the proportional parameter of the current control loop. ci Here are the integral parameters of the current control loop; L is the total inductance, L = L g +L f L f This is a filter inductor.

3. The method for judging the stability of a grid-connected converter according to claim 2, characterized in that, After a power grid fault occurs, based on the operating data of the power grid in steady state before the fault and the operating data at the first sampling time after the fault, the initial values ​​of each state variable are determined, including: Based on the grid frequency and the grid impedance, d-axis component of the output current measurement, and grid voltage when the grid is in steady state before the fault occurs, the initial value of the power angle of the grid-connected converter is determined. Based on the phase-locked loop PI parameters, grid frequency, q-axis and d-axis components of the total voltage difference before and after the fault, grid line inductance and filter inductance at the first sampling moment after the fault, grid voltage after the fault, grid voltage before the fault, and the d-axis component and grid line inductance of the output current measurement at the last sampling moment before the fault, determine the initial value of the first derivative of the power angle of the grid-type converter. The initial value of the d-axis component difference is determined based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling time after the fault. Based on the proportional parameters of the current control loop and the initial values ​​of the d-axis component differences, the initial values ​​of the first derivatives of the d-axis component differences are determined.

4. The method for judging the stability of a grid-connected converter according to claim 3, characterized in that, Based on the grid frequency, the grid impedance when the grid is in steady state before the fault, the d-axis component of the output current measurement, and the grid voltage, the initial value of the power angle of the grid-connected converter is determined, including: Using the power angle calculation formula for a grid-connected converter, the initial value of the power angle of the grid-connected converter is calculated based on the grid frequency, the grid impedance when the grid is in steady state before the fault, the d-axis component of the output current measurement, and the grid voltage. The power angle calculation formula for the grid-connected converter includes: Where δ0 is the initial value of the power angle of the grid converter; ω n L' is the power grid frequency; g I' is the grid impedance when the grid is in steady state before the fault occurs. d The d-axis component of the output current measurement value of the power grid in steady state before it reaches zero; U' g This is the grid voltage when the grid is in a steady state before the fault occurs.

5. The method for judging the stability of a grid-connected converter according to claim 4, characterized in that, Based on the phase-locked loop (PLL) PI parameters, grid frequency, q-axis and d-axis components of the total voltage difference before and after the fault, grid line inductance and filter inductance at the first sampling moment after the fault, grid voltage after the fault, grid voltage before the fault, and the d-axis component of the output current measurement at the last sampling moment before the fault, and grid line inductance, determine the initial value of the first derivative of the power angle of the grid-connected converter, including: Using the formula for calculating the first derivative of the power angle of a grid-connected converter, based on the phase-locked loop (PLL) PI parameters, grid frequency, the q-axis and d-axis components of the total voltage difference before and after a fault, the grid line inductance and filter inductance at the first sampling moment after the fault, the grid voltage after the fault, the grid voltage before the fault, and the d-axis component of the output current measurement and the grid line inductance at the last sampling moment before the fault, the initial value of the first derivative of the power angle of the grid-connected converter is calculated. The formula for calculating the first derivative of the power angle of the grid-connected converter includes: in, The initial value of the first derivative of the power angle of the grid converter; A and B are intermediate variables; ΔU q_0 L is the q-axis component of the total voltage difference before and after the fault. g+ L represents the inductance of the power grid line at the first sampling moment after the fault occurs. + L represents the total inductance at the first sampling moment after the fault occurs. + =L g+ +L f+ L f+ The filter inductance at the first sampling moment after the fault occurs; U g The grid voltage before the fault occurred; I d- The d-axis component of the output current measurement at the last sampling moment before the fault occurred; L g- The inductance of the power grid line at the last sampling moment before the fault occurred; ΔU d_0 This represents the d-axis component of the total voltage difference before and after the fault occurred.

6. The method for judging the stability of a grid-connected converter according to claim 5, characterized in that, Based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling moment after the fault, the initial value of the d-axis component difference is determined, including: Using the d-axis component difference calculation formula, the initial value of the d-axis component difference is calculated based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid-connected converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling moment after the fault. The d-axis component difference calculation formula includes: ΔI d_0 =I d_0 -I dref+ ; Where, ΔI d_0 The initial value of the d-axis component difference; I d_0 The d-axis component of the calculated output current at the first sampling moment after the fault occurs; I dref+ The d-axis component of the output current reference value at the first sampling moment after the fault occurs.

7. The method for judging the stability of a grid-connected converter according to claim 6, characterized in that, Based on the initial values ​​of the proportional parameters of the current control loop and the initial values ​​of the d-axis component differences, the initial values ​​of the first derivatives of the d-axis component differences are determined, including: Using the formula for calculating the first derivative of the d-axis component difference, the initial value of the first derivative of the d-axis component difference is calculated based on the proportional parameters of the current control loop and the initial value of the d-axis component difference. The formula for calculating the first derivative of the d-axis component difference includes: in, is the initial value of the first derivative of the difference between the d-axis components.

8. A stability assessment system for a grid-connected converter, characterized in that, include: The data acquisition module is used to acquire operational data in real time at each sampling moment during the operation of the power grid. The operating data includes: grid voltage, grid line inductance, filter inductance, d-axis component of output current measurement, d-axis component of output current reference value, q-axis component of output current reference value, grid frequency, phase-locked loop (PLL) PI parameters, and current control loop (DCL) PI parameters; the PLL PI parameters include: proportional and integral parameters of the PLL, and the DCL PI parameters include: proportional and integral parameters of the DCL. An initialization module is used to determine the initial values ​​of each state variable after a grid fault occurs, based on the operating data of the grid when it was in steady state before the fault and the operating data at the first sampling time after the fault. The state variables include: the power angle of the grid-connected converter, the first derivative of the power angle of the grid-connected converter, the d-axis component difference, and the first derivative of the d-axis component difference. The d-axis component difference is the difference between the calculated output current value and the reference output current value of the grid-connected converter. The solution module is used to solve the fourth-order nonlinear transient model with the fault occurrence time as the initial prediction time, based on the initial values ​​of each state variable, and according to a preset time step, within a preset time period after the initial prediction time, to obtain the predicted values ​​of each state variable at each prediction time within the preset time period; the fourth-order nonlinear transient model is an ordinary differential equation about the state variables and the operating data. The curve plotting module is used to plot the time-varying curve of the power angle of the grid-connected converter based on the predicted values ​​of the power angle of the grid-connected converter at each prediction time; the horizontal axis of the time-varying curve of the power angle of the grid-connected converter is the prediction time, and the vertical axis is the predicted value of the power angle of the grid-connected converter. The stability determination module is used to determine the stability of the grid-connected converter within a preset time period based on the power angle time-varying curve of the grid-connected converter; the grid-connected converter is connected to the power grid, and the stability is either stable or unstable.

9. The stability judgment system for a grid-connected converter according to claim 8, characterized in that, The fourth-order nonlinear transient model includes: Where x1 is the first state variable; x2 is the second state variable; x3 is the third state variable; x4 is the fourth state variable; and δ is the power angle of the grid converter. The first derivative of the power angle of the grid converter; ΔI d This represents the difference between the d-axis components. It is the first derivative of the difference between the d-axis components; The first derivative of x1; The first derivative of x²; The first derivative of x³; K is the first derivative of x⁴; p U is the proportional parameter of the phase-locked loop; f ω is the grid voltage after the fault occurs; n For grid frequency; L g For power grid line inductance; I dref The d-axis component of the output current reference value; K i K represents the integral parameters of the phase-locked loop. cp K is the proportional parameter of the current control loop. ci Here are the integral parameters of the current control loop; L is the total inductance, L = L g +L f L f This is a filter inductor.

10. The stability judgment system for a grid-connected converter according to claim 9, characterized in that, The initialization module includes: The first initialization unit is used to determine the initial value of the power angle of the grid-connected converter based on the grid frequency, the grid impedance when the grid is in steady state before the fault occurs, the d-axis component of the output current measurement value, and the grid voltage. The second initialization unit is used to determine the initial value of the first derivative of the power angle of the grid-type converter based on the phase-locked loop PI parameters, grid frequency, q-axis and d-axis components of the total voltage difference before and after the fault, grid line inductance and filter inductance at the first sampling time after the fault, grid voltage after the fault, grid voltage before the fault, and the d-axis component and grid line inductance of the output current measurement at the last sampling time before the fault. The third initialization unit is used to determine the initial value of the d-axis component difference based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid converter, and the d-axis component of the grid line inductance, filter inductance and output current reference value at the first sampling time after the fault. The fourth initialization unit is used to determine the initial value of the first derivative of the d-axis component difference based on the initial value of the proportional parameter of the current control loop and the initial value of the d-axis component difference.

11. The stability judgment system for a grid-connected converter according to claim 10, characterized in that, Based on the grid frequency, the grid impedance when the grid is in steady state before the fault, the d-axis component of the output current measurement, and the grid voltage, the initial value of the power angle of the grid-connected converter is determined, including: Using the power angle calculation formula for a grid-connected converter, the initial value of the power angle of the grid-connected converter is calculated based on the grid frequency, the grid impedance when the grid is in steady state before the fault, the d-axis component of the output current measurement, and the grid voltage. The power angle calculation formula for the grid-connected converter includes: Where δ0 is the initial value of the power angle of the grid converter; ω n L' is the power grid frequency; g I' is the grid impedance when the grid is in steady state before the fault occurs. d The d-axis component of the output current measurement value of the power grid in steady state before it reaches zero; U' g This is the grid voltage when the grid is in a steady state before the fault occurs.

12. The stability judgment system for a grid-connected converter according to claim 11, characterized in that, Based on the phase-locked loop (PLL) PI parameters, grid frequency, q-axis and d-axis components of the total voltage difference before and after the fault, grid line inductance and filter inductance at the first sampling moment after the fault, grid voltage after the fault, grid voltage before the fault, and the d-axis component of the output current measurement at the last sampling moment before the fault, and grid line inductance, determine the initial value of the first derivative of the power angle of the grid-connected converter, including: Using the formula for calculating the first derivative of the power angle of a grid-connected converter, based on the phase-locked loop (PLL) PI parameters, grid frequency, the q-axis and d-axis components of the total voltage difference before and after a fault, the grid line inductance and filter inductance at the first sampling moment after the fault, the grid voltage after the fault, the grid voltage before the fault, and the d-axis component of the output current measurement and the grid line inductance at the last sampling moment before the fault, the initial value of the first derivative of the power angle of the grid-connected converter is calculated. The formula for calculating the first derivative of the power angle of the grid-connected converter includes: in, The initial value of the first derivative of the power angle of the grid converter; A and B are intermediate variables; ΔU q_0 L is the q-axis component of the total voltage difference before and after the fault. g+ L represents the inductance of the power grid line at the first sampling moment after the fault occurs. + L represents the total inductance at the first sampling moment after the fault occurs. + =L g+ +L f+ L f+ The filter inductance at the first sampling moment after the fault occurs; U g The grid voltage before the fault occurred; I d- The d-axis component of the output current measurement at the last sampling moment before the fault occurred; L g- The inductance of the power grid line at the last sampling moment before the fault occurred; ΔU d_0 This represents the d-axis component of the total voltage difference before and after the fault occurred.

13. The stability judgment system for a grid-connected converter according to claim 12, characterized in that, Based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling moment after the fault, the initial value of the d-axis component difference is determined, including: Using the d-axis component difference calculation formula, the initial value of the d-axis component difference is calculated based on the d-axis component of the total voltage difference before and after the fault, the grid frequency, the initial value of the first derivative of the power angle of the grid-connected converter, and the d-axis components of the grid line inductance, filter inductance, and output current reference value at the first sampling moment after the fault. The d-axis component difference calculation formula includes: ΔI d_0 =I d_0 -I dref+ ; Where, ΔI d_0 The initial value of the d-axis component difference; I d_0 The d-axis component of the calculated output current at the first sampling moment after the fault occurs; I dref+ The d-axis component of the output current reference value at the first sampling moment after the fault occurs.

14. The stability judgment system for a grid-connected converter according to claim 12, characterized in that, Based on the initial values ​​of the proportional parameters of the current control loop and the initial values ​​of the d-axis component differences, the initial values ​​of the first derivatives of the d-axis component differences are determined, including: Using the formula for calculating the first derivative of the d-axis component difference, the initial value of the first derivative of the d-axis component difference is calculated based on the proportional parameters of the current control loop and the initial value of the d-axis component difference. The formula for calculating the first derivative of the d-axis component difference includes: in, is the initial value of the first derivative of the difference between the d-axis components.

15. A computer device comprising: A memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that the processor executes the computer program to implement the stability determination method for a grid-connected converter according to any one of claims 1-7.

16. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the computer program implements the stability determination method for the grid-type converter as described in any one of claims 1-7.

17. A computer program product, comprising a computer program, characterized in that, When executed by a processor, the computer program implements the stability determination method for the grid-type converter as described in any one of claims 1-7.

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